Short-Term Molecular-Level Effects of Silver Nanoparticle Exposure On the Earthworm, Eisenia Fetida.

Short-term changes in levels of expression of nine stress response genes and oxidative damage of proteins were examined in earthworms, Eisenia fetida, exposed to polyvinylpyrrolidone (PVP) coated silver nanoparticles (Ag-NP) and AgNO₃ in natural soils. The worms were exposed at nominal Ag concentration of 100 mg kg^-1 for 0, 1, 3, and 7 days. The responses varied significantly among days with seven out of nine analyzed genes having day as the most significant variable determining changes in gene expression. Among the days, the third day showed the highest number of significant changes in levels of gene expression and protein carbonyls relative to controls. Gene expression patterns for Ag-NPs and AgNO₃ were similar, suggesting that ions are primarily responsible for the observed responses. Significant correlations of decreased gene expression with increased Ag soil concentration were observed for CAT and HSP70 on day three and for MT on day seven suggesting that genetic responses of these genes are determined by Ag soil concentration independent of the NP size and from whether the worms were exposed to ions or particles. Significant increases in the levels of protein carbonyls on day three were observed for all treatments indicating that E. fetida is experiencing oxidative stress three days after exposure to both particles and ions. Decreased expression and activity of CAT, one of the enzymes responsible for detoxification of highly reactive oxygen species H₂O_2, was also observed on day three.  Decreases in CAT may lead to accumulation of H₂O₂, perhaps explaining the increases in protein carbonyls on day three.  The results from this study suggest that Ag ions are primarily responsible for Ag NP toxicity in E. fetida. However, given that <15% of Ag in the NPs was oxidized in these soils, dissolution of Ag-NP is likely to occur after or during their uptake.