Physiological and biochemical changes reveal stress-associated photosynthetic carbon partitioning into triacylglycerol in the oleaginous marine alga Nannochloropsis oculata

Abstract

Abiotic stresses such as high light (HL) and/or nitrogen deficiency (−N) might induce triacylglycerol (TAG) biosynthesis in microalgae. However, little is known about the changes of lipid classes and the mechanism underlying the enhanced lipid production as imposed by irradiance and nitrogen starvation. In this work HL and −N were applied individually or in combination to Nannochloropsis oculata to investigate the underlying mechanism for stress-associated TAG synthesis. Stress conditions, HL−N in particular, inhibited cell growth and stimulated the accumulation of neutral lipids, predominantly in the form of TAG (402mg/gDW). The membrane polar lipids, however, were attenuated to differential degrees depending on the stress conditions. Over 90% of TAG sn-2 position was occupied by C16 fatty acids, suggesting that N. oculata may involve the prokaryotic pathway mostly for TAG synthesis. Key enzymes of acetyl-CoA carboxylase, diacylglycerol acyltransferase and NADPH involved in lipid biosynthesis were up-regulated approximately 3–13 times by HL−N. Our results altogether demonstrated that the photosynthetically fixed carbon was partitioned into TAG as the major energy reservoir, and irradiance and nitrogen were the two important engineering factors that enhanced lipid production by pushing photosynthetic carbon flux to fatty acid biosynthesis and pulling fatty acids to TAG assembly.