Carbon dioxide removal (CDR) is a term used to describe anthropogenic activities that directly or indirectly remove carbon dioxide (CO2) from the atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products. Marine carbon dioxide removal (mCDR) is a subset of CDR approaches that leverage the ocean to remove CO2 and/or store captured CO2 in ocean reservoirs.  

Macroalgae, or seaweed, are fast-growing marine organisms that use photosynthesis to incorporate carbon from seawater into their living tissue. Macroalgae flourish in coastal and some pelagic environments and a portion of the seaweed produced is exported naturally to the deep sea where it may be buried or remineralized by marine food webs (Wu et al., 2023; Krause-Jensen & Duarte, 2016). Slow turnover times in most deep ocean environments will sequester carbon released to bottom waters for centuries or longer (Siegel et al., 2021). Recently, there have been proposals to cultivate macroalgae and then sink its biomass to the deep ocean to sequester carbon from the atmosphere. In theory, this could amplify the natural process of seaweed carbon sequestration and could remove additional carbon dioxide from the atmosphere (GESAMP 2019; NASEM 2022) however, significantly more research is needed to understand the potential efficacy and impacts from both environmental and social standpoints.

The cultivation of seaweed has been practiced for centuries. Asia continues to dominate seaweed mariculture, producing 99.5% of global yields, with the vast majority coming from just four countries: China, Indonesia, the Republic of Korea, and the Philippines. In 2015 the global production of seaweed totaled 30.4 million tonnes, of which over 1.1 million tonnes were wild harvest and 29.4 million tonnes were cultivated (in freshwater, brackish, and saltwater). Chile, China, and Norway were the leading producers of wild species while China, Indonesia, the Republic of Korea, and the Philippines led in cultivated species. Today, the global seaweed industry is worth over USD 6 billion per annum. In 2018, 32.4 million tonnes of aquatic algae were wild-collected and cultivated with a majority of macroalgal farming taking place in coastal waters (FAO 2018). Macroalgae is used in a vast array of food and non-food items globally but has yet to be farmed specifically for CDR purposes. Although several start-ups are proposing this with some beginning field trials and demonstration projects.  While historically dominated by a small number of countries, algae farming is becoming increasingly popular in Europe and North America, with a particular interest in climate change mitigation and contribution to the blue economy. There are wide knowledge and technology disparities across the globe with Asian countries, the European Union, and the United States leading the way with the most advanced farming practices including integrated multi-trophic aquaculture (Chopin & Tacon, 2020; García-Poza et al., 2020). History shows successful long-term cultivation and farming of seaweeds, and a revived interest in this aquatic organism promises great potential for innovation and climate change mitigation solutions (Duarte et al., 2017).

1. Carbon Capture – The ultimate CDR potential of large-scale macroalgal cultivation and sequestration is as yet difficult to determine. Recent consensus reports cite the following as possible carbon dioxide removal potential from macroalgae cultivation and sequestration:
   • 0.1 – 1.0 Gt CO₂/year (NASEM 2022)
   • 0.1 – 0.6 Gt CO₂/ year (NOAA 2023)

Theoretically, macroalgal cultivation and sequestration  could be scaled to between 0.1 – 1.0 Gt CO₂/year (NASEM 2022, NOAA 2023), but realized CDR may be much lower due to a number of factors, including:

• • Competition for space with existing ocean stakeholders – commercial shipping,  commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc. 
  • Challenges with expanding to offshore environments – engineering of moorings  and farm structures, challenges with operations and maintenance in distant environments (Bak et al., 2020; Lovatelli et al., 2010), and the likely associated cost increases (at least at first)  associated with offshore operations
  • Most open ocean environments require nutrient additions to sustain macroalgal yields. If these nutrients come from the deep ocean, they will be accompanied by high CO₂ water that can be released and will offset macroalgal carbon sequestration to some degree (which needs to be quantified) (Pan et al., 2016; Oschlies et al., 2009).
    • If macroalgal farms are located closer to the coastal zone, excess anthropogenic nitrogen inputs could support ~3.5 million km2 of macroalgal cultivation, corresponding with an estimated CDR range of 5- 10 Gt CDR annually (NASEM workshop comments from Carlos Duarte, 2021), alleviating competition for “natural” nutrient supplies with phytoplankton and alleviating the need for an upwelled supply of nutrients (and the associated risks of releasing CO₂ from upwelled deep ocean waters).
  • It is hypothesized that macroalgae-based pathways could supply CDR at moderate cost ($25-125 per ton of CO₂ removed) (Energy Futures Initiative, 2020; Capron et al., 2020; NASEM, 2022), but these cost estimates are preliminary and subject to change as macroalgae-based pathways advance in technological readiness.
    • Researchers at UC Irvine have developed a biophysical-economic model (G-MACMODS) to better understand the geophysical, biological, and economic sensitivities of macroalgae-based CDR pathways (DeAngelo et al., 2022). Also, see the affiliated site suitability tool.

2. Sequestration Permanence -For cultivated macroalgae to contribute to CDR, the carbon captured in the macroalgal tissue needs to be sequestered to prevent its decomposition and return to the atmosphere. Sequestration pathways include:

• • Intentionally sinking the macroalgae (pre- or post-processing) into the deep ocean (>1000 meters depth) where it can be sequestered from contact with the atmosphere for hundreds to a few thousand years (if remineralized in the deep ocean) or for thousands-to-millions of years (if buried in marine sediments) (GESAMP 2019).
    • There are natural analogs to this process already. In macroalgal systems, already ~50% of natural macroalgal production breaks off and becomes dissolved or particulate organic matter. Some of the particulate organic matter is transported to the deep ocean, where the carbon is sequestered (Krause-Jensen & Duarte, 2016).
  • Relying on sedimentary burial below or adjacent to the farm (typically more applicable for farms not located over deep water) (Duarte et al., 2023)
  • Harvesting the macroalgae for:
    • Bioenergy (Hughes et al., 2013)
      • Combining combustion pathways with carbon capture and storage (thousands-to-millions of years if stored in a geologic reservoir) (Mereira & Pires, 2016)
      • Pyrolysis, resulting in biochar (hundreds to thousands of years) (Zhang et al., 2020; Roberts et al., 2015)
    • Production of long-lived bioproducts, such as bioplastics, that are capable of permanent (>100 years) sequestration (Albright & Fujita, 2023; Chia et al., 2020)

3. Differentiating from Avoided Emissions – Macroalgae can provide a number of other services that either reduce or replace greenhouse gas emissions, but do not remove legacy carbon dioxide pollution from the atmosphere. One example of reducing emissions: red algal supplements added to cattle feed may play a key role in reducing enteric methane emissions from cattle by over 50% (Roque et al., 2019; Vijn et al., 2020). Given methane’s potency as a greenhouse gas, such emissions reductions may play an outsized role in slowing planetary temperature increases through the reduction of greenhouse gases. Macroalgae biomass may also be converted into short-lived, high-value bioproducts, such as food and nutritional supplements (Biris-Dorhoi et al., 2020). This process may contribute to avoided emissions if it substitutes for carbon-intensive feedstocks, but it does not represent CDR because of the short timescale over which these products sequester carbon before it returns to the atmosphere. Although the focus of this roadmap is on macroalgae pathways for CDR, many of the obstacles faced, development needs, and near-term opportunities for advancement are relevant to these associated pathways for avoided greenhouse gas emissions.

Chemical 

• Localized buffering/reductions in ocean acidification due to CO₂ uptake. (Koweek et al., 2016; Hirsh et al., 2020; Kapsenberg & Cyronak 2019; Fernández et al., 2019; Xiao et al., 2021)

Biological

• Nutrient remediation and metal uptake in eutrophied, polluted coastal waters (Neveux et al., 2018)
• Building macroalgae cultivation facilities near shellfish or fish aquaculture facilities may alleviate negative impacts from such activities (NASEM 2022) (e.g., deoxygenation, eutrophication)

Physical

• Macroalgae farms may attenuate wave energy (Mork, 1996)
• Creation of habitat with resulting nurseries for fish and other marine life (Smale et al., 2013)

Other

• Carbon capture at sea may reduce the demand for terrestrial-based CDR alternatives that compete for land and/or freshwater

Chemical

• Macroalgae are known to release bromoform and other halomethanes (Carpenter et al., 2009; Mehlmann et al., 2020) , and thus, large-scale macroalgae cultivation seems likely to increase the release of these substances. This needs additional research as the natural marine sources of these gases are currently estimated to be responsible for around 9% of stratospheric ozone loss, including depletion due to anthropogenic causes (Tegtmeier et al., 2015).
• Production of methane, nitrous oxide, and other potentially hazardous gases by the macroalgae
• The potential for CO₂ outgassing from pumping deep water to the surface  (artificial upwelling) to supply needed nutrients for the macroalgae (Pan et al., 2015).
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific
• The remineralization of sunken seaweed may lead to oxygen depletion and acidification (Wu et al., 2023, Ocean Visions 2022)

Biological

• Impacts to biodiversity and ecosystem function from large-scale cultivation operations, including (Campbell et al., 2019):
  • Enhanced disease and parasite risk
  • Alteration of population genetics
  • Introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO₂ release
• Reduced phytoplankton production in and around large macroalgae farms due to competition for nutrients and light
• Ecological and biogeochemical (especially acidification and hypoxia) impacts in the deep sea from sinking large quantities of macroalgae into the deep ocean
  • Such effects are likely to be location-specific

Physical

• Changes in light and nutrient availability (including possible changes in ocean albedo)
• In near-coastal zones, changes in circulation patterns due to drag by the seaweed could change residence time in nearshore environments
  • Changes in coastal residence time may affect the prevalence and intensity of harmful algal blooms (HABs)
• Entanglement of marine megafauna (e.g., whales)

Other

Addition of noise pollution due to vessel traffic and machinery

Enhance the Blue Economy: There is increased interest in the potential for seaweed aquaculture and its potential to increase economic opportunities for seaweed farmers if practices are modified to increase climate benefits and if these benefits are sufficiently monetized (Duarte et al., 2017).

• Job Creation: Seaweed farming could also present new job opportunities for fishers whose work is threatened by climate change (e.g., seaweed farming at Atlantic Sea Farms).
• Value-Added Products: High-value bioproducts (pigments, lipids, proteins, etc) can replace more carbon-intensive alternatives in feed, food, fuel, and other commodities (Albright & Fujita, 2023).

1. Competition for Space: Competition for space with existing ocean stakeholders – commercial shipping, commercial fishing, indigenous rights, marine protected areas, non-seaweed aquaculture, etc.
   • Potential for entanglement of macroalgae cultivation equipment with shipping, commercial fishing gear, aquaculture farms etc, particularly if the macroalgae farm is free floating
   • Protected sites include marine protected areas, world heritage sites, culturally significant areas, treaty-protected resources, ecologically or biologically significant marine areas, sensitive or vulnerable marine ecosystems, marine protected areas, and other effective area-based conservation measures.
2. Competition for Food: Given that macroalgae can be converted to nutrient dense food stuffs, there may also be social resistance to this method as it involves the willful destruction of viable food sources that could be used in furtherance of human food security (Stedt et al., 2022)
3. Financing: Financing any CDR approach brings with it the risk of creating inequity and decreasing social welfare (Cooley et al., 2022). With macroalgae cultivation, this may be especially applicable to coastal dwelling communities who rely on the ocean for their livelihoods, food, and cultural meaning.