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 CO ₂ delivery to algal cultures. To assess the impact of culture pH on growth rate and uptake of CO ₂, 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 CO ₂ uptake ability at extreme as at neutral pH values. Cultures of the alkaliphilic species captured a higher proportion of CO ₂ from the gas stream than the acidophilic species; removing 50 to 65% of CO ₂ from air compared to only 38% removed by acidophilic species (or 10–24% from CO ₂ enriched air). Alkaliphilic species did not become carbon limited when fed CO ₂ at the concentration provided by air (0.04% CO ₂), but produced less biomass and captured less total CO ₂ (0.06 to 0.08 g CO ₂ per day) than the acidophilic species (0.6–0.8 g CO ₂ day ⁻¹) which required CO ₂ enriched air to avoid carbon limitation. Bicarbonate feeding reduced the loss of CO ₂ to the environment, compared to feeding gaseous CO ₂, but with a potential cost in reduced specific growth rate or biomass production.