Maximizing CO2 biofixation and lipid productivity of oleaginous microalga Graesiella sp. WBG­1 via CO2-regulated pH in indoor and outdoor open reactors

Carbon dioxide (CO2) and pH are two interdependent factors that greatly impact the growth and lipid accumulation of microalgae. However, the effects of these two factors are usually studied separately. The use of exogenous CO2, such as flue gas derived, to regulate pH in the large-scale cultivation of microalgae provides an ideal means for combining CO2 biofixation and biodiesel production. In this study, the CO2 biofixation and lipid production of oleaginous microalga Graesiella sp. WBG­1 was explored for four pH levels regulated by exogenous 15% CO2 (flue gas concentration) in 10L circular culture ponds and 5m2 open raceway reactors. Results revealed that pH8.0–9.0 was the optimum pH for CO2 fixation and lipid production, attaining the highest CO2 fixation rates of 0.26gL−1day−1 and 18.9gm−2day−1, respectively, lipid contents of 46.28% and 32.38%, and lipid productivities of 64.8mgL−1day−1 and 3.14gm−2day−1. A positive correlation between CO2 utilization efficiency and pH in open reactors was also suggested in this research, and thus provides direction for screening of CO2 fixation by microalgae. The present study provides an excellent strategy for coupling CO2 fixation and lipid production via microalgae in large-scale cultivation. [Display omitted] •The lipid production of WBG1 under 4 CO2-regulated pH levels was evaluated.•The culture has a maximum CO2 fixation rate under pH8.0–9.0.•Highest biomass, lipid content and productivity were obtained under pH8.0–9.0.•There is a positive correlation between CO2 utilization efficiencies and pH values.•Provide a strategy for coupling and maximizing the CO2 fixation and lipid production.