Use of flux balance analysis to promote lipid productivity in Chlorella sorokiniana

Chlorella is a potential sustainable resource for biodiesel due to its outstanding ability of accumulating lipids under different nutrient limitations. However, one of the major technological bottlenecks for its industrialization is biomass and lipid production by the algae. In this study, the heterotrophic culture and metabolic flux analysis were performed for Chlorella lipid accumulation. The cell density of Chlorella sorokiniana reached the maximum in heterotrophic culture after 12 days. The lipid content of the Chlorella increased with high glucose and low NaNO3 concentration. The highest lipid content of the Chlorella was 320 mg g−1 dry cell weight under 35 g L−1 glucose, 0.125 g L−1 NaNO3, and 12 days of culture. A metabolic network of 34 reactions including Embden–Meyerhof–Parnas pathway (EMP), hexose monophosphate pathway (HMP), tricarboxylic acid cycle (TCA), and fatty acid biosynthesis was constructed. The fluxes of EMP, HMP, and TCA increased with the decrease of NaNO3 concentration. Comparison of fluxes among different culture stages suggested that the flux of fatty acid biosynthesis at the final stage was higher than that at the initial stage. As a result, the lipid content and lipid productivity reached 389.46 mg g−1 dry cell weight and 0.6713 mg mL−1 under the optimal combination of NaNO3, glucose concentration, and harvesting time at day 12 of culture. This study depicted a constructive network of Chlorella lipid biosynthesis and elucidates the mechanism of lipid biosynthesis under NaNO3 limitation. Furthermore, our results showed that the Chlorella lipid has a favorable fatty acid composition for biodiesel.