Bioreactor-based optimization of photoautotrophic nannochloropsis gaditana culture

Abstract

Oleaginous microalga Nannochloropsis gaditana (N. gaditana) is a promising candidate as a biofuel precursor owing to its fast growth rate and high lipid content. In this study, stepwise development of a novel photobioreactor-based cultivation system is established to increase lipid productivity of N. gaditana. At first, profiles of solar irradiation wavelength were adjusted using three different fluorescent dyes to enhance blue, red, and green light spectrum. Enhancement of blue (400-500 um) and red (550-700 um) wavelengths was discovered to be effective in increasing biomass productivity by 35.1 % and 40.3 %, respectively. Lipid productivity was also increased more than 25 % using fluorescent dyes for blue- and red-targeted. Meanwhile, when incident light was suddenly switched from enriched red to green light, lipid content immediately increased by 24.2 % compared to control. Secondly, a new concept of cultivation system was developed by connecting four flat-panel photobioreactors (PBRs) in serial arrangement, thereby operating as one integrated chemostat reactor. This multi-stage continuous cultivation system, as a whole, naturally creates gradients of nutrient/biomass/lipid concentrations. Thus, a fourth reactor was able to discharge a concentrated biomass (1.11 g L^{-1}) along with high lipid content (30.0 % FAME). Moreover, cells grown under this system showed 46.1 % higher lipid productivity than that of conventional single-reactor system. Finally, the wavelength shift strategy was applied to multi-stage continuous cultivation PBRs. Each PBR operated as growth-promoting/lipid-promoting/lipid-accumulating unit. Growth promotion of N.gaditana was established by enriched red light, resulted in 24.2 % higher biomass and enriched green light allowed 45.9 % total lipid as a product. This novel cultivation system developed in this study marked 1.96-fold enhanced lipid productivity compared to that of conventional PBR, suggesting a potential application for biofuel production industry.