Exploration of two-stage cultivation strategies using nitrogen starvation to maximize the lipid productivity in Chlorella sp. HS2.

In this work, the two-stage cultivation of Chlorella sp. HS2 for enhancing the lipid productivity was optimized by adjusting the duration of nitrogen-replete (N+) and -deplete (N-) stages within a 9 day period using urea as nitrogen source. The highest lipid content of 36.7% and productivity of 216.9 mg L-1 d-1 were obtained under five days of N+ followed by four days of N- conditions. Replenishing phosphorus and other nutrients (N-P+O+) at the beginning of the nutrient-starvation resulted in 1.55 and 1.68-folds improvement in lipid productivities compared to the single stage and zero nutrient controls (N-P-O-), respectively. The estimated biodiesel properties based on the fatty acid profiles met all criteria of international standards. The findings of this study indicate that properly adjusting the period of nitrogen availability as well as the presence of other nutrients is highly important in order to maximize the biofuel productivity in two-stage microalgal cultivation.