Harmful effects of silver nanoparticles on a complex detrital model system
The rapid proliferation of silver nanoparticles (AgNP) in industry and the environment requires realistic toxicity assessments based on approaches that consider the biological complexity of ecosystems. Here we assessed the acute toxicity of carbonate-coated AgNP and, for comparison, AgNO3 (Ag+) by using a model system consisting of decomposing plant litter and the associated fungal and bacterial decomposers as central players in the functioning of stream ecosystems. Little variation in size and surface charge during the experiment indicated that the AgNP used were essentially stable. AgNP disrupted bacterial growth (<= 83% reduction in protein biosynthesis, EC50 = 0.3 mu M), clearly affected fungal growth (<= 61% reduction in ergosterol synthesis, EC50 = 47 mu M) with both endpoints more sensitive to AgNP than to Ag+. Fungal reproduction, in contrast, was stimulated by AgNP, but not Ag+, at concentrations up to 25 mu M. Both AgNP and Ag+ also stimulated extracellular alkaline phosphatase but reduced leucine aminopeptidase, whereas beta-glucosidase was stimulated by AgNP and reduced by Ag+. Importantly, the provision of cysteine, a chelating ligand that complexes free Ag+, failed to alleviate AgNP toxicity to microbial growth, clearly demonstrating particle-mediated toxicity independent of the presence of ionic silver. This contrasts with the observed inhibition of leucine aminopeptidase by Ag+, which accounted for 2-6% of the total silver in treatments receiving AgNP. These results show that although outcomes of AgNP and Ag+ exposure assessed by different functional endpoints vary widely, AgNP strongly interferes with bacterial growth and a range of other microbial processes, resulting in severe consequences for natural microbial communities and ecosystem functioning.