Toxicity and biokinetics of silver nanoparticles in daphnia magna

Silver nanoparticles (AgNPs) have wide applications in our daily life, but their influence on aquatic ecosystem is still unknown. The present study investigated toxicity and biokinetics of AgNPs in freshwater cladoceran Daphnia magna. AgNPs were not toxic to daphnids even at 500 μg/L, after soluble Ag released from AgNPs was detoxified by cysteine. The LC50s of AgNPs with different surface coatings were in large variation. However, LC50s of soluble Ag released from AgNPs were comparable to AgNO3. This indicated that soluble Ag should be responsible for observed AgNPs toxicity. During 21-d chronic exposure, AgNPs had significant inhibition on both growth and reproduction, with the lowest observed effective concentration of 5 μg/L and 50 μg/L, respectively. AgNPs also caused ionoregulatory dysfunction of sodium (Na) and calcium (Ca) in daphnids through soluble Ag release. Under AgNPs exposure, Na influx was inhibited and efflux was elevated. In contrast, AgNPs only increased Ca influx without effect on Ca efflux. The AgNPs bioaccumulation in daphnids reached to ppm level after 48 h exposure. More than 60% of AgNPs were distributed in the gut of daphnids, indicating that ingestion was the dominant uptake pathway. The uptake of AgNPs from water was dependent on concentration, size and water chemistry. The uptake rate constant (ku) was lower than that of AgNO3 at low AgNPs concentrations, and increased dramatically at high concentration. The AgNPs with small particle size had higher influx rate because of larger aggregation formed. The higher assimilation efficiency and lower efflux rate constant suggested the difficulty of eliminating AgNPs by the daphnids. Water excretion was the main elimination route for both AgNPs and AgNO3. The biokinetic model showed that more than 70% of AgNPs accumulated in the daphnids was through ingestion of algae, highlighting the importance of AgNPs transport along the food chain. Our study provides the basic understanding on toxicity and biokinetics of AgNPs, which is important for environmental risk assessment.