(460a) Effects of Dissolved Inorganic Carbon and Mixing On Growth and Neutral Lipid Formation of Chlorella Vulgaris for Open Pond Cultivation

The autotrophic growth of microalgae in an open pond system is an economical and sustainable option for the utilization of CO₂ gas and next generation biofuels production.  Carbon dioxide (CO₂(aq)) and bicarbonate ion (HCO₃^-) are the two inorganic carbon species used for the autotrophic growth.  However, an effective supply of the two dissolved inorganic carbon (DIC) species in the culture medium is critical for the cultivation in open pond systems when sparingly soluble CO₂ gas is added within a narrow pH window suitable for the growth.  In this study, sodium bicarbonate (NaHCO₃) was found to be an excellent buffer that can keep the DIC concentration without any significant loss for open systems within a benign pH window suitable for the growth.  The use of NaHCO₃ demonstrated its capability to significantly enhance the growth rate under agitation by increasing the DIC concentration in the open culture media.  A mass-transfer study based on a mass balance model exhibits that the external mass-transfer resistance between the bulk medium and the cell surface is insignificant regardless of the different initial DIC concentrations (15 and 144 mg carbon/L) and mixing speeds (0, 125, and 550 rpm) used in the study.  The photosynthesis reaction was found to be a rate-limiting step under the conditions, and to be strongly dependent on the DIC concentrations.  This result shows that high DIC concentrations can override the carbon concentrating mechanism required for the growth in natural settings.  The result also supports the use of NaHCO₃ for fast microalgal growth and effective CO₂ supply to the medium.  The NaHCO₃ concentrations between ~40 and 60 mM showed the maximum growth of C. vulgaris and neutral lipid content did not show any significant increase for all cultures in nutrient-sufficient conditions.