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

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 72 h (mimicking inhibition tests) or high concentrations for 4 h (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 96 h. 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