Melatonin facilitates the coordination of cell growth and lipid accumulation in nitrogen-stressed Chlamydomonas reinhardtii for biodiesel production

In microalgae, the lipid accumulation under nitrogen stress is always accompanied by low photosynthetic activity and low growth rate, which adversely affects the total biomass and the lipid productivity. It is highly desirable to develop a practicable strategy for promoting lipid accumulation meanwhile minimizing the adverse effects under nitrogen stress. In this research, we investigate whether melatonin (MT, an antioxidant that protects against abiotic stress in plant cells) can mitigate the oxidative stress and enhance the lipid production in Chlamydomonas reinhardtii starchless mutants under nitrogen stress. The results of physiological and biochemical analysis indicate that exogenous MT weakens the nitrogen stress-induced oxidative damage by delaying the chlorophyll loss, activating antioxidases, reducing the lipid peroxidation. In addition, MT stimulates the biosynthesis of functional membrane lipids (monogalactosyldiacylglycerol, digalactosyldiacylglycerol and diacylglyceryl-trimethylhomoserine) and polyunsaturated fatty acids, increasing the cell response to nitrogen stress and enhancing the whole lipid biosynthesis. The 5 mu M MT induces the increase of the total fatty acid methyl ester-lipid content by 35.4% (from 49.2% to 66.7% of dry weight), the biomass by 7.4% (to 0.92 g L-1), and the lipid productivity by 42.7% compared with the control groups without MT treatment. Moreover, we investigated the biodiesel properties of the lipids derived from MT-treated nitrogen-stressed cells, and the results suggest that the variation in fatty acid proportion induced by MT has little impact on the lipid's biodiesel properties. Thus, exogenous MT can facilitate the accumulation of lipids by enhancing the cellular antioxidant capability and mitigating the adverse effects of nitrogen stress.