Characterisation of bioaccumulation dynamics of three differently coated silver nanoparticles and aqueous silver in a simple freshwater food chain

Abstract

Environmental context Nanoparticles may be passed from primary producers to predators higher up the food chain, but little is currently known about this transfer. We studied the accumulation dynamics of silver nanoparticles by algae, and then from algae to zooplankton. Using the biodynamic approach, we reconstructed the accumulation process to show that diet is the primary route of uptake for silver nanoparticles. Abstract This study investigated the bioaccumulation dynamics of silver nanoparticles (Ag NPs) with different coatings (polyvinyl pyrrolidone, polyethylene glycol and citrate), in comparison with aqueous Ag (added as AgNO3), in a simplified freshwater food chain comprising the green alga Chlorella vulgaris and the crustacean Daphnia magna. Algal uptake rate constants (ku) and membrane transport characteristics (binding site density, transporter affinity and strength of binding) were determined after exposing algae to a range of either aqueous Ag or Ag NP concentrations. In general, higher ku values were related to higher toxicity in the algae. Transmission electron microscopy images were used to investigate the internalisation of Ag NPs in algal cells following exposure to low concentrations for 72h (mimicking inhibition tests) or high concentrations for 4h (mimicking preparation for daphnia dietary exposure). Ag NPs were only visualised in algal cells exposed to high Ag NP concentrations. To establish D. magna biodynamic model constants, organisms were fed Ag-contaminated algae and depurated for 96h. Assimilation efficiencies ranged from 10 to 25% and the elimination of accumulated Ag followed a two-compartmental model, indicating lower loss rate constants for polyvinyl pyrrolidone-, and polyethylene glycol-coated Ag NPs. Biodynamic model results revealed that in most cases, food is the dominant pathway of Ag uptake in D. magna. Despite the predicted low steady-state body burdens in D. magna, dietary uptake of Ag was possible from aqueous and particulate forms of Ag.