A kinetic model of heterotrophic and mixotrophic cultivation of the potential biofuel organism microalgae Chlorella sorokiniana

Better microalgal growth performance under heterotrophic and mixotrophic conditions can advance production of environmentally sustainable, lipid-based third-generation biofuels. In this study, an unstructured kinetic growth model was developed by extending a Haldane-based model to account and predict the enhancing and/or inhibitory effects of substrate (i.e. initial glucose concentration, nitrogen concentration, and light intensity) on the growth and lipid accumulation rates of the microalgae Chlorella sorokiniana during its heterotrophic and mixotrophic cultivation. The model considered a nitrogen-deprived phase of cultivation during which lipid synthesis and accumulation rates increase. Model-derived estimations for glucose and nitrogen consumption, cell growth, and lipid production under heterotrophic and mixotrophic conditions, respectively, were validated and in good agreement with experimental measurements, with R2 correlation values ranging from 0.83 to 0.99. Sensitivity analysis showed that the reaction rate constant of nitrogen consumption (K-NX) has a "significant effect " on both lipid production and nitrogen consumption, suggesting that rapid depletion of nitrogen is favorable to increase lipid content instead of cell growth. The rate of cell growth declined on the 4th day during heterotrophic cultivation but remained constant after the 4th day during mixotrophic cultivation under initial glucose concentrations of 2.29, 4.32, and 6.19 g L-1. Our model further confirmed experimental observations that the best growth performance and highest lipid production are obtained under mixotrophic conditions compared to heterotrophic conditions.