Zinc oxide nanoparticles toxicity to <i>Daphnia magna</i>: size-dependent effects and dissolution

As the production of zinc oxide nanoparticles (ZnO-NPs) and other metal oxides is exponentially increasing, it is important to investigate potential environmental and health impacts of such nanoparticles. Nanoparticles' properties (e.g., size, dissolution rate) may change in different water media, and their characterization is essential to derive conclusions about toxicity results. Therefore, an aquatic model organism, Daphnia magna, was used to investigate the effect of ZnO-NPs with 2 different particle sizes (30 nm and 80-100 nm) and then compare these effects with ZnO microsized particles (>200 nm) and the ionic counterpart (in the form of ZnCl2 ) on immobilization, feeding inhibition, and reproduction endpoints. The 48-h median lethal concentration (LC50) for immobilization ranged between 0.76 mg Zn L(-1) for the ionic zinc and 1.32 mg Zn L(-1) for ZnO-NPs of 80 nm to 100 nm. For the chronic exposures, the reproduction output was impaired similarly among zinc exposures and possibly driven mainly by the zinc ionic form. The concentrations used showed a total dissolution after 48 h. On the other hand, feeding activity was more affected by the 30 nm ZnO-NPs than by the ionic zinc, showing that the particulate form was also playing an important role in the feeding inhibition of D. magna. Dissolution and particle size in the daphnia test media were found to be essential to derive conclusions on toxicity. Therefore, they can possibly be considered critical for evaluating nanoparticles' toxicity and fate.