Differential sensitivity of nitrifying bacteria to silver nanoparticles in activated sludge

Nitrification is known as one of the most sensitive processes affected when activated sludge is exposed to antimicrobial silver nanoparticles (AgNPs). The impact of AgNPs and their released silver ions (Ag⁺) on the abundance, activity, and diversity of different nitrifying bacteria in wastewater treatment plants (WWTPs), however, is poorly understood. The present study investigated the impacts of 2 sizes of AgNPs (5nm and 35nm) and Ag⁺ ions on the nitrifier community in activated sludge, including both ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). Ammonia-oxidizing bacteria were more sensitive to AgNPs than the NOB; a 5-d and 7-d exposure of activated sludge to 35nm AgNPs (40ppm) significantly reduced AOB abundance to 24% and 19%, respectively. This finding was confirmed further by a decrease in activated sludge ammonia oxidation activity measured by ¹⁴C-labeled bicarbonate uptake. In contrast, neither AgNPs (up to 40ppm) nor Ag⁺ (1ppm) affected the abundance of NOB. Both 5nm and 35nm AgNPs decreased the diversity of AOB, as indicated by denaturing gradient gel electrophoresis with ammonia monooxygenase gene (amoA) primers, although some unknown Nitrosomonas species were relatively resistant to AgNPs. The generally greater resistance of NOB than AOB to AgNPs suggests that the accumulation of bacteriostatic nitrite in WWTPs is unlikely to be exacerbated due to the accidental or incidental release of AgNPs.