Chemically enhanced lipid production from microalgae under low sub-optimal temperature

Abstract

Low sub-optimal temperatures are one of the major environmental factors that significantly affects growth and lipid production of microalgae. However, until now little research has been done to estimate and reduce the effects of low sub-optimal temperatures (typically 15°C–20°C) on productivity of algal lipids, which serve as feedstock for biodiesel. We addressed this issue by growing the microalga Chlorella sorokiniana under a low sub-optimal temperature (18°C) (the optimal temperature was 26°C, which was used as a control). Algal growth was inhibited at 18°C compared to 26°C, resulting in overall decreased lipid productivity; however, decreased temperature increased the total lipid content of the algal cell. Importantly, decreased lipid productivity under the low sub-optimal temperature was improved by an application of exogenous glycine betaine at 500mg/L compared to the control treatment of 26°C. Quantitative PCR analysis showed that exogenous glycine betaine enhanced expression of genes encoding Rubisco, one of the key enzymes involved in carbon fixation through photosynthesis. The expression of other key genes involved in the Calvin cycle was up-regulated by treatment with glycine betaine at both 18°C and 26°C. In addition, glycine betaine treatment had little influence on algal fatty acid profiling, hence it did not affect the cetane number that is a key parameter of biodiesel quality. Taken together, our evidence supports a chemically enhanced lipid production of microalgae under low sub-optimal temperatures whose lipid output was comparable to those at optimal temperatures. The novel application of glycine betaine could strengthen the algal biodiesel industry by extending the geographical range for cultivation into areas with cooler climates.