Effects of dissolved inorganic carbon and mixing on autotrophic growth of Chlorella vulgaris

•NaHCO3 can be used for fast microalgal growth and effective CO2(g) supply to medium.•High DIC concentration with mixing greatly enhances autotrophic growth.•Use of NaHCO3 under mixing helps overcome DIC limitation for photosynthesis.•Photosynthesis is limited by DIC supply followed by light during autotrophic growth.•Use of NaHCO3 is limited by salinity generated by Na+ for freshwater algae. Carbon dioxide (CO2(aq)) and bicarbonate ion (HCO3−) are the two inorganic carbon species used for the autotrophic growth of microalgae. An effective supply of the two dissolved inorganic carbon (DIC) species in the culture medium is critical when sparingly soluble CO2 gas is added within a narrow pH window suitable for the growth. In this study, sodium bicarbonate (NaHCO3) was used as an excellent buffer that can keep the DIC concentration high without its significant loss for open systems within a benign pH window for the growth of Chlorella vulgaris. The use of NaHCO3 along with CO2 gas under agitation could significantly enhance the growth rate by overcoming the DIC limitation for photosynthesis. The photosynthesis reaction was found to be limited by DIC concentration at an initial growth stage and subsequently by light availability at a later growth stage. A high concentration of NaHCO3 helps increase a DIC concentration for photosynthesis, but was found to be limited by its salinity generated by Na+ for this freshwater green alga, C. vulgaris.