Are Fish Populations at Risk? Metformin Disrupts Zebrafish Development and Reproductive Processes at Chronic Environmentally Relevant Concentrations

The antidiabetic drug Metformin (MET), one of the most prevalent pharmaceuticals in the environment, is currently detected in surface waters in the range of ng/L to low μg/L. As current knowledge regarding the long-term effects of environmentally relevant concentrations of MET in nontarget organisms is limited, the present study aimed at investigating the generational effects of MET, in concentrations ranging from 390 to 14 423 ng/L in the model organism Danio rerio (up to 9 mpf), including the effects on its nonexposed offspring (until 60 dpf). We integrate several apical end points, i.e., embryonic development, survival, growth, and reproduction, with qRT-PCR and RNA-seq analyses to provide additional insights into the mode of action of MET. Reproductive-related parameters in the first generation were particularly sensitive to MET. MET parental exposure impacted critical molecular processes involved in the metabolism of zebrafish males, which in turn affected steroid hormone biosynthesis and upregulated male vtg1 expression by 99.78- to 155.47-fold at 390 and 14 432 MET treatment, respectively, pointing to an estrogenic effect. These findings can potentially explain the significant decrease in the fertilization rate and the increase of unactivated eggs. Nonexposed offspring was also affected by parental MET exposure, impacting its survival and growth. Altogether, these results suggest that MET, at environmentally relevant concentrations, severely affects several biological processes in zebrafish, supporting the urgent need to revise the proposed Predicted No-Effect Concentration (PNEC) and the Environmental Quality Standard (EQS) for MET.