Effects of exposure to zinc oxide nanoparticles in freshwater mussels in the presence of municipal effluents

Zinc oxide (nano-ZnO) nanoparticles are used in the production of transparent sunscreens and cosmetics, which are released into surface waters and municipal wastewater effluent. The purpose of this study was to examine the toxicity of nano-ZnO in the presence of municipal effluents to freshwater mussels Elliptio complanata. Mussels were exposed for 21 days at 15 o C to nano-ZnO and ZnCl2 in the presence of 10 % dilution of primary-treated municipal effluent. After the exposure period and 24-h depuration step, mussels were analyzed for total Zn in gills and digestive gland, free Zn, metallothioneins (MT), oxidative stress (glutathione S-transferase and LPO), endoplasmic reticulum stress (heat shock proteins and protein ubiquitination) and genotoxicity. The data revealed that although total Zn loadings did not change with these treatments, Zn levels in digestive gland were elevated in mussels exposed to nano-ZnO but not with ZnCl2 in the presence of municipal effluent. Free Zn levels in the gills were elevated in mussels exposed to the municipal effluent, but decreased in mussels exposed to nano-ZnO. MT in digestive gland showed a similar pattern and was negatively associated with free and total Zn. GST activity was significantly reduced by both nano-ZnO and municipal effluent and was negatively correlated with MT levels, suggesting the involvement of MT in the sequestration of reactive oxygen species. Discriminant function analysis showed that the municipal effluent related effects differed from the unexposed mussels and nano-ZnO exposed mussels in terms of the following responses: free Zn in gills and digestive gland and GST activity. Nano-ZnO related effects also involved GST activity, MT and protein ubiquitination, which suggests a combination of oxidative stress and reticulum endoplasmic stress. In respect with oxidative stress, the oxidative properties of nano-ZnO and ZnCl2 are dampened in the presence of the municipal effluent.