Dynamics of carbon dioxide capture in two halotolerant strains of Chlorella vulgaris

Microalgae-based systems provide a renewable approach for carbon capture to address the escalating challenge of rising global carbon dioxide (CO2) levels. This study examined carbon availability and uptake in two halotolerant Chlorella vulgaris strains under varying nutrient and carbon supply regimes. CO₂ supply defined saturation levels in algae-free culture media; a 2–8 fold increase in saturation was noticed for 1–100 % CO₂ in the inlet supply. In microalgae cultures, continuous carbon supply was seen to lead to carbon depletion, affecting growth and carbon uptake efficiency. Continuous air supply (0.04 % CO₂), led to a decrease of dissolved inorganic carbon (DIC) from 1.4 mM at the start to near zero by day 8. Nutrient supplementation enhanced DIC uptake and biomass productivity; an 8 fold increase in nitrates and phosphates supplied led to a 1.7 fold increase in DIC for both strains and a 2-fold increase in biomass productivity for one of the strains. Passive carbon supply regimes exhibited higher carbon fixation efficiency compared to active regimes. Nitrate significantly increased carbon capture, with a quadratic relationship identified between DIC levels and biomass production. These findings reveal for the first time within species differences in the dynamics of carbon availability and uptake, offering a basis for microalgal CO2 capture into scalable systems such as flue gas utilization or wastewater-fed photobioreactors. Whilst large-scale deployment requires further techno-economic validation, this study highlights a sustainable, biobased CO2 capture process that avoids hazardous chemicals, offering a scalable alternative aligned with green chemical engineering principles for climate mitigation.