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Silver nanoparticles (Ag NPs) are widely used as antibacterial agents. This antibacterial property carries with it a potential environmental risk once these NPs are discharged into the environment. This study investigated the impact on Pseudomonas fluorescens over a 24 h exposure of well characterized Ag NPs at pH values of 6-9, in the presence and absence of Suwannee River humic acids (SRHA). Ag NPs were characterized by size, aggregation, morphology, dissolution, and surface properties under all conditions. Solubility was low (less than 2%) for all Ag NP concentrations (2-2000 ppb) and under all conditions was less than 40 ppb (0.38 microM). SRHA caused a partial disaggregation of Ag NP aggregates by nanoscale film formation, with individual NPs stabilized by charge and entropically driven steric effects. Dissolved Ag reduced bacterial growth entirely at 2000 ppb (19 microM) under all conditions and adversely affected growth at 200 ppb (1.9 microM) under some conditions, indicating some toxicity. The Ag NPs showed similar toxicity at 2000 ppb (19 microM) in the absence of SRHA and at pH 9 only i.e. SRHA mitigated bactericidal action. Solubility and interactions with SRHA indicate that there was a specific nanoparticle effect which could not be explained by the effect of dissolved Ag.

Silver nanoparticles (Ag NPs) may present a risk to the environment due to their expected toxicity and wide exposure. The interactions between Ag NPs and laboratory-grown Pseudomonas putida biofilms were investigated under a range of environmentally relevant conditions (pH 6 and 7.5; presence and absence of Suwannee River fulvic acid (SRFA)) over 4 days. In the absence of SRFA, there was extensive sloughing of the biofilm bacteria into suspension implying NP-bacterial interactions and potential effects on NP transport in the environment. In the presence of SRFA, sloughing of cells into suspension was reduced under some conditions and Ag NPs and their aggregates were observed and quantified on and in the bacterial cells in the biofilm. Viability of the cells in all cases appear unchanged by the presence of Ag NPs. Cell viability was independent of the concentration of NPs in solution, but sloughing rates varied substantially, sometimes in a dose-dependent manner. The results suggest that biofilms are impacted by Ag NPs when SRFA was not present, and that SRFA increases uptake and bioaccumulation of Ag NPs to biofilms, perhaps resulting in longer term effects, which need further investigation.

Zinc oxide nanoparticles (ZnO NPs) are widely used in commercial products and knowledge of their environmental fate is a priority for ecological protection. Here we synthesized model ZnO NPs that were made from and thus labeled with the stable isotope (68)Zn and this enables highly sensitive and selective detection of labeled components against high natural Zn background levels. We combine high precision stable isotope measurements and novel bioimaging techniques to characterize parallel water-borne exposures of the common mudshrimp Corophium volutator to (68)ZnO NPs, bulk (68)ZnO, and soluble (68)ZnCl(2) in the presence of sediment. C. volutator is an important component of coastal ecosystems where river-borne NPs will accumulate and is used on a routine basis for toxicity assessments. Our results demonstrate that ionic Zn from ZnO NPs is bioavailable to C. volutator and that Zn uptake is active. Bioavailability appears to be governed primarily by the dissolved Zn content of the water, whereby Zn uptake occurs via the aqueous phase and/or the ingestion of sediment particles with adsorbed Zn from dissolution of ZnO particles. The high sorption capacity of sediments for Zn thus enhances the potential for trophic transfer of Zn derived from readily soluble ZnO NPs. The uncertainties of our isotopic data are too large, however, to conclusively rule out any additional direct uptake route of ZnO NPs by C. volutator.

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