Ammonia oxidizing bacteria (AOB) play an important role in the global nitrogen cycle and in the removal of nitrogen from wastewater. However, a wide range of contaminants often found in wastewater inhibit AOB. One of the emerging contaminants of concern are silver nanoparticles (AgNPs). AgNPs find wide applications as an antibacterial agent in textiles, food containers, baby toys, and personal care products and are released to wastewater. The rapid introduction of AgNPs into the market has increased the need to study their fate and behavior in wastewater treatment plants and the environment. In this study, the toxicity of AgNPs to Nitrosomonas europaea, the model AOB, was examined during 3-hour batch experiments. The batch bioreactors contained test media consisting of 2.5 mM ammonium sulfate ((NH₄)₂SO₄) and 30 mM HEPES buffer at pH 7.8. N. europaea was found to be extremely sensitive to ionic silver (Ag⁺) and 20 nm citrate capped AgNPs. AgNP toxicity is due primarily to the dissolution of Ag⁺. Results from abiotic UV-visible absorption studies indicated that the dissolution of the AgNPs is proportional to the NH₃ concentration in the test media. To better understand AgNPs toxicity under environmentally relevant conditions, the influence of a model protein, bovine serum albumin (BSA) and a model polysaccharide, alginate, on the toxicity and dissolution of AgNPs was further investigated. BSA and alginate both adsorbed onto the AgNPs surfaces, thus stabilizing the AgNPs and reducing their rate of dissolution over 3 hours from 34 ± 9 %, in pure test media to 12 ± 4 % in the presence of either BSA or alginate. With increasing concentrations of BSA and alginate, inhibition of N. europaea decreased, reaching full protection at 400 ppm and 1000 ppm, BSA and alginate, respectively. The higher level of protection offered by BSA was most likely due to its thiol groups, which were able to bind the Ag⁺ released by the AgNPs. Alginate was unable to bind the released Ag⁺ due to a lack of Ag⁺ ligands, including thiols. The strength of the BSA and alginate-produced AgNPs stabilization was tested through the addition of 730 μM MgSO₄, a divalent cation, which caused rapid agglomeration of AgNPs in test media. BSA-coated AgNPs stayed stable over 3 h; however rapid agglomeration was observed for the alginate stabilized AgNPs, indicating the adsorption to the AgNPs surfaces by van der Waals and electrostatic forces was stronger for BSA than alginate. Aqueous constituents influence the surface chemistry of the AgNPs and can lead to alterations in the AgNPs' stability (dissolution and agglomeration) and therefore the fate and toxicity of AgNPs. The DVD, an appendix to this thesis, is available for viewing at the Media Center at SDSU Library and Information Access.