Enhanced Biomass and Lipid Productivities of Outdoor Alkaliphillic Microalgae Cultures through Increased Media Alkalinity

Alkaliphillic microalgae have the ability to thrive under high pH and alkalinity conditions. The phototrophic biomass productivity of alkaliphillic microalgae can be improved by increasing availability of bicarbonate (HCO₃^-) at extreme pH conditions (pH >10.0) through increase in media alkalinity. The high alkalinity would allow the maintenance of sufficient “HCO₃^- buffer” in solution while high media pH would allow rapid transfer of atmospheric CO₂ to compensate for HCO₃^- uptake by photosynthetic mechanism. Thus, growth would not get restricted by inorganic carbon limitations. Further, extreme pH conditions would permit mixotrophic cultivation with sugars present in the medium.

In the present study, phototrophic and mixotrophic cultivation strategies were implemented under high alkalinity and pH conditions for alkaliphillic Chlorella Sp. (strain SLA-04) in 1000L outdoor raceway ponds. Under the phototrophic conditions, when the culture concentrations were below 0.6 g/L, biomass productivities of ~20 g/m²/day were obtained. While some of the HCO₃^- was utilized, ~ 100% of the carbon accumulated in biomass came from atmospheric CO₂. Under mixotrophic conditions, SLA-04 cultures grew without any measurable signs of contamination. In addition, due to availability of additional carbon source as well as the reducing equivalents, biomass productivity was significantly higher than phototrophic conditions (>57 g/m²/day). Lipid productivities were also higher for cultures grown under mixotrophic conditions (7 g/m²/day) when compared to phototrophic SLA-04 cultures (2 g/m²/day). Thus, the use of alkaliphillic microalgae for cultivation, would result in increased carbon capture from atmosphere and thereby reduces the costs accompanied with CO₂ sparging (usually associated with mesophillic cultures).