Seaweed farms enhance alkalinity production and carbon capture

[Geoengineering News]

‎https://www.nature.com/articles/s44458-025-00004-8

‎

‎Authors: Mojtaba Fakhraee & Noah J. Planavsky 

08 January 2026

Abstract

Seaweed aquaculture is increasingly being explored as a sustainable source of food and industrial processing feedstock, as well as a potential climate solution through carbon dioxide removal. In this study, we use a sediment diagenetic model to quantify how elevated organic carbon fluxes beneath seaweed farms enhance sedimentary alkalinity fluxes, contributing to long-term carbon dioxide sequestration. Our stochastic simulations suggest that enhanced alkalinity production under seaweed farms could remove an average of ~0.85 tonnes of carbon dioxide per hectare per year (range: ~0.1–2 tonnes per hectare per year). These findings highlight a previously underappreciated mechanism of ocean-based carbon dioxide removal associated with seaweed farming. Monetizing this carbon removal—through voluntary carbon markets, policy incentives, or supply-chain insetting—could provide an additional income stream, supporting both the economic viability of seaweed aquaculture and its potential to scale as a climate mitigation strategy.

Source: Communications Sustainability 

[Greg Rau]

From the article:

"...we adopt CO₂ uptake efficiency values reported in the literature, which represent the fraction of added alkalinity converted to atmospheric CO₂ drawdown under equilibrium conditions as a function of surface seawater temperature, salinity, and atmospheric pCO₂15. Specifically, we apply uptake efficiencies in the range of 0.75–0.85 (i.e., 75–85%), consistent with values reported in previous studies (e.g., ref. 15). We note that these values assume thermodynamic and gas-exchange equilibrium and thus represent an upper limit on effective atmospheric CO₂ uptake."

If the authors are advocating that direct CO2 removal from air occurs via the alkalinity generated, this fails to appreciate that the alkalinity is already fully carbonated via respiration CO2 in the sediments. The CDR efficiency then relates to how well this C is retained by the OAE, not by the efficiency with which CO2 is drawn from air  by the alkalinity. The initial air capture is done upstream by the seaweed photosynthesis, and it is the prevention of the return air of this C (via the conversion of resp CO2 to alkaline C) that effects the CDR. However, given that anerobic respiration of biomass is much lower/slower than the aerobic case, won't the bulk of the CDR be occurring via undecomposed biomass storage? Also, what about elevated GHG (CH4, N2O etc) production under anaerobiosis?  More generally why not advocate for greater anerobic storage of marine biomass of whatever source to reduce/circumvent respiration CO2 returning to air, thus effecting CDR (https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2023AV000950 )? In any case, given the complexities, the MRV will not be a trivial exercise?

Greg

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[Michael Hayes]

If the biomass is used for ethanol production, and then further refined for ethylene production, the C can be locked up via high density polyethylene production. 

HDPE has ~84% C content. 

If the HDPE is used to create bio-reactors that, in turn grow the biomass, that would create a largely self-replicating mCDR infrastructure that is also a form of C storage.

As a side note on bio-ethylene production, a patented engineered microalgae can now produce ethylene as a metabolic waste. What I discribed above is the non-patented path.

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