Characterization and Optimization of a Fermentation Process for the Production of High Cell Densities and Lipids Using Heterotrophic Cultivation of <i>Chlorella protothecoides</i>

This article reports an in-depth characterization and optimization of a fermentation process with the aim of increasing the production of high cell densities as a prerequisite for high productivity of lipids. The freshwater microalgae Chlorella protothecoides was grown heterotrophically in a bioreactor on aerated and stirred cultivation medium with glucose as a carbon source and yeast extract as a nitrogen source. The detailed effect of carbon to nitrogen (C/N) ratio on growth and lipid content of C. protothecoides was studied. Results show that with a C/N ratio lower than 12, the medium was carbon-limited, favoring cell proliferation, and under nitrogen-starvation conditions, (higher C/N ratios), lipid accumulation was enhanced. A two-stage, fed-batch process was developed and achieved a maximum of 255.3 g/L of biomass over 228 h of fermentation, from a low initial cell density (&lt; 2 g/L). The specific growth rate, measured when C/N ratio was low, was 0.078 h −1 . The lipid productivity was 16.7 g/L/day, and the maximum lipid content was 58 % (w/w) of the dry cell weight. This is the first time that such a very high cell density and lipid productivity have been achieved. The results offer potential for the production of important algae-derived commercial products.