Pyrosequencing reveals higher impact of silver nanoparticles than Ag⁺ on the microbial community structure of activated sludge

Although the antimicrobial capabilities of silver nanoparticles (AgNPs) are widely reported, their impacts on ecologically important microbial communities are not well understood. AgNPs released from consumer products will likely enter sewer systems and wastewater treatment plants, where they would encounter (and potentially upset) activated sludge (AS), a complex ecosystem containing a variety of bacteria. Herein we address the effects of AgNPs and Ag⁺ ions on the microbial community structure of AS, using pyrosequencing technology. Compared to Ag⁺ amendment, a lower AgNP concentration resulted in a more pronounced effect on AS community structure, possibly reflecting a higher propensity of Ag⁺ than AgNPs to be scavenged by inorganic ligands and organic matter. Furthermore, AgNPs decreased the abundance of nitrifying bacteria, which would hinder N removal, and damaged AS floc structure, which could affect sludge clarification and recycling. Overall, although released Ag⁺ is known to be the critical effector of the antimicrobial activity of AgNPs, the nanoparticles apparently delivered Ag⁺ to bacteria more effectively and exerted more pronounced microbial population shifts that would hinder some wastewater treatment processes.