De novo transcriptome analysis of Chlorella sorokiniana: effect of glucose assimilation, and moderate light intensity

Chlorella can produce an unusually wide range of metabolites under various nutrient availability, carbon source, and light availability. Glucose, an essential molecule for the growth of microorganisms, also contributes significantly to the metabolism of various metabolic compounds produced by Chlorella. In addition, manipulation of light intensity also induces the formation of secondary metabolites such as pigments, and carotenoids in Chlorella. This study will focus on the effect of glucose addition, and moderate light on the regulation of carotenoid, lipid, starch, and other key metabolic pathways in Chlorella sorokiniana. To gain knowledge about this, we performed transcriptome profiling on C. sorokiniana strain NIES-2168 in response to moderate light stress supplemented with glucose under mixotrophic conditions. A total of 60,982,352 raw paired-end (PE) reads 100 bp in length was obtained from both normal, and mixotrophic samples of C. sorokiniana. After pre-processing, 93.63% high-quality PE reads were obtained, and 18,310 predicted full-length transcripts were assembled. Differential gene expression showed that a total of 937, and 1124 genes were upregulated, and downregulated in mixotrophic samples, respectively. Transcriptome analysis revealed that the mixotrophic condition caused upregulation of genes involved in carotenoids production (specifically lutein biosynthesis), fatty acid biosynthesis, TAG accumulation, and the majority of the carbon fixation pathways. Conversely, starch biosynthesis, sucrose biosynthesis, and isoprenoid biosynthesis were downregulated. Novel insights into the pathways that link the enhanced production of valuable metabolites (such as carotenoids in C. sorokiniana) grown under mixotrophic conditions is presented.