Interactions Between Fe(III)-Oxides and Fe(III)-Phyllosilicates During Microbial Reduction 1: Synthetic Sediments

Fe(III)-oxides and Fe(III)-bearing phyllosilicates are the two major iron sources utilized as electron acceptors by dissimilatory iron-reducing bacteria (DIRB) in anoxic soils and sediments. Although there have been many studies on microbial Fe(III)-oxide and Fe(III)-phyllosilicate reduction with both natural and specimen materials, no controlled experimental information is available on the interaction between these two phases when both are available for microbial reduction. In this study, the model DIRB Geobacter sulfurreducens was used to examine the pathways of Fe(III) reduction in Fe(III)-oxide stripped subsurface sediment that was coated with different amounts of synthetic high surface area (HSA) goethite. Cryogenic (12K) ⁵⁷Fe Mössbauer spectroscopy was used to determine changes in the relative abundances of Fe(III)-oxide, Fe(III)-phyllosilicate, and phyllosilicate-associated Fe(II) [Fe(II)-phyllosilicate] in bioreduced samples. Analogous Mössbauer analyses were performed on samples from abiotic Fe(II) sorption experiments in which sediments were exposed to a quantity of exogenous soluble Fe(II) (FeCl₂?2H₂O) comparable to the amount of Fe(II) produced during microbial reduction. A Fe partitioning model was developed to analyze the fate of Fe(II) and assess the potential for abiotic Fe(II)-catalyzed reduction of Fe(III)-phyllosilicates. The microbial reduction experiments indicated that although reduction of Fe(III)-oxide accounted for virtually all of the observed bulk Fe(III) reduction activity, there was no significant abiotic electron transfer between oxide-derived Fe(II) and Fe(III)-phyllosilicatesilicates, with 26–87% of biogenic Fe(II) appearing as sorbed Fe(II) in the Fe(II)-phyllosilicate pool. In contrast, the abiotic Fe(II) sorption experiments showed that 41 and 24% of the added Fe(II) engaged in electron transfer to Fe(III)-phyllosilicate surfaces in synthetic goethite-coated and uncoated sediment. Differences in the rate of Fe(II) addition and system redox potential may account for the microbial and abiotic reaction systems. Our experiments provide new insight into pathways for Fe(III) reduction in mixed Fe(III)-oxide/Fe(III)-phyllosilicate assemblages, and provide key mechanistic insight for interpreting microbial reduction experiments and field data from complex natural soils and sediments.     

Influence of Biogenic Fe(II) on Bacterial Crystalline Fe(III) Oxide Reduction

Bacterial crystalline Fe(III) oxide reduction has the potential to significantly influence the biogeochemistry of anaerobic sedimentary environments where crystalline Fe(III) oxides are abundant relative to poorly crystalline (amorphous) phases. A review of published data on solid-phase Fe(III) abundance and speciation indicates that crystalline Fe(III) oxides are frequently 2- to S 10-fold more abundant than amorphous Fe(III) oxides in shallow subsurface sediments not yet subjected to microbial Fe(III) oxide reduction activity. Incubation experiments with coastal plain aquifer sediments demonstrated that crystalline Fe(III) oxide reduction can contribute substantially to Fe(II) production in the presence of added electron donors and nutrients. Controls on crystalline Fe(III) oxide reduction are therefore an important consideration in relation to the biogeochemical impacts of bacterial Fe(III) oxide reduction in subsurface environments. In this paper, the influence of biogenic Fe(II) on bacterial reduction of crystalline Fe(III) oxides is reviewed and analyzed in light of new experiments conducted with the acetate-oxidizing, Fe(III)-reducing bacterium (FeRB) Geobacter metallireducens . Previous experiments with Shewanella algae strain BrY indicated that adsorption and/or surface precipitation of Fe(II) on Fe(III) oxide and FeRB cell surfaces is primarily responsible for cessation of goethite ( f -FeOOH) reduction activity after only a relatively small fraction (generally < 10%) of the oxide is reduced. Similar conclusions are drawn from analogous studies with G. metallireducens . Although accumulation of aqueous Fe(II) has the potential to impose thermodynamic constraints on the extent of crystalline Fe(III) oxide reduction, our data on bacterial goethite reduction suggest that this phenomenon cannot universally explain the low microbial reducibility of this mineral. Experiments examining the influence of exogenous Fe(II) (20 mM FeCl 2 ) on soluble Fe(III)-citrate reduction by G. metallireducens and S. algae showed that high concentrations of Fe(II) did not inhibit Fe(III)-citrate reduction by freshly grown cells, which indicates that surface-bound Fe(II) does not inhibit Fe(III) reduction through a classical end-product enzyme inhibition mechanism. However, prolonged exposure of G. metallireducens and S. algae cells to high concentrations of soluble Fe(II) did cause inhibition of soluble Fe(III) reduction. These findings, together with recent documentation of the formation of Fe(II) surface precipitates on FeRB in Fe(III)-citrate medium, provide further evidence for the impact of Fe(II) sorption by FeRB on enzymatic Fe(III) reduction. Two different, but not mutually exclusive, mechanisms whereby accumulation of Fe(II) coatings on Fe(III) oxide and FeRB surfaces may lead to inhibition of enzymatic Fe(III) oxide reduction activity (in the absence of soluble electron shuttles and/or Fe(III) chelators) are identified and discussed in relation to recent experimental work and theoretical considerations.     

Importance of clay size minerals for Fe(III) respiration in a petroleum-contaminated aquifer

The availability of Fe(III)‐bearing minerals for dissimilatory Fe(III) reduction was evaluated in sediments from a petroleum‐contaminated sandy aquifer near Bemidji, Minnesota (USA). First, the sediments from a contaminated area of the aquifer, in which Fe(III) reduction was the predominant terminal electron accepting process, were compared with sediments from a nearby, uncontaminated site. Data from 0.5 m HCl extraction of different size fractions of the sediments revealed that the clay size fraction contributed a significant portion of the ‘bio‐available’ Fe(III) in the background sediment and was the most depleted in ‘bio‐available’ Fe(III) in the iron‐reducing sediment. Analytical transmission electron microscopy (TEM) revealed the disappearance of thermodynamically unstable Fe(III) and Mn(IV) hydroxides (ferrihydrite and Fe vernadite), as well as a decrease in the abundance of goethite and lepidocrocite in the clay size fraction from the contaminated sediment. TEM observations and X‐ray diffraction examination did not provide strong evidence of Fe(III)‐reduction‐related changes within another potential source of ‘bio‐available’ Fe(III) in the clay size fraction – ferruginous phyllosilicates. However, further testing in the laboratory with sediments from the methanogenic portion of the aquifer that were depleted in microbially reducible Fe(III) revealed the potential for microbial reduction of Fe(III) associated with phyllosilicates. Addition of a clay size fraction from the uncontaminated sediment, as well as Fe(III)‐coated kaolin and ferruginous nontronite SWa‐1, as sources of poorly crystalline Fe(III) hydroxides and structural iron of phyllosilicates respectively, lowered steady‐state hydrogen concentrations consistent with a stimulation of Fe(III) reduction in laboratory incubations of methanogenic sediments. There was no change in hydrogen concentration when non‐ferruginous clays or no minerals were added. This demonstrated that Fe(III)‐bearing clay size minerals were essential for microbial Fe(III) reduction and suggested that both potential sources of ‘bio‐available’ Fe(III) in the clay size fraction, poorly crystalline Fe(III) hydroxides and structural Fe(III) of phyllosilicates, were important sources of electron acceptor for indigenous iron‐reducing microorganisms in this aquifer.     

Adhesion of Dissimilatory Fe(III)-Reducing Bacteria to Fe(III) Minerals

The metabolism of dissimilatory iron-reducing bacteria (DIRB) may provide a means of remediating contaminated subsurface soils. The factors controlling the rate and extent of bacterial F(III) mineral reduction are poorly understood. Recent research suggests that molecular-scale interactions between DIRB cells and Fe(III) mineral particles play an important role in this process. One of these interactions, cell adhesion to Fe(III) mineral particles, appears to be a complex process that is, at least in part, mediated by a variety of surface proteins. This study examined the hypothesis that the flagellum serves as an adhesin to different Fe(III) minerals that range in their surface area and degree of crystallinity. Deflagellated cells of the DIRB Shewanella algae BrY showed a reduced ability to adhere to hydrous ferric oxide (HFO) relative to flagellated cells. Flagellated cells were also more hydrophobic than deflagellated cells. This was significant because hydrophobic interactions have been previously shown to dominate S. algae cell adhesion to Fe(III) minerals. Pre-incubating HFO, goethite, or hematite with purified flagella inhibited the adhesion of S. algae BrY cells to these minerals. Transposon mutagenesis was used to generate a flagellum-deficient mutant designated S. algae strain NF. There was a significant difference in the rate and extent of S. algae NF adhesion to HFO, goethite, and hematite relative to that of S. algae BrY. Amiloride, a specific inhibitor of Na + -driven flagellar motors, inhibited S. algae BrY motility but did not affect the adhesion of S. algae BrY to HFO. S.algae NF reduced HFO at the same rate as S. algae BrY. Collectively, the results of this study support the hypothesis that the flagellum of S. algae functions as a specific Fe(III) mineral adhesin. However, these results suggest that flagellum-mediated adhesion is not requisite for Fe(III) mineral reduction.     

Composition of Non-Microbially Reducible Fe(III) in Aquatic Sediments

The production of small quantities of Fe(II) during the initial phase of microbial Fe(III) reduction greatly increased the amount of Fe(III) that could be extracted from freshwater sediments with oxalate. This finding and other evidence suggest that the oxalate-extractable Fe(III) that is unavailable for microbial reduction in anoxic sediments is not in the form of mixed Fe(III)-Fe(II) forms, as was previously suggested, but rather is in the form of highly crystalline Fe(III) oxides.     

Growth of Geobacter sulfurreducens on Poorly Crystalline Fe(III) Oxyhydroxide Coatings

Few studies have examined the molecular to micron-scale interactions between dissimilatory Fe(III)-reducing bacteria and poorly crystalline Fe(III) phases which are frequently the most bioavailable Fe(III) sources in the subsurface. Here we describe methods for analysing these interactions using a range of chemical and spectroscopic techniques. Glass slides were coated with a synthetic poorly crystalline Fe(III) phase and then incubated in the presence of the Fe(III)-reducing bacterium Geobacter sulfurreducens and a suitable growth medium. Growth on the Fe(III)-coating was observed via cell staining and environmental scanning electron microscopy while microbial Fe(III) reduction was quantified using a colorimetric assay. However, following microbial reduction, Fe(II) could not be detected on the slide surface using X-ray photoelectron spectroscopy. Fe(II)-coated control slides showed that the mineral surface was not re-oxidised during handling or analysis. Further experiments intended to demonstrate removal of Tc(VII) and Cr(VI) from solution via abiotic reduction mediated by biogenic Fe(II) on the slide surface resulted in far lower levels of Tc(VII) and Cr(VI) reduction than expected. These data may indicate that the electrons transferred from G. sulfurreducens to poorly crystalline Fe(III) involves the deeper mineral structure, so that Fe(II) phases are not detectable on the surface. The environmental implications of this hypothesis are discussed.     

Mechanisms for accessing insoluble Fe(III) oxide during dissimilatory Fe(III) reduction by Geothrix fermentans

Mechanisms for Fe(III) oxide reduction were investigated in Geothrix fermentans, a dissimilatory Fe(III)-reducing microorganism found within the Fe(III) reduction zone of subsurface environments. Culture filtrates of G. fermentans stimulated the reduction of poorly crystalline Fe(III) oxide by washed cell suspensions, suggesting that G. fermentans released one or more extracellular compounds that promoted Fe(III) oxide reduction. In order to determine if G. fermentans released electron-shuttling compounds, poorly crystalline Fe(III) oxide was incorporated into microporous alginate beads, which prevented contact between G. fermentans and the Fe(III) oxide. G. fermentans reduced the Fe(III) within the beads, suggesting that one of the compounds that G. fermentans releases is an electron-shuttling compound that can transfer electrons from the cell to Fe(III) oxide that is not in contact with the organism. Analysis of culture filtrates by thin-layer chromatography suggested that the electron shuttle has characteristics similar to those of a water-soluble quinone. Analysis of filtrates by ion chromatography demonstrated that there was as much as 250 microM dissolved Fe(III) in cultures of G. fermentans growing with Fe(III) oxide as the electron acceptor, suggesting that G. fermentans released one or more compounds capable of chelating and solubilizing Fe(III). Solubilizing Fe(III) is another strategy for alleviating the need for contact between cells and Fe(III) oxide for Fe(III) reduction. This is the first demonstration of a microorganism that, in defined medium without added electron shuttles or chelators, can reduce Fe(III) derived from Fe(III) oxide without directly contacting the Fe(III) oxide. These results are in marked contrast to those with Geobacter metallireducens, which does not produce electron shuttles or Fe(III) chelators. These results demonstrate that phylogenetically distinct Fe(III)-reducing microorganisms may use significantly different strategies for Fe(III) reduction. Thus, it is important to know which Fe(III)-reducing microorganisms predominate in a given environment in order to understand the mechanisms for Fe(III) reduction in the environment of interest.     

The Impact of Fe(III)-reducing Bacteria on Uranium Mobility

The ability of specialist prokaryotes to couple the oxidation of organic compounds to the reduction of Fe(III) is widespread in the subsurface. Here microbial Fe(III) reduction can have a great impact on sediment geochemistry, affecting the minerals in the subsurface, the cycling of organic compounds and the mobility of a wide range of toxic metals and radionuclides. The contamination of the environment with radioactive waste is a major concern worldwide, and this review focuses on the mechanisms by which Fe(III)-reducing bacteria can affect the solubility and mobility of one of the most common radionuclide contaminants in the subsurface, uranium. In addition to discussing how these processes underpin natural biogeochemical cycles, we also discuss how these microbial activities can be harnessed for the bioremediation of uranium-contaminated environments.     

Diversion of Electron Flow from Methanogenesis to Crystalline Fe(III) Oxide Reduction in Carbon-Limited Cultures of Wetland Sediment Microorganisms

Electron flow in acetate-limited cultures of wetland sediment microorganisms was diverted from methane production to Fe(III) reduction in the presence of crystalline Fe(III) oxides at surface area loadings equivalent to that of amorphous Fe(III) oxide. The results indicate that inferences regarding the ability of microbial Fe(III) oxide reduction to compete with other terminal electron-accepting processes in anoxic soils and sediments should be based on estimates of bulk microbially available surface site abundance rather than assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.     

Diversion of electron flow from methanogenesis to crystalline Fe(III) oxide reduction in carbon-limited cultures of wetland sediment microorganisms

Electron flow in acetate-limited cultures of wetland sediment microorganisms was diverted from methane production to Fe(III) reduction in the presence of crystalline Fe(III) oxides at surface area loadings equivalent to that of amorphous Fe(III) oxide. The results indicate that inferences regarding the ability of microbial Fe(III) oxide reduction to compete with other terminal electron-accepting processes in anoxic soils and sediments should be based on estimates of bulk microbially available surface site abundance rather than assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.     

Bio-current as an indicator for biogenic Fe(II) generation driven by dissimilatory iron reducing bacteria

Microbial reduction of insoluble iron minerals by dissimilatory iron reducing bacteria (DIRB) is an important environment process in the iron biogeochemical cycle. We reported that the bio-current generated from oxidation of organic matter by these bacteria in the presence of iron oxides can be used as an indicator for microbial dissolution of insoluble iron oxides. Bioelectrochemical experiments were conducted to investigate the effects of the specific bacteria and the phase identity of iron oxides on bio-current generation by recording the current response as a result of a poised constant potential. Experimental results indicated that the bio-current generation can be greatly enhanced by iron oxide addition under all the conditions varying in the type of pure culture or iron oxide. The increase in the bio-current was linearly correlated with the increased concentration of biogenic Fe(II) detected either by chemical analysis or cyclic voltammetry (CV) tests. This can be understood based on the proposed mechanism that the Fe(II)/Fe(III) couple functions as the electron mediator shuttling electrons from the microbes to the electrodes.     

Enrichment of Geobacter Species in Response to Stimulation of Fe(III) Reduction in Sandy Aquifer Sediments

Abstract Engineered stimulation of Fe(III) has been proposed as a strategy to enhance the immobilization of radioactive and toxic metals in metal-contaminated subsurface environments. Therefore, laboratory and field studies were conducted to determine which microbial populations would respond to stimulation of Fe(III) reduction in the sediments of sandy aquifers. In laboratory studies, the addition of either various organic electron donors or electron shuttle compounds stimulated Fe(III) reduction and resulted in Geobacter sequences becoming important constituents of the Bacterial 16S rDNA sequences that could be detected with PCR amplification and denaturing gradient gel electrophoresis (DGGE). Quantification of Geobacteraceae sequences with a PCR most-probable-number technique indicated that the extent to which numbers of Geobacter increased was related to the degree of stimulation of Fe(III) reduction. Geothrix species were also enriched in some instances, but were orders of magnitude less numerous than Geobacter species. Shewanella species were not detected, even when organic compounds known to be electron donors for Shewanella species were used to stimulate Fe(III) reduction in the sediments. Geobacter species were also enriched in two field experiments in which Fe(III) reduction was stimulated with the addition of benzoate or aromatic hydrocarbons. The apparent growth of Geobacter species concurrent with increased Fe(III) reduction suggests that Geobacter species were responsible for much of the Fe(III) reduction in all of the stimulation approaches evaluated in three geographically distinct aquifers. Therefore, strategies for subsurface remediation that involve enhancing the activity of indigenous Fe(III)-reducing populations in aquifers should consider the physiological properties of Geobacter species in their treatment design. Received: 19 October 1999; Accepted: 28 December 1999; Online Publication: 25 April 2000     

Interactions between the Fe(III)-reducing bacterium Geobacter sulfurreducens and arsenate, and capture of the metalloid by biogenic Fe(II)

Previous work has shown that microbial communities in As-mobilizing sediments from West Bengal were dominated by Geobacter species. Thus, the potential of Geobacter sulfurreducens to mobilize arsenic via direct enzymatic reduction and indirect mechanisms linked to Fe(III) reduction was analyzed. G. sulfurreducens was unable to conserve energy for growth via the dissimilatory reduction of As(V), although it was able to grow in medium containing fumarate as the terminal electron acceptor in the presence of 500 muM As(V). There was also no evidence of As(III) in culture supernatants, suggesting that resistance to 500 muM As(V) was not mediated by a classical arsenic resistance operon, which would rely on the intracellular reduction of As(V) and the efflux of As(III). When the cells were grown using soluble Fe(III) as an electron acceptor in the presence of As(V), the Fe(II)-bearing mineral vivianite was formed. This was accompanied by the removal of As, predominantly as As(V), from solution. Biogenic siderite (ferrous carbonate) was also able to remove As from solution. When the organism was grown using insoluble ferrihydrite as an electron acceptor, Fe(III) reduction resulted in the formation of magnetite, again accompanied by the nearly quantitative sorption of As(V). These results demonstrate that G. sulfurreducens, a model Fe(III)-reducing bacterium, did not reduce As(V) enzymatically, despite the apparent genetic potential to mediate this transformation. However, the reduction of Fe(III) led to the formation of Fe(II)-bearing phases that are able to capture arsenic species and could act as sinks for arsenic in sediments.     

Limiting factors for microbial Fe(III) -reduction in a landfill leachate polluted aquifer (Vejen, Denmark)

Abstract Aquifer sediment samples from two locations within the anaerobic leachate plume of a municipal landfill were compared with respect to microbiology (especially Fe(III)-reduction) and geochemistry. The samples close to the landfill were characterized by low contents of Fe(III), whereas samples from the more distant cluster were rich in Fe(III)-oxides. The active microbial population seemed to be less dense in samples more distant from the landfill (measured by ATP and phospholipid fatty acids (PLFA)), but the microbial communities were very similar in the two sample clusters according to the composition of PLFA. Very little, if any, Fe(III)-reduction was observed close to the landfill, but all the more distant samples showed evident microbially mediated Fe(III)-reduction. After amendment with both acetate and Fe(III), all the samples showed a potential for Fe(III)-reduction, and the in situ Fe(III)-reduction seemed to be limited by the lack of Fe(III)-availability. It was suggested, that Fe(III)-reducing populations might be facultative, surviving by use of other electron-acceptors than Fe(III), when Fe(III) is not available for reduction.     

Microbial reduction of Fe(III) in acidic sediments: isolation of Acidiphilium cryptum JF-5 capable of coupling the reduction of Fe(III) to the oxidation of glucose.

To evaluate the microbial populations involved in the reduction of Fe(III) in an acidic, iron-rich sediment, the anaerobic flow of supplemental carbon and reductant was evaluated in sediment microcosms at the in situ temperature of 12 degrees C. Supplemental glucose and cellobiose stimulated the formation of Fe(II); 42 and 21% of the reducing equivalents that were theoretically obtained from glucose and cellobiose, respectively, were recovered in Fe(II). Likewise, supplemental H(2) was consumed by acidic sediments and yielded additional amounts of Fe(II) in a ratio of approximately 1:2. In contrast, supplemental lactate did not stimulate the formation of Fe(II). Supplemental acetate was not consumed and inhibited the formation of Fe(II). Most-probable-number estimates demonstrated that glucose-utilizing acidophilic Fe(III)-reducing bacteria approximated to 1% of the total direct counts of 4', 6-diamidino-2-phenylindole-stained bacteria. From the highest growth-positive dilution of the most-probable-number series at pH 2. 3 supplemented with glucose, an isolate, JF-5, that could dissimilate Fe(III) was obtained. JF-5 was an acidophilic, gram-negative, facultative anaerobe that completely oxidized the following substrates via the dissimilation of Fe(III): glucose, fructose, xylose, ethanol, glycerol, malate, glutamate, fumarate, citrate, succinate, and H(2). Growth and the reduction of Fe(III) did not occur in the presence of acetate. Cells of JF-5 grown under Fe(III)-reducing conditions formed blebs, i.e., protrusions that were still in contact with the cytoplasmic membrane. Analysis of the 16S rRNA gene sequence of JF-5 demonstrated that it was closely related to an Australian isolate of Acidiphilium cryptum (99.6% sequence similarity), an organism not previously shown to couple the complete oxidation of sugars to the reduction of Fe(III). These collective results indicate that the in situ reduction of Fe(III) in acidic sediments can be mediated by heterotrophic Acidiphilium species that are capable of coupling the reduction of Fe(III) to the complete oxidation of a large variety of substrates including glucose and H(2).     

Quinone-Mediated Microbial Goethite Reduction and Transformation of Redox Mediator,Anthraquinone-2,6-Disulfonate (AQDS)

The aim of current study is to identify the kinetic characteristics and elucidate the possible transformation pathways of the interaction between the redox mediator (anthraquinone-2,6-disulfonate, AQDS) and goethite during the process of microbial goethite reduction by Shewanella putrefaciens, a dissimilatory iron reduction bacterium (DIRB). Speciations of both AQDS and microbially reduced ferrous iron are used to characterize the interaction process among S. putrefaciens, AQDS and goethite. Due to the complexities of the natural environment, two pre-incubation reaction systems of the “DIRB–goethite” and the “DIRB–AQDS” are introduced to investigate the dynamics of goethite reduction and redox transformation of AQDS. Results show that the characteristics of the microbial goethite reduction and the kinetic transformation between two species of the redox mediator are different in two pre-incubation reaction systems. Both abiotic and enzymatic reactions and their coupling regulate the kinetic process for “redox mediatoriron” interaction in the presence of DIRB. This study will help to understand the characteristics and mechanism of microbial reduction of the Fe(III) oxide and transformation of redox mediator.     

Review: microbial mechanisms of accessing insoluble Fe(III) as an energy source

Of all the terminal electron acceptors, Fe(III) is the most naturally abundant in many subsurface environments. Fe(III)-reducing microorganisms are phylogenetically diverse and have been isolated from a variety of sources. Unlike most electron acceptors, Fe(III) has a very low solubility and is usually present as insoluble oxides at neutral pH. The mechanisms by which microorganisms access and reduce insoluble Fe(III) are poorly understood. Initially, it was considered that microorganisms could only reduce insoluble Fe(III) through direct contact with the oxide. However, recent studies indicate that extracellular electron shuttling or Fe(III)-chelating compounds may alleviate the need for cell–oxide contact. These include microbially secreted compounds or exogenous electron shuttling agents, mainly from humic substances. Electron shuttling via humic substances is likely a significant process for Fe(III) reduction in subsurface environments. This paper reviews the various mechanisms by which Fe(III) reduction may be occurring in pure culture and in soils and sediments.     

Fe(III) Reduction and U(VI) Immobilization by Paenibacillus sp. Strain 300A,Isolated from Hanford 300A Subsurface Sediments

A facultative iron-reducing [Fe(III)-reducing] Paenibacillus sp. strain was isolated from Hanford 300A subsurface sediment biofilms that was capable of reducing soluble Fe(III) complexes [Fe(III)-nitrilotriacetic acid and Fe(III)-citrate] but unable to reduce poorly crystalline ferrihydrite (Fh). However, Paenibacillus sp. 300A was capable of reducing Fh in the presence of low concentrations (2 μM) of either of the electron transfer mediators (ETMs) flavin mononucleotide (FMN) or anthraquinone-2,6-disulfonate (AQDS). Maximum initial Fh reduction rates were observed at catalytic concentrations (<10 μM) of either FMN or AQDS. Higher FMN concentrations inhibited Fh reduction, while increased AQDS concentrations did not. We also found that Paenibacillus sp. 300A could reduce Fh in the presence of natural ETMs from Hanford 300A subsurface sediments. In the absence of ETMs, Paenibacillus sp. 300A was capable of immobilizing U(VI) through both reduction and adsorption. The relative contributions of adsorption and microbial reduction to U(VI) removal from the aqueous phase were ∼7:3 in PIPES [piperazine-N,N′-bis(2-ethanesulfonic acid)] and ∼1:4 in bicarbonate buffer. Our study demonstrated that Paenibacillus sp. 300A catalyzes Fe(III) reduction and U(VI) immobilization and that these reactions benefit from externally added or naturally existing ETMs in 300A subsurface sediments.     

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring,Yellowstone National Park

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron‐rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H₂. Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid‐phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G. metallireducens‐like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat‐shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin–cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the first detailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe‐rich, circumneutral pH geothermal environment.     

Direct and Fe(II)-mediated reduction of technetium by Fe(III)-reducing bacteria

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 microM 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.