Cultivation of Microalgae at Extreme Alkaline pH Conditions: A Novel Approach for Biofuel Production

A major challenge to the economic viability of outdoor cultivation of microalgae is the high cost of CO2 supply, even when microalgae farms are co-located with point sources of CO2 emissions. In addition, the global capacity for algae biofuel generation is severely restricted when microalgae farm locations are constrained by proximity to CO2 sources along with the additional limitations of low slope lands and favorable climate. One potential solution to the impediments of CO2 supply cost and availability is through cultivation of microalgae in highly alkaline pH solutions (pH >10) that are effective at scavenging CO2 from the atmosphere at high rates. The extremely alkaline pH media would also mitigate culture crashes due to microbial contamination and predators. In this study, we report the indoor and outdoor phototrophic cultivation of a microalgae isolate (Chlorella sorokiniana str. SLA-04) adapted to grow in unusually high-pH environments. The isolate was cultivated in a growth medium at pH >10 without any inputs of concentrated CO2. Both indoor and outdoor studies showed biomass and lipid productivities that were comparable to those reported for other microalgae cultures cultivated in near-neutral pH media (pH 7-8.5) under similar conditions. SLA-04 cultures also showed high lipid productivity and high glucose-to-lipid conversion efficiency when cultivated mixotrophically in the presence of glucose as an organic carbon source. From the energy content (calorific value) of the lipids produced and glucose consumed, a relatively high amount of lipid calories (0.62) were produced per glucose calorie consumed. In conclusion, our results demonstrate the feasibility of microalgae cultivation in extremely high-pH media (pH >10) as a novel strategy for biofuel production without dependence on concentrated CO2 inputs.