Effects of Dissolved Inorganic Carbon, pH, and Light on Growth and Lipid Accumulation in Microalgae

A primary objective of this study is to investigate the feasibility of using sodium bicarbonate (NaHCO3) as a buffer to increase dissolved inorganic carbon (DIC) concentrations in a culture medium for the growth of microalgae and the effects of DIC concentrations, pH and light on growth and lipid accumulation in microalgae. Another objective is to investigate the feasibility of removing residual nutrients such as nitrogen and phosphorus from wastewater using microalgae. C. vulgaris was used to remove residual NH3/NH4+ and PO 43- from secondary wastewater effluent. C. vulgaris could effectively remove nitrogen and phosphorus under autotrophic growth, and the removal rate could be promoted by a high initial biomass concentration (e.g. ~350 mg/L). A Monod model was used to express the growth kinetics with a limiting substrate.;This study has found that NaHCO3 can play a critical role as an excellent buffer that can keep the DIC concentration high within an appropriate pH range for the growth of microalgae. The use of high DIC concentrations can significantly increase the growth rate of C. vulgaris and N. oleoabundans under a nutrient-sufficient condition. However, a sodium (Na+) ion concentration should not exceed ~60 mM for C. vulgaris and ~100 mM for N. oleoabundans because the high salinity derived from Na+ might limit their growths.;A high DIC concentration did not significantly impact on the lipid accumulation in N. oleoabundans under a nutrient-sufficient condition. However, under nitrogen deprivation, a high DIC concentration could help significantly increase neutral lipid accumulation in N. oleoabundans . In addition, pH control was found to play an important role in total lipid accumulation in N. oleoabundans under nitrogen deprivation condition.;The growth of C. vulgaris increased with an increase in incident light intensity (Iin). However, the specific growth rate with respect to average light intensity (I) decreased when I in increased (photoinhibition effect). The overall specific growth rate significantly decreased due to an increase in biomass concentration (photolimitation). Based the above observations, a model for the growth of C. vulgaris under a nutrient-sufficient condition was constructed by taking into account the specific growth and average light intensity. The model could predict the growth of C. vulgaris with a reasonably good accuracy in terms of incident light intensity and reactor size.