Differential alteration in reproductive toxicity of medaka fish on exposure to nanoscale zerovalent iron and its oxidation products

Abstract

Nanoscale zerovalent iron (nZVI) is a redox-active nanomaterial commonly used in remediation of soil and groundwater pollution and wastewater treatment processes. A large quantity of nZVI (e.g., >100 mg/L) accidentally released from in situ sites to nearby oxygenized aquifers could be rapidly oxidized to iron oxides (e.g., Fe3O4 or Fe2O3) and ions (e.g., Fe2+), for acute hypoxia effects to aquatic life. However, we do not know the ecotoxicological fate of nZVI and its oxidation products at lower, environmentally concentrations in surface water receiving waterborne transportation or effluent discharge in terms of exposure to aquatic vertebrate species. This study assessed the causal effect on reproductive toxicity in medaka adults (Oryzias latipes) of carboxymethyl cellulose-stabilized nZVI (CMC-nZVI), Fe2+ and iron oxide nanoparticles (nFe3O4) with 21-day aqueous exposure at 5 and 20 mg/L (Fe-equivalent). Such concentrations did not significantly change the dissolved oxygen, oxidation-reduction potential or pH values in the 3 iron solutions during the fish exposure period. Neither CMC-nZVI nor Fe2+ treated adults showed altered daily egg production (fecundity) and oxidative stress responses in observed tissues, as compared to controls. However, the fecundity in nFe3O4 (20 mg/L)-treated pairs was significantly decreased, with increased incidence of abnormal immature oocytes in the ovary. As well, nFe3O4 treatment suppressed activities of the antioxidants superoxide dismutase and expression of glutathione peroxidase (gpx) in the brain and ovary. Although nFe3O4 or Fe2+ treatments inhibited mRNA expression of hepatic estrogen receptor (er-α) in females, plasma levels of sex hormones and (Na, K)-ATPase activity in gills of treated fish did not differ from controls for both sexes. Hence, oxidation products (e.g., nFe3O4) from nZVI at lower milligram-per-liter levels may be potent in inducing nanoparticle-specific reproductive toxicity in medaka fish by inducing oxidative stress in female gonads. Main findingnZVI oxidation product nFe3O4 at lower mg/L induces nanoparticle-specific reproductive toxicity in medaka fish.