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 even in the absence of external CO₂sparging. Further, extreme pH conditions would permit mixotrophic cultivation with sugars present in the medium, with lower risk for contamination than at circumneutral pH.

In the present study, phototrophic and mixotrophic cultivation of an alkaliphillic Chlorella sp. (strain SLA-04) was investigated under high alkalinity and pH conditions in 1000L outdoor raceway ponds without CO₂ supplementation. Under the phototrophic conditions, biomass productivities of ~20 g/m²/day were obtained and carbon balance calculations indicated that ~80% of the biomass-C was derived from atmospheric CO₂, rather than from the added bicarbonate. Under mixotrophic cultivation with glucose, SLA-04 cultures grew without any measurable signs of contamination and showed significantly higher biomass productivity (>53 g/m²/day) due to availability of additional organic carbon and associated reducing equivalents. Lipid productivities were also higher for under mixotrophic conditions (7 g/m²/day) relative to phototrophic SLA-04 cultures (2 g/m²/day). Overall, our results suggest that cultivation of alkaliphilic microalgae allows for biofuel production even without co-location with a concentrated CO₂ source.