Shewanella Putrefaciens CN32 Research Articles

The Formation of Illite from Nontronite by Mesophilic and Thermophilic Bacterial Reaction

AbstractThe formation of illite through the smectite-to-illite (S-I) reaction is considered to be one of the most important mineral reactions occurring during diagenesis. In biologically catalyzed systems, however, this transformation has been suggested to be rapid and to bypass the high temperature and long time requirements. To understand the factors that promote the S-I reaction, the present study focused on the effects of pH, temperature, solution chemistry, and aging on the S-I reaction in microbially mediated systems. Fe(III)-reduction experiments were performed in both growth and non-growth media with two types of bacteria: mesophilic (Shewanella putrefaciensCN32) and thermophilic (Thermus scotoductusSA-01). Reductive dissolution of NAu-2 was observed and the formation of illite in treatment with thermophilic SA-01 was indicated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). A basic pH (8.4) and high temperature (65°C) were the most favorable conditions forthe formation of illite. A long incubation time was also found to enhance the formation of illite. K-nontronite (non-permanent fixation of K) was also detected and differentiated from the discrete illite in the XRD profiles. These results collectively suggested that the formation of illite associated with the biologically catalyzed smectite-to-illite reaction pathway may bypass the prolonged time and high temperature required for the S-I reaction in the absence of microbial activity.

Biogenic Formation and Growth of Uraninite (UO2)

Biogenic UO₂ (uraninite) nanocrystals may be formed as a product of a microbial reduction process in uranium-enriched environments near the Earth's surface. We investigated the size, nanometer-scale structure, and aggregation state of UO₂ formed by iron-reducing bacterium, Shewanella putrefaciens CN32, from a uranium-rich solution. Characterization of biogenic UO₂ precipitates by high-resolution transmission electron microscopy (HRTEM) revealed that the UO₂ nanoparticles formed were highly aggregated by organic polymers. Nearly all of the nanocrystals were networked in more or less 100 nm diameter spherical aggregates that displayed some concentric UO₂ accumulation with heterogeneity. Interestingly, pure UO₂ nanocrystals were piled on one another at several positions via UO₂-UO₂ interactions, which seem to be intimately related to a specific step in the process of growing large single crystals. In the process, calcium that was easily complexed with aqueous uranium(VI) appeared not to be combined with bioreduced uranium(IV), probably due to its lower binding energy. However, when phosphate was added to the system, calcium was found to be easily associated with uranium(IV), forming a new uranium phase, ningyoite. These results will extend the limited knowledge of microbial uraniferous mineralization and may provide new insights into the fate of aqueous uranium complexes.

Contrasting effects of Al substitution on microbial reduction of Fe(III) (hydr)oxides

Aluminum, one of the most abundant elements in soils and sediments, is commonly found co-precipitated with Fe in natural Fe(III) (hydr)oxides; yet, little is known about how Al substitution impacts bacterial Fe(III) reduction. Accordingly, we investigated the reduction of Al substituted (0–13 mol% Al) goethite, lepidocrocite, and ferrihydrite by the model dissimilatory Fe(III)-reducing bacterium (DIRB), Shewanella putrefaciens CN32. Here we reveal that the impact of Al on microbial reduction varies with Fe(III) (hydr)oxide type. No significant difference in Fe(III) reduction was observed for either goethite or lepidocrocite as a function of Al substitution. In contrast, Fe(III) reduction rates significantly decreased with increasing Al substitution of ferrihydrite, with reduction rates of 13% Al-ferrihydrite more than 50% lower than pure ferrihydrite. Although Al substitution changed the minerals’ surface area, particle size, structural disorder, and abiotic dissolution rates, we did not observe a direct correlation between any of these physiochemical properties and the trends in bacterial Fe(III) reduction. Based on projected Al-dependent Fe(III) reduction rates, reduction rates of ferrihydrite fall below those of lepidocrocite and goethite at substitution levels equal to or greater than 18 mol% Al. Given the prevalence of Al substitution in natural Fe(III) (hydr)oxides, our results bring into question the conventional assumptions about Fe (hydr)oxide bioavailability and suggest a more prominent role of natural lepidocrocite and goethite phases in impacting DIRB activity in soils and sediments.

Bioavailability of Fe(III) In Loess Sediments: An Important Source of Electron Acceptors

AbstractFe-reducing micro-organisms can change the oxidation state of structural Fe in clay minerals. The interactions with complex clays and clay minerals in natural materials remain poorly understood, however. The objective of this study was to determine if Fe(III) in loess was available as an electron acceptor and to study subsequent mineralogical changes. The loess samples were collected from St. Louis (Peoria), Missouri, USA, and Huanxia (HX) and Yanchang (YCH), in the Shanxi Province of China. The total Fe concentrations for the three samples was 1.69, 2.76, and 3.29 wt.%, respectively, and Fe(III) content was 0.48, 0.69, and 1.27 wt.%, respectively. All unreduced loess sediments contained Fe (oxyhydr)oxides and phyllosilicates. Bioreduction experiments were performed using Shewanella putrefaciens CN32 with lactate as the sole electron donor and Fe(III) in loess as the sole electron acceptor with and without anthraquinone-2, 6-disulfonate (AQDS) as an electron shuttle. Experiments were performed in non-growth (bicarbonate buffer) and growth (M1) media. The unreduced and bioreduced solids were analyzed by X-ray diffraction, Mössbauer spectroscopy, diffuse reflectance spectroscopy, and scanning electron microscopy/energy dispersive spectroscopy. Despite many similarities among the three loess samples, the extent and rate of Fe(III) reduction varied significantly. In the presence of AQDS the extent of reduction in the non-growth experiment was 25% of total Fe(III) in HX, 34% in Peoria, and 38% in YCH. The extent of reduction in the growth experiment was 72% in HX, 94% in Peoria, and 65% in YCH. The extent of bioreduction was less in the absence of AQDS. Overall, AQDS and the M1 growth medium significantly enhanced the rate and extent of bioreduction. Fe(III) in (oxyhydr)oxides and phyllosilicates was bioreduced. Siderite was absent in control samples, but was identified in bioreduced samples. The present research suggests that Fe(III) in loess sediments is an important potential source of electron acceptors that could support microbial activity under favorable conditions.

Effects of Oxyanions, Natural Organic Matter, and Bacterial Cell Numbers on the Bioreduction of Lepidocrocite (γ-FeOOH) and the Formation of Secondary Mineralization Products

Microbial reduction of Fe(III) oxides results in the production of Fe(II) and may lead to the subsequent formation of Fe(II)-bearing secondary mineralization products including magnetite, siderite, vivianite, chukanovite (ferrous hydroxy carbonate (FHC)), and green rust; however, the factors controlling the formation of specific Fe(II) phases are often not well-defined. This study examined effects of (i) a range of inorganic oxyanions (arsenate, borate, molybdate, phosphate, silicate, and tungstate), (ii) natural organic matter (citrate, oxalate, microbial extracellular polymeric substances [EPS], and humic substances), and (iii) the type and number of dissimilatory iron-reducing bacteria on the bioreduction of lepidocrocite and formation of Fe(II)-bearing secondary mineralization products. The bioreduction kinetics clustered into two distinct Fe(II) production profiles. "Fast" Fe(II) production kinetics [19-24 mM Fe(II) d(-1)] were accompanied by formation of magnetite and FHC in the unamended control and in systems amended with borate, oxalate, gellan EPS, or Pony Lake fulvic acid or having "low" cell numbers. Systems amended with arsenate, citrate, molybdate, phosphate, silicate, tungstate, EPS from Shewanella putrefaciens CN32, or humic substances derived from terrestrial plant material or with "high" cell numbers exhibited comparatively slow Fe(II) production kinetics [1.8-4.0 mM Fe(II) d(-1)] and the formation of green rust. The results are consistent with a conceptual model whereby competitive sorption of more strongly bound anions blocks access of bacterial cells and reduced electron-shuttling compounds to sites on the iron oxide surface, thereby limiting the rate of bioreduction.

XAFS Investigation of the Interactions of UVI with Secondary Mineralization Products from the Bioreduction of FeIII Oxides

Biogenic Fe(II) phases (magnetite, green rust, siderite, vivianite, etc.) provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of contaminants such as U(VI). We have examined the uptake and reduction of U(VI) in the presence of biogenic green rust (BioGR), magnetite (BioMAG), and siderite (BioSID) formed during the reduction of Fe(III) oxides by Shewanella putrefaciens CN32. Within 48 h, total solution-phase U(VI) concentrations decreased from 500 microM to 1.5 microM, 392 microM, and 472 microM in the U-BioGR, U-BioMAG, and U-BioSID systems, respectively. Analysis of the samples by U L(III) extended X-ray absorption fine structure spectroscopy (EXAFS) indicated that despite a stoichiometric excess of Fe(II), no more than 6% of U(VI) was reduced to U(IV) in the U-BioSID system, and no more than 22% of U(VI) was reduced in the U-BioMAG system. For comparison, in the U-BioGR system, >99% of U(VI) was reduced to U(IV). Uptake of U(VI) by BioGR and BioMAG was accompanied by formation of nanoparticulate uraninite. The U EXAFS data for the U-BioSID system were consistent with partial U(VI)/U(IV) substitution for Fe(II) in the surface layer of siderite particles and adsorption of U(IV).

Bioreduction of Nitrobenzene, Natural Organic Matter, and Hematite by Shewanella putrefaciens CN32

We examined the reduction of nitrobenzene by Shewanella putrefaciens CN32 in the presence of natural organic matter (NOM) and hematite. Bioreduction experiments were conducted with combinations and varied concentrations of nitrobenzene, soil humic acid, Georgetown NOM, hematite, and CN32. Abiotic experiments were conducted to quantify nitrobenzene reduction by biogenic Fe(II) and by bioreduced NOMs. We show that S. putrefaciens CN32 can directly reduce nitrobenzene. Both NOMs enhanced nitrobenzene reduction and the degree of enhancement depended on properties of the NOMs (aromaticity, organic radical content). Hematite enhanced nitrobenzene reduction by indirect reaction with biogenic-Fe(II), however, enhancement was dependent on the availability of excess electron donor. Under electron donor-limiting conditions, reducing equivalents diverted to hematite were not all transferred to nitrobenzene. In systems that contained both NOM and hematite we conclude that NOM-mediated reduction of nitrobenzene was more important than Fe(II)-mediated reduction.

Iron phase transformations resulting from the respiration of Shewanella putrefaciens on a mixed mineral phase

The initial Fe(III) minerals and the secondary mineralization products of Shewanella putrefaciens CN32 grown in the presence of dissolved phosphate and a commercial Fe(III) oxide, nominally nanoparticulate lepidocrocite, were determined using XRD and XAFS. The starting material was transformed by the bacteria from a reddish brown, rust colour mineral to a dark green phase over 90 days. Acid extraction of the bioreduced solids with 0.75 M HCl recovered 83% of the total iron as Fe(II), leaving a solid, acid-resistant phase. The latter was identified as nanoparticulate hematite by EXAFS. Subsequently, the starting Fe(III) phase was determined to be a mixture of 60% lepidocrocite, 26% ferrihydrite, and 14% hematite, using linear combination EXAFS analysis. For the acid-extractable phase, XANES and EXAFS indicated a predominantly Fe(II) valence state and a spectrum consistent with a mixture of brucite-type minerals(e.g., green rust or ferrous hydroxide) and siderite. The observed transformations suggest that in this mixed-mineral system, lepidocrocite and ferrihydrite are readily reducible to green rust and siderite, whereas hematite is less amenable to bacterial reduction. This study also demonstrates the utility of XAFS spectroscopy in the quantitative characterization of dissimilatory metal transformations, particularly in complex systems such as nanoparticulate minerals in hydrated mineral-bacteria assemblages.

Conserved synteny at the protein family level reveals genes underlying Shewanella species’ cold tolerance and predicts their novel phenotypes

Bacteria of the genus Shewanella can thrive in different environments and demonstrate significant variability in their metabolic and ecophysiological capabilities including cold and salt tolerance. Genomic characteristics underlying this variability across species are largely unknown. In this study, we address the problem by a comparison of the physiological, metabolic, and genomic characteristics of 19 sequenced Shewanella species. We have employed two novel approaches based on association of a phenotypic trait with the number of the trait-specific protein families (Pfam domains) and on the conservation of synteny (order in the genome) of the trait-related genes. Our first approach is top-down and involves experimental evaluation and quantification of the species’ cold tolerance followed by identification of the correlated Pfam domains and genes with a conserved synteny. The second, a bottom-up approach, predicts novel phenotypes of the species by calculating profiles of each Pfam domain among their genomes and following pair-wise correlation of the profiles and their network clustering. Using the first approach, we find a link between cold and salt tolerance of the species and the presence in the genome of a Na+/H+ antiporter gene cluster. Other cold-tolerance-related genes include peptidases, chemotaxis sensory transducer proteins, a cysteine exporter, and helicases. Using the bottom-up approach, we found several novel phenotypes in the newly sequenced Shewanella species, including degradation of aromatic compounds by an aerobic hybrid pathway in Shewanella woodyi, degradation of ethanolamine by Shewanella benthica, and propanediol degradation by Shewanella putrefaciens CN32 and Shewanella sp. W3-18-1.Electronic supplementary materialThe online version of this article (doi:10.1007/s10142-009-0142-y) contains supplementary material, which is available to authorized users.

Intracellular Precipitation of Pb by Shewanella putrefaciens CN32 during the Reductive Dissolution of Pb-Jarosite

Jarosites (MFe(3)(SO(4))(2)(OH)(6)) are precipitated in the Zn industry to remove impurities during the extraction process and contain metals such as Pb and Ag. Jarosite wastes are often confined to capped tailings ponds, thereby creating potential for anaerobic reductive dissolution by microbial populations. This study demonstrates the reductive dissolution of synthetic Pb-jarosite (PbFe(6)(SO(4))(4)(OH)(12)) by a subsurface dissimilatory Fe reducing bacterium (Shewanella putrefaciens CN32) using batch experiments under anaerobic circumneutral conditions. Solution chemistry, pH, Eh, and cell viability were monitored over time and illustrated the reduction of released structural Fe(III) from the Pb-jarosite to Fe(II). Inoculated samples containing Pb-jarosite also demonstrated decreased cellular viability coinciding with increased Pb concentrations. SEM images showed progressive nucleation of electron dense nanoparticles on the surface of bacteria, identified by TEM/EDS as intracellular crystalline precipitates enriched in Pb and P. The intracellular precipitation of Pb by S. putrefaciens CN32 observed in this study provides potential new insight into the biogeochemical cycling of Pb in reducing environments.

Biogenic Formation of As-S Nanotubes by Diverse Shewanella Strains

Shewanella sp. strain HN-41 was previously shown to produce novel, photoactive, As-S nanotubes via the reduction of As(V) and S(2)O(3)(2-) under anaerobic conditions. To determine if this ability was unique to this bacterium, 10 different Shewanella strains, including Shewanella sp. strain HN-41, Shewanella sp. strain PV-4, Shewanella alga BrY, Shewanella amazonensis SB2B, Shewanella denitrificans OS217, Shewanella oneidensis MR-1, Shewanella putrefaciens CN-32, S. putrefaciens IR-1, S. putrefaciens SP200, and S. putrefaciens W3-6-1, were examined for production of As-S nanotubes under standardized conditions. Of the 10 strains examined, three formed As-S nanotubes like those of strain HN-41. While Shewanella sp. strain HN-41 and S. putrefaciens CN-32 rapidly formed As-S precipitates in 7 days, strains S. alga BrY and S. oneidensis MR-1 reduced As(V) at a much lower rate and formed yellow As-S after 30 days. Electron microscopy, energy-dispersive X-ray spectroscopy, and extended X-ray absorption fine-structure spectroscopy analyses showed that the morphological and chemical properties of As-S formed by strains S. putrefaciens CN-32, S. alga BrY, and S. oneidensis MR-1 were similar to those previously determined for Shewanella sp. strain HN-41 As-S nanotubes. These studies indicated that the formation of As-S nanotubes is widespread among Shewanella strains and is closely related to bacterial growth and the reduction rate of As(V) and thiosulfate.

Biogenic nanoparticulate UO 2: Synthesis, characterization, and factors affecting surface reactivity

The surface reactivity of biogenic, nanoparticulate UO 2 with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of the presence of surface-associated organic matter on the biogenic UO 2. Synthesis of biogenic UO 2 was accomplished by reduction of aqueous uranyl ions, UO 2 2 + by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) to remove surface-associated organic matter and soluble uranyl species (NAUO2), or (2) to remove only soluble uranyl species (BIUO2). A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO 2 as a function of particle size and surface-associated organic matter. The N 2-BET surface areas of the two biogenic UO 2 samples following the washing procedures are 128.63 m 2 g −1 (NAUO2) and 92.56 m 2 g −1 (BIUO2), and the average particle sizes range from 5–10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO 2 (both NAUO2 and BIUO2) over the pH range 3–9 is the same as that of bulk. The U L III-edge EXAFS spectra for biogenic UO 2 (both NAUO2 and BIUO2) were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO 2. At pH 7, sorption of Zn(II) onto both bulk uraninite and biogenic, nanoparticulate UO 2 is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00 ± 0.20 μmol m −2 UO 2 (bulk uraninite), 2.34 ± 0.12 μmol m −2 UO 2 (NAUO2), and 2.57 ± 0.10 μmol m −2 UO 2 (BIUO2). Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO 2 indicate that Zn(II) sorption is dependent on the washing protocol. Zn–U pair correlations were observed at 2.8 ± 0.1 Å for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn–U distance, coupled with an average Zn–O distance of 2.09 ± 0.02 Å, indicates that Zn(O,OH) 6 sorbs as bidentate, edge-sharing complexes to UO 8 polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite, which is consistent with a Pauling bond-valence analysis. The absence of Zn–U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO 2 surface. Surface-associated organic matter on the biogenic UO 2 particles also inhibited particle annealing at 90 °C under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO 2 surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.

Effects of Nickel and Soil Humic Acid During Biological Hematite Reduction byShewanella putrefaciensCN32

The effect of nickel on the reductive dissolution of hematite by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens CN32 was studied in batch culture mode. Experiments were conducted under nongrowth conditions (108 cell mL−1) with hematite (2.0 g L−1) in 10 mM PIPES (pH 6.8) and H2 as the electron donor, spiked with nickel (0 to 1.63 mM) and soil humic acid (SHA; 100 mg L−1), and incubated for 5 days. Abiotic reductions using hydroxylamine HCl (HA-HCl) assays to estimate microbially reducible Fe(III), and biotic nitrate (0 to 1.43 mM) assays were also performed. A 50% inhibitory concentration (IC50) was defined as the nickel concentration that decreased the extent of electron acceptor consumption by 50% compared to a no-nickel control. Nickel addition resulted in a minimal effect on hematite bioreduction (IC50 of 132 mM), however SHA addition significantly increased the inhibitory effect of nickel (IC50 of 0.34 mM). SHA had an insignificant effect on nickel inhibition during nitrate reduction, while abiotic HA-HCl Fe(II) results suggest SHA-Ni(II) complexes may increase bioavailable Fe(III). Fe(II) and Ni(II) sorption to cells and hematite increased in the presence of SHA, suggesting that surface passivation or saturation of sorption surface area through the formation of ternary SHA-Ni(II)–hematite complexes appear responsible for the increased inhibitory effects by nickel observed.

A Comparison of the Rates of Fe(III) Reduction in Synthetic and Bacteriogenic Iron Oxides by Shewanella putrefaciens CN32

The susceptibility of various bacteriogenic iron oxides (BIOS) to bacterial Fe(III) reduction was examined. Reduction resulted in complete dissolution of the iron mineral from the surfaces of the Fe-oxidizing consortium. Reduction rates were compared to that of synthetic ferrihydrite (HFO). The reduction rate of HFO (0.162 day− 1) was significantly lower than that of Äspö (Gallionella dominated) BIOS (0.269 day− 1). Two Canadian (Leptothrix dominated) BIOS samples showed statistically equivalent rates of reduction (0.541 day−1 and 0.467 day− 1), which were higher than both Äspö BIOS and HFO. BIOS produced by different iron-oxidizing genera have different susceptibilities to microbial reduction.

Oxidative dissolution potential of biogenic and abiogenic TcO 2 in subsurface sediments

Technetium-99 (Tc) is an important fission product contaminant associated with sites of nuclear fuels reprocessing and geologic nuclear waste disposal. Tc is highly mobile in its most oxidized state [ Tc(VII) O 4 - ] and less mobile in the reduced form [Tc(IV)O 2· nH 2O]. Here we investigate the potential for oxidation of Tc(IV) that was heterogeneously reduced by reaction with biogenic Fe(II) in two sediments differing in mineralogy and aggregation state; unconsolidated Pliocene-age fluvial sediment from the upper Ringold (RG) Formation at the Hanford Site and a clay-rich saprolite from the Field Research Center (FRC) background site on the Oak Ridge Site. Both sediments contained Fe(III) and Mn(III/IV) as redox active phases, but FRC also contained mass-dominant Fe–phyllosilicates of different types. Shewanella putrefaciens CN32 reduced Mn(III/IV) oxides and generated Fe(II) that was reactive with Tc(VII) in heat-killed, bioreduced sediment. After bioreduction and heat-killing, biogenic Fe(II) in the FRC exceeded that in RG by a factor of two. More rapid reduction rates were observed in the RG that had lower biogenic Fe(II), and less particle aggregation. EXAFS measurements indicated that the primary reduction product was a TcO 2-like phase in both sediments. The biogenic redox product Tc(IV) oxidized rapidly and completely in RG when contacted with air. Oxidation, in contrast, was slow and incomplete in the FRC, in spite of similar molecular scale speciation of Tc compared to RG. X-ray microprobe, electron microprobe, X-ray absorption spectroscopy, and micro X-ray diffraction were applied to the whole sediment and isolated Tc-containing particles. These analyses revealed that non-oxidizable Tc(IV) in the FRC existed as complexes with octahedral Fe(III) within intra-grain domains of 50–100 μm-sized, Fe-containing micas presumptively identified as celadonite. The markedly slower oxidation rates in FRC as compared to RG were attributed to mass-transfer-limited migration of O 2 into intra-aggregate and intraparticle domains where Tc(IV) existed; and the formation of unique, oxidation-resistant, intragrain Tc(IV)–Fe(III) molecular species.

Preliminary characterization and biological reduction of putative biogenic iron oxides (BIOS) from the Tonga‐Kermadec Arc, southwest Pacific Ocean

Sediment samples were obtained from areas of diffuse hydrothermal venting along the seabed in the Tonga sector of the Tonga-Kermadec Arc, southwest Pacific Ocean. Sediments from Volcano 1 and Volcano 19 were analyzed by X-ray diffraction (XRD) and found to be composed primarily of the iron oxyhydroxide mineral, two-line ferrihydrite. XRD also suggested the possible presence of minor amounts of more ordered iron (hydr)oxides (including six-line ferrihydrite, goethite/lepidocrocite and magnetite) in the biogenic iron oxides (BIOS) from Volcano 1; however, Mössbauer spectroscopy failed to detect any mineral phases more crystalline than two-line ferrihydrite. The minerals were precipitated on the surfaces of abundant filamentous microbial structures. Morphologically, some of these structures were similar in appearance to the known iron-oxidizing genus Mariprofundus spp., suggesting that the sediments are composed of biogenic iron oxides. At Volcano 19, an areally extensive, active vent field, the microbial cells appeared to be responsible for the formation of cohesive chimney-like structures of iron oxyhydroxide, 2-3 m in height, whereas at Volcano 1, an older vent field, no chimney-like structures were apparent. Iron reduction of the sediment material (i.e. BIOS) by Shewanella putrefaciens CN32 was measured, in vitro, as the ratio of [total Fe(II)]:[total Fe]. From this parameter, reduction rates were calculated for Volcano 1 BIOS (0.0521 day(-1)), Volcano 19 BIOS (0.0473 day(-1)), and hydrous ferric oxide, a synthetic two-line ferrihydrite (0.0224 day(-1)). Sediments from both BIOS sites were more easily reduced than synthetic ferrihydrite, which suggests that the decrease in effective surface area of the minerals within the sediments (due to the presence of the organic component) does not inhibit subsequent microbial reduction. These results indicate that natural, marine BIOS are easily reduced in the presence of dissimilatory iron-reducing bacteria, and that the use of common synthetic iron minerals to model their reduction may lead to a significant underestimation of their biological reactivity.

Microbial Dissolution of Silver Jarosite: Examining Its Trace Metal Behaviour in Reduced Environments

Iron sulfate minerals such as jarosite-group compounds (e.g., M Fe3(SO 4 ) 2 (OH) 6 ) can be of considerable environmental importance because of their ability to scavenge trace elements and thus contribute to some degree of metal cycling. Jarosite forms in low temperature hydrothermal, acidic, sulfate-rich environments often yielding a range of elemental substituted forms: plumbojarosite (M = Pb), argentojarosite (M = Ag), jarosite (M = K), natrojarosite (M = Na), hydroniumjarosite (M = H3O), and ammoniojarosite (M = NH4). Anthropogenic sources of jarosite are common in mine waste environments, most often associated with the waste products resulting from base metal recovery. Few studies have investigated the effect that dissimilatory metal reducing bacteria may have in the presence of these compounds following the onset of reducing conditions. Jarosite reactivity may differ systematically as a function of its chemical properties. For example, the incorporation of Ag in the mineral lattice may have inhibitory affects on the growth of microbial strains. In this study the reductive dissolution of argentojarosite (M = Ag) in the presence of Shewanella putrefaciens CN32 (10 9 cells ml 1 , pH 7.1) was examined. Using the silver (argento)jarosite, Ag Fe 3 (SO 4 ) 2 (OH) 6 , as a sole terminal electron acceptor we observed the reduction of structural Fe(III) and Ag(I) by CN32 through the release of Fe(II) ions to solution. Aqueous silver concentrations were below instrumental detection. Environmental SEM (ESEM) and TEM micrographs of the microbial clusters revealed the progressive heterogeneous nucleation of Ag(0) nanoparticles within cellular structures and also on adjacent mineral grains. The results of this study are the first presented for the anaerobic dissolution of silver jarosite. This has implications for understanding the processes leading to the mobility or retention of silver in mine waste and industrial landfill environments. It also provides insight into the microbial mechanisms of silver resistance and nanoparticle formation with potential applications for bioleaching and/or biotechnology.

Effects of Electron Transfer Mediators on the Bioreduction of Lepidocrocite (γ-FeOOH) by Shewanella putrefaciens CN32

Electron transfer mediators (ETMs) such as low-molecular-mass quinones (e.g., juglone and lawsone) and humic substances are believed to play a role in many redox reactions involved in contaminant transformations and the biogeochemical cycling of many redox-active elements (e.g., Fe and Mn) in aquatic and terrestrial environments. This study examines the effects of a series of compounds representing major classes of natural and synthetic organic ETMs, including low-molecular-mass quinones, humic substances, phenazines, phenoxazines, phenothiazines, and indigo derivatives, on the bioreduction of lepidocrocite (gamma-FeOOH) by the dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens CN32. Although S. putrefaciens CN32 was able to reduce lepidocrocite in the absence of exogenous ETMs, the addition of exogenous ETMs enhanced the bioreduction of lepidocrocite. In general, the rate of Fe(II) production correlated well with the reduction potentials of the ETMs. The addition of humic acids or unfractionated natural organic matter at concentrations of 10 mg organic CL(-1) resulted in, at best, a minimal enhancement of lepidocrocite bioreduction. This observation suggests that electron shuttling by humic substances is not likely to play a major role in Fe(lll) bioreduction in oligotrophic environments such as subsurface sediments with low organic C contents.

Characterization of uraninite nanoparticles produced by Shewanella oneidensis MR-1

The reduction of uranium(VI) by Shewanella oneidensis MR-1 was studied to examine the effects of bioreduction kinetics and background electrolyte on the physical properties and reactivity to re-oxidation of the biogenic uraninite, UO 2(s). Bioreduction experiments were conducted with uranyl acetate as the electron acceptor and sodium lactate as the electron donor under resting cell conditions in a 30 mM NaHCO 3 buffer, and in a PIPES-buffered artificial groundwater (PBAGW). MR-1 was cultured in batch mode in a defined minimal medium with a specified air-to-medium volume ratio such that electron acceptor (O 2) limiting conditions were reached just when cells were harvested for subsequent experiments. The rate of U(VI) bioreduction was manipulated by varying the cell density and the incubation temperature (1.0 × 10 8 cell ml −1 at 20 °C or 2.0 × 10 8 cell ml −1 at 37 °C) to generate U(IV) solids at “fast” and “slow” rates in the two different buffers. The presence of Ca in PBAGW buffer altered U(VI) speciation and solubility, and significantly decreased U(VI) bioreduction kinetics. High resolution transmission electron microscopy was used to measure uraninite particle size distributions produced under the four different conditions. The most common primary particle size was 2.9–3.0 nm regardless of U(VI) bioreduction rate or background electrolyte. Extended X-ray absorption fine-structure spectroscopy was also used to estimate uraninite particle size and was consistent with TEM results. The reactivity of the biogenic uraninite products with dissolved oxygen was tested, and neither U(VI) bioreduction rate nor background electrolyte had any statistical effect on oxidation rates. With MR-1, uraninite particle size was not controlled by the bioreduction rate of U(VI) or the background electrolyte. These results for MR-1, where U(VI) bioreduction rate had no discernible effect on uraninite particle size or oxidation rate, contrast with our recent research with Shewanella putrefaciens CN32, where U(VI) bioreduction rate strongly influenced both uraninite particle size and oxidation rate. These two studies with Shewanella species can be viewed as consistent if one assumes that particle size controls oxidation rates, so the similar uraninite particle sizes produced by MR-1 regardless of U(VI) bioreduction rate would result in similar oxidation rates. Factors that might explain why U(VI) bioreduction rate was an important control on uraninite particle size for CN32 but not for MR-1 are discussed.

Role of Microbial Fe(III) Reduction and Solution Chemistry in Aggregation and Settling of Suspended Particles in the Mississippi River Delta Plain, Louisiana, USA

AbstractRiver-dominated delta areas are primary sites of active biogeochemical cycling, with productivity enhanced by terrestrial inputs of nutrients. Particle aggregation in these areas primarily controls the deposition of suspended particles, yet factors that control particle aggregation and resulting sedimentation in these environments are poorly understood. This study was designed to investigate the role of microbial Fe(III) reduction and solution chemistry in aggregation of suspended particles in the Mississippi Delta. Three representative sites along the salinity gradient were selected and sediments were collected from the sediment-water interface. Based on quantitative mineralogical analyses 88–89 wt.% of all minerals in the sediments are clays, mainly smectite and illite. Consumption of SO42−\$\end{document} and the formation of H2S and pyrite during microbial Fe(III) reduction of the non-sterile sediments by Shewanella putrefaciens CN32 in artificial pore water (APW) media suggest simultaneous sulfate and Fe(III) reduction activity. The pHPZNPC of the sediments was ⩽3.5 and their zeta potentials at the sediment-water interface pH (6.9–7.3) varied from −35 to −45 mV, suggesting that both edges and faces of clay particles have negative surface charge. Therefore, high concentrations of cations in pore water are expected to be a predominant factor in particle aggregation consistent with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Experiments on aggregation of different types of sediments in the same APW composition revealed that the sediment with low zeta potential had a high rate of aggregation. Similarly, addition of external Fe(II) (i.e. not derived from sediments) was normally found to enhance particle aggregation and deposition in all sediments, probably resulting from a decrease in surface potential of particles due to specific Fe(II) sorption. Scanning and transmission electron microscopy (SEM, TEM) images showed predominant face-to-face clay aggregation in native sediments and composite mixtures of biopolymer, bacteria, and clay minerals in the bioreduced sediments. However, a clear need remains for additional information on the conditions, if any, that favor the development of anoxia in deep- and bottom-water bodies supporting Fe(III) reduction and resulting in particle aggregation and sedimentation.