1. Open systems where the open ocean is directly manipulated to enhance biological production, capture atmospheric CO₂, and export the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production (Williamson et al., 2012). Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system (Beal et al., 2018). This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea (Zhai et al., 2020; Kim et al., 2016; Zhu et al., 2017).

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhance biological production, capture atmospheric CO₂, and export the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production (Williamson et al., 2012). Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system (Beal et al., 2018). This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea (Zhai et al., 2020; Kim et al., 2016; Zhu et al., 2017).

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhance biological production, capture atmospheric CO₂, and export the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production ^[2]Williamson, Phillip, et al. “Ocean Fertilization for Geoengineering: A Review of Effectiveness, Environmental Impacts and Emerging Governance.” Process Safety and Environmental Protection, vol. 90, no. 6, Nov. 2012, pp. 475–88, https://doi.org/10.1016/j.psep.2012.10.007. . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system ^[3]Beal, C., et al. “Marine microalgae commercial production improves sustainability of global fisheries and aquaculture.” Scientific reports, vol. 8, no.1, 2018b, pp. 1-8. . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea ^[4]Zhai, Xiaoqian, et al. “Seawater Supplemented with Bicarbonate for Efficient Marine Microalgae Production in Floating Photobioreactor on Ocean: A Case Study of Chlorella Sp.” Science of The Total Environment, vol. 738, Oct. 2020, p. 139439, https://doi.org/10.1016/j.scitotenv.2020.139439. ^[5]Kim, Z.-Hun, et al. “Development of a Floating Photobioreactor with Internal Partitions for Efficient Utilization of Ocean Wave into Improved Mass Transfer and Algal Culture Mixing.” Bioprocess and Biosystems Engineering, vol. 39, no. 5, May 2016, pp. 713–23, https://doi.org/10.1007/s00449-016-1552-6. ^[6]Zhu, Chenba, et al. “Bicarbonate-Based Carbon Capture and Algal Production System on Ocean with Floating Inflatable-Membrane Photobioreactor.” Journal of Applied Phycology, vol. 30, no. 2, Apr. 2018, pp. 875–85, https://doi.org/10.1007/s10811-017-1285-1. .

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhance biological production, capture atmospheric CO₂, and export the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea .

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhanced biological production, capture of atmospheric CO₂, and export of the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhanced biological production, capture of atmospheric CO₂, and export of the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored.).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhanced biological production, capture of atmospheric CO₂, and export of the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production ^[2]Williamson, Phillip, et al. “Ocean Fertilization for Geoengineering: A Review of Effectiveness, Environmental Impacts and Emerging Governance.” Process Safety and Environmental Protection, vol. 90, no. 6, Nov. 2012, pp. 475–88, https://doi.org/10.1016/j.psep.2012.10.007. . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored.).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system ^[3]Beal, C., et al. “Marine microalgae commercial production improves sustainability of global fisheries and aquaculture.” Scientific reports, vol. 8, no.1, 2018b, pp. 1-8. . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea ^[4]Zhai, Xiaoqian, et al. “Seawater Supplemented with Bicarbonate for Efficient Marine Microalgae Production in Floating Photobioreactor on Ocean: A Case Study of Chlorella Sp.” Science of The Total Environment, vol. 738, Oct. 2020, p. 139439, https://doi.org/10.1016/j.scitotenv.2020.139439. ^[5]Kim, Z.-Hun, et al. “Development of a Floating Photobioreactor with Internal Partitions for Efficient Utilization of Ocean Wave into Improved Mass Transfer and Algal Culture Mixing.” Bioprocess and Biosystems Engineering, vol. 39, no. 5, May 2016, pp. 713–23, https://doi.org/10.1007/s00449-016-1552-6. ^[6]Zhu, Chenba, et al. “Bicarbonate-Based Carbon Capture and Algal Production System on Ocean with Floating Inflatable-Membrane Photobioreactor.” Journal of Applied Phycology, vol. 30, no. 2, Apr. 2018, pp. 875–85, https://doi.org/10.1007/s10811-017-1285-1.

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

1. Open systems where the open ocean is directly manipulated to enhanced biological production, capture of atmospheric CO₂, and export of the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored.).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

Visualizing CDR Potential & Technology Readiness Level

1. Open systems where the open ocean is directly manipulated to enhanced biological production, capture of atmospheric CO₂, and export of the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production ^[2]Williamson, Phillip, et al. “Ocean Fertilization for Geoengineering: A Review of Effectiveness, Environmental Impacts and Emerging Governance.” Process Safety and Environmental Protection, vol. 90, no. 6, Nov. 2012, pp. 475–88, https://doi.org/10.1016/j.psep.2012.10.007. . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored.).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system ^[3]Beal, C., et al. “Marine microalgae commercial production improves sustainability of global fisheries and aquaculture.” Scientific reports, vol. 8, no.1, 2018b, pp. 1-8. . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea ^[4]Zhai, Xiaoqian, et al. “Seawater Supplemented with Bicarbonate for Efficient Marine Microalgae Production in Floating Photobioreactor on Ocean: A Case Study of Chlorella Sp.” Science of The Total Environment, vol. 738, Oct. 2020, p. 139439, https://doi.org/10.1016/j.scitotenv.2020.139439. ^[5]Kim, Z.-Hun, et al. “Development of a Floating Photobioreactor with Internal Partitions for Efficient Utilization of Ocean Wave into Improved Mass Transfer and Algal Culture Mixing.” Bioprocess and Biosystems Engineering, vol. 39, no. 5, May 2016, pp. 713–23, https://doi.org/10.1007/s00449-016-1552-6. ^[6]Zhu, Chenba, et al. “Bicarbonate-Based Carbon Capture and Algal Production System on Ocean with Floating Inflatable-Membrane Photobioreactor.” Journal of Applied Phycology, vol. 30, no. 2, Apr. 2018, pp. 875–85, https://doi.org/10.1007/s10811-017-1285-1.

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

Visualizing CDR Potential & Technology Readiness Level

1. Open systems where the open ocean is directly manipulated to enhanced biological production, capture of atmospheric CO₂, and export of the captured carbon to the deep ocean. In open systems, CO₂ fixation is facilitated by the addition of limiting macronutrients (e.g., phosphorus, nitrogen, silica) and/or micronutrients (e.g., iron) to the ocean’s surface to augment biological production ^[2]Williamson, Phillip, et al. “Ocean Fertilization for Geoengineering: A Review of Effectiveness, Environmental Impacts and Emerging Governance.” Process Safety and Environmental Protection, vol. 90, no. 6, Nov. 2012, pp. 475–88, https://doi.org/10.1016/j.psep.2012.10.007. . Open system techniques accelerate natural processes already occurring in the ocean. Most approaches in the open ocean fall into the following two categories (some proposals that do not fit into these categories are also explored.).
   1. Surface nutrient addition: the direct addition of nutrients (macro or micro) into ocean waters in situ to increase microalgal growth 
   2. Nutrient upwelling: artificial upwelling of nutrient-rich deep ocean waters to the surface to increase microalgal growth
2. Closed systems where inputs and growth conditions are controlled, and outputs (microalgae) are harvested within the confines of a pond or a photobioreactor. In closed systems, CO₂ fixation is facilitated by the mixing of required inputs (sunlight, nutrients, CO₂, water) and the introduction of microalgae culture with the intention of reproduction and continuous fixation in a contained system ^[3]Beal, C., et al. “Marine microalgae commercial production improves sustainability of global fisheries and aquaculture.” Scientific reports, vol. 8, no.1, 2018b, pp. 1-8. . This can be accomplished on shore in cultivation ponds or photobioreactors, or afloat in photobioreactors either stationary or towed at sea ^[4]Zhai, Xiaoqian, et al. “Seawater Supplemented with Bicarbonate for Efficient Marine Microalgae Production in Floating Photobioreactor on Ocean: A Case Study of Chlorella Sp.” Science of The Total Environment, vol. 738, Oct. 2020, p. 139439, https://doi.org/10.1016/j.scitotenv.2020.139439. ^[5]Kim, Z.-Hun, et al. “Development of a Floating Photobioreactor with Internal Partitions for Efficient Utilization of Ocean Wave into Improved Mass Transfer and Algal Culture Mixing.” Bioprocess and Biosystems Engineering, vol. 39, no. 5, May 2016, pp. 713–23, https://doi.org/10.1007/s00449-016-1552-6. ^[6]Zhu, Chenba, et al. “Bicarbonate-Based Carbon Capture and Algal Production System on Ocean with Floating Inflatable-Membrane Photobioreactor.” Journal of Applied Phycology, vol. 30, no. 2, Apr. 2018, pp. 875–85, https://doi.org/10.1007/s10811-017-1285-1.

1. 1. Onshore: encompasses more established methods of microalgae cultivation, including photobioreactors, cultivation ponds, and hybrid onshore configurations. In these systems, all inputs are tightly controlled and regulated, and outputs must be directly managed through either storage or utilization of byproducts. While cultivation techniques are well-established and show high technological readiness, storage and utilization pathways remain underdeveloped and scale is a major consideration. Social and environmental risks for closed onshore systems are easier to monitor and mitigate due to the controlled nature of the system. 
   2. Offshore: includes floating photobioreactors (PBRs) that are incorporated into a floating platform which can be stationary or towed behind a ship. In these systems, cultivation occurs in the photobioreactor, inputs are regulated, and outputs can be actively managed or directed. While at sea, these photobioreactors operate much like their onshore counterparts to cultivate microalgae, however nutrients and energy are provided by the ocean water and wave action, respectively. After microalgae are cultivated, they can be sunk into the deep ocean for sequestration or hauled to shore to be used as biomass. This is an area that has garnered much attention in startup communities (see this Y Combinator request for startups), however, little is available about the technologies in the open-sourced or peer reviewed literature.

Visualizing CDR Potential & Technology Readiness Level

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Carbon fixation rates and storage capacity of the microalgae species
  • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae (Li et al., 2022). 

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers (Hazlebeck & Rickman, 2019). Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting (Muylaert et al., 2017). 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds (Morales et al., 2020). Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae (Morales et al., 2020). New techniques with biofilms are in development (Mantzorou & Ververidis, 2018).

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability (Siegel et al., 2021). Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific (Siegel et al., 2021). Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least (Nowicki et al., 2022). Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
  • A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China). Also note work being done to remediate harmful algal blooms and the potential applicability of findings on the efficacy of sinking algal biomass to mCDR research (see BlueGreen Water Technologies as an example).
  • In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
    • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean (Bates et al., 2019). The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate the conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae (Avnimelech et al., 1982; Chen et al., 2018).

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal (Bi & He, 2022; Lee et al., 2020; Wang et al., 2023).
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration (OpenAir, 2022).

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Carbon fixation rates and storage capacity of the microalgae species
  • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae (Li et al., 2022). 

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers (Hazlebeck & Rickman, 2019). Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting (Muylaert et al., 2017). 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds (Morales et al., 2020). Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae (Morales et al., 2020). New techniques with biofilms are in development (Mantzorou & Ververidis, 2018).

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability (Siegel et al., 2021). Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific (Siegel et al., 2021). Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least (Nowicki et al., 2022). Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
  • A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China). Also note work done to remediate harmful algal blooms and the potential applicability of findings on the efficacy of sinking algal biomass to mCDR research (see BlueGreen Water Technologies as an example).
  • In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
    • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean (Bates et al., 2019). The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate the conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae (Avnimelech et al., 1982; Chen et al., 2018).

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal (Bi & He, 2022; Lee et al., 2020; Wang et al., 2023).
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration (OpenAir, 2022).

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Carbon fixation rates and storage capacity of the microalgae species
  • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae (Li et al., 2022). 

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers (Hazlebeck & Rickman, 2019). Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting (Muylaert et al., 2017). 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds (Morales et al., 2020). Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae (Morales et al., 2020). New techniques with biofilms are in development (Mantzorou & Ververidis, 2018).

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability (Siegel et al., 2021). Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific (Siegel et al., 2021). Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least (Nowicki et al., 2022). Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
  • A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
  • In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
    • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean (Bates et al., 2019). The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate the conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae (Avnimelech et al., 1982; Chen et al., 2018).

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal (Bi & He, 2022; Lee et al., 2020; Wang et al., 2023).
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration (OpenAir, 2022).

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Carbon fixation rates and storage capacity of the microalgae species
  • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae (Li et al., 2022). 

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers (Hazlebeck & Rickman, 2019). Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting (Muylaert et al., 2017). 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds (Morales et al., 2020). Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae (Morales et al., 2020). New techniques with biofilms are in development (Mantzorou & Ververidis, 2018).

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ^[7]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ^[8]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ^[9]Nowicki, M., DeVries, T., & Siegel, D. A. (2022). Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles, 36, e2021GB007083. https://doi.org/10.1029/2021GB00708. . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
  • A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
  • In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
    • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ^[10]Bates, E., et al. Role of Clay-Gel Hybrids in Enhancing Carbon Sequestration. Dec. 2019, pp. GC31I-1325, https://ui.adsabs.harvard.edu/abs/2019AGUFMGC31I1325B. . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ^[11]Avnimelech, Yoram, Bill W. Troeger, and Lester W. Reed. "Mutual flocculation of algae and clay: evidence and implications." Science 216.4541 (1982): 63-65. ^[12]Chen C, Pan G, Shi W, Xu F, Techtmann SM, Pfiffner SM and Hazen TC (2018) Clay Flocculation Effect on Microbial Community Composition in Water and Sediment. Front. Environ. Sci. 6:60. doi: 10.3389 fenvs.2018.00060 .

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ^[13]Bi, Z., & He, B. B. (2022). Biochar from microalgae. 3rd Generation Biofuels, 613-637. https://doi.org/10.1016/B978-0-323-90971-6.00025-5 ^[14]Lee, Xin Jiat, et al. “State of Art Review on Conventional and Advanced Pyrolysis of Macroalgae and Microalgae for Biochar, Bio-Oil and Bio-Syngas Production.” Energy Conversion and Management, vol. 210, 2020, p. 112707., https://doi.org/10.1016/j.enconman.2020.112707. ^[15]Wang, Liuwei, et al. “Role of Biochar toward Carbon Neutrality.” Carbon Research, vol. 2, no. 1, 2023, https://doi.org/10.1007/s44246-023-00035-7. .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ^[16]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s. .

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Carbon fixation rates and storage capacity of the microalgae species
  • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae . 

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae . New techniques with biofilms are in development .

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
  • A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
  • In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
    • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae .

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration .

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Carbon fixation rates and storage capacity of the microalgae species
  • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae ^[6]Li, Shengnan, et al. “How to Enhance Carbon Capture by Evolution of Microalgal Photosynthesis?” Separation and Purification Technology, vol. 291, June 2022, p. 120951, https://doi.org/10.1016/j.seppur.2022.120951. . 

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers ^[1]Hazlebeck, David, and Rickman, William. Zobi® based processes for ultra-low energy algal harvesting and dewatering. United States: N. p., 2019. . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting ^[2]Muylaert, K., Bastiaens, L., Vandamme, D., & Gouveia, L. (2017). Harvesting of microalgae: Overview of process options and their strengths and drawbacks. Microalgae-Based Biofuels and Bioproducts, 113-132. https://doi.org/10.1016/B978-0-08-101023-5.00005-4 . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds ^[3]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae ^[4]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . New techniques with biofilms are in development ^[5]Mantzorou A, Ververidis F. Microalgal biofilms: A further step over current microalgal cultivation techniques. Sci Total Environ. 2019 Feb 15;651(Pt 2):3187-3201. doi: 10.1016/j.scitotenv.2018.09.355. Epub 2018 Oct 1. PMID: 30463168. .

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ^[7]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ^[8]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ^[9]Nowicki, M., DeVries, T., & Siegel, D. A. (2022). Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles, 36, e2021GB007083. https://doi.org/10.1029/2021GB00708. . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ^[10]Bates, E., et al. Role of Clay-Gel Hybrids in Enhancing Carbon Sequestration. Dec. 2019, pp. GC31I-1325, https://ui.adsabs.harvard.edu/abs/2019AGUFMGC31I1325B. . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ^[11]Avnimelech, Yoram, Bill W. Troeger, and Lester W. Reed. "Mutual flocculation of algae and clay: evidence and implications." Science 216.4541 (1982): 63-65. ^[12]Chen C, Pan G, Shi W, Xu F, Techtmann SM, Pfiffner SM and Hazen TC (2018) Clay Flocculation Effect on Microbial Community Composition in Water and Sediment. Front. Environ. Sci. 6:60. doi: 10.3389 fenvs.2018.00060 .

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ^[13]Bi, Z., & He, B. B. (2022). Biochar from microalgae. 3rd Generation Biofuels, 613-637. https://doi.org/10.1016/B978-0-323-90971-6.00025-5 ^[14]Lee, Xin Jiat, et al. “State of Art Review on Conventional and Advanced Pyrolysis of Macroalgae and Microalgae for Biochar, Bio-Oil and Bio-Syngas Production.” Energy Conversion and Management, vol. 210, 2020, p. 112707., https://doi.org/10.1016/j.enconman.2020.112707. ^[15]Wang, Liuwei, et al. “Role of Biochar toward Carbon Neutrality.” Carbon Research, vol. 2, no. 1, 2023, https://doi.org/10.1007/s44246-023-00035-7. .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ^[16]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s. .

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae . New techniques with biofilms are in development . 

• Carbon fixation rates and storage capacity of the microalgae species

• • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae . 

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae .

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration .

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or in on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae . New techniques with biofilms are in development . 

• Carbon fixation rates and storage capacity of the microalgae species

• • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae . 

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae .

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration .

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers ^[1]Hazlebeck, David, and Rickman, William. Zobi® based processes for ultra-low energy algal harvesting and dewatering. United States: N. p., 2019. . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting ^[2]Muylaert, K., Bastiaens, L., Vandamme, D., & Gouveia, L. (2017). Harvesting of microalgae: Overview of process options and their strengths and drawbacks. Microalgae-Based Biofuels and Bioproducts, 113-132. https://doi.org/10.1016/B978-0-08-101023-5.00005-4 . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or in on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds ^[3]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae ^[4]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . New techniques with biofilms are in development ^[5]Mantzorou A, Ververidis F. Microalgal biofilms: A further step over current microalgal cultivation techniques. Sci Total Environ. 2019 Feb 15;651(Pt 2):3187-3201. doi: 10.1016/j.scitotenv.2018.09.355. Epub 2018 Oct 1. PMID: 30463168. . 

• Carbon fixation rates and storage capacity of the microalgae species

• • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae ^[6]Li, Shengnan, et al. “How to Enhance Carbon Capture by Evolution of Microalgal Photosynthesis?” Separation and Purification Technology, vol. 291, June 2022, p. 120951, https://doi.org/10.1016/j.seppur.2022.120951. . 

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1,000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ^[7]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ^[8]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ^[9]Nowicki, M., DeVries, T., & Siegel, D. A. (2022). Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles, 36, e2021GB007083. https://doi.org/10.1029/2021GB00708. . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ^[10]Bates, E., et al. Role of Clay-Gel Hybrids in Enhancing Carbon Sequestration. Dec. 2019, pp. GC31I-1325, https://ui.adsabs.harvard.edu/abs/2019AGUFMGC31I1325B. . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ^[11]Avnimelech, Yoram, Bill W. Troeger, and Lester W. Reed. "Mutual flocculation of algae and clay: evidence and implications." Science 216.4541 (1982): 63-65. ^[12]Chen C, Pan G, Shi W, Xu F, Techtmann SM, Pfiffner SM and Hazen TC (2018) Clay Flocculation Effect on Microbial Community Composition in Water and Sediment. Front. Environ. Sci. 6:60. doi: 10.3389 fenvs.2018.00060

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ^[13]Bi, Z., & He, B. B. (2022). Biochar from microalgae. 3rd Generation Biofuels, 613-637. https://doi.org/10.1016/B978-0-323-90971-6.00025-5 ^[14]Lee, Xin Jiat, et al. “State of Art Review on Conventional and Advanced Pyrolysis of Macroalgae and Microalgae for Biochar, Bio-Oil and Bio-Syngas Production.” Energy Conversion and Management, vol. 210, 2020, p. 112707., https://doi.org/10.1016/j.enconman.2020.112707. ^[15]Wang, Liuwei, et al. “Role of Biochar toward Carbon Neutrality.” Carbon Research, vol. 2, no. 1, 2023, https://doi.org/10.1007/s44246-023-00035-7. .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ^[16]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m³, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m³ and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers ^[1]Hazlebeck, David, and Rickman, William. Zobi® based processes for ultra-low energy algal harvesting and dewatering. United States: N. p., 2019. . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting ^[2]Muylaert, K., Bastiaens, L., Vandamme, D., & Gouveia, L. (2017). Harvesting of microalgae: Overview of process options and their strengths and drawbacks. Microalgae-Based Biofuels and Bioproducts, 113-132. https://doi.org/10.1016/B978-0-08-101023-5.00005-4 . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or in on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds ^[3]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae ^[4]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . New techniques with biofilms are in development ^[5]Mantzorou A, Ververidis F. Microalgal biofilms: A further step over current microalgal cultivation techniques. Sci Total Environ. 2019 Feb 15;651(Pt 2):3187-3201. doi: 10.1016/j.scitotenv.2018.09.355. Epub 2018 Oct 1. PMID: 30463168. . 

• Carbon fixation rates and storage capacity of the microalgae species

• • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae ^[6]Li, Shengnan, et al. “How to Enhance Carbon Capture by Evolution of Microalgal Photosynthesis?” Separation and Purification Technology, vol. 291, June 2022, p. 120951, https://doi.org/10.1016/j.seppur.2022.120951. . 

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ^[7]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ^[8]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ^[9]Nowicki, M., DeVries, T., & Siegel, D. A. (2022). Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles, 36, e2021GB007083. https://doi.org/10.1029/2021GB00708. . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ^[10]Bates, E., et al. Role of Clay-Gel Hybrids in Enhancing Carbon Sequestration. Dec. 2019, pp. GC31I-1325, https://ui.adsabs.harvard.edu/abs/2019AGUFMGC31I1325B. . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ^[11]Avnimelech, Yoram, Bill W. Troeger, and Lester W. Reed. "Mutual flocculation of algae and clay: evidence and implications." Science 216.4541 (1982): 63-65. ^[12]Chen C, Pan G, Shi W, Xu F, Techtmann SM, Pfiffner SM and Hazen TC (2018) Clay Flocculation Effect on Microbial Community Composition in Water and Sediment. Front. Environ. Sci. 6:60. doi: 10.3389 fenvs.2018.00060

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ^[13]Bi, Z., & He, B. B. (2022). Biochar from microalgae. 3rd Generation Biofuels, 613-637. https://doi.org/10.1016/B978-0-323-90971-6.00025-5 ^[14]Lee, Xin Jiat, et al. “State of Art Review on Conventional and Advanced Pyrolysis of Macroalgae and Microalgae for Biochar, Bio-Oil and Bio-Syngas Production.” Energy Conversion and Management, vol. 210, 2020, p. 112707., https://doi.org/10.1016/j.enconman.2020.112707. ^[15]Wang, Liuwei, et al. “Role of Biochar toward Carbon Neutrality.” Carbon Research, vol. 2, no. 1, 2023, https://doi.org/10.1007/s44246-023-00035-7. .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ^[16]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Regardless of approach or technology, there are two fundamental questions that make up the “CDR Potential” for a given pathway:

1) How much carbon can this approach sequester?  2) How long can it stay sequestered?

How much carbon can be sequestered?

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m3, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m3 and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers ^[1]Hazlebeck, David, and Rickman, William. Zobi® based processes for ultra-low energy algal harvesting and dewatering. United States: N. p., 2019. . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting ^[2]Muylaert, K., Bastiaens, L., Vandamme, D., & Gouveia, L. (2017). Harvesting of microalgae: Overview of process options and their strengths and drawbacks. Microalgae-Based Biofuels and Bioproducts, 113-132. https://doi.org/10.1016/B978-0-08-101023-5.00005-4 . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or in on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds ^[3]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae ^[4]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . New techniques with biofilms are in development ^[5]Mantzorou A, Ververidis F. Microalgal biofilms: A further step over current microalgal cultivation techniques. Sci Total Environ. 2019 Feb 15;651(Pt 2):3187-3201. doi: 10.1016/j.scitotenv.2018.09.355. Epub 2018 Oct 1. PMID: 30463168. . 

• Carbon fixation rates and storage capacity of the microalgae species

• • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae ^[6]Li, Shengnan, et al. “How to Enhance Carbon Capture by Evolution of Microalgal Photosynthesis?” Separation and Purification Technology, vol. 291, June 2022, p. 120951, https://doi.org/10.1016/j.seppur.2022.120951. . 

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ^[7]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ^[8]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ^[9]Nowicki, M., DeVries, T., & Siegel, D. A. (2022). Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles, 36, e2021GB007083. https://doi.org/10.1029/2021GB00708. . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ^[10]Bates, E., et al. Role of Clay-Gel Hybrids in Enhancing Carbon Sequestration. Dec. 2019, pp. GC31I-1325, https://ui.adsabs.harvard.edu/abs/2019AGUFMGC31I1325B. . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ^[11]Avnimelech, Yoram, Bill W. Troeger, and Lester W. Reed. "Mutual flocculation of algae and clay: evidence and implications." Science 216.4541 (1982): 63-65. ^[12]Chen C, Pan G, Shi W, Xu F, Techtmann SM, Pfiffner SM and Hazen TC (2018) Clay Flocculation Effect on Microbial Community Composition in Water and Sediment. Front. Environ. Sci. 6:60. doi: 10.3389 fenvs.2018.00060

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ^[13]Bi, Z., & He, B. B. (2022). Biochar from microalgae. 3rd Generation Biofuels, 613-637. https://doi.org/10.1016/B978-0-323-90971-6.00025-5 ^[14]Lee, Xin Jiat, et al. “State of Art Review on Conventional and Advanced Pyrolysis of Macroalgae and Microalgae for Biochar, Bio-Oil and Bio-Syngas Production.” Energy Conversion and Management, vol. 210, 2020, p. 112707., https://doi.org/10.1016/j.enconman.2020.112707. ^[15]Wang, Liuwei, et al. “Role of Biochar toward Carbon Neutrality.” Carbon Research, vol. 2, no. 1, 2023, https://doi.org/10.1007/s44246-023-00035-7. .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ^[16]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

How much carbon can be sequestered?

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m3, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m3 and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers ^[1]Hazlebeck, David, and Rickman, William. Zobi® based processes for ultra-low energy algal harvesting and dewatering. United States: N. p., 2019. . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting ^[2]Muylaert, K., Bastiaens, L., Vandamme, D., & Gouveia, L. (2017). Harvesting of microalgae: Overview of process options and their strengths and drawbacks. Microalgae-Based Biofuels and Bioproducts, 113-132. https://doi.org/10.1016/B978-0-08-101023-5.00005-4 . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or in on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds ^[3]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae ^[4]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . New techniques with biofilms are in development ^[5]Mantzorou A, Ververidis F. Microalgal biofilms: A further step over current microalgal cultivation techniques. Sci Total Environ. 2019 Feb 15;651(Pt 2):3187-3201. doi: 10.1016/j.scitotenv.2018.09.355. Epub 2018 Oct 1. PMID: 30463168. . 

• Carbon fixation rates and storage capacity of the microalgae species

• • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae ^[6]Li, Shengnan, et al. “How to Enhance Carbon Capture by Evolution of Microalgal Photosynthesis?” Separation and Purification Technology, vol. 291, June 2022, p. 120951, https://doi.org/10.1016/j.seppur.2022.120951. . 

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ^[7]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ^[8]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ^[9]Nowicki, M., DeVries, T., & Siegel, D. A. (2022). Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles, 36, e2021GB007083. https://doi.org/10.1029/2021GB00708. . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ^[10]Bates, E., et al. Role of Clay-Gel Hybrids in Enhancing Carbon Sequestration. Dec. 2019, pp. GC31I-1325, https://ui.adsabs.harvard.edu/abs/2019AGUFMGC31I1325B. . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ^[11]Avnimelech, Yoram, Bill W. Troeger, and Lester W. Reed. "Mutual flocculation of algae and clay: evidence and implications." Science 216.4541 (1982): 63-65. ^[12]Chen C, Pan G, Shi W, Xu F, Techtmann SM, Pfiffner SM and Hazen TC (2018) Clay Flocculation Effect on Microbial Community Composition in Water and Sediment. Front. Environ. Sci. 6:60. doi: 10.3389 fenvs.2018.00060

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ^[13]Bi, Z., & He, B. B. (2022). Biochar from microalgae. 3rd Generation Biofuels, 613-637. https://doi.org/10.1016/B978-0-323-90971-6.00025-5 ^[14]Lee, Xin Jiat, et al. “State of Art Review on Conventional and Advanced Pyrolysis of Macroalgae and Microalgae for Biochar, Bio-Oil and Bio-Syngas Production.” Energy Conversion and Management, vol. 210, 2020, p. 112707., https://doi.org/10.1016/j.enconman.2020.112707. ^[15]Wang, Liuwei, et al. “Role of Biochar toward Carbon Neutrality.” Carbon Research, vol. 2, no. 1, 2023, https://doi.org/10.1007/s44246-023-00035-7. .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ^[16]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

How much carbon can be sequestered?

• Scalability of the approach (for example, offshore cultivation has fewer spatial imitations than onshore cultivation) 
  • The expense of acquiring, transporting, and delivering nutrients, as well as the bioavailability of those nutrients, and the resulting carbon-to-nutrient ratio in the microalgae
  • Additional considerations for closed systems include:
    • Harvesting Considerations: The energy necessary for harvesting algae from cultivation set-ups (e.g., raceway ponds) can be an impediment to scaling. Current estimates from the US Department of Energy cite energy usage as ~1.4 kWh/m3, and high capital costs, ~$152,000/million gallons per day. Automated harvesters, such as the Zobi Harvester, have the potential to cut time and costs significantly, using less than 0.14 kwh/m3 and a capital cost of less than $106,000 per million gallons per day of capacity, unlocking harvesting barriers ^[1]Hazlebeck, David, and Rickman, William. Zobi® based processes for ultra-low energy algal harvesting and dewatering. United States: N. p., 2019. . Utilizing gravity-based systems is another way of increasing energy efficiency for harvesting ^[2]Muylaert, K., Bastiaens, L., Vandamme, D., & Gouveia, L. (2017). Harvesting of microalgae: Overview of process options and their strengths and drawbacks. Microalgae-Based Biofuels and Bioproducts, 113-132. https://doi.org/10.1016/B978-0-08-101023-5.00005-4 . 
    • Planktonic Cultivation vs Biofilms: Microalgae can be cultivated in two forms: planktonic or in on biofilms. Planktonic microalgae are suspended in water and are highly diluted (less than 1%). Planktonic microalgae have been more extensively studied and can be cultivated in photobioreactors or ponds ^[3]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . Biofilms are aggregates of microorganisms attached to a surface and are much more concentrated than planktonic cultivation. The areal productivity for biofilms of microalgae is about two-fold the one for planktonic microalgae ^[4]Morales, Marjorie, et al. “Rotating Algal Biofilm versus Planktonic Cultivation: LCA Perspective.” Journal of Cleaner Production, vol. 257, 2020, p. 120547., https://doi.org/10.1016/j.jclepro.2020.120547. . New techniques with biofilms are in development ^[5]Mantzorou A, Ververidis F. Microalgal biofilms: A further step over current microalgal cultivation techniques. Sci Total Environ. 2019 Feb 15;651(Pt 2):3187-3201. doi: 10.1016/j.scitotenv.2018.09.355. Epub 2018 Oct 1. PMID: 30463168. . 

• Carbon fixation rates and storage capacity of the microalgae species

• • The capacity of microalgae to fix carbon differs across species and selecting appropriate species for different approaches remains an area of study. Genetic modification of carbon fixation rates and carbon storage capacity may expand the CDR potential for microalgae ^[6]Li, Shengnan, et al. “How to Enhance Carbon Capture by Evolution of Microalgal Photosynthesis?” Separation and Purification Technology, vol. 291, June 2022, p. 120951, https://doi.org/10.1016/j.seppur.2022.120951. . 

How durable is the carbon sequestration? 

• Deep ocean: wherein microalgae, and their embedded carbon, cultivated via ocean fertilization or on land in photobioreactors or ponds are stored in the ocean for long-term storage. If purposely placed or injected into the water column at depth (>1,000 meters), many parts of the global ocean can offer 100 to 1000 years-scale permanence. Sequestration durability is reduced to 10 to 100 years-scale for microalgae participating in the biological carbon pump due to high rates of remineralization in the upper ocean and during transit to the deep ocean as microalgae are grazed or decompose. There is extensive basin-scale variability in sequestration durability ^[7]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Areas of the ocean best suited to long carbon storage are places where surface waters and sinking matter can reach depths greater than 1,000 meters. The Southern Ocean south of the biogeochemical divide separating the Antarctic from the sub-Antarctic would be one such suitable location as well as much of the North Pacific ^[8]Siegel, D A, et al. 2021b. “Assessing the Sequestration Time Scales of Some Ocean-Based Carbon Dioxide Reduction Strategies.” Environmental Research Letters, vol. 16, no. 10, 2021b, p. 104003., https://doi.org/10.1088/1748-9326/ac0be0. . Findings also suggest the North Atlantic and North Pacific oceans have the greatest carbon storage potential, while subtropical oceans have the least ^[9]Nowicki, M., DeVries, T., & Siegel, D. A. (2022). Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles, 36, e2021GB007083. https://doi.org/10.1029/2021GB00708. . Continued thorough measurements and models are needed to quantify the durability of sequestration for any given site, accounting for the fact that durability is likely best defined as a probability distribution given the various transit times of water masses.
• A number of large-scale campaigns are underway to improve our understanding of deep ocean carbon storage. These include EXPORTS (USA), COMICS (UK), and ONCE (China).
• In addition, there are a number of studies underway to investigate ways to accelerate deep ocean carbon storage, among them:

• • Investigating Carbon Storage in Microalgae Polysaccharides: Scientists at the MARUM MPG Bridge Group Marine Glycobiology recently won the European Research Council’s Consolidator Grant to research carbon storage in marine algal polysaccharides and their role in the marine carbon cycle. 
  • Work from Mukul Sharma’s lab at Dartmouth College is investigating the use of clay minerals to enhance the removal of carbon from the euphotic zone and sequester it in the deep ocean ^[10]Bates, E., et al. Role of Clay-Gel Hybrids in Enhancing Carbon Sequestration. Dec. 2019, pp. GC31I-1325, https://ui.adsabs.harvard.edu/abs/2019AGUFMGC31I1325B. . The principal idea behind this research is the formation of clay-gel hybrids when clay comes into contact with dissolved organic matter produced by microalgae and bacteria. These hybrids may facilitate conversion of dissolved organic matter to particulate organic matter and rapid export out of the mixed layer. While clay additions are very underexplored in open ocean environments, they have long been recognized to interact with, and flocculate, in the presence of microalgae ^[11]Avnimelech, Yoram, Bill W. Troeger, and Lester W. Reed. "Mutual flocculation of algae and clay: evidence and implications." Science 216.4541 (1982): 63-65. ^[12]Chen C, Pan G, Shi W, Xu F, Techtmann SM, Pfiffner SM and Hazen TC (2018) Clay Flocculation Effect on Microbial Community Composition in Water and Sediment. Front. Environ. Sci. 6:60. doi: 10.3389 fenvs.2018.00060

• Long-lived bioproducts: wherein carbon sequestered in microalgae is incorporated into products such as bioplastics for long-term storage.
• Biochar: Microalgal biomass can be processed via pyrolysis, gasification, or hydrothermal carbonization and turned into biochar. Biochar is a carbon-rich product that can be used in soils for agriculture (helping to prevent leaching of pesticides and nutrients) or as absorbents in air/water treatments for pollutant removal ^[13]Bi, Z., & He, B. B. (2022). Biochar from microalgae. 3rd Generation Biofuels, 613-637. https://doi.org/10.1016/B978-0-323-90971-6.00025-5 ^[14]Lee, Xin Jiat, et al. “State of Art Review on Conventional and Advanced Pyrolysis of Macroalgae and Microalgae for Biochar, Bio-Oil and Bio-Syngas Production.” Energy Conversion and Management, vol. 210, 2020, p. 112707., https://doi.org/10.1016/j.enconman.2020.112707. ^[15]Wang, Liuwei, et al. “Role of Biochar toward Carbon Neutrality.” Carbon Research, vol. 2, no. 1, 2023, https://doi.org/10.1007/s44246-023-00035-7. .
• Burial & land-based sequestration: Algal biomass that is sufficiently salty, dry, acidic, and rich in natural preservatives can be stored on land (buried) for long-term sequestration ^[16]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients (NASEM, 2022).
2. Factors affecting carbon sequestration that remain uncertain (NASEM, 2022):
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust (Martin et al., 1990). John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis (Yoon et al., 2018), rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration (Martin et al., 1988; Morel & Price, 2003; ExOIS, 2022) and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump (Martin et al., 1990; Martin & Fitzwater, 1988). Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique (NASEM, 2022). 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner (OpenAir, 2022).

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients (NASEM, 2022).
2. Factors affecting carbon sequestration that remain uncertain (NASEM, 2022):
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust (Martin et al., 1990). John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis (Yoon et al., 2018), rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration (Martin et al., 1988; Morel & Price, 2003; ExOIS, 2022) and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump (Martin et al., 1990; Martin & Fitzwater, 1988). Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique (NASEM, 2022). 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients (NASEM, 2022).
2. Factors affecting carbon sequestration that remain uncertain (NASEM, 2022):
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust (Martin et al., 1990). John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis (Yoon et al., 2018), rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration (Martin et al., 1988; Morel & Price, 2003; ExOIS, 2022) and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump (Martin et al., 1990; Martin & Fitzwater, 1988). Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ^[18]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients (NASEM, 2022).
2. Factors affecting carbon sequestration that remain uncertain (NASEM, 2022):
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust ^[11]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis ^[12]Yoon, Joo-Eun, et al. "Reviews and syntheses: Ocean iron fertilization experiments–past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project." Biogeosciences 15.19 (2018): 5847-5889. , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration ^[13]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. ^[14]Morel, F. M., and N. M. Price. 2003. The biogeochemical cycles of trace metals in the oceans. Science 300(5621):944-947. doi:10.1126/science.1083545. ^[15]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/ and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump ^[16]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. ^[17]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ^[18]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients ^[9]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. .
2. Factors affecting carbon sequestration that remain uncertain ^[10]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust ^[11]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis ^[12]Yoon, Joo-Eun, et al. "Reviews and syntheses: Ocean iron fertilization experiments–past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project." Biogeosciences 15.19 (2018): 5847-5889. , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration ^[13]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. ^[14]Morel, F. M., and N. M. Price. 2003. The biogeochemical cycles of trace metals in the oceans. Science 300(5621):944-947. doi:10.1126/science.1083545. ^[15]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/ and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump ^[16]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. ^[17]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ^[18]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients .
2. Factors affecting carbon sequestration that remain uncertain :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients ^[9]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. .
2. Factors affecting carbon sequestration that remain uncertain ^[10]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust ^[11]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis ^[12]Yoon, Joo-Eun, et al. "Reviews and syntheses: Ocean iron fertilization experiments–past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project." Biogeosciences 15.19 (2018): 5847-5889. , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration ^[13]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. ^[14]Morel, F. M., and N. M. Price. 2003. The biogeochemical cycles of trace metals in the oceans. Science 300(5621):944-947. doi:10.1126/science.1083545. ^[15]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/ and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump ^[16]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. ^[17]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ^[18]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Cultivation and Fertilization: Technologies and methods related to cultivating microalgae, in some cases this entails fertilization events. Cultivation and fertilization can happen both offshore and onshore, with some proposed technologies utilizing energy or inputs from other processes or systems. In general, all methods need significantly more research into their ability to sequester carbon durably for net negative carbon dioxide removal.

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients .
2. Factors affecting carbon sequestration that remain uncertain :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• • Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels , it may be viable for CDR purposes.  

• • Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Cultivation and Fertilization: Technologies and methods related to cultivating microalgae, in some cases this entails fertilization events. Cultivation and fertilization can happen both offshore and onshore, with some proposed technologies utilizing energy or inputs from other processes or systems. In general, all methods need significantly more research into their ability to sequester carbon durably for net negative carbon dioxide removal.

Nutrient Fertilization

1. The depth of remineralization for carbon, iron, and other macronutrients .
2. Factors affecting carbon sequestration that remain uncertain :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels , it may be viable for CDR purposes.  

• Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Cultivation and Fertilization: Technologies and methods related to cultivating microalgae, in some cases this entails fertilization events. Cultivation and fertilization can happen both offshore and onshore, with some proposed technologies utilizing energy or inputs from other processes or systems. In general, all methods need significantly more research into their ability to sequester carbon durably for net negative carbon dioxide removal.

Nutrient Fertilization

CDR Potential: >1 Gt CO₂/year ^[1]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. ^[2]GESAMP (2019). “High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 98, 144 p. , 0.1 – 1.0+ Gt CO₂/year ^[3]NOAA, 2022. A White Paper documenting a Potential NOAA CDR Science Strategy as an element of NOAA’s Climate Mitigation Portfolio. https://sciencecouncil.noaa.gov/Draft-CDR-Strategy. Technology Readiness: Field studies still necessary to determine fate of carbon, net carbon sequestration, and sequestration durability ^[4]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. ^[5]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/

1. The depth of remineralization for carbon, iron, and other macronutrients ^[9]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. .
2. Factors affecting carbon sequestration that remain uncertain ^[10]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• • Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust ^[11]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis ^[12]Yoon, Joo-Eun, et al. "Reviews and syntheses: Ocean iron fertilization experiments–past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project." Biogeosciences 15.19 (2018): 5847-5889. , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration ^[13]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. ^[14]Morel, F. M., and N. M. Price. 2003. The biogeochemical cycles of trace metals in the oceans. Science 300(5621):944-947. doi:10.1126/science.1083545. ^[15]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/ and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump ^[16]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. ^[17]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ^[18]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. . 

Artificial Upwelling

CDR Potential: <0.1 Gt CO₂/year. There is low confidence that artificial upwelling will be effective, durable, or scalable for CDR ^[19]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. Technology Readiness: The technology necessary to conduct field trials is not ready for deployment ^[20]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. .

Floating Photobioreactors

CDR Potential: unknown  Technology Readiness: Many designs and configurations have been proposed, but more field trials are needed to assess carbon capture efficiency and scalability, as well as address the issue of carbon supply. 

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

CDR Potential: Largely unknown, Brilliant Planet projects 0.5 Gt CO₂/year ^[30]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s. Technology Readiness: Techniques and methods are well studied, and many are well established (though very few have been intended for CDR). Raceway ponds are largely ready to be scaled. 

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

CDR Potential:  unknown  Technology Readiness: Early stages of development

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Cultivation and Fertilization: Technologies and methods related to cultivating microalgae, in some cases this entails fertilization events. Cultivation and fertilization can happen both offshore and onshore, with some proposed technologies utilizing energy or inputs from other processes or systems. In general, all methods need significantly more research into their ability to sequester carbon durably for net negative carbon dioxide removal.

Nutrient Fertilization

CDR Potential: >1 Gt CO₂/year , 0.1 – 1.0+ Gt CO₂/year Technology Readiness: Field studies still necessary to determine fate of carbon, net carbon sequestration, and sequestration durability

1. The depth of remineralization for carbon, iron, and other macronutrients .
2. Factors affecting carbon sequestration that remain uncertain :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• • Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels , it may be viable for CDR purposes.  

• Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Cultivation and Fertilization: Technologies and methods related to cultivating microalgae, in some cases this entails fertilization events. Cultivation and fertilization can happen both offshore and onshore, with some proposed technologies utilizing energy or inputs from other processes or systems. In general, all methods need significantly more research into their ability to sequester carbon durably for net negative carbon dioxide removal.

Nutrient Fertilization

CDR Potential: >1 Gt CO₂/year ^[1]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. ^[2]GESAMP (2019). “High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 98, 144 p. , 0.1 – 1.0+ Gt CO₂/year ^[3]NOAA, 2022. A White Paper documenting a Potential NOAA CDR Science Strategy as an element of NOAA’s Climate Mitigation Portfolio. https://sciencecouncil.noaa.gov/Draft-CDR-Strategy.
Technology Readiness: Field studies still necessary to determine fate of carbon, net carbon sequestration, and sequestration durability ^[4]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. ^[5]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/

1. The depth of remineralization for carbon, iron, and other macronutrients ^[9]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. .
2. Factors affecting carbon sequestration that remain uncertain ^[10]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• • Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust ^[11]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis ^[12]Yoon, Joo-Eun, et al. "Reviews and syntheses: Ocean iron fertilization experiments–past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project." Biogeosciences 15.19 (2018): 5847-5889. , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration ^[13]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. ^[14]Morel, F. M., and N. M. Price. 2003. The biogeochemical cycles of trace metals in the oceans. Science 300(5621):944-947. doi:10.1126/science.1083545. ^[15]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/ and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump ^[16]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. ^[17]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ^[18]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. . 

Artificial Upwelling

Floating Photobioreactors

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Cultivation and Fertilization: Technologies and methods related to cultivating microalgae, in some cases this entails fertilization events. Cultivation and fertilization can happen both offshore and onshore, with some proposed technologies utilizing energy or inputs from other processes or systems. In general, all methods need significantly more research into their ability to sequester carbon durably for net negative carbon dioxide removal.

Nutrient Fertilization

CDR Potential: >1 Gt CO₂/year , 0.1 – 1.0+ Gt CO₂/year

Technology Readiness: Field studies still necessary to determine fate of carbon, net carbon sequestration, and sequestration durability

1. The depth of remineralization for carbon, iron, and other macronutrients .
2. Factors affecting carbon sequestration that remain uncertain :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• • Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique . 

Artificial Upwelling

CDR Potential: <0.1 Gt CO₂/year. There is low confidence that artificial upwelling will be effective, durable, or scalable for CDR

Technology Readiness: The technology necessary to conduct field trials is not ready for deployment .

Floating Photobioreactors

CDR Potential: unknown 

Technology Readiness: Many designs and configurations have been proposed, but more field trials are needed to assess carbon capture efficiency and scalability, as well as address the issue of carbon supply. 

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels , it may be viable for CDR purposes.  

• Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

Cultivation and Fertilization: Technologies and methods related to cultivating microalgae, in some cases this entails fertilization events. Cultivation and fertilization can happen both offshore and onshore, with some proposed technologies utilizing energy or inputs from other processes or systems. In general, all methods need significantly more research into their ability to sequester carbon durably for net negative carbon dioxide removal.

Nutrient Fertilization

CDR Potential: >1 Gt CO₂/year ^[1]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. ^[2]GESAMP (2019). “High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 98, 144 p. , 0.1 – 1.0+ Gt CO₂/year ^[3]NOAA, 2022. A White Paper documenting a Potential NOAA CDR Science Strategy as an element of NOAA’s Climate Mitigation Portfolio. https://sciencecouncil.noaa.gov/Draft-CDR-Strategy.

Technology Readiness: Field studies still necessary to determine fate of carbon, net carbon sequestration, and sequestration durability ^[4]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. ^[5]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/

1. The depth of remineralization for carbon, iron, and other macronutrients ^[9]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. .
2. Factors affecting carbon sequestration that remain uncertain ^[10]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. :
   1. Net primary productivity in the vicinity of nutrient fertilization and downstream (to address issues associated with “nutrient robbing”). 
   2. Efficiency of export of materials out of the euphotic zone, related to the attenuation of the particle flux 
      1. Ratio of iron to carbon (Fe:C) in exported materials and how that ratio changes with depth 
3. Ecosystem / trophic level interactions 
4. The effectiveness of particular methods of delivering nutrients 
5. Pulse (one-time) versus continuous (repeated) experiments will likely change carbon capture efficiency 
6. Field-trials have yet to observe full growth cycles of microalgae including the bloom demise

• • Artificial iron fertilization: The first observations of a relationship between iron and atmospheric CO₂ came from geological records that revealed a correlation between CO₂ in ice cores and iron rich dust ^[11]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. . John Martin conducted pioneering work in the late 1980s to identify iron-limited regions of the ocean. The next decade (1993 – 2004) was spent executing studies in the Equatorial Pacific, Southern Ocean, Subarctic North Pacific, and Subarctic North Atlantic to explicitly examine the primary biological response of adding iron to surface waters. What resulted were thirteen studies in the open ocean accompanied by decades of analysis ^[12]Yoon, Joo-Eun, et al. "Reviews and syntheses: Ocean iron fertilization experiments–past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project." Biogeosciences 15.19 (2018): 5847-5889. , rendering artificial iron fertilization the most studied and well-understood approach in this road map. The application of iron to the ocean surface to investigate the relationship between iron limitation and phytoplankton growth suggests potentially high global capacity for iron fertilization to result in gigaton scale carbon dioxide sequestration ^[13]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. ^[14]Morel, F. M., and N. M. Price. 2003. The biogeochemical cycles of trace metals in the oceans. Science 300(5621):944-947. doi:10.1126/science.1083545. ^[15]ExOIS, 2022. Ocean Iron Fertilization: Assessing its potential as a climate solution. https://cafethorium.whoi.edu/ocean-iron-fertilization-assessing-its-potential-as-a-climate-solution/ and has led scientists to look to iron as a strategy to enhance the ocean’s biological carbon pump ^[16]Martin, J. H., R. M. Gordon, and S. E. Fitzwater. 1990. Iron in Antarctic waters. Nature 345(6271):156-158. doi:10.1038/345156a0. ^[17]Martin, J. H., and S. E. Fitzwater. 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331(6154):341-343. doi:10.1038/331341a0. . Most of the open ocean experiments conducted to date were largely focused on the immediate biological response to added iron in the form of a microalgae bloom and not the geochemical response (in the form of carbon fluxes), thus there is limited knowledge on the efficacy of this technique for net carbon dioxide removal. However, there are a handful of groups around the globe currently investigating the fate of carbon from net primary production. In addition to the efficacy of such methods, biogeochemical and ecological impacts remain unknown. A 2022 whitepaper by ExOIS (Exploring Ocean Iron Solutions) lays out a number of questions that remain to be answered by science including: What controls the efficacy and durability of ocean iron fertilization as a CDR strategy? How can ocean iron fertilization efficacy be accurately and efficiently monitored and verified? What are the intended and unintended ecological consequences of ocean iron fertilization and how can they be mitigated? 

• Engineered nanoparticles: Babakhani et al. (2021) propose the application of engineered nanoparticles in addition to iron fertilization in an attempt to enhance phytoplankton growth and possibly mitigate environmental risks posed by iron fertilization (e.g., nutrient robbing). Here, the goal is for the engineered nanoparticles to control the rate of nutrient release. Nanoparticles may be more expensive to deploy but may also be more effective at CDR. Focused field trials of engineered nanoparticles with artificial ocean fertilization are still needed to fully understand the impacts and efficacy of this technique. 

• • Triggering the formation of marine snow: The GEA275 Project (Croatia) is investigating if adding iron chelates to high-nitrate, low-chlorophyll regions of the ocean can induce the formation of marine snow (organic detritus) for carbon removal and storage in the deep sea. Unlike other forms of iron fertilization, this approach does not aim to stimulate phytoplankton growth but rather to accelerate the release of extracellular organic carbon by existing communities of phytoplankton and heterotrophic bacteria. 

• Macronutrient fertilization: Compared to iron fertilization, macronutrient fertilization has received less attention from the scientific community (but note Harrison 2017), with one clear disadvantage of this method being that larger quantities of nutrient material are needed per ton of carbon removal due to the higher cellular ratios of nitrogen and phosphorus to carbon (as compared to iron to carbon). However, ideal locations for macronutrient fertilization (low-nutrient, low-chlorophyll) may be more accessible than hard-to-reach locations ideal for iron fertilization, such as the Southern Ocean. In general, macronutrient fertilization presents general concerns around acquiring sufficient quantities of nutrients (some of which are nonrenewable, like phosphate), ecosystem impacts, and the fate of carbon once sequestered in phytoplankton. More work is needed to better understand macronutrient fertilization’s potential as a scalable CDR technique ^[18]NASEM (National Academies of Sciences, Engineering, and Medicine.) A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration. The National Academies Press, https://doi.org/10.17226/26278. Accessed 18 Aug. 2022. . 

Artificial Upwelling

Floating Photobioreactors

CDR Potential: unknown 

Technology Readiness: Many designs and configurations have been proposed, but more field trials are needed to assess carbon capture efficiency and scalability, as well as address the issue of carbon supply. 

• Combining cultivation with wastewater treatment: Wastewater is a nutrient-rich medium that has been explored as a nutrient source for microalgae cultivation ^[26]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 . Wastewater is rich in nitrogen, phosphorous, heavy metals, etc., which are often costly and energy-intensive to remove through traditional methods ^[27]Molazadeh M, Ahmadzadeh H, Pourianfar HR, Lyon S and Rampelotto PH (2019) The Use of Microalgae for Coupling Wastewater Treatment With CO₂ Biofixation. Front. Bioeng. Biotechnol. 7:42. doi: 10.3389/fbioe.2019.00042 . Many species of microalgae can remove these compounds by accumulating and/or using them in their cells. One challenge in this approach is providing microalgae with enough CO₂ for optimal growth.  There is potential for wastewater treatment to be paired with onshore cultivation systems such as raceway ponds or with offshore systems such as photobioreactors, however more research is needed to assess the feasibility of such set-ups. Most studies into coupling wastewater treatment with microalgae have looked at end products such as biofuels and not the permanent removal of carbon dioxide. As such, there is limited data on the actual carbon dioxide removal potential of this approach. While closed microalgae cultivation is widely considered to be an economically non-viable way to make fuels ^[28]Kumar, D., & Singh, B. (2019). Algal biorefinery: An integrated approach for sustainable biodiesel production. Biomass and Bioenergy, 131, 105398. https://doi.org/10.1016/j.biombioe.2019.105398 ^[29]Wang, S., Mukhambet, Y., Esakkimuthu, S., & Abomohra, A. E. F. (2022). Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives. Journal of Cleaner Production, 348, 131245. https://doi.org/10.1016/j.jclepro.2022.131245 , it may be viable for CDR purposes.  

• Current Projects: Algae Systems (US) grows microalgae in bags of disinfected wastewater and CO₂. The bags are deployed offshore in Mobile Bay, Alabama, and the combination of wave energy and sun create the necessary conditions for algae to grow. Currently, the algae are harvested and used for diesel and jet fuel, while the water is cleaned for re-use. The plant is currently at demo-scale and treats 40,000 gallons of wastewater per acre per day. 

Cultivation Ponds

• Current Projects: Brilliant Planet (London) began growing microalgae in 2013 on the shores of St Helena, South Africa and now have a thirty-thousand square meter production facility in the coastal desert of Morocco which has been operating for over five years. Their systems use seawater to cultivate microalgae in large ponds and are monitored with high-frequency satellites. Permanent carbon sequestration is achieved by burying resulting biomass in ultra-dry desert landfills and sealing it with a geomembrane liner ^[32]OpenAir. (December 2022) .This Is CDR Ep. 57: Brilliant Planet with Dr. Raffael Jovine. [Video]. [YouTube]. https://www.youtube.com/watch?v=6B_LLBcdOhI&t=210s.

Microalgae-based Bioenergy for Carbon Capture and Storage (MBECCS / ABECCS)

• Current Projects: The Johnson Lab at the Duke Nicholas School of the Environment was awarded a grant for a pilot demonstration of carbon capture/storage technology based on ABECCS.  The project uses a combined system that generates CO₂ from forest products (bark/sawdust) and then uses that CO₂ to grow algae for carbon sequestration in valuable bioproducts. Preliminary analysis of this process shows it to be carbon negative and economically viable. 

General Risks of Open Systems

Harmful Algal Blooms 

Land-Use

Summary Table of Potential Impacts

General Risks of Open Systems

Harmful Algal Blooms 

Land-Use

Summary Table of Potential Impacts

General Risks of Open Systems

Harmful Algal Blooms 

Land-Use

Summary Table of Potential Impacts

General Risks of Open Systems

Harmful Algal Blooms 

Land-Use

Summary Table of Potential Impacts

Social Risks 

• Lack of public support & social license: There are currently low levels of public support and/or acceptance of CDR broadly in the United States and beyond, particularly with activities viewed as “geoengineering” or “climate engineering” (Carlisle et al., 2020; Cox et al., 2020). Other factors that may affect public support include perceptions of scale and perceptions of capture versus storage. These are areas that require more focused study to better understand. Early experiments are opportunities to adopt best practices and avoid mistakes when building public support and stakeholder engagement. 
• Risk to culturally significant landscapes: CDR interventions that alter marine landscapes may lead to disruptions (e.g., native species populations) to cultural, spiritual, or cosmological landscapes and other non-tangible uses or relationships to the ocean and its inhabitants. The introduction of any new technology that interacts with natural ecosystems is likely to always be contested due to the diversity of ways in which people view the relationship between humans and the non-human world (Gannon & Hulme, 2018; Abate, 2016). 
• Human health risks: Exposure to aerosolization of toxins from harmful algal blooms (HABs) have been known to cause respiratory symptoms in humans (Stewart et al., 2006). Although HABs have not been observed during artificial iron fertilization experiments, their occurrence is not impossible across many microalgae-based CDR approaches, including shore-based cultivation operations. 

• Irreversible consequences: Negative impacts (environmental or social) of an approach that are irreversible may impact future generations, raising concerns regarding intergenerational justice (Cooley et al., 2022). This concern highlights the need for appropriate procedures so that any field trials that are approved and deployed do not have irreversible consequences. 

• Decreased equity and social welfare due to financing: The creation of financing systems like carbon markets for forest carbon sequestration caused tensions around land rights and had implications on human rights and equity for Indigenous and forest-dependent communities (Cooley et al., 2022). These issues will likely vary greatly for closed systems on land, coastal closed systems, and open ocean/deep ocean commons. 

• Location considerations: Just as there are different governance implications for activities occurring in exclusive economic zones versus the high seas, these geographical differences may also carry importance for social impacts. Environmental or ecological impacts from activities taking place in near-shore waters may have more direct social impacts than activities taking place in the open ocean. 

• Food security: Microalgae-based interventions that risk disturbing food supply chains (e.g., fisheries) may threaten food security. This is especially relevant for communities that rely on the ocean for sustenance. 

• Impacts on livelihoods from tourism and fisheries: Activities that use or interfere with near-shore environments and ecology may impact industries like tourism and fisheries, thus impacting livelihoods. This is particularly important in areas that depend heavily on these industries economically and/or are socioeconomically vulnerable in general (e.g., the Global South).  

Social Co-Benefits 

• Economic incentives: The various potential utilization pathways of microalgae could create avenues for wealth generation and emissions-free economic growth – namely economic incentives within existing carbon markets, preventing costly environmental disasters resulting from uncontrolled climate change, and converting a damaging waste product (CO₂) into a value-added product (Daneshvar et al., 2022).
• Job creation: Economic value-addition is likely to also create jobs in new and existing industries in aquaculture and beyond (Mayo-Ramsay, 2010).
• Ownership and benefits accruing from uses of genetic resources: Many microalgae-based innovations bring with them a suite of potential biotech and genomic-based approaches. It will be important to understand when and how local benefits rights holders (e.g., coastal communities, Indigenous communities) can benefit from the use of marine-based genetic resources. This is an inherently complex subject that questions ownership of marine resources and is rich with justice and equity implications.

Social Risks 

• Lack of public support & social license: There are currently low levels of public support and/or acceptance of CDR broadly in the United States and beyond, particularly with activities viewed as “geoengineering” or “climate engineering” (Carlisle et al., 2020; Cox et al., 2020). Other factors that may affect public support include perceptions of scale and perceptions of capture versus storage. These are areas that require more focused study to better understand. Early experiments are opportunities to adopt best practices and avoid mistakes when building public support and stakeholder engagement. 
• Risk to culturally significant landscapes: CDR interventions that alter marine landscapes may lead to disruptions (e.g., native species populations) to cultural, spiritual, or cosmological landscapes and other non-tangible uses or relationships to the ocean and its inhabitants. The introduction of any new technology that interacts with natural ecosystems is likely to always be contested due to the diversity of ways in which people view the relationship between humans and the non-human world (Gannon & Hulme, 2018; Abate, 2016). 
• Human health risks: Exposure to aerosolization of toxins from harmful algal blooms (HABs) have been known to cause respiratory symptoms in humans ^[7]Stewart, Ian, et al. “Recreational and Occupational Field Exposure to Freshwater Cyanobacteria – a Review of Anecdotal and Case Reports, Epidemiological Studies and the Challenges for Epidemiologic Assessment.” Environmental Health, vol. 5, no. 1, 2006, https://doi.org/10.1186/1476-069x-5-6. . Although HABs have not been observed during artificial iron fertilization experiments, their occurrence is not impossible across many microalgae-based CDR approaches, including shore-based cultivation operations. 

• Irreversible consequences: Negative impacts (environmental or social) of an approach that are irreversible may impact future generations, raising concerns regarding intergenerational justice ^[8]Cooley, Sarah R., et al. “Sociotechnical Considerations about Ocean Carbon Dioxide Removal.” Annual Review of Marine Science, vol. 15, no. 1, 2022, https://doi.org/10.1146/annurev-marine-032122-113850. . This concern highlights the need for appropriate procedures so that any field trials that are approved and deployed do not have irreversible consequences. 

• Decreased equity and social welfare due to financing: The creation of financing systems like carbon markets for forest carbon sequestration caused tensions around land rights and had implications on human rights and equity for Indigenous and forest-dependent communities ^[9]Cooley, Sarah R., et al. “Sociotechnical Considerations about Ocean Carbon Dioxide Removal.” Annual Review of Marine Science, vol. 15, no. 1, 2022, https://doi.org/10.1146/annurev-marine-032122-113850. . These issues will likely vary greatly for closed systems on land, coastal closed systems, and open ocean/deep ocean commons. 

• Location considerations: Just as there are different governance implications for activities occurring in exclusive economic zones versus the high seas, these geographical differences may also carry importance for social impacts. Environmental or ecological impacts from activities taking place in near-shore waters may have more direct social impacts than activities taking place in the open ocean. 

• Food security: Microalgae-based interventions that risk disturbing food supply chains (e.g., fisheries) may threaten food security. This is especially relevant for communities that rely on the ocean for sustenance. 

• Impacts on livelihoods from tourism and fisheries: Activities that use or interfere with near-shore environments and ecology may impact industries like tourism and fisheries, thus impacting livelihoods. This is particularly important in areas that depend heavily on these industries economically and/or are socioeconomically vulnerable in general (e.g., the Global South).  

Social Co-Benefits 

• Economic incentives: The various potential utilization pathways of microalgae could create avenues for wealth generation and emissions-free economic growth – namely economic incentives within existing carbon markets, preventing costly environmental disasters resulting from uncontrolled climate change, and converting a damaging waste product (CO₂) into a value-added product. ^[10]Daneshvar, Ehsan, et al. “Biologically-Mediated Carbon Capture and Utilization by Microalgae towards Sustainable CO₂ Biofixation and Biomass Valorization – A Review.” Chemical Engineering Journal, vol. 427, Jan. 2022, p. 130884, https://doi.org/10.1016/j.cej.2021.130884.
• Job creation: Economic value-addition is likely to also create jobs in new and existing industries in aquaculture and beyond ^[11]Mayo-Ramsay, Julia. “Environmental, Legal and Social Implications of Ocean Urea Fertilization: Sulu Sea Example.” Marine Policy, vol. 34, no. 5, 2010, pp. 831–835., https://doi.org/10.1016/j.marpol.2010.01.004. .
• Ownership and benefits accruing from uses of genetic resources: Many microalgae-based innovations bring with them a suite of potential biotech and genomic-based approaches. It will be important to understand when and how local benefits rights holders (e.g., coastal communities, Indigenous communities) can benefit from the use of marine-based genetic resources. This is an inherently complex subject that questions ownership of marine resources and is rich with justice and equity implications.

Social Risks 

• Lack of public support & social license: There are currently low levels of public support and/or acceptance of CDR broadly in the United States and beyond, particularly with activities viewed as “geoengineering” or “climate engineering” ^[3]Carlisle, Daniel P., et al. “The Public Remain Uninformed and Wary of Climate Engineering.” Climatic Change, vol. 160, no. 2, 2020, pp. 303–322., https://doi.org/10.1007/s10584-020-02706-5. ^[4]Cox, Emily, et al. “Public Perceptions of Carbon Dioxide Removal in the United States and the United Kingdom.” Nature Climate Change, vol. 10, no. 8, 2020, pp. 744–749., https://doi.org/10.1038/s41558-020-0823-z. . Other factors that may affect public support include perceptions of scale and perceptions of capture versus storage. These are areas that require more focused study to better understand. Early experiments are opportunities to adopt best practices and avoid mistakes when building public support and stakeholder engagement. 
• Risk to culturally significant landscapes: CDR interventions that alter marine landscapes may lead to disruptions (e.g., native species populations) to cultural, spiritual, or cosmological landscapes and other non-tangible uses or relationships to the ocean and its inhabitants. The introduction of any new technology that interacts with natural ecosystems is likely to always be contested due to the diversity of ways in which people view the relationship between humans and the non-human world ^[5]Gannon, Kate and Hulme, Mike (2018) Geoengineering at the 'edge of the world': exploring perceptions of ocean fertilization through the Haida Salmon Restoration Corporation. Geo: Geography and Environment, 5 (1). e00054. ISSN 2054-4049 DOI: 10.1002/geo2.54, https://rgs-ibg.onlinelibrary.wiley.com/doi/full/10.1002/geo2.54 ^[6]Abate, Randall S. “Ocean Iron Fertilization and Indigenous Peoples’ Right to Food: Leveraging International and Domestic Law Protections to Enhance Access to Salmon in the Pacific Northwest.” UCLA Journal of International Law and Foreign Affairs, vol. 20, 2016, p. 45, https://heinonline.org/HOL/Page?handle=hein.journals/jilfa20&id=49&div=&collection=. . 
• Human health risks: Exposure to aerosolization of toxins from harmful algal blooms (HABs) have been known to cause respiratory symptoms in humans ^[7]Stewart, Ian, et al. “Recreational and Occupational Field Exposure to Freshwater Cyanobacteria – a Review of Anecdotal and Case Reports, Epidemiological Studies and the Challenges for Epidemiologic Assessment.” Environmental Health, vol. 5, no. 1, 2006, https://doi.org/10.1186/1476-069x-5-6. . Although HABs have not been observed during artificial iron fertilization experiments, their occurrence is not impossible across many microalgae-based CDR approaches, including shore-based cultivation operations. 

• Irreversible consequences: Negative impacts (environmental or social) of an approach that are irreversible may impact future generations, raising concerns regarding intergenerational justice ^[8]Cooley, Sarah R., et al. “Sociotechnical Considerations about Ocean Carbon Dioxide Removal.” Annual Review of Marine Science, vol. 15, no. 1, 2022, https://doi.org/10.1146/annurev-marine-032122-113850. . This concern highlights the need for appropriate procedures so that any field trials that are approved and deployed do not have irreversible consequences. 

• Decreased equity and social welfare due to financing: The creation of financing systems like carbon markets for forest carbon sequestration caused tensions around land rights and had implications on human rights and equity for Indigenous and forest-dependent communities ^[9]Cooley, Sarah R., et al. “Sociotechnical Considerations about Ocean Carbon Dioxide Removal.” Annual Review of Marine Science, vol. 15, no. 1, 2022, https://doi.org/10.1146/annurev-marine-032122-113850. . These issues will likely vary greatly for closed systems on land, coastal closed systems, and open ocean/deep ocean commons. 

• Location considerations: Just as there are different governance implications for activities occurring in exclusive economic zones versus the high seas, these geographical differences may also carry importance for social impacts. Environmental or ecological impacts from activities taking place in near-shore waters may have more direct social impacts than activities taking place in the open ocean. 

• Food security: Microalgae-based interventions that risk disturbing food supply chains (e.g., fisheries) may threaten food security. This is especially relevant for communities that rely on the ocean for sustenance. 

• Impacts on livelihoods from tourism and fisheries: Activities that use or interfere with near-shore environments and ecology may impact industries like tourism and fisheries, thus impacting livelihoods. This is particularly important in areas that depend heavily on these industries economically and/or are socioeconomically vulnerable in general (e.g., the Global South).  

Social Co-Benefits 

• Economic incentives: The various potential utilization pathways of microalgae could create avenues for wealth generation and emissions-free economic growth – namely economic incentives within existing carbon markets, preventing costly environmental disasters resulting from uncontrolled climate change, and converting a damaging waste product (CO₂) into a value-added product. ^[10]Daneshvar, Ehsan, et al. “Biologically-Mediated Carbon Capture and Utilization by Microalgae towards Sustainable CO₂ Biofixation and Biomass Valorization – A Review.” Chemical Engineering Journal, vol. 427, Jan. 2022, p. 130884, https://doi.org/10.1016/j.cej.2021.130884.
• Job creation: Economic value-addition is likely to also create jobs in new and existing industries in aquaculture and beyond ^[11]Mayo-Ramsay, Julia. “Environmental, Legal and Social Implications of Ocean Urea Fertilization: Sulu Sea Example.” Marine Policy, vol. 34, no. 5, 2010, pp. 831–835., https://doi.org/10.1016/j.marpol.2010.01.004. .
• Ownership and benefits accruing from uses of genetic resources: Many microalgae-based innovations bring with them a suite of potential biotech and genomic-based approaches. It will be important to understand when and how local benefits rights holders (e.g., coastal communities, Indigenous communities) can benefit from the use of marine-based genetic resources. This is an inherently complex subject that questions ownership of marine resources and is rich with justice and equity implications.

Social Risks 

• Lack of public support & social license: There are currently low levels of public support and/or acceptance of CDR broadly in the United States and beyond, particularly with activities viewed as “geoengineering” or “climate engineering” . Other factors that may affect public support include perceptions of scale and perceptions of capture versus storage. These are areas that require more focused study to better understand. Early experiments are opportunities to adopt best practices and avoid mistakes when building public support and stakeholder engagement. 
• Risk to culturally significant landscapes: CDR interventions that alter marine landscapes may lead to disruptions (e.g., native species populations) to cultural, spiritual, or cosmological landscapes and other non-tangible uses or relationships to the ocean and its inhabitants. The introduction of any new technology that interacts with natural ecosystems is likely to always be contested due to the diversity of ways in which people view the relationship between humans and the non-human world . 
• Human health risks: Exposure to aerosolization of toxins from harmful algal blooms (HABs) have been known to cause respiratory symptoms in humans . Although HABs have not been observed during artificial iron fertilization experiments, their occurrence is not impossible across many microalgae-based CDR approaches, including shore-based cultivation operations. 

• Irreversible consequences: Negative impacts (environmental or social) of an approach that are irreversible may impact future generations, raising concerns regarding intergenerational justice . This concern highlights the need for appropriate procedures so that any field trials that are approved and deployed do not have irreversible consequences. 

• Decreased equity and social welfare due to financing: The creation of financing systems like carbon markets for forest carbon sequestration caused tensions around land rights and had implications on human rights and equity for Indigenous and forest-dependent communities . These issues will likely vary greatly for closed systems on land, coastal closed systems, and open ocean/deep ocean commons. 

• Location considerations: Just as there are different governance implications for activities occurring in exclusive economic zones versus the high seas, these geographical differences may also carry importance for social impacts. Environmental or ecological impacts from activities taking place in near-shore waters may have more direct social impacts than activities taking place in the open ocean. 

• Food security: Microalgae-based interventions that risk disturbing food supply chains (e.g., fisheries) may threaten food security. This is especially relevant for communities that rely on the ocean for sustenance. 

• Impacts on livelihoods from tourism and fisheries: Activities that use or interfere with near-shore environments and ecology may impact industries like tourism and fisheries, thus impacting livelihoods. This is particularly important in areas that depend heavily on these industries economically and/or are socioeconomically vulnerable in general (e.g., the Global South).  

Social Co-Benefits 

• Economic incentives: The various potential utilization pathways of microalgae could create avenues for wealth generation and emissions-free economic growth- namely economic incentives within existing carbon markets, preventing costly environmental disasters resulting from uncontrolled climate change, and converting a damaging waste product (CO₂) into a value-added product.
• Job creation: Economic value-addition is likely to also create jobs in new and existing industries in aquaculture and beyond .
• Ownership and benefits accruing from uses of genetic resources: Many microalgae-based innovations bring with them a suite of potential biotech and genomic-based approaches. It will be important to understand when and how local benefits rights holders (e.g., coastal communities, Indigenous communities) can benefit from the use of marine-based genetic resources. This is an inherently complex subject that questions ownership of marine resources and is rich with justice and equity implications.

Social Risks 

• Lack of public support & social license: There are currently low levels of public support and/or acceptance of CDR broadly in the United States and beyond, particularly with activities viewed as “geoengineering” or “climate engineering” ^[3]Carlisle, Daniel P., et al. “The Public Remain Uninformed and Wary of Climate Engineering.” Climatic Change, vol. 160, no. 2, 2020, pp. 303–322., https://doi.org/10.1007/s10584-020-02706-5. ^[4]Cox, Emily, et al. “Public Perceptions of Carbon Dioxide Removal in the United States and the United Kingdom.” Nature Climate Change, vol. 10, no. 8, 2020, pp. 744–749., https://doi.org/10.1038/s41558-020-0823-z. . Other factors that may affect public support include perceptions of scale and perceptions of capture versus storage. These are areas that require more focused study to better understand. Early experiments are opportunities to adopt best practices and avoid mistakes when building public support and stakeholder engagement. 
• Risk to culturally significant landscapes: CDR interventions that alter marine landscapes may lead to disruptions (e.g., native species populations) to cultural, spiritual, or cosmological landscapes and other non-tangible uses or relationships to the ocean and its inhabitants. The introduction of any new technology that interacts with natural ecosystems is likely to always be contested due to the diversity of ways in which people view the relationship between humans and the non-human world ^[5]Gannon, Kate and Hulme, Mike (2018) Geoengineering at the 'edge of the world': exploring perceptions of ocean fertilization through the Haida Salmon Restoration Corporation. Geo: Geography and Environment, 5 (1). e00054. ISSN 2054-4049 DOI: 10.1002/geo2.54, https://rgs-ibg.onlinelibrary.wiley.com/doi/full/10.1002/geo2.54 ^[6]Abate, Randall S. “Ocean Iron Fertilization and Indigenous Peoples’ Right to Food: Leveraging International and Domestic Law Protections to Enhance Access to Salmon in the Pacific Northwest.” UCLA Journal of International Law and Foreign Affairs, vol. 20, 2016, p. 45, https://heinonline.org/HOL/Page?handle=hein.journals/jilfa20&id=49&div=&collection=. . 
• Human health risks: Exposure to aerosolization of toxins from harmful algal blooms (HABs) have been known to cause respiratory symptoms in humans ^[7]Stewart, Ian, et al. “Recreational and Occupational Field Exposure to Freshwater Cyanobacteria – a Review of Anecdotal and Case Reports, Epidemiological Studies and the Challenges for Epidemiologic Assessment.” Environmental Health, vol. 5, no. 1, 2006, https://doi.org/10.1186/1476-069x-5-6. . Although HABs have not been observed during artificial iron fertilization experiments, their occurrence is not impossible across many microalgae-based CDR approaches, including shore-based cultivation operations. 

• Irreversible consequences: Negative impacts (environmental or social) of an approach that are irreversible may impact future generations, raising concerns regarding intergenerational justice ^[8]Cooley, Sarah R., et al. “Sociotechnical Considerations about Ocean Carbon Dioxide Removal.” Annual Review of Marine Science, vol. 15, no. 1, 2022, https://doi.org/10.1146/annurev-marine-032122-113850. . This concern highlights the need for appropriate procedures so that any field trials that are approved and deployed do not have irreversible consequences. 

• Decreased equity and social welfare due to financing: The creation of financing systems like carbon markets for forest carbon sequestration caused tensions around land rights and had implications on human rights and equity for Indigenous and forest-dependent communities ^[9]Cooley, Sarah R., et al. “Sociotechnical Considerations about Ocean Carbon Dioxide Removal.” Annual Review of Marine Science, vol. 15, no. 1, 2022, https://doi.org/10.1146/annurev-marine-032122-113850. . These issues will likely vary greatly for closed systems on land, coastal closed systems, and open ocean/deep ocean commons. 

• Location considerations: Just as there are different governance implications for activities occurring in exclusive economic zones versus the high seas, these geographical differences may also carry importance for social impacts. Environmental or ecological impacts from activities taking place in near-shore waters may have more direct social impacts than activities taking place in the open ocean. 

• Food security: Microalgae-based interventions that risk disturbing food supply chains (e.g., fisheries) may threaten food security. This is especially relevant for communities that rely on the ocean for sustenance. 

• Impacts on livelihoods from tourism and fisheries: Activities that use or interfere with near-shore environments and ecology may impact industries like tourism and fisheries, thus impacting livelihoods. This is particularly important in areas that depend heavily on these industries economically and/or are socioeconomically vulnerable in general (e.g., the Global South).  

Social Co-Benefits 

• Economic incentives: The various potential utilization pathways of microalgae could create avenues for wealth generation and emissions-free economic growth- namely economic incentives within existing carbon markets, preventing costly environmental disasters resulting from uncontrolled climate change, and converting a damaging waste product (CO₂) into a value-added product. ^[10]Daneshvar, Ehsan, et al. “Biologically-Mediated Carbon Capture and Utilization by Microalgae towards Sustainable CO₂ Biofixation and Biomass Valorization – A Review.” Chemical Engineering Journal, vol. 427, Jan. 2022, p. 130884, https://doi.org/10.1016/j.cej.2021.130884.
• Job creation: Economic value-addition is likely to also create jobs in new and existing industries in aquaculture and beyond ^[11]Mayo-Ramsay, Julia. “Environmental, Legal and Social Implications of Ocean Urea Fertilization: Sulu Sea Example.” Marine Policy, vol. 34, no. 5, 2010, pp. 831–835., https://doi.org/10.1016/j.marpol.2010.01.004. .
• Ownership and benefits accruing from uses of genetic resources: Many microalgae-based innovations bring with them a suite of potential biotech and genomic-based approaches. It will be important to understand when and how local benefits rights holders (e.g., coastal communities, Indigenous communities) can benefit from the use of marine-based genetic resources. This is an inherently complex subject that questions ownership of marine resources and is rich with justice and equity implications.

Existing Governance Structures that May be Applicable to Microalgae for CDR 

• United Nations Convention on the Law of the Sea (UNCLOS): Adopted/ratified by 167 countries and the European Union, UNCLOS defines countries’ rights and responsibilities in terms of use and management of offshore areas. Provisions that may be relevant to microalgae cultivation and/or sequestration in the open ocean include: 
  • Part XIII of UNCLOS which establishes rules for “marine scientific research” 
  • Part XII of UNCLOS which requires countries to “protect and preserve rare or fragile ecosystems as well as the habitat of depleted, threatened or endangered species and other forms of marine life” and includes a number of specific provisions on marine pollution from vessels, land-based sources, activities in the area, and dumping, among others. Without clear distinctions on what is or is not considered “pollution”, these provisions may be relevant for microalgae-based CDR interventions.
  • ​​Proelss (2022) investigates the United Nations Convention on the Law of the Sea and finds that although it covers ocean fertilization research stages, it does not apply to ocean fertilization for CDR, leaving the legality of ocean fertilization unclear under international law.  
• Convention on Biological Diversity (CBD): Adopted in 1992, the CBD aims to protect biological diversity. 195 countries and the European Union have ratified it; the United States has signed but not ratified it. In 2008 the Conference of the Parties of the Convention on Biological Diversity recommended that ocean fertilization activities not take place until there exists a sufficient scientific basis to justify it. The term “ocean fertilization” was not defined. In 2010 a decision was made to regulate “geoengineering activities” such that no geoengineering activities take place that may impact biodiversity until there is a scientific basis to justify such actions. An exception is made for small-scale scientific experiments in controlled settings (the definition of “controlled settings” may prevent research in the open ocean) after thorough assessment and justification, though key terms in the exceptions for scientific experimentation are not defined, resulting in significant uncertainty as to when it will apply. 
• London Protocol / London Convention: The 1972 Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (London Convention) and the 1996 Protocol to the Convention (London Protocol) aim to limit ocean dumping. The London Protocol was intended to eventually replace the London Convention however not all countries party to the Convention have ratified the Protocol (for example, the United States). There is a lack of clarity and general disagreement concerning what the future of policy and governance application of ocean iron fertilization may look like, particularly with the precedent of the London Protocol, which has banned nonscientific application outright (Rohr, 2019). The London Protocol has not yet ruled on closed offshore systems and represents an opportunity to bring attention back to issues of ocean utilization for carbon dioxide removal. 
  • Amendment on Ocean CDR: This amendment, not yet entered into force, states: “Contracting Parties shall not allow the placement of matter into the sea from vessels, aircraft, platforms or other man-made structures at sea for marine geoengineering activities listed in annex 4 (an amendment that regulates the placement of matter into the ocean for marine geoengineering) unless the listing provides that the activity or the subcategory of an activity may be authorized under a permit.”
  • Statement on Marine Geoengineering: In October of 2022, parties to the treaties that regulate marine dumping adopted a statement calling out the need for evaluation of four marine geoengineering techniques that have the potential to mitigate the effects of climate change but could also threaten marine ecosystems. The four techniques given priority are: ocean alkalinity enhancement, macroalgae cultivation and sequestration (including artificial upwelling), marine cloud brightening, and microbubbles/reflective particles/materials. The London Protocol and London Convention Parties are conducting a technical and legal analysis to evaluate the next steps and future regulation. (See the International Maritime Organization’s briefing) 
  • GESAMP (Group of Experts on the Scientific Aspects of Marine Environmental Protection) Working Group 41: Ocean Interventions for Climate Change (formerly Marine Geoengineering): In early 2022, GESAMP’s Working Group 41 formally advised the London Protocol Parties to consider marine geoengineering techniques to list in the new annex 4 of the protocol. These geoengineering techniques are those that may assist in mitigating or reversing the impacts of climate change. Included in this list are approaches that utilize microalgae to sequester carbon for long-term storage. (Timeline of Ocean Fertilization under the London Protocol)