Silver nanoparticles temporarily retard NO2− production without significantly affecting N2O release by Nitrosomonas europaea

Nitrifying bacteria are highly susceptible to silver nanoparticles (AgNPs). However, the effect of sublethal exposure to AgNPs after their release of nitrogenous compounds of environmental concern (e.g., the greenhouse gas nitrous oxide [N2O] and the common water pollutant nitrite [NO2−]) has not been systematically investigated. The present study reports the effect of AgNPs (and potentially released silver ions [Ag+]) on NO2− and N2O production by Nitrosomonas europaea, and on the transcription of the associated genes. The release of NO2− was more negatively affected than the production of N2O. For example, exposure to AgNPs at 0.075 mg/L temporarily enhanced N2O production (by 12%) without affecting nitrite release, whereas higher AgNP concentrations (>0.25 mg/L) inhibited NO2− release (by >12%) but not N2O production. Transcriptomic analyses corroborated these trends; AgNPs at 0.075 mg/L increased the expression of the nitric oxide reductase gene (norQ) associated with N2O production (by 5.3‐fold to 12.8‐fold), whereas both 0.075 mg/L of Ag+ and 0.75 mg/L of AgNPs down‐regulated the ammonia monooxygenase gene (amoA2; by 0.08‐fold to 0.15‐fold and 0.32‐fold to 0.64‐fold, respectively), the nitrite reductase gene (nirK; by 0.01‐fold to 0.02‐fold and 0.22‐fold to 0.44‐fold, respectively), and norQ (by 0.11‐fold to 0.15‐fold and 0.32‐fold to 0.57‐fold, respectively). These results suggest that AgNP release to sewage treatment plants and land application of AgNP‐containing biosolids should be minimized because of their potential temporary stimulation of N2O release and interference with nitrification. Environ Toxicol Chem 2015;34:2231–2235. © 2015 SETAC