Effects of Silver Nanoparticles on Soil Microbial Communities and Bacterial Nitrification in Suburban Vegetable Soils

Silver nanoparticles(AgNPs) are widely used antimicrobial compounds; however, they may pose a threat to non-targeted bacteria in the environment. In this study high-throughput sequencing was used to investigate the effects of different concentrations of AgNPs(10, 50, and 100 mg kg-1) on soil microbial community structure during short-term(7 d) exposure. The amounts of Acidobacteria,Actinobacteria, Cyanobacteria, and Nitrospirae significantly decreased with increasing AgNP concentration; meanwhile, several other phyla(e.g., Proteobacteria and Planctomycetes) increased and dominated. Nitrosomonas europaea, a well-characterized ammoniaoxidizing bacterium, was used to study the sensitivity of bacteria to Ag NPs and ionic silver(Ag~+). Flow cytometry was used to monitor the toxicity of low(1 mg L), middle(10 mg L), and high concentrations(20 mg L) of AgNPs, as well as Ag~+(1 mg L) released into the medium from 20 mg Lconcentration of AgNPs, towards N. europaea. After 12 h of exposure, the survival rate of N. europaea treated with 1 mg LAg~+ was significantly lower than those treated with low(1 mg L) and middle concentrations(10 mg L) of AgNPs, but the survival rate in the treatment with high concentration(20 mg L) of AgNPs was much lower. Additionally, necrosis rates were higher in the treatment with 20 mg LAgNPs. Electron microscopy showed that Ag+caused serious damage to the cell wall of N. europaea, disintegrated the nucleoids, and condensed next to the cell membrane; however,dissolved Ag~+ is only one of the antibacterial mechanisms of AgNPs.