Increased biomass productivity and atmospheric CO2 capture by alkaliphilic microalgae under higher alkalinity conditions

Highly alkaline solutions (pH ≥ 10) absorb CO₂ rapidly from atmosphere. Hence, alkaliphilic microalgal strains, which thrive and grow in alkaline media, constitute an efficient means of atmospheric CO₂ capture. In media with pH > 10, the dissolved carbon is mainly in the form of HCO₃^– and CO₃^–2 species, with the proportion CO₃^–2 relative to HCO₃^– increasing as the pH rises. Since HCO₃^– is the only species assimilated by algae and the pH of the medium increases with algal growth, the amount of HCO₃^– available for growth becomes limited when the alkalinity of the medium (i.e., buffer concentration (Na₂CO₃/NaHCO₃) is low. Because of low diffusivity of CO₂ in water maintaining pH is expensive process for open pond algal cultivation. In the present study effect of increasing the media alkalinity on algal growth and the resulting improvement in CO₂ sequestration was evaluated by cultivating alkaliphilic microalgae Chlorella Spp. in open ponds (25 L and an initial pH of 9.9) under pH uncontrolled conditions. Biomass productivity of 32 g/m²/day was observed for cultures grown at an alkalinity of 86 mM compared to 14 g/m²/day seen at an alkalinity of 11 mM. This improved growth rate correlated well with better atmospheric CO₂ capture: carbon capture of 8 g/m²/day was observed at 86 mM alkalinity vs. 4.2 g/m²/day at 11 mM alkalinity. These results indicate that maintaining higher alkalinity for growth medium during open-pond cultivation of alkaliphilic algae is a valuable strategy that leads improved biomass productivity as well as superior atmospheric CO₂ capture.