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Silver nanoparticles (NPs) are the largest and fastest growing category of nanotechnology-based medicines and consumer products. Silver can have great toxicity to some aquatic organisms and, as a biocidal agent, may also damage or alter the most abundant and vulnerable beneficial microorganisms in the environment, such as Gordonia sp. However, considering the complex chemical background of naturalwaters, silver NPs can have complicated interactions with background chemicals such as chloride, surfactants, and dissolved natural organic matters (NOM). The results of this study show that the average particle size and dispersivity of silver NPs and the surface characteristics play an important role in the toxicity of silver NPs. Aggregation was enhanced for silver NPs in 10 mM NaNO3, but not much in 10 mM NaCl due to reactions with chloride. However, the presence of 3 mM sodium dodecyl sulfate (SDS) or 8 mgC/L Suwannee River (SR) NOM appeared to reduce the aggregation of silver NPs. Regarding the bactericidal effect of silver NPs, solubility analysis suggests silver NPs inactivate Gordonia sp. differently from Ag+ and/or a slow release of Ag+ from silver NPs. When the silver NP concentration was raised from 7.3 to 29.2 mg/L in DI water, the log inactivation rate of Gordonia sp. increased from 0.16 +/- 0.04 to 0.45 +/- 0.13. However, with 29.2 mg/L silver NPs the log inactivation rate reached 1.40 +/- 0.26 in 3 mM SDS. The presence of SRNOM mitigated the bactericidal efficacy of silver NPs due to surface coating/adsorption. On the other hand, 10 mM NaCl reduced the log inactivation rate to 0.07 +/- 0.07 due to the formation of likely less toxic silver chloride species, such as AgCl, AgCl2-, AgCl32-, and AgCl43-. (C) 2016 Elsevier B.V. All rights reserved.