Understanding lipid metabolism in high-lipid-producing Chlorella vulgaris mutants at the genome-wide level

In this study, physical and chemical mutagenesis methods were applied to enhance lipid productivity in Chlorella vulgaris. Then, de novo RNA-seq was performed to observe lipid metabolism changes at the genome-wide level. Characterization of two mutants, UV-715 and EMS-25, showed marked increases in lipid contents, i.e., 42% and 45%, respectively. In addition, the biomass productivity of the UV-715 cells was 9% higher than that of wildtype cells. Furthermore, gas chromatography-mass spectrophotometry analysis showed that both mutants have higher fatty acid methyl ester (FAME) contents than wild-type cells. To understand the effect of mutations that caused yield changes in UV-715 and EMS-25 cells at a genome-wide level, we carried out de novo RNA-seq. As expected, the transcriptional levels of the lipid biosynthesis genes were up-regulated, while the transcriptional levels of genes involved in lipid catabolism were down-regulated. Surprisingly, the transcriptional levels of the genes involved in nitrate assimilation and detoxification of reactive oxygen species (ROS) were significantly increased in the mutants. The genome-wide analysis results highlight the importance of nitrate metabolism and detoxification of ROS for high biomass and lipid productivity.