Impacts ofAgNPs on ammonia oxidizing bacteria (AOB). The impacts of 0.2, 2 and 20 ppm AgNPs coated with citrate, gum arabic, and polyvinylpyrrolidone (PVP) were investigated on the model AOB Nitrosomonas europaea. Specifically, the inhibition of nitrite production was quantified and follow up studies were undertaken to determine the mechanisms of inhibition for each AgNP. Nitrite production was lowered by 68 ± 4 and 96 ± 1 % for citrate and gum arabic coated AgNPs, respectively, at a concentration of 2 ppm while no significant effect was observed at 0.2 ppm. At 20 ppm of citrate and gum arabic coated AgNPs, a reduction in nitrite production was measured of 85 ± 1 and 93 ± 1.4 %, respectively. Citrate AgNP treatments were statistically different between 2 and 20 ppm, while gum arabic AgNPs were not. For PVP coated AgNPs, nitrite production was not as greatly affected until reaching 20 ppm, where a 89 ± 1% reduction was measured. Lower concentrations tested of PVP AgNPs did not exhibit statistical differences from no-silver control samples. The membrane disruption of N. europaea exposed to citrate and PVP coated AgNPs was present at concentrations, which exhibited nitrite production inhibition, while gum arabic coated AgNP did not affect membrane integrity even at concentrations where inhibition of nitrite production was observed. The addition of L-cysteine relieved all AgNP toxicity in nitrite inhibition and membrane integrity assays, with the exception of the PVP AgNPs. Gene expression studies on amoA, hao, gshB, and merA showed that the only gene that was impacted by AgNPs was merA, which was up-regulated in the presence of all three AgNPs. These results suggest that the infiltration of some AgNPs could negatively impact nitrification and that coating plays a critical role in determining the ultimate nanoparticle toxicity.
Ecological impacts of AgNPs on soil microbial communities following biosolid land application. To examine the effects of AgNPs on microbial community structure, we compared the composition of bacterial 16S-rRNA genes present using Terminal Restriction Fragment Length Polymorphisms on 0-1cm soils immediately prior to biosolid amendment, immediately following addition and 50 days later. As expected, prior to amendment the composition was similar in all mesocosms. Immediately following addition, there was a large shift in all biosolid treatments away from the Controls likely due to biosolid associated microbes. In addition to all three biosolid treatments being significantly different from Controls, the three slurry treatments were significantly different from one. The differences among treatments decreased by day 50, with controls remaining distinct from slurry treatments, but slurry treatments converging on one another by the end of the experiment, suggesting that the effects of biosolids and silver on community composition decreased over longer time scales.