Regulation of carbon metabolic fluxes in response to CO2 supplementation in phototrophic Chlorella vulgaris: a cytomic and biochemical study

As one of the promising species of microalgae for biofuel production, Chlorella vulgaris CS-42 was cultivated phototrophically in two cylindrical photobioreactors with aeration of 5 % (v/v) CO2 or air for 13 days to evaluate the effects of CO2 supplementation on biomass, CO2 fixation performance, and biochemical content. Significant increases of specific growth rate and total carbon content in biomass resulting in a higher CO2 fixation rate were found with 5 % CO2. The maximum biomass concentration, carbohydrate and fatty acid contents with 5 % CO2 were significantly higher than those with air, while carbohydrate biosynthesis was most affected as compared to other biochemical components. Cytomic analysis revealed a rapid accumulation of neutral lipid in the late growth phase with more lipid bodies visualized by confocal laser scanning microscopy (CLSM), when nitrate consumption was accelerated with CO2 supplementation. Gas chromatography mass spectrometry (GC-MS) analysis indicated that 5 % CO2 favored the formation of C18:2, which led to a decrease in the degree of lipid unsaturation (DLU). These results proved that CO2 supplementation was one of the most efficient methods to significantly prompt the growth of microalgae and increase the C/N ratio in the medium, which in turn regulated the carbon metabolic flux to enhance neutral lipid and fatty acid production in C. vulgaris.