Comparative transcriptome analysis of wild type and an oleaginous mutant strain of Desmodesmus sp. reveals a unique reprogramming of lipid metabolism under high light

A mutant generated via ethylmethane sulfonate mutagenesis, Desmodesmus sp. G3, exhibited greater biomass and neutral lipid production over the wild type (WT) strain Desmodesmus sp. G41 in our previous study (Zhang et al., Bioresour Technol 207:268-275, 2016). G3 possessed a higher growth rate and lipid production than WT, with a biomass yield and total lipid content of 1302.86 mg L-1 and 48.58% respectively, which was promoted by 20.50% and 18.84%, compared to WT. Comparative transcriptome analysis was performed to elucidate the mechanism supporting enhanced biomass and lipid production in G3. A total of 1488 differentially expressed genes (DEGs) was identified comparing G3 and WT sequencing datasets, of which 753 and 735 genes were upregulated and downregulated respectively in G3. Pathway enrichment analysis indicated that 'photosynthesis,' 'starch and sucrose metabolism,' 'fatty acid elongation,' and 'pyruvate metabolism' were the notable represented DEGs-enriched pathways that might affect cell growth and lipid metabolism. Light harvesting capture was enhanced in G3. To protect PSII from photodamage, light harvesting complex-dependent non-photochemical quenching and state transitions was employed as the photoprotective strategy. De novo fatty acids (FAs) synthesis in chloroplast was downregulated in G3 while mitochondrion localized FAs elongation was enhanced to recycle carbon skeletons for lipid biosynthesis. Pyruvate mechanism was activated in G3 to generate acetyl-CoA through a pyruvate dehydrogenase bypass pathway, as the predominant pathway to provide precursor for lipids biosynthesis.