Microbial community response to silver nanoparticles and Ag⁺ in nitrifying activated sludge revealed by ion...

Silver nanoparticles (AgNPs), which are known to act as biocides, are incorporated into medical and consumer products including athletic clothing, stuffed animals, liquid dietary supplements, and more. The increasing use of AgNPs in these products is likely to lead to their entry into both natural and engineered systems, which has the potential to disrupt bacterial processes including those involved in nutrient cycling in wastewater treatment. In the present study, sequencing batch reactors (SBR) mimicking secondary wastewater treatment were operated to determine the effects of AgNPs on the microbial communities contained within activated sludge of wastewater treatment plants (WWTP). SBRs were treated with 0.2 and 2ppm of either gum Arabic (GA)-coated AgNPs, citrate (Ca)-coated AgNPs, or Ag⁺ as AgNO₃. Cell samples were collected and DNA isolated periodically throughout SBR operation. DNA was used for Ion Torrent Next Gen Sequencing of the V3 region of the 16S rDNA gene. Subsequent analyses revealed that the microbial community both shifted and recovered quickly in response to Ag⁺. Both AgNP treatments resulted in slower initial community shifts than that observed with the Ag⁺ treatment. GA-AgNPs elicited the longest lasting effect. Additional examination of nitrogen removal bacteria suggested the possibility of an increase in sludge bulking species with increased concentrations of AgNPs in WWTPs. This study supports the hypothesis that Ag⁺ release from AgNPs is largely coating-dependent and thus a key driver in dictating AgNP toxicity.