Analysis of the effects of BP-3 and BP-4 on the transcriptome of Chlamydomonas reinhardtii: An RNA-Seq approach

Abstract

Benzophenones (BPs) are one of the most widely used UV-filters and previous flow cytometric studies have shown that these aquatic emerging pollutants alter the physiology of the freshwater microalga Chlamydomonas reinhardtii. In order to obtain a more detailed study of the different cellular metabolic pathways affected, changes caused by BPs in the transcriptome of C. reinhardtii were investigated using RNA-Seq analysis after 24 h of exposure. Each benzophenone at its corresponding 96 h-EC50 value for growth provoked alterations in the gene expression of this microalga, although BP-3-exposed cells showed a higher number of differentially expressed genes than cells exposed to BP-4. GO enrichment analyses suggested that both compounds affected the same cellular metabolic pathways. Transcripts encoding for light-harvesting and chlorophyll-binding proteins were highly reduced. In addition, an overexpression of genes related to amino acid catabolism was also detected, suggesting that C. reinhardtii cells oxidize amino acids to obtain energy when photosynthesis was damaged by the pollutants. Regarding the oxidative damage provoked by the contaminants, genes encoding main antioxidant enzymes and involved in glutathione-associated metabolism were upregulated. Moreover, sulphur metabolism could have some relevance to explain the mechanism of action of BP-4 and its lower toxicity on microalgae, since the sulfonic acid group is the major structural difference between both BPs. Obtained results suggest that photosynthesis was impaired on cells exposed to the UV-filters, leading microalgae to obtain energy via a heterotrophic metabolism to survive. Thus, the occurrence of these sunscreens in freshwater ecosystems could trigger a worrying reduction in global CO2 fixation.