Biokinetic uptake and efflux of silver nanoparticles in Daphnia magna

Silver nanoparticles (AgNP) are widely used as antibacterial products, and there are increasing concerns for their potential environmental risks in aquatic ecosystems. The biokinetics of AgNP in aquatic organisms has not yet been determined. In the present study, we employed a radiotracer methodology to quantify the biokinetics of AgNP in a freshwater cladoceran Daphnia magna, including the uptake from water, dietary assimilation, and elimination of AgNP. We found that the uptake of AgNP was concentration dependent and governed by two phases. The uptake rate constant (k(u)) was 0.060 L/g/h at low AgNP concentrations (2, 10, and 40 μg/L), which was 4.3 times lower than that of the Ag free ion. At a higher AgNP concentration (160 and 500 μg/L), the uptake rate increased disproportionately, likely as a result of direct ingestion of these nanoparticles by the daphnids. When the AgNP were associated with the algal food, their dietary assimilation efficiency (AE) was in the range of 22-45%, which was much higher than the dietary assimilation of Ag quantified under the same food conditions. The efflux rate constants of AgNP in daphnids were also much lower than those of the Ag, again suggesting the difficulty of eliminating AgNP by the daphnids. Water excretion was the main elimination route for both AgNP and Ag, but a higher percentage of AgNP was lost through fecal production. Finally, we used a kinetic equation to compare the importance of aqueous and dietary uptake of AgNP using the quantified kinetic parameters. The biokinetic model showed that more than 70% of AgNP accumulated in the daphnids was through ingestion of algae, highlighting the importance of AgNP transport along the food chain. Our present study showed the unique characteristic of AgNP biokinetics and suggested that more attention should be paid to the dietborne AgNP toxicity in aquatic ecosystems.