Kinetic modelling of growth and storage molecule production in microalgae under mixotrophic and autotrophic conditions

• Algal cultures were modelled under mixotrophic and autotrophic growth conditions.
• The kinetic model was developed as a function of nitrogen, carbon and light supply.
• The model is based on the Droop model and carbon balance within an algal cell.
• Starch and TAG are the main storage components produced under nitrogen limitation.
• Biomass concentration and storage content were highest under mixotrophic condition.

In order to improve algal biofuel production on a commercial-scale, an understanding of algal growth and fuel molecule accumulation is essential. A mathematical model is presented that describes biomass growth and storage molecule (TAG lipid and starch) accumulation in the freshwater microalga Chlorella vulgaris, under mixotrophic and autotrophic conditions. Biomass growth was formulated based on the Droop model, while the storage molecule production was calculated based on the carbon balance within the algal cells incorporating carbon fixation via photosynthesis, organic carbon uptake and functional biomass growth. The model was validated with experimental growth data of C. vulgaris and was found to fit the data well. Sensitivity analysis showed that the model performance was highly sensitive to variations in parameters associated with nutrient factors, photosynthesis and light intensity. The maximum productivity and biomass concentration were achieved under mixotrophic nitrogen sufficient conditions, while the maximum storage content was obtained under mixotrophic nitrogen deficient conditions.