The effects of 5-azacytidine and cadmium on global 5-methylcytosine content and secondary metabolites in the freshwater microalgae Chlamydomonas reinhardtii and Scenedesmus quadricauda

Epigenetic changes are important mechanisms in the regulation of chromatin structure and gene expression. Cytosine methylation is one of the major epigenetic modifications, mediated by DNA methyltransferases, which transfer methyl groups from S-adenosyl-L-methionine (SAM) to the fifth carbon of cytosine. Various external environmental conditions can change the global hypo/hypermethylation pattern of DNA. These alterations may affect the organism's response to stress conditions. In this study, for the first time, we investigated the effects of 5-azacytidine, a DNA methyltransferase inhibitor, and cadmium, a toxic metal and environmental pollutant, on the growth, biosynthesis of secondary metabolites (phenols, flavonoids, carotenoids), SAM, S-adenosylhomocysteine, 5 '-methylthioadenosine and global 5-methylcytosine (5-mC) in the green microalgae Chlamydomonas reinhardtii and Scenedesmus quadricauda. The studied species showed major differences in 5-mC content, secondary metabolite content, and antioxidant activity. Cadmium increased GSH (glutathione) content in C. reinhardtii by 60% whereas 5-azacytidine did not affect GSH. The biosynthesis of GSH in S. quadricauda in response to the stressors was the opposite. Global 5-mC content of C. reinhardtii was 1%-1.5%, and the content in S. quadricauda was 3.5%. Amount of some investigated methionine cycle metabolites (SAM, S-adenosyl homocysteine [SAH], methionine) in S. quadricauda distinctly exceeded C. reinhardtii as well. However, chlorophylls a and b, carotenoids, total phenolic content, total flavonoid content and, antioxidant activity were significantly higher in C. reinhardtii than S. quadricauda. Therefore, in further studies it would be advisable to verify whether methylation of cytosine affects the expression of genes encoding certain secondary metabolites.