Cobalt-induced genotoxicity in male zebrafish (Danio rerio), with implications for reproduction and expression of DNA repair genes

Although cobalt (Co) is an environmental contaminant of surface waters in both radioactive (e.g. (60)Co) and non-radioactive forms, there is relatively little information about Co toxicity in fishes. The objective of this study was to investigate acute and chronic toxicity of Co in zebrafish, with emphasis on male genotoxicity and implications for reproductive success. The lethal concentration for 50% mortality (LC(50)) in larval zebrafish exposed (96 h) to 0-50 mg l(-1) Co was 35.3 ± 1.1 (95%C.I.) mg l(-1) Co. Adult zebrafish were exposed (13 d) to sub-lethal (0-25 mg l(-1)) Co and allowed to spawn every 4 d and embryos were collected. After 12-d exposure, fertilisation rate was reduced (6% total eggs fertilised, 25 mg l(-1)) and embryo survival to hatching decreased (60% fertilised eggs survived, 25 mg l(-1)). A concentration-dependent increase in DNA strand breaks was detected in sperm from males exposed (13 d) to Co, and DNA damage in sperm returned to control levels after males recovered for 6 d in clean water. Induction of DNA repair genes (rad51, xrcc5, and xrcc6) in testes was complex and not directly related to Co concentration, although there was significant induction in fish exposed to 15 and 25 mg l(-1) Co relative to controls. Induction of 4.0 ± 0.9, 2.5 ± 0.7, and 3.1 ± 0.7-fold change (mean ± S.E.M. for rad51, xrcc5, and xrcc6, respectively) was observed in testes at the highest Co concentration (25 mg l(-1)). Expression of these genes was not altered in offspring (larvae) spawned after 12-d exposure. Chronic exposure to Co resulted in DNA damage in sperm, induction of DNA repair genes in testes, and indications of reduced reproductive success.