Silver release from silver nanoparticles in natural waters

Silver nanoparticles (AgNPs) are used increasingly in consumer products for their antimicrobial properties. This increased use raises ecological concern because of the release of AgNPs into the environment. Once released, zero-valent silver may be oxidized to Ag+ and the cation liberated or it may persist as AgNPs. The chemical form of Ag has implications for its toxicity; it is therefore crucial to characterize the persistence of AgNPs to predict their ecotoxicological potential. In this study, we evaluated the release of Ag from AgNPs of various sizes exposed to river and lake water for up to 4 months. Several AgNP-capping agents were also considered: polyvinylpyrrolidone (PVP), tannic acid (Tan), and citric acid (Cit). We observed a striking difference between 5, 10, and 50 nm AgNPs, with the latter being more resistant to dissolution in oxic water on a mass basis. However, the difference decreased when Ag was surface-area-normalized, suggesting an important role of the surface area in determining Ag loss. We propose that rapid initial Ag+ release was attributable to desorption of Ag+ from nanoparticle surfaces. We also observed that PVP- and Tan-AgNPs are more prone to Ag+ release than Cit-AgNPs. In addition, it is likely that oxidative dissolution also occurs but at a slower rate. This study clearly shows that small AgNPs (5 nm, PVP and Tan) dissolve rapidly and almost completely, while larger AgNPs (50 nm) have the potential to persist for an extended period of time and could serve as a continuous source of Ag ions

The small RNA RyhB is a regulator of cytochrome expression in Shewanella oneidensis

Shewanella oneidensis produces an extensive electron transfer network that results in metabolic flexibility. A large number of c-type cytochromes are expressed by S. oneidensis and these function as the fundamental electron transport chain proteins. Although several S. oneidensis cytochromes have been well-characterized, little is known about how their expression is regulated. In this study, we investigate the role of the ferric uptake regulator (Fur) and the sRNA RyhB in regulation. Our results demonstrate that loss of Fur leads to diminished growth and an apparent decrease in heme-containing proteins. Remarkably, deleting the Fur-repressed ryhB gene almost completely reverses these physiological changes, indicating that the phenotypes resulting from loss of Fur are (at least partially) dependent on RyhB. RNA sequencing identified a number of possible RyhB repressed genes. A large fraction of these encode c-type cytochromes, among them two of the most abundant periplasmic cytochromes CctA (also known as STC) and ScyA. We show that RyhB destabilizes the mRNA of four of its target genes, cctA, scyA, omp35, and nrfA and this requires the presence of the RNA chaperone Hfq. Iron limitation decreases the expression of the RyhB target genes cctA and scyA and this regulation relies on the presence of both Fur and RyhB. Overall, this study suggests that controlling cytochrome expression is of importance to maintain iron homeostasis and that sRNAs molecules are important players in the regulation of fundamental processes in S. oneidensis MR-1

Environmental Mineralogy: New challenges, new materials

The close links between mineralogy and materials science are leading to major developments in how society deals more effectively with energy and environmental challenges. The fast expanding field of "environmental mineralogy" helps mitigate major environmental issues related to the impact of anthropic activities on the global ecosystem. Focusing on energy-related materials and environmental cleanup, this article shows how minerals inspire us to design new materials for advanced technologies needed for energy production, managing contaminated areas, and disposing of nuclear waste. We illustrate the environmental importance of nanomaterials, non- and poorly crystalline phases, and the interactions between minerals and ubiquitous microbial activity

Membrane vesicles as a novel strategy for shedding encrusted cell surfaces

Surface encrustation by minerals, which impedes cellular metabolism, is a potential hazard for microbes. The reduction of U(VI) to U(IV) by Shewanella oneidensis strain MR-1 leads to the precipitation of the mineral uraninite, as well as a non-crystalline U(IV) product. The wild-type (WT) strain can produce extracellular polymeric substances (EPS), prompting precipitation of U some distance from the cells and precluding encrustation. Using cryo-transmission electron microscopy and scanning transmission X-ray microscopy we show that, in the biofilm-deficient mutant Delta mxdA, as well as in the WT strain to a lesser extent, we observe the formation of membrane vesicles (MVs) as an additional means to lessen encrustation. Additionally, under conditions in which the WT does not produce EPS, formation of MVs was the only observed mechanism to mitigate cell encrustation. Viability studies comparing U-free controls to cells exposed to U showed a decrease in the number of viable cells in conditions where MVs alone are detected, yet no loss of viability when cells produce both EPS and MVs. We conclude that MV formation is a microbial strategy to shed encrusted cell surfaces but is less effective at maintaining cell viability than the precipitation of U on EPS

The response of Desulfotomaculum reducens MI-1 to U(VI) exposure: a transcriptomic study

Desulfotomaculum reducens is the first Gram-positive sulfate- and metal-reducing bacterium for which the transcriptomic response to uranium exposure has been evaluated. The genes upregulated during fermentative growth in the presence of U(VI) as compared to its absence included those encoding for proteins involved in respiration such as NADH quinone oxidoreductase and heterodisulfide reductase. This finding suggested that electrons were shuttled to the electron transport chain during fermentation and points to the reduction of U(VI) as a metabolic process. Although U(IV) is typically insoluble and readily removable by filtration, U(IV) produced during active growth was not retained by a 0.2 mu m pore size filter and filtration was not sufficient to differentiate between U(VI) and U(IV). In addition, genes involved in iron homeostasis were upregulated in the presence of uranium, which was consistent with the upregulation of genes involved in c-type cytochrome biogenesis. Despite the upregulation of cytochrome biosynthesis genes, the sole c-type cytochrome encoded in the genome was not differentially expressed. Finally, genes encoding metal efflux pumps were also upregulated indicating the toxic nature of uranium. Analysis of the time-dependent gene expression showed that sporulation was the dominant process at the early stationary phase and that the presence of U at that stage did not impact expression

Rates of microbial hydrogen oxidation and sulfate reduction in Opalinus Clay rock

Hydrogen gas (H2) may be produced by the anoxic corrosion of steel components in underground structures, such as geological repositories for radioactive waste. In such environments, hydrogen was shown to serve as an electron donor for autotrophic bacteria. High gas overpressures are to be avoided in radioactive waste repositories and, thus, microbial consumption of H2 is generally viewed as beneficial. However, to fully consider this biological process in models of repository evolution over time, it is crucial to determine the in situ rates of microbial hydrogen oxidation and sulfate reduction. These rates were estimated through two distinct in situ experiments, using several measurement and calculation methods. Volumetric consumption rates were calculated to be between 1.13 and 1.93 μmol cm−3 day−1 for H2, and 0.14 and 0.20 μmol cm−3 day−1 for sulfate. Based on the stoichiometry of the reaction, there is an excess of H2 consumed, suggesting that it serves as an electron donor to reduce electron acceptors other than sulfate, and/or that some H2 is lost via diffusion. These rate estimates are critical to evaluate whether biological H2 consumption can negate H2 production in repositories, and to determine whether sulfate reduction can consume sulfate faster than it is replenished by diffusion, which could lead to methanogenic conditions

SunCHem: an integrated process for the hydrothermal production of methane from microalgae and CO2 mitigation

We describe a potential novel process (SunCHem) for the production of bio-methane via hydrothermal gasification of microalgae, envisioned as a closed-loop system, where the nutrients, water, and CO2 produced are recycled. The influence on the growth of microalgae of nickel, a trace contaminant that might accumulate upon effluent recycling, was investigated. For all microalgae tested, the growth was adversely affected by the nickel present (1, 5, and 10 ppm). At 25 ppm Ni, complete inhibition of cell division occurred. Successful hydrothermal gasification of the microalgae Phaeodactylum tricornutum to a methane-rich gas with high carbon gasification efficiency (68-74%) and C1-C3 hydrocarbon yields of 0.2 gC1-C3/gDM (DM, dry matter) was demonstrated. The biomass-released sulfur was shown to adversely affect Ru/C catalyst performance. Liquefaction of P. tricornutum at short residence times around 360°C was possible without coke formatio

Rapid mobilization of noncrystalline U(IV) coupled with FeS oxidation

The reactivity of disordered, noncrystalline U(IV) species remains poorly characterized despite their prevalence in biostimulated sediments. Because of the lack of crystalline structure, noncrystalline U(IV) may be susceptible to oxidative mobilization under oxic conditions. The present study investigated the mechanism and rate of oxidation of biogenic noncrystalline U(IV) by dissolved oxygen (DO) in the presence of mackinawite (FeS). Previously recognized as an effective reductant and oxygen scavenger, nanoparticulate FeS was evaluated for its role in influencing U release in a flow through system as a function of pH and carbonate concentration. The results demonstrated that noncrystalline U(IV) was more susceptible to oxidation than uraninite (UO2) in the presence of dissolved carbonate. A rapid release of U occurred immediately after FeS addition without exhibiting a temporary inhibition stage, as was observed during the oxidation of UO2, although FeS still kept DO levels low. X-ray photoelectron spectroscopy (XPS) characterized a transient surface Fe(III) species during the initial FeS oxidation, which was likely responsible for oxidizing noncrystalline U(IV) in addition to oxygen. In the absence of carbonate, however, the release of dissolved U was significantly hindered as a result of U adsorption by FeS oxidation products. This study illustrates the strong interactions between iron sulfide and U(IV) species during redox transformation and implies the lability of biogenic noncrystalline U(IV) species in the subsurface environment when subjected to redox cycling events

Fe(III) reduction during pyruvate fermentation by Desulfotomaculum reducens strain MI-1

Desulfotomaculum reducens MI-1 is a Gram-positive, sulfate-reducing bacterium also capable of reducing several metals, among which is Fe(III). Very limited knowledge is available on the potential mechanism(s) of metal reduction among Gram-positive bacteria, despite their preponderance in the microbial communities that inhabit some inhospitable environments (e.g., thermal or hyperthermal ecosystems, extreme pH or salinity environments, heavy metal or radionuclide contaminated sediments). Here, we show that in the presence of pyruvate, this micro-organism is capable of reducing both soluble Fe(III)-citrate and solid-phase hydrous ferric oxide, although growth is sustained by pyruvate fermentation rather than Fe(III) respiration. Despite the fact that Fe(III) reduction does not support direct energy conservation, D.reducens uses it as a complementary means of discarding excess reducing equivalent after H-2 accumulation in the culture headspace renders proton reduction unfavorable. Thus, Fe(III) reduction permits the oxidation of greater amounts of pyruvate than fermentation alone. Fe(III) reduction by D.reducens is mediated by a soluble electron carrier, most likely riboflavin. Additionally, an intracellular electron storage molecule acts as a capacitor and accumulates electrons during pyruvate oxidation for slow release to Fe(III). The reductase responsible for the transfer of electrons from the capacitor to the soluble carrier has not been identified, but data presented here argue against the involvement of c-type cytochromes