RNAseq analysis of whole zebrafish (Danio rerio) larvae revealed the main cellular biological effects of geosmin and microcystin exposure at environmentally relevant concentrations

Cyanobacterial blooms are common events that releases secondary metabolites into water posing considerable threats to the environment, wildlife, and public health. Some of these metabolites, such as microcystin, have been extensively studied and associated with harmful effects in mammals and aquatic organisms, while the biological effects of others, like geosmin, remain much less investigated. Enhancing our understanding of cyanotoxins effects on organisms is especially relevant facing the complex scenarios projected due to global warming. The aim of this study was to assess the transcriptional modulation in whole zebrafish (Danio rerio) larvae (n = 9) in response to a 7-days immersion exposure to 3 μg L^-1 MCLR or 5 μg L^-1 geosmin. No mortality or differences in length gain were observed in zebrafish larvae exposed to environmentally realistic doses of both cyanotoxins. The exposure to MCLR and to geosmin caused the differential expression of 164 and 172 genes respectively, being 23 upregulated by MCLR and 98 upregulated by geosmin. Among the upregulated genes, 16 were shared, while 42 were shared among the downregulated genes. Over-representation analysis identified three enriched GO terms only among the genes upregulated by geosmin: organic hydroxy compound metabolic process (1901615), small molecule biosynthetic process (0044283), and lipid metabolic process (0006629). In fact, the expression of 12 of the 13 genes directly involved in the synthesis of cholesterol from acetyl-CoA was upregulated by geosmin. A chronic upregulation of cholesterol biosynthetic pathway is linked to several diseases and metabolic disorders, including alterations in sex-related hormones. Moreover, our results indicate that geosmin and MCLR acts through different mechanisms. Geosmin does not appear to provoke short-term adverse effects as MCLR but could disrupt the endocrine system by altering the lipid and steroid metabolism.