Seaweed farming at scale

In plain language

Seaweed farming means growing large marine algae on ropes, lines or nets in coastal water and then harvesting them. FAO counts algae within its global fisheries and aquaculture statistics, and reported 37.8 million tonnes of algae production in 2022.

Two ideas have drawn attention beyond food. The first is that seaweed takes carbon dioxide out of seawater while it grows, and that the carbon might be locked away if the seaweed is buried in sediment or sinks to the deep sea. The second is that a red seaweed called Asparagopsis, added in small amounts to cattle feed, may sharply cut the methane that cows burp out. Scientists dispute the carbon idea. The methane idea rests on early trial results for one red seaweed.

Technical explanation

Seaweed fixes carbon through photosynthesis, so a farm draws down CO2 as it grows. Whether that uptake becomes a long-term climate benefit depends on what happens to the biomass. Duarte and colleagues (2017) propose that carbon from seaweed farms may be buried in sediments or exported to the deep sea, so that the farm acts as a CO2 sink. Gallagher and colleagues (2022), as described in a 2023 comment, suggested that seaweed ecosystems are net heterotrophic carbon sources, because they respire organic carbon from outside the system. Filbee-Dexter and colleagues (2023) reply that the 2022 conclusion rests on 18 studies whose net ecosystem production measurements are highly variable, and that this does not statistically support it. They argue that most seaweed forests are net autotrophic, meaning they are carbon sinks, and they acknowledge that substantial uncertainties remain.

For livestock, Vijn and colleagues (2020), in a workshop perspective, summarise a steer trial by Kinley and colleagues (2020). In that trial, Asparagopsis at 0.10% and 0.20% of dietary dry matter, fed for 90 days, reduced methane production in steers by up to 40% and up to 98%. The perspective calls for replication by other research teams. This profile could not check the primary Kinley paper, so the diet, the measurement method and the effects on intake and rumen function are not stated here. The high figure should not be read as a general cattle result.

At industrial scale, the FAO reported 37.8 million tonnes of algae within 223.2 million tonnes of global fisheries and aquaculture production in 2022. Scaling carbon removal would need very large areas. David Siegel of UC Santa Barbara, quoted by Latitude Media, says that scaling seaweed sinking is still hypothetical and that farms would need to grow to at least a million square kilometres. Latitude Media also quotes researchers who question whether alkalinity enhancement or biomass storage can be measured accurately, and who are concerned that proper verification could greatly raise costs.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• Whether seaweed ecosystems are net carbon sinks or sources is disputed. The 2022 and 2023 papers reach opposite conclusions, and the 2023 comment says the 2022 measurements are highly variable.
• The 98% methane reduction comes from one trial, reported at 0.20% of dietary dry matter over 90 days. This profile could not confirm the diet type, measurement method or animal performance effects from the primary paper. The 40% figure at 0.10% has the same single-trial basis.
• Measurement, reporting and verification for sinking or storing seaweed biomass is hard to do accurately and may be costly, according to researchers quoted by Latitude Media.
• FAO statistics group seaweed with other algae, so the sources reviewed here do not give a separate, verified figure for farmed seaweed alone.
• Storms, disease and warm water can damage crops. The sources reviewed here did not give verified loss figures for most farming regions.
• Bromoform, a compound found in Asparagopsis, is linked to the methane effect. This profile has not checked the safety or residue evidence on bromoform.

Possible future uses

Timeline

1. 12 Apr 2017Duarte and colleagues publish a Perspective in Frontiers in Marine Science on whether seaweed farming can contribute to climate change mitigation and adaptation. Sources: [2]
2. 23 Dec 2020Vijn and colleagues publish a workshop perspective that summarises the Asparagopsis steer trial by Kinley and colleagues, reporting methane reductions of up to 40% at 0.10% and up to 98% at 0.20% of dietary dry matter over 90 days. Sources: [4]
3. 2022Gallagher and colleagues suggest that seaweed ecosystems are net heterotrophic carbon sources, as described in the 2023 comment that disputes them. Sources: [3]
4. 6 Jul 2023Filbee-Dexter and colleagues publish a comment in ICES Journal of Marine Science disputing the 2022 conclusion. Sources: [3]
5. 2024FAO's SOFIA 2024 newsroom release reports 37.8 million tonnes of algae in 2022 production. Sources: [1]
6. 2024Running Tide, an ocean carbon-removal company whose main activity was sinking wood, announces its shutdown and blames financing. Latitude Media reports that its later pitch included a shift toward seaweed. Sources: [5]

Futures this could help bring about

How far away?

Cutting cattle methane with seaweed feed

Pilots · 2 of 7 steps done

Related breakthroughs

• Pelleted Asparagopsis seaweed cut methane 37.7% on average in grazing steers (2 Dec 2024)

Glossary

Blue carbon

Carbon captured by coastal and marine ecosystems, such as seagrass, mangroves and salt marshes, and seaweed, that can be stored in sediments.

Net ecosystem production

The balance between carbon taken up by photosynthesis and carbon released by respiration across an ecosystem. A positive value means the ecosystem is a net carbon sink.

Autotrophic and heterotrophic

Autotrophic systems produce more organic carbon than they use, so they take up CO2 overall. Heterotrophic systems use more than they produce and release CO2 overall.

Enteric methane

Methane produced by microbes in the digestive system of ruminants such as cattle, released mainly when they burp.

Sources

1. government report
   FAO report: Global fisheries and aquaculture production reaches a new record high (SOFIA 2024)
   Food and Agriculture Organization of the United Nations, 2024. Accessed 21 Oct 2026.
2. peer reviewed paper
   Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation?
   Carlos M. Duarte, Jiaping Wu, Xi Xiao et al.. Frontiers in Marine Science, 12 Apr 2017. DOI: 10.3389/fmars.2017.00100. Accessed 21 Oct 2026.
3. peer reviewed paper
   Seaweed forests are carbon sinks that may help mitigate CO2 emissions: a comment on Gallagher et al. (2022)
   Karen Filbee-Dexter, Albert Pessarrodona, Carlos M. Duarte et al.. ICES Journal of Marine Science, 6 Jul 2023. DOI: 10.1093/icesjms/fsad107. Accessed 21 Oct 2026.
4. peer reviewed paper
   Key Considerations for the Use of Seaweed to Reduce Enteric Methane Emissions From Cattle
   Sandra Vijn, Devan Paulus Compart, Nikki Dutta et al.. Frontiers in Veterinary Science, 23 Dec 2020. DOI: 10.3389/fvets.2020.597430. Accessed 21 Oct 2026.
5. news
   What Running Tide's demise means for carbon removal's future
   Maeve Allsup. Latitude Media, 24 Jun 2024. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.