Microalgal CO₂ capture at extreme pH values

Although algae are often grown at pH values between 6 and 8, shifting to more alkali or acidic conditions may benefit CO₂ delivery to algal cultures. To assess the impact of culture pH on growth rate and uptake of CO₂, we grew three relatively fast growing acidophilic (Coccomyxa sp., Euglena mutabilis and Euglena gracilis) and three alkaliphilic (Thalassiosira pseudonana, Phaeodactylum tricornutum, Chlamydomonas sp.) algal species at pH values near neutral and near the extreme of their growth range. All six species showed similar growth and CO₂ uptake ability at extreme as at neutral pH values. Cultures of the alkaliphilic species captured a higher proportion of CO₂ from the gas stream than the acidophilic species; removing 50 to 65% of CO₂ from air compared to only 38% removed by acidophilic species (or 10–24% from CO₂ enriched air). Alkaliphilic species did not become carbon limited when fed CO₂ at the concentration provided by air (0.04% CO₂), but produced less biomass and captured less total CO₂ (0.06 to 0.08 g CO₂ per day) than the acidophilic species (0.6–0.8 g CO₂ day⁻¹) which required CO₂ enriched air to avoid carbon limitation. Bicarbonate feeding reduced the loss of CO₂ to the environment, compared to feeding gaseous CO₂, but with a potential cost in reduced specific growth rate or biomass production.