Oceanic Algal Growth for CO₂ Removal: Why the Concept Excels on Paper but Falls Short in Practice
Abstract
Marine algal cultivation has often been presented as a natural extension of blue carbon strategies and an apparently limitless route to atmospheric carbon dioxide (CO₂) removal. While the biological principles are sound - photosynthetic uptake of dissolved inorganic carbon (DIC) and conversion into organic biomass - the translation of these processes into genuine, certifiable greenhouse gas removals remains unresolved. This paper reviews the underlying mechanisms, the scientific and engineering constraints, and the reasons why open-ocean or sub-aquatic algal systems are unlikely to deliver verifiable CO₂ removal at scale under current frameworks.
1. Introduction
The idea of using marine algae to mitigate climate change is intuitively appealing. Oceans cover more than 70 per cent of the planet, macroalgae such as kelp can grow at extraordinary rates, and seawater already contains dissolved carbon that can be photosynthetically fixed. A growing number of start-ups and research initiatives have proposed large-scale algal farming, open-ocean fertilisation, or sub-aquatic installations as potential carbon dioxide removal (CDR) pathways.
However, when the chemistry, physics, and verification requirements of greenhouse gas removal are examined in detail, most of these approaches face fundamental barriers. The challenge is not that algae cannot grow — but that growth alone does not equal atmospheric CO₂ removal.
2. Mechanism of carbon uptake
Algae — micro or macro — do not absorb gaseous CO₂ directly from the atmosphere. Instead, they draw on the dissolved inorganic carbon present in seawater, primarily bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.
When algae photosynthesise, they lower the local concentration of DIC and hence the partial pressure of CO₂ (pCO₂) in the surrounding water. In theory, this local drawdown can cause additional CO₂ to diffuse across the air–sea interface to restore equilibrium. The degree to which this occurs depends on temperature, salinity, wind mixing, and other factors that govern gas exchange.
In practice, only a fraction of the carbon fixed by marine algae represents new atmospheric CO₂ removal; the rest simply redistributes carbon within the ocean’s existing inorganic pool.
3. Why the concept looks attractive
From a systems perspective, oceanic algal growth has several features that appear favourable:
Abundant area: vast ocean surfaces theoretically available for cultivation.
No land conversion: avoids direct competition with agriculture.
Low input medium: seawater provides the growth environment and major nutrients can, in principle, be recycled.
Visual simplicity: the notion of “growing carbon sinks” is easy to communicate.
These advantages make oceanic algae an attractive research topic and a recurring theme in popular discussions of negative emissions. Yet they mask the chemical and logistical realities of converting oceanic photosynthesis into measurable removals.
4. Why it fails as a greenhouse gas removal method
4.1 Not direct atmospheric CO₂ removal
Marine algal photosynthesis draws on DIC rather than directly capturing atmospheric CO₂. The subsequent equilibration between seawater and air is partial and slow, and depends on environmental conditions that are highly variable. Quantifying how much atmospheric CO₂ is actually displaced therefore requires complex coupled ocean–atmosphere modelling.
Without this quantification, the process cannot meet the accounting standards required for inclusion in regulated GGR or carbon credit systems.
4.2 Uncertain permanence
Even when marine biomass production is high, the carbon is seldom stored permanently. If algae decompose near the surface or are consumed by marine organisms, the carbon quickly returns to seawater and, ultimately, to the atmosphere.
Long-term storage would require either:
deliberate sinking of biomass to depths greater than 1 000 m, or
conversion of harvested material into stable forms such as biochar or bio-oil on land.
Both options introduce energy, cost, and environmental risks. Deep-ocean deposition, in particular, raises ecological and legal concerns under the London Convention on ocean dumping.
4.3 Poor scalability and resource intensity
Modelling studies suggest that to remove even 1 Gt CO₂ yr⁻¹ through macroalgal sinking, vast ocean areas — on the order of hundreds of thousands of square kilometres — would be required, along with large nutrient inputs and extensive infrastructure for mooring, harvesting, or towing. The logistical footprint and cost exceed most realistic deployment scenarios.
4.4 Lack of certifiable measurement, reporting, and verification (MRV)
Under voluntary and emerging compliance markets, a removal pathway must demonstrate additionality, verifiability, and permanence. For open-ocean systems, none of these can currently be guaranteed.
Monitoring air–sea CO₂ fluxes and deep-ocean carbon burial requires continuous chemical and isotopic measurements that are infeasible at operational scale. The UK Government’s Independent Review of Greenhouse Gas Removals (2025) acknowledges that “marine CDR remains scientifically intriguing but is not yet suitable for inclusion in national inventories” due to unresolved MRV and ecological impact issues.
5. Sub-aquatic installations and near-shore concepts
Several organisations are developing sub-aquatic algae installations within ports, harbours, or coastal zones, often with additional aims such as water quality improvement or biodiversity enhancement. These systems may provide local environmental benefits, but from a carbon accounting standpoint they remain subject to the same limitations:
Uptake is from seawater, not directly from the atmosphere.
Biomass decomposition typically returns carbon within short timeframes.
Quantitative verification of net CO₂ removal is not yet possible.
Such projects therefore contribute to mitigation or adaptation rather than to certified carbon removal.
6. Pathways toward credible alternatives
Progress in this field is not without merit. Marine biology and coastal restoration play important roles in ecosystem health and carbon cycling. The key lesson is that controlled, measurable systems are required for true removals accounting.
Closed or semi-closed photobioreactor systems — operating on seawater media but with defined process boundaries and stable end-of-life carbon storage — offer a more defensible route to permanence. These approaches maintain the advantages of marine feedstocks while avoiding the uncertainties of open-ocean operations.
7. Conclusions
Oceanic algal growth represents an elegant concept that leverages nature’s largest carbon reservoir, yet it does not currently satisfy the scientific or regulatory criteria for greenhouse gas removal. The process is indirect, difficult to measure, and unlikely to provide permanent storage without significant intervention.
Until measurement technologies and regulatory frameworks advance substantially, oceanic algae should be regarded as a valuable component of marine ecosystem management, not as a scalable or certifiable pathway for carbon dioxide removal.
References
Boyd, P. W. et al. (2024). Macroalgae open-ocean mariculture and sinking (MOS) as a potential CO₂ removal pathway: constraints and uncertainties. Earth System Dynamics, 14, 185–210.
Krause-Jensen, D. & Duarte, C. M. (2016). Substantial role of macroalgae in marine carbon sequestration. Science, 354(6319), 70–74.
Department for Energy Security and Net Zero (2025). Independent Review of Greenhouse Gas Removals. GOV.UK, March 2025.
Keller, D. P. et al. (2023). The status of marine carbon dioxide removal research and readiness. Annual Review of Marine Science, 16, in press.
MBARI (2024). Measurements of oceanic CO₂ uptake following phytoplankton bloom development. Technical memorandum.
