Physiochemical and gene expression analyses reveal differential responses of the marine oleaginous alga Nannochloropsis salina under different lipid-induction conditions

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Microalgae can synthesize and accumulate lipid upon abiotic stresses such as high light (HL) and nitrogen deficiency (ND). In this work, HL and ND were applied to the oleaginous alga Nannochloropsis salina, individually or in combination, in order to investigate the transcriptional regulation on lipid metabolism. An impaired cell growth was observed under all three stress conditions, with HL + ND leading to the lowest biomass concentration. HL + ND gave rise to the greatest total lipid content (46.5% dry weight), 64% higher than the control. By contrast, the highest lipid productivity was achieved under HL condition. Transcriptional analyses using quantitative real-time PCR suggested that the augmented accumulation of lipids including triacylglycerol (TAG) upon stresses correlated with the upregulation of pyruvate dehydrogenase complex, diacylglycerol acyltransferase, malic enzyme, and glucose-6-phosphate dehydrogenase and the downregulation of pyruvate dehydrogenase kinase, acetyl-CoA carboxylase (ACCase), and fatty acid elongase. The presence of Quizalofop, an inhibitor of ACCase, caused a considerable decline in TAG accumulation, yet having little effect on membrane lipids. Possible carbon metabolic pathways related to TAG biosynthesis are proposed. Our results facilitate the understanding of TAG accumulation and its transcriptional regulation and provide implications into future genetic engineering for improving lipid production by N. salina.