Distinct biokinetic behavior of ZnO nanoparticles in Daphnia magna quantified by synthesizing ⁶⁵Zn tracer.

ZnO nanoparticles (nZnO) are widely used in different fields and there are increasingly concerns for their hazards in the environment. The biokinetic behavior of nZnO in aquatic organisms however remains essentially unknown. The aim of this study was to separate the uptake and depuration behavior of nZnO and Zn ions in a freshwater cladoceran Daphnia magna. We for the first time radio-synthesized the nZnO and followed its uptake and depuration in D. magna. Two concentrations (0.5 mg/L and 2 mg/L) of nZnO were employed in this study, and the releases of nZnO into soluble Zn were 20-30% during the experiments. We found that the uptake of nZnO by D. magna was related to nZnO concentration. The uptake of ionic Zn released from nZnO by D. magna followed a linear increase during the exposure period (40 min or 8 h). The nanoparticles could enter the body of daphnids and reached a peak within a short time, followed by a rapid release. Uptake of nanoparticles was mainly by direct ingestion, with negligible nZnO absorption onto the carapace. The depuration of nZnO was also rapid and controlled by nZnO dissolution in the body of D. magna. Our study showed a distinctive uptake mode of nZnO and suggested that both dissolved Zn and nanoparticles should be considered in studying the toxicology of nZnO.