Palmelloid-like structure formation mechanism in Chlorella sorokiniana under ethanol stress

Although the formation of "palmelloid-like" cells as a response to environmental stress has been sporadically reported in Chlorella sp., the association between morphological and molecular indices has been poorly understood. Hence, this study investigated the morphological and molecular effects of ethanol stress on C. sorokiniana by providing 0.0%, 0.025%, and 0.1% (v/v) ethanol. The results showed that cell growth, chlorophyll a, and photosynthetic efficiency were promoted under 0.025% ethanol. In contrast, the cells under 0.1% ethanol treatment were highly stressed; cell growth and physiological activities were inhibited, the content of cellular lipid, carbohydrate, reactive oxygen species, and the cell volume increased, and palmelloid-like structures with copious cell envelopes and higher cell wall carbohydrate contents were observed. The transcriptomic gene set enrichment analysis showed that chitin binding and organelle organization were upregulated while the developmental process was downregulated. Genes for actin-related-2, auxin-biding 1, phosphatidylinositol 4-kinase alpha1, phosphatidylinositol 4-phosphate 5-kinase 2 isoform A, and cytokinesis dedicator 4 were downregulated, whereas polysaccharide export, putative polygalacturonase, carbohydrate deacetylase, chitin, cellulose biosynthesis, and unsaturation of fatty acids were upregulated, implying polysaccharide was incorporated into the cell wall, and the rigidity of the cell membrane was promoted. These results suggest the suppression of the developmental process and cytokinesis and the overexpression of microtubules and cell-envelope genes could be the driving force for palmelloid-like structure formation, which could enhance the survival of cells under stress conditions by reducing cell surface area, promoting the production of protective cover and settleability, and adjusting cell rigidity.