The impact of zinc oxide nanoparticles in freshwater mussels exposed to municipal effluents

Zinc oxide nanoparticles (nano-ZnO) are used in the production of transparent sunscreens and cosmetics, which are released into the environment through municipal effluents. The purpose of this study was to examine the toxicity of nano-ZnO to freshwater mussels (Elliptio complanata) in the presence of municipal effluents. Mussels were exposed for 21 days at 15 o C to 1 and 10 µg/L nanoZnO, and ZnCl2 in the presence of a physico-chemically treated municipal effluent (3 and 10 % v/v). After the exposure period and a 24 h depuration step, mussels were analyzed for free Zn in gills, metallothioneins (MT), oxidative stress (production of malondialdehyde (MDA) during lipid peroxidation), gonad alkali-labile phosphate (ALP) levels and genotoxicity. Gill MT levels were increased at 10 µg/L nano-ZnO and ZnCl2 and in the presence of the municipal effluent. MT levels were positively correlated with free Zn in gills and negatively correlated with MDA levels, indicating its involvement in the prevention of oxidative stress. However, MDA levels were significantly related to DNA damage in gills, indicating that MT induction did not prevent oxidative-mediated damage in cells. Gonad ALP levels were increased by exposure to ZnCl2 and to the highest concentration of municipal effluent. DNA strand breaks were increased in mussels treated to nano-ZnO indepentely of municipal effluent. Multivariate discriminant function analysis revealed that control mussels differed from mussels exposed to the municipal effluent and from those exposed to nano-ZnO or ZnCl2 alone. When the municipal effluent was added, changes in MDA, MT and labile Zn were produced and formed another cluster, suggesting a change in the toxicity of the municipal effluent in the presence of nano-ZnO.