Contributions of Nitrogen and Carbon Metabolic Pathways to Lipid Metabolism Clarified in Lipid Producing Microalgae
Nitrogen starvation is usually used to enhance lipid biosynthesis in microalgae. However, the underlying mechanisms of lipid biosynthesis in microalgae are unknown, particularly the metabolic changes induced by nitrogen starvation. The Research Group of Algal Biochemistry at Institute of Hydrobiology (IHB) of Chinese Academy of Sciences has expounded the metabolic changes in lipid producing microalgae Chlorella based on the analysis of metabolomics data, and revealed that nitrogen and carbon metabolic pathways contributed to lipid metabolism.
Researchers analyzed the global metabolic profiles of three Chlorella strains to identify the contributions of key metabolic pathways to lipid metabolism. These strains (C1, C2, and C3) are genetically closely related but with significant different lipid productivity. By analyzing various Chlamydomonas mutants deficient in glutamate synthase/NADH-dependent, glutamate synthase/Fd-dependent, glutamine synthetase, aspartate aminotransferase, alanine aminotransferase, pyruvate kinase, and citrate synthase, the contributions of key metabolic pathways were then substantiated in the model green alga Chlamydomonas reinhardtii.
The results showed that nitrogen obtained from amino acid catabolism was assimilated via the glutamate-glutamine pathway, and then stored as amino acids and intermediate molecules via the corresponding metabolic pathways, which led to carbon-nitrogen disequilibrium. Excess carbon obtained from photosynthesis or glycolysis was re-distributed into carbon-containing compounds, and then diverted into lipid metabolism for the production of storage lipids via the gamma-aminobutyrate pathway, glycolysis, and the tricarboxylic acid cycle. Based on the results, a scenario, in which a series of metabolic pathways contribute to lipid metabolism, was proposed.