Cadmium sub-lethal effects on Daphnia magna – A new take on an old subject

Rising production and use of electronic products leads to higher cadmium usage, increasing the risk of exposure to this dangerous metal. Despite being a classic environmental contaminant, cadmium's full toxicity spectrum and mechanisms of action towards aquatic biota are still in need of further clarification. This work contributes to such clarification by comprehensively characterizing cadmium mechanisms of toxic action resulting from a seven-day exposure to a sub-lethal concentration (EC20; 4.5 µg L^-1) on the freshwater species Daphnia magna. A holistic approach starting at molecular level responses with epigenetic endpoints (total DNA methylation and DNA methyltransferases activity) and transcriptomic analysis (RNA-Seq), then working up to phenotypic effects at the sub-cellular and individual level was applied. Cadmium exposure caused global genome hypomethylation and altered DNA methyltransferases activity. These methylome changes were accompanied by significant changes in gene expression, that correlate with the observed phenotypic effects. A significant increase in the activity of the main antioxidant enzymes, lipid peroxidation levels and DNA damage was recorded, along with a reduction in heart rate and somatic growth in exposed organisms. These findings provide new insights into cadmium lesser-known effects spanning multiple levels of biological organization in D. magna even at a low environmentally relevant concentration.