Chronic toxicity effects of ZnSO4 and ZnO nanoparticles in Daphnia magna

The chronic toxicity of ZnSO4 and ZnO nanoparticles has been studied in Daphnia magna also considering the life cycle parameters beyond the standard 21-day exposure time. Specimens have been individually followed until the natural end of their life, and some of them sampled for microscopic analyses at 48 h, 9 and 21 days. Despite the low level of exposure (0.3 mg Zn/L), ultrastructural analyses of the midgut epithelial cells revealed efficient internalization of nanoparticles between 48 h and 9 d, and translocation to other tissues as well. At 21 d, the most affected fields have been recorded for both compounds; in particular samples exposed to ZnO nanoparticles showed swelling of mitochondria, while those exposed to ZnSO4 had a great number of autophagy vacuoles. The life cycle parameters resulted altered as well, with a significant inhibition of reproduction in both groups, when compared to controls. After the 21-day exposure, some interesting results were obtained: animals, previously exposed to nanoZnO at low concentrations, showed a complete recovery of the full reproduction potential, while those previously exposed to ZnSO4 presented a dose-dependent and compound-specific reduction in lifespan. Based on the results from the present research and the effects of the same chemicals at higher doses, it can be concluded that the soluble form plays a key role in ZnO nanoparticle cytotoxicity, and that the nanoparticulate form is able to locally increase the amount of Zn inside the cell, even within the ovary. It's worth noting that ZnO nanoparticles have been internalized despite the very low concentration used: this raises concern about the possible environmental implications which may derive from their use, and which in turn must be carefully considered.