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Microbial metabolism plays a pivotal role in controlling the solubility and mobility of radionuclides in waters contaminated by nuclear waste. The distribution and activity of dissimilatory Fe(III)-reducing bacteria are of particular importance because they can alter the solubility of radionuclides via direct enzymatic reduction or by indirect mechanisms catalyzed via a range of electron shuttling compounds. Using a combination of the techniques of microbiology, biochemistry, and molecular biology, we have characterized the mechanisms of electron transfer to key radionuclides by Fe(III)-reducing bacteria. The mechanisms of enzyme-mediated reduction of problematic actinides, principally U(VI) but including Pu(IV) and Np(V), are described in this review. In addition, the mechanisms by which the fission product Tc(VII) is reduced are also discussed. Direct enzymatic reductions of Tc(VII), catalyzed by microbial hydrogenases, are described along with indirect mechanisms catalyzed by microbially produced Fe(II...

The presence of uranium in groundwater at nuclear sites can be controlled by microbial processes. Here we describe the results from stimulating microbial reduction of U(VI) in sediment samples obtained from a nuclear-licensed site in the UK. A variety of different lithology sediments were selected to represent the heterogeneity of the subsurface at a site underlain by glacial outwash deposits and sandstone. The natural sediment microbial communities were stimulated via the addition of an acetate/lactate electron donor mix and were monitored for changes in geochemistry and molecular ecology. Most sediments facilitated the removal of 12 ppm U(VI) during the onset of Fe(III)-reducing conditions; this was reflected by an increase in the proportion of known Fe(III)- and U(VI)-reducing species. However U(VI) remained in solution in two sediments and Fe(III)-reducing conditions did not develop. Sequential extractions, addition of an Fe(III)-enrichment culture and most probable number enumerations revealed that a lack of bioavailable iron or low cell numbers of Fe(III)-reducing bacteria may be responsible. These results highlight the potential for stimulation of microbial U(VI)-reduction to be used as a bioremediation strategy at UK nuclear sites, and they emphasise the importance of both site-specific and borehole-specific investigations to be completed prior to implementation. (C) 2014 The Authors. Published by Elsevier Ltd.

Environmental context The use of groundwater with elevated concentrations of arsenic for drinking, cooking or irrigation has resulted in the worst mass poisoning in human history. This study shows that organic compounds that can be found in arsenic rich subsurface sediments may be used by indigenous microorganisms, contributing to the release of arsenic from the sediments into the groundwater. This study increases our understanding of the range of organic substrates (and their sources) that can potentially stimulate arsenic mobilisation into groundwaters. Abstract Microbial activity is generally accepted to play a critical role, with the aid of suitable organic carbon substrates, in the mobilisation of arsenic from sediments into shallow reducing groundwaters. The nature of the organic matter in natural aquifers driving the reduction of AsV to AsIII is of particular importance but is poorly understood. In this study, sediments from an arsenic rich aquifer in Cambodia were amended with two 13C-labelled organic substrates. 13C-hexadecane was used as a model for potentially bioavailable long chain n-alkanes and a 13C-kerogen analogue as a proxy for non-extractable organic matter. During anaerobic incubation for 8 weeks, significant FeIII reduction and AsIII mobilisation were observed in the biotic microcosms only, suggesting that these processes were microbially driven. Microcosms amended with 13C-hexadecane exhibited a similar extent of FeIII reduction to the non-amended microcosms, but marginally higher AsIII release. Moreover, gas chromatography–mass spectrometry analysis showed that 65% of the added 13C-hexadecane was degraded during the 8-week incubation. The degradation of 13C-hexadecane was microbially driven, as confirmed by DNA stable isotope probing (DNA-SIP). Amendment with 13C-kerogen did not enhance FeIII reduction or AsIII mobilisation, and microbial degradation of kerogen could not be confirmed conclusively by DNA-SIP fractionation or 13C incorporation in the phospholipid fatty acids. These data are, therefore, consistent with the utilisation of long chain n-alkanes (but not kerogen) as electron donors for anaerobic processes, potentially including FeIII and AsV reduction in the subsurface.

Although there is consensus that microorganisms significantly influence uranium speciation and mobility in the subsurface under circumneutral conditions, microbiologically mediated U(VI) redox cycling under alkaline conditions relevant to the geological disposal of cementitious intermediate level radioactive waste, remains unexplored. Here, we describe microcosm experiments that investigate the biogeochemical fate of U(VI) at pH 10-10.5, using sediments from a legacy lime working site, stimulated with an added electron donor, and incubated in the presence and absence of added Fe(III) as ferrihydrite. In systems without added Fe(III), partial U(VI) reduction occurred, forming a U(IV)-bearing non-uraninite phase which underwent reoxidation in the presence of air (O2) and to some extent nitrate. By contrast, in the presence of added Fe(III), U(VI) was first removed from solution by sorption to the Fe(III) mineral, followed by bioreduction and (bio)magnetite formation coupled to formation of a complex U(IV)-bearing phase with uraninite present, which also underwent air (O2) and partial nitrate reoxidation. 16S rRNA gene pyrosequencing showed that Gram-positive bacteria affiliated with the Firmicutes and Bacteroidetes dominated in the post-reduction sediments. These data provide the first insights into uranium biogeochemistry at high pH and have significant implications for the long-term fate of uranium in geological disposal in both engineered barrier systems and the alkaline, chemically disturbed geosphere.

Conservation of energy by Fe(III)-reducing species such as Shewanella oneidensis could potentially control the redox potential of environments relevant to the geological disposal of radioactive waste and radionuclide contaminated land. Such environments will be exposed to ionizing radiation so characterization of radiation alteration to the mineralogy and the resultant impact upon microbial respiration of iron is essential. Radiation induced changes to the iron mineralogy may impact upon microbial respiration and, subsequently, influence the oxidation state of redox-sensitive radionuclides. In the present work, Mössbauer spectroscopy and electron microscopy indicate that irradiation (1 MGy gamma) of 2-line ferrihydrite can lead to conversion to a more crystalline phase, one similar to akaganeite. The room temperature Mössbauer spectrum of irradiated hematite shows the emergence of a paramagnetic Fe(III) phase. Spectrophotometric determination of Fe(II) reveals a radiation-induced increase in the rate and extent of ferrihydrite and hematite reduction by S. oneidensis in the presence of an electron shuttle (riboflavin). Characterization of bioreduced solids via XRD indicate that this additional Fe(II) is incorporated into siderite and ferrous hydroxy carbonate, along with magnetite, in ferrihydrite systems, and siderite in hematite systems. This study suggests that mineralogical changes to ferrihydrite and hematite induced by radiation may lead to an increase in bioavailability of Fe(III) for respiration by Fe(III)-reducing bacteria.

The magnetic moments of magnetite nanoparticles are dramatically enhanced through the addition of zinc in a microbiologically driven synthesis procedure. The particles are produced through the reduction of Fe(III)-compounds containing Zn(II) by the iron reducing bacterium Geobacter sulfurreducens. Results indicate a significant increase in the saturation magnetization by over 50% compared to magnetite at both room and low temperatures for relatively minor quantities of zinc substitution. A maximum saturation magnetization of nearly 100 emu g-1 of sample is measured at room temperature. Analysis of the cation site ordering reveals a complex dependence on the Zn content, with the combined effect of Zn substitution of Fe3+ ions on tetrahedral sites, together with Fe2+ cation oxidation, leading to the observed magnetization enhancement for low Zn doping levels. The improved magnetic properties give superior performance in MRI applications with an MRI contrast enhancement among the largest values reported, being more than 5 times larger than a commercial contrast agent (Feridex) measured under identical conditions. The synthesis technique applied here involves an environmentally benign route and offers the potential to tune the magnetic properties of magnetic nanoparticles, with increased overall magnetization desirable for many different commercial applications. A range of zinc doped magnetite nanoparticles are produced through the reduction of zinc-iron oxyhydroxide precursors by the bacterium Geobacter sulfurreducens. These materials exhibit significant increases in saturation magnetization at low zinc concentrations in comparison to stoichiometric magnetite. The enhanced magnetic properties are tested as potential MRI contrast agents and show significant MRI contrast enhancement over a commercially available agent. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This chapter describes the biochemical basis of microbial-metalloid interactions with emphasis on energy-yielding redox biotransformations that cycle between the As(V) and As(III) oxidation states. A particular focus is the microbial reduction of As(V) that is sorbed onto subsurface sediments, and the subsequent mobilization of As(III) into water that is abstracted for drinking and irrigation. The reduction of As(V) by Geobacter uraniireducens clearly has a number of important environmental implications. This organism is the first member of the family Geobacteraceae to be shown to reduce As(V) and this activity is supported by the identification of genes potentially encoding a respiratory arsenate reductase. Genetic studies are needed to confirm the role of these genes in arsenic metabolism in G. uraniireducens, and explore the diversity of As(V)-reducing Geobacter species and the ecophysiology of organisms in arsenic-impacted subsurface sediments. Recent studies highlight that metal-reducing bacteria, especially dissimilatory As(V)-respiring bacteria, may play an important role in the microbial process. The microbiological focus of research in this area is shifting now toward the unequivocal identification of the organisms involved, which will in turn lead to studies on the underlying mechanisms at a molecular (genetic) level and the factors that result in the activation of these physiological processes in situ. Recent studies suggest that sulfate reduction can be supported even in very low carbon aquifer sediments that contain low concentrations of electron donor, resulting in removal of arsenic mobilized by dissimilatory metal reduction.

This chapter provides an overview of metal-microbe interactions and describes how they could be harnessed to clean up metal-contaminated water, soil, and land. Biosorption of metals has been reviewed extensively, and this chapter notes only some salient points and recent developments of interest. Biosensors may prove very useful in identifying the need for metal remediation, which will be dictated in many cases by the concentration of bioavailable toxic metals in a given soil or water matrix, and also in defining the end point for bioremediation efforts. The biodegradation of toxic organotin compounds used as biocides and antifouling agents has also received recent attention. Laboratory tests have indicated that the immobilization of metals and radionuclides by bioremediation could be very effective, with removal of contaminants from the mobile aqueous phase to below critical values. Nevertheless, contaminant immobilization caused by bioremediation must be regarded as the retardation of contaminant migration rather than as a permanent solution to the problem. However, driven by the realization that large areas of land contaminated with metal and radionuclides cannot be economically remediated by conventional chemical approaches, significant resources have become available for this research area. Supported by genomics-enabled studies ongoing in many laboratories worldwide, one can expect this research area to develop further in the near future, delivering more robust technologies for the bioremediation of metal-contaminated waters and land.

This study investigates the application of a hybrid system combining hollow-fibre membrane technology with the reductive abilities of magnetic nanoparticles for the remediation of toxic Cr(VI) and the azo dye, Remazol Black B. Nano-scale biogenic magnetite (Fe3O4), formed by microbial reduction of the mineral ferrihydrite, has a high reductive capacity due to the presence of Fe(II) in the mineral structure. The magnetic nanoparticles (approximately 20 nm) can be arrayed with Pd0 nanoparticles (approximately 5 nm) making a catalytically active nanomaterial. Membrane units, with and without nanoparticles, were challenged with either Cr(VI) or azo dye and some were supplemented with sodium formate, as an electron donor for contaminant reduction promoted by the Pd. The combination of Pd-magnetite with formate resulted in the most effective remediation strategy for both contaminants and the lifetime of the membrane unit was also increased, with 55% (19 days) and 70% (23 days) removal of the azo dye and Cr(VI), respectively. Low flow rates of 0.1 ml/min resulted in improved efficiencies due to increased contact time with the membrane/nanoparticle unit, with 70-75% removal of each contaminant. Chemical analyses of the nanoparticles post-exposure to Cr(VI) in the membrane modules indicated Pd to be more oxidized when Cr removal was maximized, and that the Cr was partially reduced to Cr(III) at the surface of the magnetite. These results have demonstrated that hollow-fibre membrane units can be enhanced for the removal of soluble, redox sensitive contaminants by incorporation of a layer of palladized biogenic nanoparticulate magnetite.

Magnetic hyperthermia uses AC stimulation of magnetic nanoparticles to generate heat for cancer cell destruction. Whilst nanoparticles produced inside magnetotactic bacteria have shown amongst the highest reported heating to date, these particles are magnetically blocked so that strong heating occurs only for mobile particles, unless magnetic field parameters are far outside clinical limits. Here, nanoparticles extracellularly produced by the bacteria Geobacter sulfurreducens are investigated that contain Co or Zn dopants to tune the magnetic anisotropy, saturation magnetization and nanoparticle sizes, enabling heating within clinical field constraints. The heating mechanisms specific to either Co or Zn doping are determined from frequency dependent specific absorption rate (SAR) measurements and innovative AC susceptometry simulations that use a realistic model concerning clusters of polydisperse nanoparticles in suspension. Whilst both particle types undergo magnetization relaxation and show heating effects in water under low AC frequency and field, only Zn doped particles maintain relaxation combined with hysteresis losses even when immobilized. This magnetic heating process could prove important in the biological environment where nanoparticle mobility may not be possible. Obtained SARs are discussed regarding clinical conditions which, together with their enhanced MRI contrast, indicate that biogenic Zn doped particles are promising for combined diagnostics and cancer therapy.

The reduction of Tc(VII), Cr(VI) Se(IV) and Te(IV) by representatives of three genera of sulphate-reducing bacteria was studied with respect to the specificity of electron donor and acceptor. Tc(VII) and Cr(VI) were reduced by different mechanisms involving a hydrogenase. Cr(VI) reduction was achieved using a new isolate with lactate as the electron donor or by using H 2 in the presence of bicarbonate ion. Te(IV) and Se(IV) were reduced to base metals. The removal of Se(IV) was enhanced under sulphidogenic conditions, with metal sulphide identified by energy dispersive X-ray microanalysis. The order of preference of the electron acceptors was Te(IV)>S(VI)>Se(IV), which is in sharp contrast to that predicted by the redox potentials alone.

The dissimilatory Fe(III)-reducing bacterium Geobacter sulfurreducens reduced and precipitated Tc(VII) by two mechanisms. Washed cell suspensions coupled the oxidation of hydrogen to enzymatic reduction of Tc(VII) to Tc(IV), leading to the precipitation of TcO 2 at the periphery of the cell. An indirect, Fe(II)-mediated mechanism was also identified. Acetate, although not utilized efficiently as an electron donor for direct cell-mediated reduction of technetium, supported the reduction of Fe(III), and the Fe(II) formed was able to transfer electrons abiotically to Tc(VII). Tc(VII) reduction was comparatively inefficient via this indirect mechanism when soluble Fe(III) citrate was supplied to the cultures but was enhanced in the presence of solid Fe(III) oxide. The rate of Tc(VII) reduction was optimal, however, when Fe(III) oxide reduction was stimulated by the addition of the humic analog and electron shuttle anthaquinone-2,6-disulfonate, leading to the rapid formation of the Fe(II)-bearing mineral magnetite. Under these conditions, Tc(VII) was reduced and precipitated abiotically on the nanocrystals of biogenic magnetite as TcO 2 and was removed from solution to concentrations below the limit of detection by scintillation counting. Cultures of Fe(III)-reducing bacteria enriched from radionuclide-contaminated sediment using Fe(III) oxide as an electron acceptor in the presence of 25 μM Tc(VII) contained a single Geobacter sp. detected by 16S ribosomal DNA analysis and were also able to reduce and precipitate the radionuclide via biogenic magnetite. Fe(III) reduction was stimulated in aquifer material, resulting in the formation of Fe(II)-containing minerals that were able to reduce and precipitate Tc(VII). These results suggest that Fe(III)-reducing bacteria may play an important role in immobilizing technetium in sediments via direct and indirect mechanisms.

Resting cells of Desulfovibrio desulfuricans coupled the oxidation of a range of electron donors to Tc(VII) reduction. The reduced technetium was precipitated as an insoluble low-valence oxide. The optimum electron donor for the biotransformation was hydrogen, although rapid rates of reduction were also supported when formate or pyruvate was supplied to the cells. Technetium reduction was less efficient when the growth substrates lactate and ethanol were supplied as electron donors, while glycerol, succinate, acetate, and methanol supported negligible reduction. Enzyme activity was stable for several weeks and was insensitive to oxygen. Transmission electron microscopy showed that the radionuclide was precipitated at the periphery of the cell. Cells poisoned with Cu(II), which is selective for periplasmic but not cytoplasmic hydrogenases, were unable to reduce Tc(VII), a result consistent with the involvement of a periplasmic hydrogenase in Tc(VII) reduction. Resting cells, immobilized in a flowthrough membrane bioreactor and supplied with Tc(VII)-supplemented solution, accumulated substantial quantities of the radionuclide when formate was supplied as the electron donor, indicating the potential of this organism as a biocatalyst to treat Tc-contaminated wastewaters.

The principal goal of this work was to establish the feasibility of two biosensor technologies with enhanced specificity and selectivity for the detection of several bioavailable heavy metals in environmental samples. Two parallel strategies have been followed. The first approach was to construct whole cell bacterial biosensors that emit a bioluminescent or fluorescent signal in the presence of a biologically available heavy metal. The molecular basis of σ-54 promoters as sensing elements of environmental pollutants has been determined and a number of metal-induced promoter regions have been identified, sequenced and cloned as promoter cassettes. The specificity of the promoter cassettes has been determined using luxCDABE reporter systems. Whole cell-biosensors containing metal-induced lux reporter systems have been incorporated into different matrices for their later immobilisation on optic fibres and characterised in terms of their sensitivity and storage capacity. The second type of sensors was based on the direct interaction between metal-binding proteins and heavy metal ions. In this case, the capacitance changes of the proteins, such as synechoccocal metallothionein (as a GST-SmtA fusion protein) and the mercury regulatory protein, MerR, were detected in the presence of femtomolar to millimolar metal ion concentrations.

Escherichia coli and Desulfovibrio desulfuricans reduce Tc(VII) (TcO(4)(-)) with formate or hydrogen as electron donors. The reaction is catalyzed by the hydrogenase component of the formate hydrogenlyase complex (FHL) of E. coli and is associated with a periplasmic hydrogenase activity in D. desulfuricans. Tc(VII) reduction in E. coli by H(2) and formate was either inhibited or repressed by 10 mM nitrate. By contrast, Tc(VII) reduction catalyzed by D. desulfuricans was less sensitive to nitrate when formate was the electron donor, and unaffected by 10 mM or 100 mM nitrate when H(2) was the electron donor. The optimum pH for Tc(VII) reduction by both organisms was 5.5 and the optimum temperature was 40 degrees C and 20 degrees C for E. coli and D. desulfuricans, respectively. Both strains had an apparent K(m) for Tc(VII) of 0.5 mM, but Tc(VII) was removed from a solution of 300 nM TcO(4)(-) within 30 h by D. desulfuricans at the expense of H(2). The greater bioprocess potential of D. desulfuricans was shown also by the K(s) for formate (>25 mM and 0.5 mM for E. coli and D. desulfuricans, respectively), attributable to the more accessible, periplasmic localization of the enzyme in the latter. The relative rates of Tc(VII) reduction for E. coli and D. desulfuricans (with H(2)) were 12.5 and 800 micromol Tc(VII) reduced/g biomass/h, but the use of an E. coli HycA mutant (which upregulates FHL activities by approx. 50%) had a similarly enhancing effect on the rate of Tc reduction. The more rapid reduction of Tc(VII) by D. desulfuricans compared with the E. coli strains was also shown using cells immobilized in a hollow-fiber reactor, in which the flow residence times sustaining steady-state removal of 80% of the radionuclide were 24.3 h for the wild-type E. coli, 4.25 h for the upregulated mutant, and 1.5 h for D. desulfuricans.

Sensors based on proteins (GST-SmtA and MerR) with distinct binding sites for heavy metal ions were developed and characterized. A capacitive signal transducer was used to measure the conformational change following binding. The proteins were overexpressed in Escherichia coli, purified, and immobilized in different ways to a self-assembled thiol layer on a gold electrode placed as the working electrode in a potentiostatic arrangement in a flow analysis system. The selectivity and the sensitivity of the two protein-based biosensors were measured and compared for copper, cadmium, mercury, and zinc ions. The GST-SmtA electrodes displayed a broader selectivity (sensing all four heavy metal ions) compared with the MerR-based ones, which showed an accentuated selectivity for mercury ions. Metal ions could be detected with both electrode types down to femtomolar concentration. The upper measuring limits, presumably due to near saturation of the proteins' binding sites, were around 10(-10) M. Control electrodes similarly constructed but based on bovine serum albumin or urease did not yield any signals. The electrodes could be regenerated with EDTA and used for more than 2 weeks with about 40% reduction in sensitivity.

Resting cells of Escherichia coli, immobilized in a flow-through bioreactor, coupled the oxidation of formate or hydrogen to Tc(VII) reduction and removal from solution. Cells, pregrown anaerobically in a hollow-fiber membrane bioreactor, were challenged with 50 microM Tc(VII) in a carrier solution of phosphate-buffered saline. The radionuclide accumulated within the membrane component of the reactor, corresponding to the localization of the cells. Negligible Tc removal was noted in a reactor containing a mutant deficient in active Tc(VII) reductase, when supplied with formate as an electron donor. Formate or hydrogen was supplied as the electron donor for Tc(VII) reduction to cells immobilized in reactors operated in transverse (crossflow) and direct (dead-end filtration) modes, respectively. Flow-rate activity relationships were used to compare the performance of the reactors. A flow rate of 2.4 mL h(-1) supported the removal of 50% of the Tc from solution in a reactor operated in transverse mode with formate as an electron donor. In contrast, a flow rate of 0.7 mL h(-1), supported comparable Tc removal when hydrogen was introduced to a reactor operated in direct mode. The reduced reactor efficiency, when hydrogen was used as an electron donor, could be attributed, in part, to poor delivery of the gas to the cells. The biocatalyst was highly stable in the reactor; no loss in activity was noted over 200 h of continuous use.

Bioreactor selection is important for maximising the productivity of recombinant organisms. In this paper a comparison is made between growth and recombinant protein synthesis in three types of bioreactor containing a marine Vibrio capable of heterologous expression and secretion of the non-toxic B-subunit pentamer of Escherichia coli heat-labile enterotoxin, EtxB. The heterologous gene was located on the plasmid pMMB68. Resistance to carbenicillin was used to select for plasmid-containing cells. In batch and continuous culture. Volumetric productivities were highest when cells were grown in the presence of carbenicillin. Without antibiotic selection, the highest volumetric productivity (9.4 mg EtxB-1 h-1) was observed in hollow-fibre bioreactors, and the production phase could be maintained for over 50 h. The highest specific productivity under these conditions was found in batch culture, but the maximal production phase was only of 5 h duration. In hollow-fibre reactors the type of fibre used significantly affected productivity, both with regards to the maintenance of reactor integrity and by allowing passage of the recombinant toxoid through the selectively permeable membrane. Where contamination of the product with carbenicillin is to be avoided, these bioreactors are superior to batch or continuous culture.

Anaerobically grown cells of Escherichia coli were immobilised within a range of entrapment matrices and packed into a column under standard conditions, and the ability of the immobilised cells to reduce nitrite (0.5 mM) was measured at a range of flow rates using sodium formate (20 mM) as the electron donor for nitrite reduction. A flow-rate/activity plot was constructed for each flow-through reactor and RA1/2 values (residence time corresponding to 50 % nitrite removal) calculated for each reactor type. Cells immobilised in flat and hollow-fibre membranes were the most effective (RA1/2 = 0.35 h and 0.47 h respectively), with cells entrapped by dialysis membrane (1.53 h), alginate beads (1.93 h), Hypol foam (2.31 h) and polyacrylamide gel (50 % nitrite not removed at maximum residence time tested: 4.9 h) performing progressively less effectively. Cells grown as a biofilm on a range of support materials were also tested in comparable packed-bed reactors. Cell loss from these supports was extensive and contributed to poor performance of the reactors despite high initial biomass loadings (RA1/2 values using raschig rings, coke and activated-carbon supports: 1.6 h, 2.3 h and 1.0 h respectively). Biofilms grown on Pharmacia microcarrier supports and used in packed and also fluidised beds were more stable and the performance of these reactors was superior to that of biofilm reactors using other supports, and comparable to that of the membrane reactors (RA1/2 values for Cytoline 2, Cytopore 2 and Cytodex 3: 0.76 h, 0.56 h, 0.68 h respectively).

Anaerobic, but not aerobic, cultures of Escherichia coli accumulated Tc(VII) and reduced it to a black insoluble precipitate. Tc was the predominant element detected when the precipitate was analyzed by proton-induced X-ray emission. Electron microscopy in combination with energy-dispersive X-ray analysis showed that the site of Tc deposition was intracellular. It is proposed that Tc precipitation was a result of enzymatically mediated reduction of Tc(VII) to an insoluble oxide. Formate was an effective electron donor for Tc(VII) reduction which could be replaced by pyruvate, glucose, or glycerol but not by acetate, lactate, succinate, or ethanol. Mutants defective in the synthesis of the transcription factor FNR, in molybdenum cofactor (molybdopterin guanine dinucleotide [MGD]) synthesis, or in formate dehydrogenase H synthesis were all defective in Tc(VII) reduction, implicating a role for the formate hydrogenlyase complex in Tc(VII) reduction. The following observations confirmed that the hydrogenase III (Hyc) component of formate hydrogenlyase in both essential and sufficient for Tc(VII) reduction: (i) dihydrogen could replace formate as an effective electron donor for Tc(VII) reduction by wild-type bacteria and mutants defective in MGD synthesis; (ii) the inability of fnr mutants to reduce Tc(VII) can be suppressed phenotypically by growth with 250 microM Ni2+ and formate; (iii) Tc(VII) reduction is defective in a hyc mutant; (iv) the ability to reduce Tc(VII) was repressed during anaerobic growth in the presence of nitrate, but this repression was counteracted by the addition of formate to the growth medium; (v) H2, but not formate, was an effective electron donor for a Sel- mutant which is unable to incorporate selenocysteine into any of the three known formate dehydrogenases of E. coli. This appears to be the first report of Hyc functioning as an H2-oxidizing hydrogenase or as a dissimilatory metal ion reductase in enteric bacteria.

Microbially mediated arsenic release from Holocene and Pleistocene Cambodian aquifer sediments was investigated using microcosm experiments and substrate amendments. In the Holocene sediment, the metabolically active bacteria, including arsenate-respiring bacteria, were determined by DNA stable-isotope probing. After incubation with (13) C-acetate and (13) C-lactate, active bacterial community in the Holocene sediment was dominated by different Geobacter spp.-related 16S rRNA sequences. Substrate addition also resulted in the enrichment of sequences related to the arsenate-respiring Sulfurospirillum spp. (13) C-acetate selected for ArrA related to Geobacter spp. whereas (13) C-lactate selected for ArrA which were not closely related to any cultivated organism. Incubation of the Pleistocene sediment with lactate favoured a 16S rRNA-phylotype related to the sulphate-reducing Desulfovibrio oxamicus DSM1925, whereas the ArrA sequences clustered with environmental sequences distinct from those identified in the Holocene sediment. Whereas limited As(III) release was observed in Pleistocene sediment after lactate addition, no arsenic mobilization occurred from Holocene sediments, probably because of the initial reduced state of As, as determined by X-ray Absorption Near Edge Structure. Our findings demonstrate that in the presence of reactive organic carbon, As(III) mobilization can occur in Pleistocene sediments, having implications for future strategies that aim to reduce arsenic contamination in drinking waters by using aquifers containing Pleistocene sediments.

Microcosms containing sediment from an aquifer in Cambodia with naturally elevated levels of arsenic in the associated groundwater were used to evaluate the effectiveness of microbially mediated production of iron minerals for in situ As remediation. The microcosms were first incubated without amendments for 28 days, and the release of As and other geogenic chemicals from the sediments into the aqueous phase was monitored. Nitrate or a mixture of sulfate and lactate was then added to stimulate biological Fe(II) oxidation or sulfate reduction, respectively. Without treatment, soluble As concentrations reached 3.9 � 0.9 μM at the end of the 143-day experiment. However, in the nitrate- and sulfate-plus-lactate-amended microcosms, soluble As levels decreased to 0.01 and 0.41 � 0.13 μM, respectively, by the end of the experiment. Analyses using a range of biogeochemical and mineralogical tools indicated that sorption onto freshly formed hydrous ferric oxide (HFO) and iron sulfide mineral phases are the likely mechanisms for As removal in the respective treatments. Incorporation of the experimental results into a one-dimensional transport-reaction model suggests that, under conditions representative of the Cambodian aquifer, the in situ precipitation of HFO would be effective in bringing groundwater into compliance with the World Health Organization (WHO) provisional guideline value for As (10 ppb or 0.13 μM), although soluble Mn release accompanying microbial Fe(II) oxidation presents a potential health concern. In contrast, production of biogenic iron sulfide minerals would not remediate the groundwater As concentration below the recommended WHO limit.

International audience; Batch microcosms were setup to determine the impact of different sized zero valent iron (Fe-0) particles on microbial sulfate reduction during the in situ bio-precipitation of metals. The microcosms were constructed with aquifer sediment and groundwater from a low pH (3.1), heavy-metal contaminated aquifer. Nano (nFe(0)), micro (mFe(0)) and granular (gFe(0)) sized Fe-0 particles were added to separate microcosms. Additionally, selected microcosms were also amended with glycerol as a C-source for sulfate-reducing bacteria. In addition to metal removal, Fe-0 in microcosms also raised the pH from 3.1 to 6.5, and decreased the oxidation redox potential from initial values of 249 to 226 mV, providing more favorable conditions for microbial sulfate reduction. mFe(0) and gFe(0) in combination with glycerol were found to enhance microbial sulfate reduction. However, no sulfate reduction occurred in the controls without Fe-0 or in the microcosm amended with nFe(0). A separate dose test confirmed the inhibition for sulfate reduction in presence of nFe(0). Hydrogen produced by Fe-0 was not capable of supporting microbial sulfate reduction as a lone electron donor in this study. Microbial analysis revealed that the addition of Fe-0 and glycerol shifted the microbial community towards Desulfosporosinus sp. from a population initially dominated by low pH and metal-resisting Acidithiobacillus ferrooxidans. (C) 2013 Elsevier Ltd. All rights reserved.

Marinobacter santoriniensis NKSG1 T originates from metalliferous marine sediment. It can respire and redox cycle arsenic species and perform mixotrophic, nitrate-dependent Fe(II) oxidation. The genome sequence, reported here, will help further elucidate the genetic mechanisms underlying these and other potential biogeochemically relevant functions, such as arsenic and mercury resistance and hydrocarbon degradation.

To determine whether biologically mediated Fe(III) reduction is possible under alkaline conditions in systems of relevance to geological disposal of radioactive wastes, a series of microcosm experiments was set up using hyperalkaline sediments (pH similar to 11.8) surrounding a legacy lime working site in Buxton, United Kingdom. The microcosms were incubated for 28 days and held at pH 10. There was clear evidence for anoxic microbial activity, with consumption of lactate (added as an electron donor) concomitant with the reduction of Fe(III) as ferrihydrite (added as the electron acceptor). The products of microbial Fe(III) reduction were black and magnetic, and a range of analyses, including X-ray diffraction, transmission electron microscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism confirmed the extensive formation of biomagnetite in this system. The addition of soluble exogenous and endogenous electron shuttles such as the humic analogue anthraquinone-2,6-disulfonate and riboflavin increased both the initial rate and the final extent of Fe(III) reduction in comparison to the nonamended experiments. In addition, a soluble humic acid (Aldrich) also increased both the rate and the extent of Fe(III) reduction. These results show that microbial Fe(III) reduction can occur in conditions relevant to a geological disposal facility containing cement-based wasteforms that has evolved into a high pH environment over prolonged periods of time (> 100,000 years). The potential impact of such processes on the biogeochemistry of a geological disposal facility is discussed, including possible coupling to the redox conditions and solubility of key radionuclides. Williamson, Adam J. Morris, Katherine Shaw, Sam Byrne, James M. Boothman, Christopher Lloyd, Jonathan R.

Primary metabolites can be efficiently produced in hollow-fiber bioreactors. In contrast, much poorer productivities have been reported for secondary metabolites. Synthesis of the secondary metabolite, ethylene, under oxygen-limited conditions in batch culture or in hollow-fiber bioreactors operating with low aeration was barely detectable. The need for oxygen has been demonstrated in the conversion of the intermediate 2-oxo 4-methylthiobutyrate (KMBA) to ethylene. By selecting direct mode operation and increasing the air flow to immobilized cells, productivities comparable to aerated batch culture could be achieved. The pattern of fermentation products detected by these cultures indicated that the supply of oxygen is the major factor influencing the product yields under the conditions used. When nitrate was employed as an alternative electron acceptor to oxygen, the E. coli strain grew well but only converted small amounts of methionine to intermediates on the ethylenogenic pathway. No ethylene could be detected in these cultures. Growth with methionine as the sole source of nitrogen was only possible when batch cultures were aerated, indicating that oxygen is needed for other aspects of methionine metabolism in addition to the conversion of KMBA to ethylene.

The ability of microbes to reductively transform a variety of metals has wide-reaching implications for controlling the mobility of contaminants in the subsurface, resulting in the degradation of toxic organics or the reductive immobilization of metals. For example, soluble toxic metal contaminants, including Cr(VI), Hg(II), V(V), Co(III), U(VI), Tc(VII) and Np(V) can be reduced directly and removed from solution by enzymatic processes, often being used as terminal electron acceptors during anoxic respiration. In many cases these transformations can also be mediated indirectly via reactive end products of metal reduction, including biogenic Fe(II) or Pd(0). Similar indirect mechanisms for the reductive transformations of organics, such as chlorinated solvents, are also possible, as is the enzymatic oxidation of several organic “xenobiotics”, coupled directly to the reduction of metals such as Fe(III). Many of these processes occur naturally within contaminant systems, and the ability to accelerate them during bioremediation applications has attracted much recent interest. Over the past two decades, studies have sought to understand these processes from the molecular level while applying them at field or industrial scale. This review seeks to summarise these findings and address areas of current and prospective progress in the bioremediation field.

Denitrification driven by pyrite oxidation can play a major role in the removal of nitrate from groundwater systems. As yet, limited information is available on the interactions between the micro-organisms and aqueous and mineral phases in aquifers where pyrite oxidation is occurring. In this study, we examine the groundwater and sediment composition along a well-characterized redox gradient in a heavily nitrate-polluted pyritic sandy aquifer in Oostrum (the Netherlands) to identify the sequence of steps involved in denitrification coupled to pyrite oxidation. Multi-isotope analyses (δ15N-NO3-, δ18O-NO3-, δ34S-SO42-, δ18O-SO42- and δ34Spyrite) confirm that pyrite is the main electron donor for denitrification at this location. Enrichment factors derived from the observed changes in nitrate isotopic composition range from -2.0 to -10.9‰ for ε15N and from -2.0 to -9.1‰ for ε18O. The isotopic data indicate that pyrite oxidation accounts for approximately 70% of the sulfate present in the zone of denitrification. Solid-phase analyses confirm the presence of pyrite- and organic matter-rich clay lenses in the subsurface at Oostrum. In addition, sulfur XANES and iron XAS results suggest the presence of a series of intermediate sulfur species (elemental sulfur and SO32-) that may be produced during denitrification. Consistent with geochemical analysis, 16S rRNA gene sequencing revealed the presence of bacteria capable of sulfide oxidation coupled to nitrate reduction and that are tolerant to high aqueous metal concentrations. © 2012 Elsevier B.V.

Microorganisms have been producing nanoparticles for billions of years and by controlling and tuning this productivity they have the potential to provide novel materials using environmentally friendly manufacturing pathways. Metal-reducing bacteria are a particularly fertile source of nanoparticles and their reduction of Fe (III) oxides leads to the formation of ferrite spinel nanoparticles, especially magnetite, Fe3O4. The high yields produced by extracellular biomineralising processes make them commercially attractive, and the production of these bionano ferrite spinels can be tuned by doping the precursor Fe(III) phase with Co, Ni, Zn, Mn and V. The oxidation state of the cations and the sites of substitution are determined by X-ray absorption spectroscopy (XAS), especially by examination of metal L-edge spectra and X-ray magnetic circular dichroism (XMCD). Vanadium substitution in bionano ferrite spinels is revealed for the first time, and substitution in the octahedral site as V(III) confirmed. Bionanomagnetite is shown to be effective in the remediation of azo dyes with the complete breakdown of Remazol Black B to colourless amines and acids. XMCD shows this to involve oxidation of the surface Fe(III) and the potential for regeneration of the nanoparticles.

Recent work has suggested that the electrical self-potential (SP) geophysical technique may be used to non-invasively map redox conditions associated with contaminant plumes or bioremediation schemes. The proposed mechanism linking SP response and redox involves the generation of a current source and sink in the subsurface whereby electrons are transferred between anoxic and oxic environments via a conductive biofilm and/or biominerals, creating a biogeobattery. Here, we seek direct experimental evidence for the biogeobattery hypothesis, which has so far remained elusive, using a flow-through column experimental set-up specifically designed for this purpose. We successfully created contrasting redox zones, with an oxic section containing clean sand transitioning into an Fe(III)-reducing section. In experiments, Fe(III)-reduction was mediated both by a natural microbial community and by a pure culture of the model organism Shewanella oneidensis MR-1 in two separate column experiments. Visual observations and electron microscopy showed that ferrihydrite was sequentially transformed to goethite and magnetite; despite this, no SP signal was generated in either column. Electron microscopy suggested that in the pure culture column, S. oneidensis MR-1 cells did not form a continuous, interconnected biofilm but rather interacted with the iron (oxyhydr)oxide surfaces as individual cells. We thus conclude that (i) there is still no unambiguous direct proof of the biogeobattery hypothesis and (ii) development of Fe(III)-reducing conditions does not necessarily lead to SP signal generation. Overall, this study shows that key redox variations are not necessarily accompanied by a large SP response, suggesting that SP cannot be used in isolation to monitor subsurface biogeochemical conditions.

The widespread spread use of nuclear materials over the past 60 years has lead to anthropogenic release of radionuclides into the environment. The release of such contaminants is currently of great public concern and scientific interest worldwide. Contamination has arisen on sites involved in both military and civilian uses of nuclear material through leakages, spills, controlled discharges and munitions use. The management of this nuclear legacy is a global priority as governments seek to decommission and reclaim land contaminated by the use of nuclear facilities. The scale of contamination presents a serious financial burden with the cleanup of US sites expected to cost up to a trillion dollars. In the UK, the problem exists on a smaller but significant scale with associated cleanup costs estimated to be in the order of £100 billion. A wide range of disciplines are required to understand the behaviour of radionuclides and co-contaminants in these contaminated environments in order for effective remediation techniques to be utilised. Potential remediation strategies cover a range of biological, chemical and physical methods which can be used to treat the complex contamination scenarios found at nuclear sites. A number of these remediation techniques have been trialled at several sites managed by the United States Department of Energy with some success in treating radionuclide contamination.

Mercuric chloride solutions have historically been used as pesticides to prevent bacterial, fungal and insect degradation of herbarium specimens. The University of Manchester museum herbarium contains over a million specimens from numerous collections, many preserved using HgCl(2) and its transformation to Hg(v)(0) represents a health risk to herbarium staff. Elevated mercury concentrations in work areas (similar to 1.7 mu g m(-3)) are below advised safe levels (

International audience; Aggregates derived from senescent phytoplankton populations and associated microbial assemblages were incubated aerobically in the dark to assess the compositional changes in lipids during the degradation of artificial marine snow. The prevalence of saturated over unsaturated fatty acids indicated that aggregates were already degraded when incubation started. Nonetheless, the lipids in the artificial aggregates were quickly further degraded as indicated by a depletion in short-chain (20) saturated fatty acids fluctuated, suggesting that some of these lipids could have been produced in situ by marine microorganisms rather than deriving from higher plant debris. In addition, a bacterial branched monounsaturated fatty acid (11-methyloctadecenoic acid), which has not been found previously in marine particles was present in artificial aggregates. Molecular (16S rRNA gene) analyses indicate that the bacterial community attached to aggregates is dominated by (predominantly uncultured) alpha-Proteobacteria. gamma-Proteobacteria and Planctomycetes. Of these, a Roseobacter sp. which was present in artificial aggregates analysed in a parallel study (Balzano et al. 2009, Aquatic Microbial Ecology 54:291-303), can contain 11-methyloctadecenoic acid, and other bacteria present in artificial aggregates have the potential to produce long-chain saturated fatty acids. Thus, the fatty acid assemblage appears to reflect both organic matter degradation, including selective preservation, but also changes in the microbial assemblage. (C) 2010 Elsevier B.V. All rights reserved.

A microbial community showing diverse respiratory processes was identified within an arsenic-rich, ferruginous shallow marine hydrothermal sediment (20-40°C, pH 6.0-6.3) in Santorini, Greece. Analyses showed that ferric iron reduction with depth was broadly accompanied by manganese and arsenic reduction and FeS accumulation. Clone library analyses indicated the suboxic-anoxic transition zone sediment contained abundant Fe(III)- and sulfate-reducing Deltaproteobacteria, whereas the overlying surface sediment was dominated by clones related to the Fe(II)-oxidizing zetaproteobacterium, Mariprofundus ferroxydans. Cultures obtained from the transition zone were enriched in bacteria that reduced Fe(III), nitrate, sulfate and As(V) using acetate or lactate as electron donors. In the absence of added organic carbon, bacteria were enriched that oxidized Fe(II) anaerobically or microaerobically, sulfide microaerobically and aerobically and As(III) aerobically. According to 16S rRNA gene analyses, enriched bacteria represented a phylogenetically wide distribution. Most probable number counts indicated an abundance of nitrate-, As(V)- and Fe(III) (s,aq) -reducers, and dissolved sulfide-oxidizers over sulfate-reducers, and FeS-, As(III)- and nitrate-dependent Fe(II)-oxidisers in the transition zone. It is noteworthy that the combined community and geochemical data imply near-surface microbial iron and arsenic redox cycling were dominant biogeochemical processes. © 2010 International Society for Microbial Ecology. All rights reserved.

INTRODUCTION Although it has been known for over a century that micro-organisms have the potential to reduce metals, more recent observations showing that a diversity of specialist bacteria and archaea can use such activities to conserve energy for growth under anaerobic conditions have opened up new and fascinating areas of research with potentially exciting practical applications (Lloyd, 2003). Micro-organisms have also evolved metal-resistance processes that often incorporate changes in the oxidation state of toxic metals. Several such resistance mechanisms, which do not support anaerobic growth, have been studied in detail by using the tools of molecular biology. Three obvious examples include resistance to Hg(II), As(V) and Ag(II) (Bruins et al ., 2000). The molecular bases of respiratory metal-reduction processes have not, however, been studied in such fine detail, although rapid advances are expected in this area with the imminent availability of complete genome sequences for key metal-reducing bacteria, in combination with genomic, proteomic and metabolomic tools. This research is being driven forward both by the need to understand the fundamental basis of a range of biogeochemical cycles, and also by the possibility of harnessing such activities for a range of biotechnological applications. These include the bioremediation of metal-contaminated land and water (Lloyd & Lovley, 2001), the oxidation of xenobiotics under anaerobic conditions (Lovley & Anderson, 2000), metal recovery in combination with the formation of novel biocatalysts (Yong et al ., 2002a) and even the generation of electricity from sediments (Bond et al ., 2002).

The influence of Fe(III) starting material on the ability of magnetically recoverable biogenic magnetites produced by Geobacter sulfurreducens to retain metal oxyanion contaminants has been investigated. The reduction/removal of aqueous Cr(VI) was used to probe the reactivity of the biomagnetites. Nanomagnetites produced by the bacterial reduction of schwertmannite powder were more efficient at reducing Cr(VI) than either ferrihydrite "gel"-derived biomagnetite or commercial nanoscale Fe(3)O(4). Examination of post-exposure magnetite surfaces indicated both Cr(III) and Cr(VI) were present. X-ray magnetic circular dichroism (XMCD) studies at the Fe L(2,3)-edge showed that the amount of Fe(III) "gained" by Cr(VI) reduction could not be entirely accounted for by "lost" Fe(II). Cr L(2,3)-edge XMCD spectra found Cr(III) replaced approximately 14%-20% of octahedral Fe in both biogenic magnetites, producing a layer resembling CrFe(2)O(4). However, schwertmannite-derived biomagnetite was associated with approximately twice as much Cr as ferrihydrite-derived magnetite. Column studies using a gamma-camera to image a (99)mTc(VII) radiotracer were performed to visualize the relative performances of biogenic magnetites at removing aqueous metal oxyanion contaminants. Again, schwertmannite-derived biomagnetite proved capable of retaining more (approximately 20%) (99)mTc(VII) than ferrihydrite-derived biomagnetite, confirming that the production of biomagnetite can be fine-tuned for efficient environmental remediation through careful selection of the Fe(III) mineral substrate supplied to Fe(III)-reducing bacteria.

The high financial and environmental costs associated with remediation of land contaminated through 60 years of global nuclear activity has underpinned the development of new passive in situ bioremediation processes for sites contaminated with nuclear waste. Many of these processes rely on successfully harnessing newly discovered natural biogeochemical cycles to manage contamination from key radionuclides. Of particular note are strategies that involve enzymatic and indirect redox transformations of actinides such as uranium, neptunium and plutonium and fission products such as technetium. This chapter will discuss the recent advances that have been made in understanding the microbial colonization of radioactive environments and the biological basis of microbial transformations of radioactive waste in these settings. In addition, the impact of co-contaminants such as nitrate on both the microbial ecology of sediments and radionuclide speciation will also be discussed.

High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal-reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters.

The anaerobic oxidation of methane (AOM) is a major methane sink in marine sediments and plays a crucial role in mitigating methane fluxes to the overlying water column. We investigated biomarker distributions and compound specific isotopic signatures in sediments from sites on the Northern European continental margin that are characterized by a diffusive flux of methane. At all sites, the organic matter (OM) is predominantly derived from terrestrial higher plants, with subordinate abundances of algal biomarkers, but biomarkers for archaea and bacteria are also present. The co-occurrence of the archaeal lipids archaeol, sn-2-hydroxyarchaeol and 2,6,10,15,19-pentamethylicosane (PMI) with non-isoprenoidal glycerol diethers inferred to derive from sulfate-reducing bacteria (SRB) is similar to microbial biomarker assemblages observed at cold seeps. The archaeal and inferred SRB biomarker concentrations typically reach maxima close to the sulfate-methane transition zone (SMTZ), where archaeal biomarkers are depleted in 13C. The 16S rRNA gene sequences from the SMTZ of the Aarhus Bay sediment core indicate the occurrence of ANME-1 archaea, consistent with inferences derived from biomarker distributions. The observations suggest that AOM in these diffusive settings is mediated by consortia of archaea and bacteria similar to those found at many seep and methane hydrate sites around the world.

Anaerobically grown cells of Shewanella strain J18 143 were able to bio-bleach the color from cotton fabric that was dyed with Remazol Black B (C.I. Reactive Black 5), a common diazo reactive dye. This bio-bleaching process, involving a bacterial catalyst, offers potential benefits to the color industry as the removal of color from dyed fabric opens up the potential for fabric re-use. Growing cells removed the color with greater efficiency than that achieved using pre-grown "resting" cells. Assays of resting cells were used to determine the effect of cell concentration and depth of shade of the dyeing on the color removal process. Further resting cell assays were carried out to ascertain if an electron donor was required for the color removal process, and suggested that the cotton substrate could supply some reducing power to the biocatalyst, although dye reduction rates were maximal with added electron donor (formate). The Shewanella cells were also able to remove the color from dyed cotton fabric that was isolated inside a dialysis membrane to prevent contact with the cells. This indicates that Shewanella strain J18 143 is able to synthesize and excrete endogenous extracellular electron shuttles, eliminating the need for direct contact between the intracellular electron transport components and the extracellular terminal electron acceptors. The dyed cotton fabric was assessed visually, and by reflectance spectroscopy and environmental scanning electron microscopy. © The Author(s), 2010.

Dynamic gamma-camera imaging of radiotracer technetium ((99m)Tc) was used to assess the impact of biostimulation of metal-reducing bacteria on technetium mobility at 10(-12) mol L(-1) concentrations in sediments. Addition of the electron donor acetate was used to stimulate a redox profile in sediment columns, from oxic to Fe(III)-reducing conditions. When (99m)Tc was pumped through the columns, real-time gamma-camera imaging combined with geochemical analyses showed technetium was localized in regions containing biogenic Fe(II). In parallel experiments, electron microscopy with energy-dispersive X-ray (EDX) mapping confirmed sediment-bound Tc was associated with iron, while X-ray absorption spectroscopy (XAS) confirmed reduction of Tc(VII) to poorly soluble Tc(IV). Molecular analyses of microbial communities in these experiments supported a direct link between biogenic Fe(II) accumulation and Tc(VII) reductive precipitation, with Fe(III)-reducing bacteria more abundant in technetium immobilization zones. This offers a novel approach to assessing radionuclide mobility at ultratrace concentrations in real-time biogeochemical experiments, and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilizing technetium.

Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying nitrate and carbonate concentrations added to synthetic groundwater to assess the influence ofpHand nitrate on bioreduction processes. The systems studied include unamended-, carbonate buffered-, low nitrate-, and high nitrate-groundwaters. During anaerobic incubation, terminal electron accepting processes (TEAPs) in the circumneutral pH, carbonate buffered system progressed to sulfate reduction, and Tc(VII) was removed from solution during Fe(III) reduction. In the high-nitrate system, pH increased during denitrification (pH 5.5 to 7.2), then TEAPs progressed to sulfate reduction. Again, Tc(VII) removal was associated with Fe(III) reduction. In both systems, XAS confirmed reduction to hydrous Tc(IV)O2 like phases on Tc removal from solution. In the unamended and low-nitrate systems, the pH remained low, Fe(III) reduction was inhibited, and Tc(VII) remained in solution. Thus, nitrate can have complex influences on the development of the metal reducing conditions required for radionuclide treatment. High nitrate concentrations stimulated denitrification and caused pH neutralization facilitating Fe(III) reduction and Tc(VII) removal; acidic, low nitrate systemsshowed no Fe(III)-reduction. These results have implications for Tc-cycling in contaminated environments where nitrate has been considered undesirable, but where it may enhance Fe(III)-reduction via a novel pH "conditioning" step. © 2010 American Chemical Society.

The metal-reducing bacteria Geobacter sulfurreducens, Shewanella oneidensis and Veillonella atypica, use different mechanisms to transform toxic, bioavailable sodium selenite to less toxic, non-mobile elemental selenium and then to selenide in anaerobic environments, offering the potential for in situ and ex situ bioremediation of contaminated soils, sediments, industrial effluents, and agricultural drainage waters. The products of these reductive transformations depend on both the organism involved and the reduction conditions employed, in terms of electron donor and exogenous extracellular redox mediator. The intermediary phase involves the precipitation of elemental selenium nanospheres and the potential role of proteins in the formation of these structures is discussed. The bionanomineral phases produced during these transformations, including both elemental selenium nanospheres and metal selenide nanoparticles, have catalytic, semiconducting and light-emitting properties, which may have unique applications in the realm of nanophotonics. This research offers the potential to combine remediation of contaminants with the development of environmentally friendly manufacturing pathways for novel bionanominerals. © 2009 Taylor & Francis.

High levels of naturally occurring arsenic are found in the shallow reducing aquifers of West Bengal, Bangladesh, and other areas of Southeast Asia. These aquifers are used extensively for drinking water and irrigation by the local population. Mechanisms for its release are unclear, although increasing evidence points to a microbial control. The type of organic matter present is of vital importance because it has a direct impact on the rate of microbial activity and on the amount of arsenic released into the ground water. The discovery of naturally occurring hydrocarbons in an arsenic-rich aquifer from West Bengal provides a source of potential electron donors for this process. Using microcosm-based techniques, seven sediments from a site containing naturally occurring hydrocarbons in West Bengal were incubated with synthetic ground water for 28 d under anaerobic conditions without the addition of an external electron donor. Arsenic release and Fe(III) reduction appeared to be microbially mediated, with variable rates of arsenic mobilization in comparison to Fe(III) reduction, suggesting that multiple processes are involved. All sediments showed a preferential loss of petroleum-sourced n-alkanes over terrestrially sourced sedimentary hydrocarbons n-alkanes during the incubation, implying that the use of petroleum-sourced n-alkanes could support, directly or indirectly, microbial Fe(III) reduction. Samples undergoing maximal release of As(III) contained a significant population of Sulfurospirillum sp., a known As(V)-reducing bacterium, providing the first evidence that such organisms may mediate arsenic release from West Bengali aquifers.

The testing of armor-piercing depleted uranium (DU) "penetrators" has resulted in the deposition of DU in the sediments of the Solway Firth, UK. In this study, DU-amended, microcosm experiments simulating Solway Firth sediments under high (31.5) and medium (16.5) salinity conditions were used to investigate the effect of salinity and biogeochemical conditions on the corrosion and fate of DU, and the impact of the corroding DU on the microbial population. Under suboxic conditions, the average corrosion rates were the same forthe 31.5 and 16.5 salinity systems at 0.056 ± 0.006 g cm-2 y-1, implying that complete corrosion of a 120 mm penetrator would take approximately 540 years. Under sulfate-reducing conditions, corrosion ceased due to passivation of the surface. Corroding DU resulted in more reducing conditions and decreased microbial diversity as indicated by DNA sequencing and phylogenetic analysis. The lack of colloidal and particulate DU corrosion products, along with measurable dissolved U and a homogeneous association of U with the sediment suggest that U was transported from the penetrator surface into the surrounding environment through dissolution of U(VI), with subsequent interactions resulting in the formation of secondary uranium species in the sediment © 2009 American Chemical Society.

Marinobacter santoriniensis NKSG1(T) is a mesophilic, dissimilatory arsenate-reducing and arsenite-oxidizing bacterium isolated from an arsenate-reducing enrichment culture. The inoculum was obtained from arsenic-rich shallow marine hydrothermal sediment from Santorini, Greece, with evidence of arsenic redox cycling. Growth studies demonstrated M. santoriniensis NKSG1(T) is capable of conserving energy from the reduction of arsenate [As(V)] with acetate or lactate as the electron donor, and of oxidizing arsenite [As(III)] heterotrophically with oxygen as the electron acceptor. The oxidation of As(III) coincided with the expression of the aoxB gene encoding for the catalytic molybdopterin subunit of the heterodimeric arsenite oxidase operon, indicating the reaction is enzymatically controlled, and M. santoriniensis NKSG1(T) is a heterotrophic As(III)-oxidizing bacterium. Although it is clear that this organism also performs dissimilatory As(V) reduction, no amplification of the arrA arsenate reductase gene was attained using a range of primers and PCR conditions. Marinobacter santoriniensis NKSG1(T) belongs to a genus of bacteria widely occurring in marine environments, including hydrothermal sediments, and is among the first marine bacteria shown to be capable of either anaerobic As(V) respiration or aerobic As(III) oxidation.

Nanoscale ferrimagnetic particles have a diverse range of uses from directed cancer therapy and drug delivery systems to magnetic recording media and transducers. Such applications require the production of monodisperse nanoparticles with well-controlled size, composition, and magnetic properties. To fabricate these materials purely using synthetic methods is costly in both environmental and economical terms. However, metalreducing microorganisms offer an untapped resource to produce these materials. Here, the Fe(III)-reducing bacterium Geobacter sulfurreducens is used to synthesize magnetic iron oxide nanoparticles. A combination of electron microscopy, soft X-ray spectroscopy, and magnetometry techniques was employed to show that this method of biosynthesis results in high yields of crystalline nanoparticles with a narrow size distribution and magnetic properties equal to the best chemically synthesized materials. In particular, it is demonstrated here that cobalt ferrite (CoFe2O4) nanoparticles with low temperature coercivity approaching 8 kOe and an effective anisotropy constant of ∼ 106 erg cm-3 can be manufactured through this biotechnological route. The dramatic enhancement in the magnetic properties of the nanoparticles by the introduction of high quantities of Co into the spinel structure represents a significant advance over previous biomineralization studies in this area using magnetotactic bacteria. The successful production of nanoparticulate ferrites achieved in this study at high yields could open up the way for the scaled-up industrial manufacture of nanoparticles using environmentally benign methodologies. ©2009 American Chemical Society.