• Proof-of-concept field experiments have not been conducted in open ocean conditions to test growth rates, sequestration potential, and environmental impacts of macroalgal CDR pathways (Accelerate Design and Permitting of Controlled Field Trials)
• Siting analyses are needed to identify optimal nutrient, light, and wave conditions for growth; as well as potential conflicts with other marine industries (such as the NOAA Coastal Aquaculture Siting and Sustainability toolkit) (Develop New Modeling Tools to Support Design and Evaluation)
• A suite of tools and methodologies to estimate productivity, carbon capture, export, and sequestration, including direct and remote sensing approaches^[1]Bell TW, Nidzieko NJ, Siegel DA, Miller RJ, Cavanaugh KC, Nelson NB, Reed DC, Fedorov D, Moran C, Snyder JN, Cavanaugh KC, Yorke CE and Griffith M (2020) The Utility of Satellites and Autonomous Remote Sensing Platforms for Monitoring Offshore Aquaculture Farms: A Case Study for Canopy Forming Kelps. Front. Mar. Sci. 7:520223. doi: 10.3389/fmars.2020.520223 needs to be built (Develop New Modeling Tools to Support Design and Evaluation, Measure the Scale and Impacts of CDR via Macroalgae Sinking, Develop New In-Water Tools for Autonomous CDR Operations). 
  • For sinking pathways, the challenges of following the carbon from source (farm) to deposition (seafloor) in energetically active environments (horizontal and vertical currents, turbulent areas, etc.) 
• We do not understand enough about the net CDR benefit from a life cycle perspective (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials, Develop New In-Water Tools for Autonomous CDR Operations, Develop CDR Monitoring and Verification Protocols)
• Challenges exist verifying additional CO₂ uptake from the atmosphere to the ocean in a dynamic background as a result of macroalgae cultivation and sequestration CDR pathways (Hurd et al., 2023; Burger et al., 2023)(Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials, Develop New In-Water Tools for Autonomous CDR Operations)
• Inclusion of various macroalgal CDR into integrated assessment models to simulate and predict complex, global responses to macroalgal CDR. Examples include changes in CO₂ fluxes in other ecosystems via teleconnections (connections in Earth processes and non-continuous geographic regions, which are often caused by processes not immediately apparent from first principles), and to estimate permanence via the various macroalgal CDR pathways (Develop New Modeling Tools to Support Design and Evaluation)
• Increased knowledge base regarding the physiology (including heat tolerance) and genomics of a broader array of potential cultivars – beyond the ten most common – to understand their growth and carbon sequestration potential is needed (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials) 
• Better understanding of how large-scale macroalgae cultivation affects the partitioning of carbon between particulate and dissolved phases, and its implications for CDR (Develop New Modeling Tools to Support Design and Evaluation, Accelerate Design and Permitting of Controlled Field Trials)

*See Ocean Visions’ framework for global research on macroalgae cultivation and sinking which further outlines these knowledge gaps.

• Advances are needed in offshore mooring design, as well as viable, durable, and cost-effective farming systems for offshore cultivation technology (Bak et al., 2020).  (Develop New In-Water Tools for Autonomous CDR Operations)
• Current observational technologies (sensors, ROVs, AUVs, etc.) and modeling tools are not widespread or easily available to support field trials with the necessary spatial and temporal frequency of monitoring and sampling (Develop New In-Water Tools for Autonomous CDR Operations)
• Technologies to accelerate macroalgal sinking are not well explored or understood (Accelerate Design and Permitting of Controlled Field Trials, Develop New In-Water Tools for Autonomous CDR Operations)
• Wave-powered devices (both for electrical power and upwelling) are in their infancy (Develop New In-Water Tools for Autonomous CDR Operations)
  • Upwelling systems must be designed and tested to access deeper water nutrients and keep them in the euphotic zone where they can support macroalgal growth (as opposed to immediately sinking out of the euphotic zone)
• Harvesting and processing technologies that minimize environmental impact and energy use are needed (Develop New In-Water Tools for Autonomous CDR Operations)
  • More efficient means of drying the harvested crop (for non-sinking pathways)
• Technologies to scale shore-based hatcheries to produce more juvenile kelp ready for outplanting are needed (Develop New In-Water Tools for Autonomous CDR Operations)

• Training, skills development, and technology transfers are needed globally to expand available workforce for macroalgal cultivation and sequestration (Accelerate RD&D Through New Partnerships, Broaden Funding Base for RD&D, Growing & Maintaining Public Support) 
• Stable, increased research and development funding to build capacity in this sector, such as has been seen with the ARPA-E MARINER program in the US (Broaden Funding Base for RD&D)

• Certification standards for macroalgal carbon sequestration are needed to support developing markets for macroalgae CO₂ sequestration (Develop CDR Monitoring and Verification Protocols) 
• Scalable business models and markets to support demand for the multitude of potential products that can be derived from macroalgae (nutritional supplements, high value food items, additives, biochar, bioenergy, etc.) are needed to support this industry (Accelerate RD&D Through New Partnerships, Broaden Funding Base for RD&D, Growing & Maintaining Public Support) 
• A standard for mCDR is still needed. Evidence of a move towards a standard for seaweed can be seen in Verra’s Seascape Carbon Initiative.

While interests and awareness amongst researchers and technologists is growing (i.e., Pessarrodona et al., 2023; DeAngelo et al., 2022; Troell et al., 2024), public awareness and support remain low. Many of the obstacles and needs around building and maintaining public support are not specific to macroalgal cultivation and sequestration, but there are a few points of interest specific to macroalgal cultivation pathways (Growing & Maintaining Public Support):

• Industrial-scale farms could have a negative public connotation (e.g. corn fields/monoculture in the ocean)
• If genetically modified macroalgae were to be used to increase cultivation yields and sequestration potential, especially in the context of industrial-scale macroalgae farms, that might be perceived negatively by the public given public views on genetically modified organisms 
• Public perceptions of sinking macroalgal may differ from perceptions about conversion of cultivated macroalgae into high value bio products