Title: "The Oleaginous Astaxanthin-producing Alga Chromochloris zofingiensis: Potential from Production to Model for Studying Lipid Metabolism and Carotenogenesis"

Speaker: Dr. Jin Liu, Peking University

Abstract: Chromochloris zofingiensis, capable of accumulating triacylglycerol (TAG) and the value-added keto-carotenoid astaxanthin simultaneously, has been cited as a promising single-cell factory for oil and astaxanthin production. In order to unravel the mechanisms of oleaginous and astaxanthin-rich phenotype of this alga, multi-omics were performed in response to nitrogen deprivation (ND), a common inducer for both TAG and astaxanthin. The drastic TAG increase upon ND involved primarily the simultaneous up-regulation of 1) pyruvate dehydrogenase complex (PDHC) and PDHC bypass pathways for producing acetyl-CoAs, the carbon precursors for de novo fatty acid synthesis, 2) de novo fatty acid synthesis, fatty acid activation and desaturation, 3) glycerol-3-phosphate dehydrogenase and glycerol kinase genes for glycerol-3-phosphate production, and 4) TAG assembly in Kennedy pathway particularly diacylglycerol acyltransferase (DGAT) genes (seven out of ten DGAT genes were up-regulated). Moreover, down-regulated protein synthesis and up-regulated starch catabolism and membrane lipid turnover supported the carbon shunt from protein, starch and membrane lipids for meeting the enhanced demand of TAG synthesis. Secondary carotenoids particularly astaxanthin were considerably promoted by ND likely at the expense of primary carotenoids, due to the up-regulated β-carotene synthesis (lycopeneβ-cyclase), β-carotene hydroxylation (β-carotene hydroxylase) and ketolation (β-carotene ketolase), and down-regulated lutein synthesis. The energy needed for the elevated synthesis of fatty acids and carotenoids were mainly derived from up-regulated glycolysis and tricarboxylic acid cycle, while the reductant NADPH were from the up-regulation of oxidative pentose phosphate pathway and a chloroplastic NADP-malic enzyme gene. Potential transcription factors were predicated as global regulators of lipid metabolism and carotenogenesis. Together with the development of genetic tools and characterization of several key functional genes, our findings illuminate the regulatory mechanisms of TAG and astaxanthin biosynthesis in C. zofingiensis, and will benefit more rational manipulation of algae for improved production of these two metabolites.