Ecotoxicology of silver nanoparticles to the soil nematode Caenorhabditis elegans: From molecular mechanisms to population consequences

As the number of applications for engineered nanoparticles (ENPs) is increasing, so inevitably is the rate of their release into the environment. The novel properties (size, reactivity) that make their use and the development of applications appealing are also the basis for concerns about their environmental risks. Here the soil nematode Caenorhabditis elegans was used to assess the toxicity of various silver nanoparticle types with respect to gene expression, individual life history traits and population sensitivity. Exposures were carried out reflecting different environmentally relevant conditions that affected ENP fate and bioavailabilty. Characterisation of ENP fate in the media showed that while agglomeration/aggregation and dissolution processes occurred regardless of ENP type and exposure medium, their properties did alter the extent and dynamics of these processes. The toxicity of different Ag-ENPs and ionic silver was investigated and compared. Concentration-dependent decreases in reproduction were found for all silver forms with Ag+ being the most toxic by mass throughout. The effects of environmentally-relevant exposure media on ENP fate and reproductive toxicity to C. elegans was assessed, first by conducting tests in artificial media with varied composition, and second by using pore waters from soils with different properties. In both cases media pH had the greatest impact on observed toxicity. Multigenerational exposure to pristine and aged (sulfidised) Ag-ENPs revealed significant increases in sensitivity towards ionic and pristine Ag-ENP exposures in the second offspring generation (a novel finding), from which populations did not recover; aged ENPs did not induce similar increased sensitivity. Finally, using microarray analysis clear differences in gene expression after exposure to size and surface property-variant ENPs were found, yet changes in neurological functions were identified as the main underlying mode of toxic action regardless of ENP properties. This PhD thus provides a comprehensive assessment of Ag-ENP exposure, mechanisms of toxicity and long-term effects in a soil dwelling species.