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.   

The study indicated that nitrogen and carbon assimilation and distribution pathways contributed to lipid biosynthesis, and provided valuable information that can be used in efforts to enhance microalgal biofuel production. This work would be of great interest not only to scientists examining fundamental processes in microorganisms, such as lipid biosynthesis, stress responses, biological evolution, photosynthesis and metabolic reactions, but will be appealing to a general audience, related to microalgae based renewable energy production and synthetic biology as well. 
This work was supported by the State Key Laboratory of Freshwater Ecology and Biotechnology (Y11901-1-F01) and the Science and Technology Service Network Initiative of the CAS (KFJ-SW-STS-163).

The distribution and contribution of nitrogen/carbon metabolism for lipid metabolism pathway. Black words, the metabolic pathways which are related to lipid synthesis; gray words, the metabolic pathways which are not related to lipid synthesis. Blue arrow, N flow pathways; red arrow, C flow pathways; green arrow, N and C flow pathways. Solid arrow, one step of metabolic flow; dotted arrow, more than one step of metabolic flow. (Figure by IHB)