Environmentally relevant concentrations of nickel and imidacloprid induce reproductive toxicity in earthworm (Eisenia fetida fetida)

The current research investigates individual and combined toxicity effects of nickel (Ni) and imidacloprid (IMI) on earthworm species Eisenia fetida fetida. Employing standardized toxicity parameters, we assessed the impact of environmentally relevant concentrations (ERC) of Ni, IMI, and their mixtures on key biomarkers and reproductive fitness of earthworms. Our findings reveal concentration-dependent responses with discernible adverse effects on physiological parameters. The ERC obtained for Ni was 0.095 ppm, and for imidacloprid was 0.01 ppm. Two concentrations (ERC and 1/5th) of both toxicants (individually and in combinations) were further given for 14 days, and parameters like avoidance behaviour, antioxidants, histology, and metabolomic profile were observed. The behaviour of earthworms was noted, where at 24-48 h, it was found to be in control soil, while later, at 72-96 h, they migrated to toxicants-treated soil. Levels of antioxidants (superoxide dismutase, catalase, reduced glutathione, ascorbic acid), lipid peroxidation, and lactate dehydrogenase were elevated in the testis, spermatheca, ovary, and prostate gland in a high concentration of Ni + IMI. Histological studies showed more vacuolization and disruption of epithelium that was increased in the prostate gland of the Ni + IMI high group, decreased number of spermatids, and damaged cell architecture was noted in testis and spermatheca of the Ni + IMI high group. The highest number of metabolites was found in Ni exposed group (181), followed by IMI (131) and Control (125). Thus, this study sheds light on the ecotoxicological effects of combinational exposure of these contaminants on an essential soil-dwelling organism, where IMI was more toxic than Ni, and both toxicants decreased earthworm reproductive fecundity.