Quantitative study on lipid productivity of Euglena gracilis and its biodiesel production according to the cultivation conditions

Biodiesel is a carbon neutral liquid fuel that is derived from transesterification of lipids in biomass. Microalgae could be a promising raw feedstock for biodiesel production thanks to high lipid content in line with the rapid carbon fixation capability. Nonetheless, lipid productivity from microalgae is sensitive to microalgal species and cultivation conditions. Hence, to seek cultivation parameters maximizing lipid productivity, Euglena gracilis (E. gracilis) was kept under the controlled conditions as a case study. Non-catalytic transesterification was also adopted for quantitative/qualitative analyses of lipid in E. gracilis due to its analytical reliability. E. gracilis was cultured in the aerobic condition for 7 d, and the cultivation mode was changed into the anaerobic condition for 3 d to experimentally confirm wax ester fermentation mechanisms. The lipid content in E. gracilis reached up to 24.81 wt% (dry basis) under the heterotrophic condition, which was in good agreement with the wax fermentation mechanism. It was concluded that the high dose of nutrients (glucose, nitrogen, and phosphorus) under the oxygen-free environment led to a favorable condition for lipid accumulation. Nevertheless, this study offered that lipid productivity in 6 d (0.131 g _lipid L⁻¹ d⁻¹) under aerobic condition was higher than that in 10 d (0.092 g _lipid L⁻¹ d⁻¹) under anaerobic condition due to substantial carbon loss arising from the wax ester fermentation stage under the anaerobic condition.