Seaweed &
Macroalgae Cultivation
THS v1.0 - Annex M
Seaweed and macroalgae cultivation grows large brown, red, or green algae species on engineered offshore or nearshore infrastructure - longlines, rafts, or anchored cultivation arrays - capturing atmospheric CO₂ through photosynthesis at growth rates far exceeding most terrestrial biomass. Depending on the final disposition of the harvested biomass - sunk to the deep ocean floor, processed into durable products, or converted to biochar - this pathway can achieve durable carbon sequestration while also supplying valuable co-products like food, animal feed, or biostimulants.
How this pathway works
Seaweed and macroalgae - kelp, sargassum, and other large marine algae - grow extremely rapidly under favourable nutrient and light conditions, some species capable of growing over half a metre per day. Cultivated on engineered longline, raft, or ring-buoy infrastructure anchored offshore, macroalgae capture atmospheric CO₂ through photosynthesis at a rate that, aggregated across large cultivation arrays, can represent a meaningful carbon flux. The durability of the resulting carbon benefit depends entirely on what happens to the harvested biomass: sunk to the deep ocean floor beyond the depth where decomposition releases carbon back to surface waters, it can achieve durable Class II sequestration; processed into food, feed, or biostimulant products, the carbon benefit is generally shorter-lived (Class I) unless converted to biochar or another stable form.
Under the Teravent Hybrid Carbon Standard (THS v1.0) Annex M, this pathway is classified as hybrid because reliable crediting depends equally on the biological cultivation system and the engineered infrastructure - moorings, harvest vessels, and the sinking or processing technology determining the biomass's ultimate carbon fate. Teravent Hybrid Credits (THC) are issued for verified net tonnes of CO₂-equivalent sequestered, net of full lifecycle project emissions.
THS v1.0 - Annex M
This pathway is governed exclusively by the Teravent Hybrid Carbon Standard (THS v1.0). No external registry, standard, or methodology is referenced or incorporated.
Three approved methodology variants
THS v1.0 Annex M approves three methodology types, differentiated by the final disposition of harvested biomass.
Cultivated macroalgae is harvested and deliberately sunk - either naturally through weighted attachment or via engineered sinking mechanisms - to a confirmed depth below which decomposition products (CO₂) are effectively isolated from the surface ocean-atmosphere carbon exchange for multi-century timescales. Sinking depth, ocean circulation patterns at the site, and biomass composition are all critical variables determining whether the sequestration achieves genuine long-term durability.
- Harvested biomass mass and carbon content, metered per harvest event
- Sinking depth confirmation via tracked sensors or weighted marker buoys
- Ocean circulation model validated for the specific cultivation and sinking site, confirming isolation from surface exchange
- Deep-sea ecosystem impact monitoring at the sinking location
Harvested macroalgae biomass is dried and pyrolysed into biochar, following the same H/Corg stability protocol used for terrestrial biochar pathways, then applied to agricultural soil or, in some designs, used as a soil amendment in coastal restoration projects. This methodology extends the marine biomass's carbon durability well beyond the raw sinking timeline, at the cost of the additional drying and pyrolysis energy input.
- Harvested biomass mass and drying/pyrolysis energy per batch
- Biochar H/Corg atomic ratio tested per production batch
- Biochar application rate metered at ±5% accuracy, with salt content assessed for agricultural soil compatibility
- Cultivation infrastructure and harvest vessel operations documented
Some macroalgae species yield alginate or other polymers suitable for processing into bioplastics, packaging materials, or textile fibres - an emerging application where the harvested carbon is embedded in a manufactured product with a documented service life, similar in concept to synthetic carbon materials or bio-based building products, though the durability here depends heavily on the specific product category and its eventual disposal or degradation pathway.
- Harvested biomass mass and processing yield to finished product
- Independent engineering or materials science service-life assessment for the specific product category
- Product distribution and end-of-life disposition tracking
- Cultivation infrastructure operations and biomass carbon content per harvest
Which emission sources must be counted
Measurement, reporting
& verification
Harvested biomass is directly weighed at harvest, giving very high confidence in gross carbon input. For deep-ocean sinking (SWD-M01), the defining MRV challenge is confirming sinking depth and ocean circulation isolation - requiring tracked sensor confirmation and a site-validated ocean circulation model. For biochar (SWD-M02), the standard H/Corg stability protocol applies. For durable products (SWD-M03), an independent service-life assessment specific to the product category is required.
Demonstrating additionality
Leakage types & deductions
Buffer pool & NPRR assessment
| Methodology | Durability Class | Buffer Pool Rate | Primary Reversal Risks |
|---|---|---|---|
| SWD-M01 Deep-Ocean Sinking | Class II | 10–25% | Ocean circulation model uncertainty; shallower-than-planned sinking |
| SWD-M02 Biochar Conversion | Class II | 7–20% | Incomplete pyrolysis; application-site erosion or salt toxicity |
| SWD-M03 Durable Products | Class I (default) / Class II (verified) | 10–35% | Premature product disposal; early-stage material performance uncertainty |
Key registration criteria
Sustainable Development
Goal alignment
Deployment scope: Coastal and offshore regions with suitable water depth, temperature, and nutrient conditions for macroalgae growth - currently active in East Asia, the Pacific coast of the Americas, and parts of Northern Europe.
Ready to register your
seaweed cultivation project?
Submit a Project Concept Note under THS v1.0 Annex M to begin your registration. Select the SWD-M code matching your biomass disposition, confirm your monitoring approach, and appoint an accredited VVB to validate your PDD.