Algae-Based Carbon Capture: Quantifying Feasibility Through Cost and Scalability

Introduction

Fig. 1: Annual Increases in Carbon Dioxide. [1] (Source: Wikimedia Commons)

Rising atmospheric carbon dioxide (CO₂) concentrations represent one of the defining environmental challenges of the 21st century. According to data from the World Meteorological Organization (WMO), the global mean atmospheric CO₂ level reached approximately 420 parts per million (ppm) in 2024, an increase of more than 140 ppm since pre-industrial times. Shown in Fig. 1, this rate continues to grow at about 2 to 3 ppm per year. [1] The accumulation of CO₂ in the atmosphere is driving climate change, motivating the search for scalable carbon dioxide removal (CDR) strategies that can complement emissions reductions.

Most existing CDR initiatives focus on terrestrial systems, such as reforestation, bioenergy with carbon capture and storage, and soil carbon management. However, marine photosynthetic organisms such as microalgae and macroalgae (seaweeds) efficiently fix atmospheric CO₂ through photosynthesis and can convert it into biomass that may be stored or utilized for bioenergy production. This analysis provides a quantitative comparison of two carbon capture approaches: microalgae cultivation and seaweed farming. The goal is to analyze each individually, evaluating its technical feasibility, scalability, and long-term sequestration potential. Using data from peer-reviewed studies, we analyze each methods carbon fixation rate, areal productivity, costs, and major engineering and ecological constraints.

Microalgae-Based Carbon Capture

Microalgae are unicellular photosynthetic organisms capable of converting solar energy into chemical energy with high efficiency. Typical species can transform 9 to 10% of incident solar radiation into biomass. [2] Industrial-scale cultivation in photobioreactors or open ponds can yield 30 to 77 g m^-2 day^-1 of dry biomass, corresponding to approximately 110 to 280 tons per hectare per year. [2] Given that one kilogram of algal biomass fixes about 1.83 kilograms of CO₂, a 1 km microalgae cultivation facility would capture roughly 20,000 to 51,000 tons of CO₂ per year under typical operating conditions. [2]

Microalgae possess multiple characteristics favorable for sustainable carbon capture. They do not require arable land or freshwater, can thrive in saline or wastewater conditions, and do not compete with food crops. Moreover, their composition, typically 50 to 70% lipids, with some species such as B. braunii reaching 80% oil content, makes them ideal for biofuel production. [2] Microalgae-based biofuels can substitute for fossil fuels, offering a closed carbon loop when coupled with CO₂ recycling. Additionally, algae can be co-located with industrial facilities to utilize CO₂ as a feedstock, enhancing overall mitigation efficiency. [2] The combustion of algal biofuels releases previously captured CO₂, resulting in carbon neutrality if the systems life-cycle emissions are minimized.

Economic assessments estimate that microalgae-based fuels become competitive when crude oil prices exceed $100 per barrel, with modeled production costs between $2.95 and $5.00 per gallon of gasoline equivalent. [2] These costs are influenced by nutrient supply, harvesting, and dewatering processes, which collectively account for more than half of total operational expenses. However, coupling algal systems with wastewater treatment or nutrient recycling can substantially lower these costs while providing additional environmental benefits.

Despite their high productivity, large-scale implementation of microalgae systems faces several obstacles. Cultivation requires substantial energy input for mixing, lighting, and temperature control, which can offset a portion of captured CO₂. [2] The energy return on investment remains low unless systems are optimized for passive sunlight and natural nutrient sources. The feasibility threshold for global impact is notably steep. Offsetting 1% of annual CO₂ emissions (~35 Gt CO₂) would require thousands of square kilometers of continuous microalgae cultivation, significant capital investment, and extensive infrastructure. Overall, microalgae cultivation offers a high theoretical carbon capture rate per unit area, but its scalability is constrained by economic and energetic inefficiencies rather than biological potential.

Seaweed Farming and Macroalgal Carbon Sequestration

Seaweed aquaculture is currently the fastest-growing sector of global food production. In 2014, it has an annual production exceeding 27 million tons and an average growth rate of 8% per year. [3] Seaweeds, which include kelps and other macroalgae, are highly autotrophic and absorb CO₂ at rates that can exceed 1.5 Pg C yr^-1 globally when wild and cultivated stocks are considered. [3] The carbon content of seaweed dry biomass averages 24.8%, and typical aquaculture yields are approximately 1,604 tons dry weight per km² per year, corresponding to a CO₂ sequestration intensity of about 1,500 tons CO₂ km^-2 yr^-1. [3] Although this represents only a fraction of global emissions, it is a high sequestration density compared with other crops. Seaweed farming also covers a very small spatial footprint, roughly 1,600 km² worldwide, or 0.004% of agricultural land, indicating substantial room for expansion. [3] Macroalgae already plays an important role within the broader Blue Carbon framework, a worldwide initiative aimed at restoring carbon-rich coastal ecosystems. Krause-Jensen and Duarte estimated that about 173 Tg C yr^-1 is ultimately sequestered in sediments or the deep ocean, comparable to the sequestration from all other Blue Carbon habitats combined. [4]

Despite promising sequestration intensity, seaweed farming is not without constraints. Expansion is limited by suitable coastal area availability, nutrient supply, and engineering challenges associated with offshore cultivation in high-energy environments. [3] Additionally, increasing production could depress market prices, potentially discouraging investment. If global production were scaled by a factor of 100, seaweed aquaculture could sequester on the order of 0.30 Gt CO₂ per year, still below the gigaton-level removals needed for substantial global impact. However, when combined with ecosystem co-benefits, seaweed farming represents the most immediately scalable and verifiable marine carbon capture pathway available today.

Comparative Discussion

To evaluate the relative feasibility of these methods, it is useful to compare their quantitative characteristics. In the equations below, A_algae and A_seaweed are metrics for areal productivity (dry biomass). C_algae and C_seaweed are metrics for carbon capture rate. The masses are reported as dry weight: [3]

The CO₂-per-biomass factors used for microalgae and seaweed follow directly from their measured carbon content. Microalgae contain roughly 50% carbon by dry weight due to their high lipid and protein fractions, as documented in Khan et al. [2] Applying the stoichiometric ratio (44 g CO₂ per 12 g C) gives: 0.50 44/12 ≈ 1.8 kg CO₂ per kg biomass. Seaweeds, by contrast, contain much lower carbon fractions (24.8% on average) because a large portion of their dry mass is inorganic ash (20 to 49% dw). [3] There is also a large fraction of structural polysaccharides, as quantified in the recent biochemical analysis in Ferreira et al. [5] Using the same conversion yields: 0.248 44/12 ≈ 0.9 kg CO₂ per kg biomass. These two values therefore arise directly from the experimentally measured carbon content of each biomass type, and are consistent with field-scale CO₂/biomass ratios reported in the literature.

The quantitative comparison shows that microalgae substantially outperform seaweed farming in both areal biomass productivity and CO₂ capture potential, with values an order of magnitude higher in each category. Seaweed aquaculture, while measurably productive, operates at a far lower flux of biomass and carbon per unit area and therefore yields a more modest sequestration potential. These differences reflect the inherent biological and physiological growth limits of each system rather than downstream processing choices. However, the operational and economic context shifts this comparison. Microalgae cultivation requires continuous energy input for mixing, aeration, and harvesting, and is limited by high capital and operating costs associated with dewatering and biomass processing. [2] Seaweed farming, on the other hand, benefits from passive nutrient delivery, lower infrastructure demands, and naturally occurring pathways for long term carbon storage via deep-sea export and sediment burial. [3] These practical considerations mean that microalgae, despite superior per-area performance, face significant scalability constraints, while seaweed provides a more operationally feasible though less potentapproach to marine-based carbon capture.

Conclusion

Overall, no single marine method currently offers the gigaton-scale removal required for significant climate stabilization. However, integrating these approaches into broader Blue Carbon initiatives can contribute meaningfully to regional mitigation and adaptation strategies. Future research should prioritize life-cycle carbon assessments, offshore engineering innovations, and standardized monitoring systems to quantify long- term sequestration. As atmospheric CO₂ levels continue to climb, advancing such scalable, solutions will be essential for a comprehensive climate response.

To avoid ambiguity in the expression of large masses: A teragram (Tg) is defined as 10¹² g, which is equivalent to 10⁹ kg or 10⁶ tonnes (a megatonne). A petagram (Pg) is 10¹⁵ g, equal to 10¹² kg or 10⁹ tonnes (a gigatonne). Likewise, in climate literature a gigatonne (Gt) of CO₂ is often used interchangeably with 1 Pg of CO₂, corresponding to 10¹⁵ g or 10¹² kg.

© Lauren Korsnick. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "WMO Greenhouse Gas Bulletin, No. 21," World Meteorological Organization, October 2025.

[2] M. I. Khan, J. H. Shin, and J. D. Kim, "The Promising Future of Microalgae: Current Status, Challenges, and Optimization of a Sustainable and Renewable Industry For Biofuels, Feed, and Other Products," Microb. Cell Fact. 17, 36 (2018).

[3] C. M. Duarte et al., "Can Seaweed Farming Play a Role in Climate Change Mitigation and Adaptation?" Front. Mar. Sci. 4, 100 (2017)

[4] D. Krause-Jensen and C. M. Duarte, "Substantial Role of Macroalgae in Marine Carbon Sequestration," Nat. Geosci. 9, 737 (2016).

[5] H. S. Ferreira et al., "Assessing High-Value Bioproducts From Seaweed Biomass: A Comparative Study of Wild, Cultivated and Residual Pulp Sources," Appl. Sci. 15, 5745 (2025).