Aggregation and dissolution kinetics are important environmental properties of silver nanoparticles (AgNPs), and characterization of the interplay between these two processes is critical to understanding the environmental fate, transport, and biological impacts of AgNPs. Time-resolved dynamic light scattering was employed to measure the aggregation kinetics of AgNPs over a range of monovalent electrolyte (NaCl) concentrations. The fractal dimensions (Df) obtained from aggregation kinetics and static light scattering were found to be dependent on the aggregation mechanism and, in accord with expectation, varied from 1.7 for diffusion-limited cluster aggregation to 2.3 for reaction-limited cluster aggregation. An aggregation-dissolution model, in which the proportion of accessible reactive sites on primary particles as well as the aggregate size and Df are assumed to be key determinants of reactivity, is found to provide an excellent description of the decline of normalized rate of dissolution of AgNPs during aggregation for different NaCl concentrations. This model also provides fundamental insights into the factors accounting for the observed change in rate of dissolution of AgNPs on injection into seawater thereby facilitating improved prediction of the likely toxicity of AgNPs in the marine environment.

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Increasing concentrations of dissolved silicate progressively retard Fe(II) oxidation kinetics in the circum-neutral pH range 6.0-7.0. As Si:Fe molar ratios increase from 0 to 2, the primary Fe(III) oxidation product transitions from lepidocrocite to a ferrihydrite/silica-ferrihydrite composite. Empirical results, supported by chemical kinetic modeling, indicated that the decreased heterogeneous oxidation rate was not due to differences in absolute Fe(II) sorption between the two solids types or competition for adsorption sites in the presence of silicate. Rather, competitive desorption experiments suggest Fe(II) was associated with more weakly bound, outer-sphere complexes on silica-ferrihydrite compared to lepidocrocite. A reduction in extent of inner-sphere Fe(II) complexation on silica-ferrihydrite confers a decreased ability for Fe(II) to undergo surface-induced hydrolysis via electronic configuration alterations, thereby inhibiting the heterogeneous Fe(II) oxidation mechanism. Water samples from a legacy radioactive waste site (Little Forest, Australia) were shown to exhibit a similar pattern of Fe(II) oxidation retardation derived from elevated silicate concentrations. These findings have important implications for contaminant migration at this site as well as a variety of other groundwater/high silicate containing natural and engineered sites that might undergo iron redox fluctuations.

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The hepatotoxin microcystin (MCYST) is produced by a variety of freshwater cyanobacterial species, including Microcystis aeruginosa. Interestingly, MCYST-producing M. aeruginosa strains have been shown to outcompete their nontoxic counterparts under iron-limiting conditions. However, the reasons for this are unclear. Here we examined the proteomic response of M. aeruginosa PCC 7806 continuous cultures under different iron and growth regimes. Iron limitation was correlated with a global reduction in levels of proteins associated with energy metabolism and photosynthesis. These proteomic changes were consistent with physiological observations, including reduced chlorophyll a content and reduced cell size. While levels of MCYST biosynthesis proteins did not fluctuate during the study period, both intra-and extracellular toxin quotas were significantly higher under iron-limiting conditions. Our results support the hypothesis that intracellular MCYST plays a role in protecting the cell against oxidative stress. Further, we propose that extracellular MCYST may act as a signaling molecule, stimulating MCYST production under conditions of iron limitation and enhancing the fitness of bloom populations. IMPORTANCEMicrocystin production in water supply reservoirs is a global public health problem. Understanding the ecophysiology of hepatotoxic cyanobacteria, including their responses to the presence of key micronutrient metals such as iron, is central to managing harmful blooms. To our knowledge, this was the first study to examine proteomic and physiological changes occurring in M. aeruginosa continuous cultures under conditions of iron limitation at different growth rates. C yanobacteria ("blue-green algae") proliferate in warm stratified water bodies rich in nitrogen and phosphorus (1, 2). Cyanobacterial blooms can have a negative impact on the appearance, taste, and odor of water, and their subsequent decay can lead to oxygen depletion and fish kills (3). However, of greatest concern to public health is their production of potent toxins. Microcystis aeruginosa (Kützing) Lemmermann is a common bloom-forming cyanobacterial species that produces hepatotoxic microcystins (MCYSTs) (1). These nonribosomally synthesized peptides inhibit eukaryotic protein phosphatases, leading to liver necrosis in acute doses and hepatocellular carcinoma in chronic low doses (4, 5).There is strong evidence that the production of MCYST is a direct function of cell division (6). It is therefore not surprising that parameters affecting cyanobacterial growth rate (e.g., nutrients, trace metals, temperature, pH, and light) have been correlated with fluctuating MCYST levels in batch culture experiments (7-9).Recent molecular studies suggest that iron (Fe) and nitrogen (N) also play a role in regulating the expression of MCYST biosynthesis (mcy) genes. In M. aeruginosa, for example, the mcy promoter region contains binding sites for the ferric uptake regulator (Fur) and the global nitrogen regulator (NtcA) (10, 11). Despite these genetic clu...

Soil-transmitted helminths (STHs) are medically important parasites that infect 1. 5 billion humans globally, causing a substantial disease burden. These parasites infect the gastrointestinal tract (GIT) of their host where they co-exist and interact with the host gut bacterial flora, leading to the coevolution of the parasites, microbiota, and host organisms. However, little is known about how these interactions change through time with the progression of infection. Strongyloidiasis is a human parasitic disease caused by the nematode Strongyloides stercoralis infecting 30–100 million people. In this study, we used a closely related rodent parasite Strongyloides venezuelensis and mice as a model of gastrointestinal parasite infection. We conducted a time-course experiment to examine changes in the fecal microbiota from the start of infection to parasite clearance. We found that bacterial taxa in the host intestinal microbiota changed significantly as the infection progressed, with an increase in the genera Bacteroides and Candidatus Arthromitus, and a decrease in Prevotella and Rikenellaceae . However, the microbiota recovered to the pre-infective state after parasite clearance from the host, suggesting that these perturbations are reversible. Microarray analysis revealed that this microbiota transition is likely to correspond with the host immune response. These findings give us an insight into the dynamics of parasite-microbiota interactions in the host gut during parasite infection.

The mode of Fe uptake by the cyanobacterium Microcystis aeruginosa cultured in Fraquil* (pH 8) containing Suwannee River fulvic acid (SRFA) was examined using short-term radiolabeled (55)Fe uptake assays and a kinetic model that describes extracellular Fe transformations. Both Fe(II) and Fe(III) uptake rates decreased substantially with increasing SRFA concentration as the availability of unchelated Fe decreased due to complexation by SRFA. Fe uptake rates under illuminated conditions were comparable to or slightly higher than those observed in the dark at the same Fe:SRFA concentration ratio, in contrast to results for systems containing ethylenediaminetetraacetic acid where Fe uptake rates were much greater under illumination than in the dark. The limited effect of light principally resulted from the relatively high rates of thermal dissociation and dark reduction of Fe(III) bound to SRFA and complexation of photogenerated Fe(II) by SRFA. Our findings imply that Fe uptake by M. aeruginosa at a fixed total Fe concentration of 200 nM is close to saturation when fulvic acid is present at concentrations near those typically found in natural waters (< ∼5 mg·L(-1)), with cellular growth likely to be limited by Fe availability only when natural organic matter is present at very high concentrations (>25 mg·L(-1)).