Glucose feeding recalibrates carbon flux distribution and favours lipid accumulation in Chlorella protothecoides through cell energetic management

Heterotrophic culture of algal cells is now known as an efficient avenue for lipid production. However, the mechanisms involved are not clearly understood, especially when feeding glucose concurrently to autotrophic culture condition. In this work, the time course dynamics of central carbon metabolites of Chlorella protothecoides was studied in autotrophic, heterotrophic and mixotrophic cultures to elucidate the effect of glucose on the cell metabolic reorganization. Results show that assimilated CO2 mainly goes to the synthesis of upstream carbohydrate-based metabolites under autotrophic condition, while supplementing glucose recalibrates the metabolism towards downstream metabolites and lipids, rather than carbohydrates accumulation. The analysis of the lipid class shows, under glucose supplementation, that cells accumulate neutral lipids as storage rather than as membrane polar lipids, while fatty acid composition changes from polyunsaturated to saturated and monosaturated, which shows improving the quality of biodiesel precursors. The metabolic flux rearrangement seemed being regulated by a high cell energetic state that was maintained by a glucose metabolism. A high initial ATP-to-ADP ratio was observed after adding glucose, suggesting cell energetics as a biomarker of a metabolic shift from starch to lipid accumulation. These findings thus bring novel data on the regulation of carbon flow in microalgal cells, and enhance our understanding of microalgae as a lipid production platform.