Effects of silver ions and nanoparticles on suspended cells and biofilms of Nitrosomonas europaea

Graduation date: 2015Silver nanoparticles are increasingly being incorporated into consumer products due to their broad-spectrum antimicrobial properties. The resulting influx of silver nanoparticles into wastewater may pose a threat to bacteria involved in biological wastewater treatment. Ammonia-oxidizing bacteria, which convert ammonia to nitrite in the first step of nitrification, are highly sensitive to contamination, and inhibition of these bacteria by silver nanoparticles may complicate the removal of nitrogen from wastewater. This study examined the effects of silver ions and silver nanoparticles on biofilms, suspended cells, and resuspended biofilms of Nitrosomonas europaea, a model ammonia-oxidizing bacterium. Intact biofilms were exposed to varying concentrations of silver ions and nanoparticles. Nitrite production, bound silver, protein content, and effluent silver concentrations were monitored. Suspended batch cells were exposed to silver ions or nanoparticles to study nitrification inhibition and cell death resulting from ion or nanoparticle exposure. Resuspended biofilms were exposed to silver ion in batch tests to compare inhibition and cell death to suspended cells and intact biofilms. Experiments confirmed that silver ions inhibit nitrification in both N. europaea biofilms and suspended cells to a greater degree than silver nanoparticles. Intact biofilms were found to be more resistant to silver ion inhibition than suspended\ud cells, but resuspended biofilms were no more resistant than suspended cells. Silver sorption tests on both suspended cells and resuspended biofilms confirmed that both adsorb silver on a similar protein basis, indicating that extracellular polymeric substances in biofilms do not bind silver ions. The combination of inhibition and sorption results suggests that the increased resistance of N. europaea biofilms compared to suspended cells is not attributable to interference from EPS, but rather to mass-transfer limitations resulting from the structure of the biofilm. Cell lysis was found to be responsible for some but not all observed nitrification inhibition in suspended cells and biofilms. Results suggest that loss of nitrifying activity in biofilms may result from a combination of enzyme-specific inhibition, cell death, and sloughing of biofilm cells. The slow recovery of nitrification activity in biofilms after exposure to silver ions supports these observations. Toxicity of silver nanoparticles was found to stem from silver ion release, which occurred slowly in the presence of high-ionic-strength media. Findings from this study suggest that silver nanoparticles are unlikely to impact ammonia oxidation in wastewater treatment plants