Hepatic effects of fathead minnows exposed to zinc oxide nanoparticles and municipal effluent

Nanotechnology is an area of intense commercial development, and concerns have been raised about the possible impacts to the environment. Zinc oxide nanoparticles (nanoZnO) are used in transparent sunscreens and may find their way into the environment through wastewater treatment plants and through skin contact with water. The purpose of this study was to examine the bioavailability of Zn in fathead minnows exposed to either dietary nanoZnO or ZnCl2 and to increasing concentrations of a typical chemically processed municipal effluent. Male and female fathead minnows were exposed to food spiked with 100 μg Zn/kg of either nanoZnO or ZnCl2 for 21 d at 25°C. After the exposure period, the following biomarkers were determined: metal levels in fish carcasses, oxidative stress (lipid peroxidation), DNA damage, mitochondrial electron transport, metallothioneins, cytochrome P4501A and 3A, and glutathione S-transferase activity. The results showed that although fish did not significantly accumulate Zn from dietary nanoZnO or ZnCl2, there was a trend of higher Zn levels in females than in males, and the difference was more significant when fish were exposed to either dietary nanoZnO or ZnCl2 in the presence of municipal effluent. Levels of metallothioneins were also higher in females than in males when fish were exposed to the highest concentration of municipal effluent, to dietary ZnCl2 alone, and to the municipal effluent and Zn-enriched diet (nanoZnO and ZnCl2). Cytochrome P4501A activity was increased only by the municipal effluent, but the increases were greater in the presence of either form of Zn in the diet. Fish expended more metabolic energy (mitochondrial activity) and sustained more DNA damage (females) at the highest tested concentration of municipal effluent (20% v/v), and this effect was exacerbated by the addition of either form of dietary Zn. Lipid peroxidation tended to be lower when dietary ZnCl2 was present but was elevated at low municipal effluent concentrations. Multivariate analysis revealed that the effects could be grouped in the following clusters: controls, dietary nanoZnO/ZnCl2, and municipal effluent with dietary Zn. Responses to dietary nanoZnO and ZnCl2 were similar, suggesting that nanoZnO toxicity involved cationic Zn. Exposure to municipal effluent led to more significant changes in metallothioneins, mitochondrial activity, Cu levels and GSI, while exposure to either form of dietary Zn led to more significant changes in lipid peroxidation, tissue Zn levels, metallothioneins and cytochrome P4501A activity. In conclusion, the effects of dietary nanoZnO and ZnCl2 appeared similar but differed from the overall effects of municipal effluent alone. In combination, different interactions were observed when the fish were exposed to nanoZnO or ZnCl2 in the presence of municipal effluent.