Transcriptome and key gene expression related to carbon metabolism and fatty acid synthesis of Chlorella vulgaris under a nitrogen starvation and phosphorus repletion regime

For the comprehensive understanding of regulated carbon metabolism associated with lipid accumulation in microalgae Chlorella sp. cultivated with nitrogen starvation and phosphorus repletion (N-P+), the discrepancy among gene expression levels in carbon flow from CO2 to fatty acid synthesis were characterized through de novo sequencing, transcriptome assembly, annotation, and differential expression analysis. The biomass production of 1900 mg L-1 and lipid content of 677.55 mg (10(10) cells)(-1) in Chlorella sp. under N-P+ condition were 1.46-fold and 1.49-fold of those under nitrogen starvation and phosphorus limitation (N-Plim) condition, respectively. The expression levels of many enzymes related to carbon metabolism under N-P+ condition were upregulated, including carbon fixation, pyruvate metabolism, glycolysis, TCA cycle and fatty acid synthesis pathways. CO2 fixation accelerated through the upregulated Calvin cycle and glycolysis pathways. The rapid synthesis of pyruvate and upregulation of key rate-limiting enzymes involved in fatty acid synthesis favored lipid accumulation.