Enhanced Lipid Productivity in Low-Starch Mutant of Monoraphidium sp. for Biodiesel Production

Starch and lipids are two major carbon storage compounds in algal cells. Theoretically, switching the carbon flow from starch to lipid synthesis may be a promising approach to realize the overproduction of microalgae-derived biodiesel. However, the results in previous studies vary across algal strains. To test the hypothesis, Monoraphidium sp. mutants were generated using ethyl methane sulfonate (EMS) as a random chemical mutagen. Among the 10% survival cells after exposure to EMS (4 mM) for 30 min, 23 low-starch mutants were selected, of which only 9 strains exhibited enhanced relative fluorescence unit (RFU) compared to wild type, implying the complexity in metabolic steps underlying the re-allocation of starch and lipid in Monoraphidium. Amid these mutants, EM25 showed increased lipid content (25.1%) and moderate biomass productivity (71 mg L day), resulting in a higher lipid productivity (17.8 mg L day) than that of wild type. The alteration of carbon storage compounds in the mutant was attributed to changes in the activities of enzymes (AGPase, ACCase, PEPC, DGAT) related to starch and lipid synthesis. Additionally, the fatty acid composition of the mutant complied with most of the parameters defined by ASTM D6751 and EN 14,214, with a sharp decrease in cloud point compared to wild type, suggesting its application in wide geographic area and the potential to be developed as a promising candidate for biodiesel production.