High Productivity Cultivation of Microalgae without Concentrated CO2 Input

Outdoor cultivation of microalgae for biofuel production is not yet economically feasible, in part due to costs associated with CO2 supply. Large point sources of CO2 are also rarely available in proximity to desired large-scale algae cultivation sites (nonagricultural, barren, and flat lands). The cost and location constraints posed by concentrated CO2 supply can be avoided by cultivation of microalgae that have the ability to grow under extreme alkaline pH conditions (pH > 10). These alkaline solutions are effective at scavenging atmospheric CO2 at high rates and, in addition, afford natural protection for desired alkaliphilic microalgae cultivars against common competing organisms and predators. In this study, we report the cultivation of an extreme alkaliphilic microalgae isolate, Chlorella sorokiniana str. SLA-04 in a high pH (pH > 10) medium that also contains a high carbonate/bicarbonate alkalinity (>100 mequiv·L–1). This medium design simultaneously provides (i) a high driving force to effectively scavenge atmospheric CO2 due to the high medium pH and (ii) nonlimiting bicarbonate concentrations (>20 mM) for photosynthetic carbon fixation. We demonstrated high biomass productivities (>16 g·m–2·d–1) in 4.2 m2 raceway ponds by using this medium and without addition of concentrated CO2 (i.e., using atmospheric CO2 alone). Experimental measurements and theoretical mass transfer calculations show that rates of CO2 uptake into the medium from the ambient atmosphere matched or exceeded the rates of dissolved inorganic carbon uptake due to photosynthesis. Further, measured lipid productivities were either higher or comparable to those reported for other raceway pond microalgae cultivations that used concentrated CO2 sparging. This is the first study to report high biomass and lipid productivities of phototrophic microalgae cultures sustained with atmospheric CO2 alone.