Effects of Dissolved Inorganic Carbon and Mixing on Autotrophic Growth of Chlorella vulgaris

An open pond system is an economical and sustainable option for the autotrophic growth of microalgae using CO₂ gas.  Carbon dioxide (CO₂(aq)) and bicarbonate ion (HCO₃^-) species are the two dissolved inorganic carbon (DIC) species used in the aqueous phase for the autotrophic growth.  From this perspective, the two DIC species should be effectively supplied for the cultivation in open systems when sparingly soluble CO₂ gas is added to the aqueous phase in a narrow pH window suitable for the growth.  In this study, sodium bicarbonate (NaHCO₃) was used as an excellent buffer than can keep the DIC concentration high without any significant loss for open systems within a benign pH window suitable for the growth.  The use of NaHCO₃ along with CO₂ gas under mixing could significantly promote the growth rate by overcoming the DIC limitation for photosynthesis in the open systems.  Our mass-transfer study shows that the external mass-transfer resistance between the bulk medium and the cell surface is insignificant regardless of the different initial DIC concentrations and mixing speeds.  The autotrophic growth is was found to be limited by photosynthesis and/or internal mass transfer at an initial growth stage and subsequently by light availability at a later growth stage.  This result shows that high DIC concentrations can override the well-known carbon concentrating mechanism required for the growth in natural settings.  A high concentration of NaHCO₃ helps keep the DIC concentration high for photosynthesis, but the concentration was found to be limited by its salinity generated by sodium ion (Na⁺) for Chlorella vulgaris.