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 ammonia-oxidizing bacterium, was used to study the sensitivity of bacteria to AgNPs and ionic silver (Ag+). Flow cytometry was used to monitor the toxicity of low (1 mg L−1), middle (10 mg L−1), and high concentrations (20 mg L−1) of AgNPs, as well as Ag+ (1 mg L−1) released into the medium from 20 mg L−1 concentration of AgNPs, towards N. europaea. After 12 h of exposure, the survival rate of N. europaea treated with 1 mg L−1 Ag+ was significantly lower than those treated with low (1 mg L−1) and middle concentrations (10 mg L−1) of AgNPs, but the survival rate in the treatment with high concentration (20 mg L−1) of AgNPs was much lower. Additionally, necrosis rates were higher in the treatment with 20 mg L−1 AgNPs. 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.