Cultivated kelp take up large quantities of carbon in its dissolved inorganic form (DIC), releasing oxygen and providing ecosystem services locally.

Cultivated Seaweed and Blue Carbon: Hype or Real Promise for Climate Fight?

SINTEF Research Unveils Potential of Seaweed Cultivation as Carbon Sink, While Questions Remain on Quantification

OSLO – Growing attention to "blue carbon" from seaweed cultivation has brought focus to its potential to mitigate climate change. New research led by SINTEF, the Norwegian research organization, delves into how cultivated macroalgae can contribute to carbon capture, shedding light on the complexities of quantifying this process and its additional benefits.

While the idea of macroalgae as natural carbon dioxide (CO2) sinks was first recognized over 40 years ago, the discussion about carbon flows and sequestration opportunities from cultivated algae is more recent. Although seaweed farming alone won't solve climate change, "as a crop we are still learning to use, it has a good head start," SINTEF's research indicates.

Saccharina latissima growth in April (left) and bryozoan colonies spreading on the lamina in June (right), making the kelp brittle and easily eroded from the cultivation facility. Photo: SINTEF.

Seaweed is a versatile, rapidly growing biomass that requires no fertilizers, pesticides, or fresh water. It offers a multitude of applications, from food and feed to bioplastics and carbon dioxide removal (CDR) products. Furthermore, seaweed farms attract high biodiversity and filter excess inorganic nutrients from seawater, releasing oxygen through photosynthesis.

Challenges in Quantifying Carbon Loss

SINTEF's study focuses on quantifying how much organic carbon is naturally lost from a seaweed farm, in what form it occurs (particulate or dissolved, labile or recalcitrant), and how this can be incorporated into cultivation strategies.

Schematic representation of the main processes investigated in this study: carbon (and nitrogen) uptake and storage in stipe (S), meristem (ME), mid lamina (ML) and distal lamina (DL); kelp length growth, erosion (POC losses) and exudation (DOC losses). Modified from Broch et al., 2019. Source: Particulate and dissolved organic carbon losses in high latitude seaweed farms

According to the research, seaweed biomass continuously loses small fragments of biomass and biomass-bound carbon to the surrounding environment. An ongoing PhD work aims to quantify these losses. Data from autumn and winter deployments of Saccharina latissima (sugar kelp), monitored at SINTEF Ocean’s research facility in Skarvøya, Hitra, for two consecutive years, show that by June-July, as much as 45-65% of the CO2 taken up by the kelp was passively released into the ocean as kelp particles and dissolved organic compounds (DOC).

Dr. Sissel Albrektsen, Senior Researcher at Nofima, explained in previous studies that the loss of particulate and dissolved carbon depends on the timing of seedling deployment and biomass harvest. On average, for each ton of kelp harvested in April, June, and July, 8, 18, and 28 kg of carbon, respectively, were lost to the environment.

Studying DOC release over a 24-hour period from a Winter deployment (top row), an Autumn deployment (middle) and seawater controls (bottom). Photo: SINTEF.

The Holy Grail: Recalcitrant Dissolved Organic Carbon (RDOC)

The key to carbon sequestration lies in recalcitrant dissolved organic carbon (RDOC), a form of carbon resistant to degradation that can remain out of atmospheric exchange for hundreds or thousands of years. Microbes break down particulate and dissolved organic carbon into these more recalcitrant forms.

According to scientific literature, 5-78% of the DOC released by macroalgae is refractory (RDOC), and an additional 0.12% to 0.3% of particulate organic carbon may be converted into RDOC. SINTEF's research is generating the first data on the DOC composition of Saccharina latissima, one of Europe's most commonly farmed seaweeds, an area with significant variability and non-standardized methods.

Particulate and dissolved organic carbon losses in high latitude seaweed farms with 500 t production capacity. Values are averages for 2022–2023 relative to C-NPP (%) and biomass volumes, for different harvest times targeting different markets. Tonnes C in cultivated biomass is the carbon remaining in kelp tissue after losses are accounted for in one cultivation cycle. Source: Neves et al., 2025. Click on the image to enlarge it

Implications for Policy and Industry's Future

These findings are crucial not only for understanding natural processes and monitoring environmental impact but also for informing decision-makers and recognizing the active contribution of the seaweed farming industry to society and the environment.

Japan has already officially incorporated seaweeds into its national greenhouse gas accounting, with "J-Blue credits" valued at around 400 USD per ton of CO2. These credits are not exclusively carbon credits but are "stackable" to account for other ecosystem services that, although known to exist, are difficult to quantify.

The question now is whether Norway can follow Japan's example to create a coherent and credible carbon(+) accounting system, and whether the political will to support financial incentives that flow back to seaweed farmers and those who restore natural forests will materialize, helping these activities grow sustainably.

This work is part of the PhD thesis "Seaweed Cultivation as a Climate Positive Solution" (RCN 323324), funded by the Research Council of Norway.

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