Process Engineering for High-Cell-Density Cultivation of Lipid Rich Microalgal Biomass of Chlorella sp. FC2 IITG

Abstract

In the present study, process engineering strategy was applied to achieve lipid-rich biomass with high density of Chlorella sp. FC2 IITG under photoautotrophic condition. The strategy involved medium optimization, intermittent feeding of limiting nutrients, dynamic change in light intensity, and decoupling growth and lipid induction phases. Medium optimization was performed using combinations of artificial neural network or response surface methodology with genetic algorithm (ANN-GA and RSM-GA). Further, a fed-batch operation was employed to achieve high cell density with intermittent feeding of nitrate and phosphate along with stepwise increase in light intensity. Finally, mutually exclusive biomass and lipid production phases were decoupled into two-stage cultivation process: biomass generation in first stage under nutrient sufficient condition followed by lipid enrichment through nitrogen starvation. The key findings were as follows: (i) ANN-GA resulted in an increase in biomass titer of 157 % (0.95 g L⁻¹) in shake flask and 42.8 % (1.0 g L⁻¹) in bioreactor against unoptimized medium at light intensity of 20 μE m⁻² s⁻¹; (ii) further optimization of light intensity in bioreactor gave significantly improved biomass titer of 5.6 g L⁻¹ at light intensity of 250 μE m⁻² s⁻¹; (iii) high cell density of 13.5 g L⁻¹ with biomass productivity of 675 mg L⁻¹ day⁻¹ was achieved with dynamic increase in light intensity and intermittent feeding of limiting nutrients; (iv) finally, two-phase cultivation resulted in biomass titer of 17.7 g L⁻¹ and total lipid productivity of 313 mg L⁻¹ day⁻¹ which was highest among Chlorella sp. under photoautotrophic condition.