Fe Reduction Rates Research Articles

Potential for Iron-reduction and Iron-cycling in Iron Oxyhydroxide-rich Microbial Mats at Loihi Seamount

The summit of Loihi Seamount (∼1000 mbsl) is host to active hydrothermalism with vent fluids that are rich in Fe(II), resulting in copious deposition of microbial iron mats. A variety of freshly collected mat samples were assessed for their potential to support Fe-reduction, by incubating them under anaerobic conditions with 5 mM acetate. Ten of eleven samples showed active Fe-reduction within 5 to 10 days. Most probable number (MPN) analysis for Fe-reducing bacteria indicated they were present, typically around 103 cells . ml−1, but composed well under 1% of the total bacterial population. A novel, obligately anaerobic Fe-reducing bacterium, Geothermobacter, strain HR-1, was isolated from one of the mats, and was used in experiments to test Fe-reduction rates under different conditions. Strain HR-1 initiated growth more rapidly on biogenic oxides than synthetically prepared ferrihydrite, furthermore magnetite production was not observed in cells grown on biogenic oxides, but was present when cells were grown on synthetic ferrihydrite. When Fe-mats were inoculated with HR-1 and 5 mM acetate rapid iron reduction occurred; in mat samples not amended with acetate the rates and amounts of Fe-reduction were significantly less, suggesting that the system was limited for C. Similar results, with even more pronounced differences between acetate-amended and unamended samples, were found when HR-1 was inoculated into biogenic oxides formed by a pure culture of Mariprofundus ferrooxydans, a common Fe-oxidizing bacterium in the Loihi Fe mats. Together these results suggest that Fe-reducing bacteria (FeRB) are present in the iron mats at Loihi, albeit at low numbers, and that in situ they contribute only in a minor way to the iron budget at Loihi. The results are discussed in the context of bacterial iron cycling and the implications for understanding the genesis of banded iron formations.

Isotopically Exchangeable Concentrations of Elements Having Multiple Oxidation States: The Case of Fe(II)/Fe(III) Isotope Self-Exchange in Coastal Lowland Acid Sulfate Soils

Isotope exchange techniques have been used to probe isotopically exchangeable concentrations of Fe (E value) that are in dynamic equilibrium between the aqueous- and solid-phase of coastal lowland acid sulfate soils. Isotope self-exchange between Fe(II) and Fe(III) was rapid and complete in <1 min (p < 0.05) indicating that this reaction was initially occurring solely in the aqueous-phase and the surface of the soil solid-phase. It is further demonstrated that accurate and valid measurements of isotopically exchangeable concentrations of Fe do not require corrections for Fe speciation. This also holds for any element existing in two or more oxidation states which are completely isotopically self-exchangeable in soils. As isotope self-exchange between Fe(II) and Fe(III) is rapid, the distribution coefficient (Kd) and E value determined via this methodology are, therefore, truly representative of Fe regardless of the relative importance of Fe(II) or Fe(III) to the isotopically exchangeable pool of Fe. In the 21 soil samples examined, isotopically exchangeable concentrations of Fe varied from 90 mg/kg to values as high as 3610 mg/kg in acidic, saturated samples collected below the groundwater table from the transition soil horizon. The combination of low Evalues and extremely high Kd values in the upper oxidized layers of these soils indicate that these soil horizons are a relatively insignificant source of transportable (labile) Fe. As such, given our knowledge on the general rates of microbial Fe(III) reduction in, and the hydraulic properties of, the coastal lowland acid sulfate soils of this region, only those soils adjacent to agricultural drains are likely to contribute to the load of Fe entering surrounding aquatic systems.

Screening Citrus Rootstocks and Related Selections in Soil and Solution Culture for Tolerance to Low-iron Stress

A broad range of plant selections across the orange subfamily Aurantioideae were screened in solution and soil culture for their tolerance to low iron (Fe) stress. Young seedlings grown in soil were transferred to tubs of +Fe nutrient solution, which was later replaced after a brief period with a –Fe solution. Over several trials, ≈20 white root tips were harvested periodically from the plants in each tub and assayed for their ability to reduce Fe3+. The procedure was miniaturized to determine if a fewer number of root tips could be assayed to screen individual plants and to estimate the required sample size. For solution screening, seven root tips were estimated to be adequate for representing a single plant. Seedlings of a few selections were also grown in small containers of soil amended with 0% to 5.9% CaCO3. The results in solution and soil culture were consistent with each other and with previous assessments of the various selections. Based on a summary of the solution and soil responses, the citrus selections were grouped in descending order of Fe3+ reduction rates as Volkamer lemon/Rangpur/sour orange selections/Citrus macrophylla &gt; mandarins and mandarin hybrids &gt; citranges &gt; citrumelos &gt; trifoliate orange. Of the citrus relatives tested in solution culture only, those in the genera Glycosmis, Citropsis, Clausena, and Murraya had high Fe reduction rates with good seedling growth and new leaves developed a light yellow color or showed no loss of greenness. Other citrus relatives in the genera Severinia, Atalantia, and Fortunella and most somatic hybrids had low seedling vigor and produced too few root tips to be properly assessed. The results are useful because of the breadth of selections screened, the identification of various citrus relatives as potential sources of low-Fe stress tolerance in breeding new rootstocks, and the apparent positive relationship between the Fe3+ reduction responses, soil screening responses, and field experiences with carbonate-induced Fe chlorosis responses.

Mineralogical and morphological constraints on the reduction of Fe(III) minerals by Geobacter sulfurreducens

The biologically-mediated reduction of synthetic samples of the Fe(III)-bearing minerals hematite, goethite, lepidocrocite, feroxhyte, ford ferrihydrite, akaganeite and schwertmannite by Geobacter sulfurreducens has been investigated using microbiological techniques in conjunction with X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). This combination of approaches offers unique insights into the influence of subtle variations in the crystallinity of a given mineral on biogeochemical processes, and has highlighted the importance of (oxyhydr)oxide crystallite morphology in determining the changes occurring in a given mineral phase. Problems arising from normalising the biological Fe(III) reduction rates relative to the specific surface areas of the starting materials are also highlighted. These problems are caused primarily by particle aggregation, and compounded when using spectrophotometric assays to monitor reduction. For example, the initial rates of Fe(III) reduction observed for two synthetic feroxyhytes with different crystallinities (as shown by XRD and TEM studies) but almost identical surface areas, differ substantially. Both microbiological and high-resolution TEM studies show that hematite and goethite are susceptible to limited amounts of Fe(III) reduction, as evidenced by the accumulation of Fe(II) during incubation with G. sulfurreducens and the growth of nodular structures on crystalline goethite laths during incubation. Lepidocrocite and akaganeite readily transform into mixtures of magnetite and goethite, and XRD data indicate that the proportion of magnetite increases within the transformation products as the crystallinity of the starting material decreases. The presence of anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle increases both the initial rate and longer term extent of biological Fe(III) reduction for all of the synthetic minerals examined. High-resolution XPS indicates subtle but measurable differences in the Fe(III):Fe(II) ratios at the mineral surfaces following extended incubation. For example, for a poorly crystalline schwertmannite, deconvolution of the Fe2p 3/2 peak suggests that the Fe(III):Fe(II) ratio of the near-surface regions varies from 1.0 in the starting material to 0.9 following 144 h of incubation with G. sulfurreducens, and to 0.75 following the same incubation period in the presence of 10 μM AQDS. These results have important implications for the biogeochemical cycling of iron.

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

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

Strontium desorption from bacteriogenic iron oxides (BIOS) subjected to microbial Fe(III) reduction

The effect of microbial reduction of bacteriogenic iron oxides (BIOS) on the release of sorbed Sr was examined using a naturally low-Sr BIOS (n-BIOS) collected from Chalk River, Canada, and a naturally high-Sr BIOS (Äspö-BIOS) collected from Äspö, Sweden. EXAFS analysis suggested that Sr was bound to the Äspö-BIOS as an outer-sphere complex of hydrated Sr 2+. During reduction, Sr desorption from n-BIOS was concomitant with the production of Fe(II), resulting in solubilization of 80% of the total Sr. In contrast, nearly 100% of the Sr was solubilized from Äspö-BIOS, although Sr desorption was delayed, relative to Fe(II) production. When n-BIOS were artificially saturated with Sr, there was a significant decrease in the rate and extent of Fe(III) reduction and an induced delay in Sr desorption (similar to that observed for Äspö-BIOS). Saturation of synthetic hydrous ferric oxide with Sr had no such effect on Fe(III) reduction rate, although a delay in Sr desorption was again observed. In all instances, the levels of desorbed aqueous Sr remained constant, suggesting that re-sorption or co-precipitation of Sr with secondary mineral precipitates (vivianite) did not occur under the experimental conditions. In situ porewater analysis of the Chalk River BIOS indicated positive correlations between soluble-phase concentrations of Fe(II) and Sr at depth, suggesting that anaerobic reduction of Fe(III) releases Sr back into the groundwater flow at the site. Further, in areas where BIOS deposition is occurring within the groundwater discharge zone, levels of soluble Sr are 2 to 3 times lower than in areas where BIOS is absent. The data indicate that phase partitioning of Sr within BIOS is governed largely by Fe oxidation state, and that BIOS will act as effective sorbents of Sr as long as oxidizing conditions are maintained. However reduction of the BIOS will likely result in significant desorption and remobilization of the bound Sr.

Bioreduction of hematite nanoparticles by the dissimilatory iron reducing bacterium Shewanella oneidensis MR-1

We examined the reduction of different size hematite (α-Fe 2O 3) nanoparticles (average diameter of 11, 12, 30, 43, and 99 nm) by the dissimilatory iron reducing bacteria (DIRB), Shewanella oneidensis MR-1, to determine how S. oneidensis MR-1 may utilize these environmentally relevant solid-phase electron acceptors. The surface-area-normalized-bacterial Fe(III) reduction rate for the larger nanoparticles (99 nm) was one order of magnitude higher than the rate observed for the smallest nanoparticles (11 nm). The Fe(III) reduction rates for the 12, 30, and 43 nm nanoparticles fell between these two extremes. Whole-cell TEM images showed that the mode of Fe 2O 3 nanoparticle attachment to bacterial cells was different for the aggregated, pseudo-hexagonal/irregular and platey 11, 12, and 99 nm nanoparticles compared to the non-aggregated 30 and 43 nm rhombohedral nanoparticles. Due to differences in aggregation, the 11, 12, and 99 nm nanoparticles exhibited less cell contact and less cell coverage than did the 30 and 43 nm nanoparticles. We hypothesize that S. oneidensis MR-1 employs both indirect and direct mechanisms of electron transfer to Fe(III)-oxide nanoparticles and that the bioreduction mechanisms employed and Fe(III) reduction rates depend on the nanoparticles’ aggregation state, size, shape and exposed crystal faces.

Microbial Reduction of Fe(III) in Hematite Nanoparticles by Geobacter sulfurreducens

The rates of microbial Fe(III) reduction of three sizes of hematite nanoparticles by Geobacter sulfurreducens were measured under two H2 partial pressures (0.01 and 1 atm) and three pH (7.0, 7.5, and 8.0) conditions. Hematite particles with mean primary particle sizes of 10, 30, and 50 nm were synthesized by a novel aerosol method that allows tight control of the particle size distribution. The mass-normalized reduction rates of the 10 and 30 nm particles were comparable to each other and higher than the rate for the 50 nm particles. However, the surface area-normalized rate was highest for the 30 nm particles. Consistent with a previously published model, the reduction rates are likely to be proportional to the bacteria-hematite contact area and not to the total hematite surface area. Surface area-normalized iron reduction rates were higher than those reported in previous studies, which may be due to the sequestration of Fe(II) through formation of vivianite. Similar initial reduction rates were observed under all pH and H2 conditions studied.

Fe(III), Cr(VI), and Fe(III) mediated Cr(VI) reduction in alkaline media using a Halomonas isolate from Soap Lake, Washington

Hexavalent chromium is one of the most widely distributed environmental contaminants. Given the carcinogenic and mutagenic consequences of Cr(VI) exposure, the release of Cr(VI) into the environment has long been a major concern. While many reports of microbial Cr(VI) reduction are in circulation, very few have demonstrated Cr(VI) reduction under alkaline conditions. Since Cr(VI) exhibits higher mobility in alkaline soils relative to pH neutral soils, and since Cr contamination of alkaline soils is associated with a number of industrial activities, microbial Cr(VI) reduction under alkaline conditions requires attention.Soda lakes are the most stable alkaline environments on earth, and contain a wide diversity of alkaliphilic organisms. In this study, a bacterial isolate belonging to the Halomonas genus was obtained from Soap Lake, a chemically stratified alkaline lake located in central Washington State. The ability of this isolate to reduce Cr(VI) and Fe(III) was assessed under alkaline (pH = 9), anoxic, non-growth conditions with acetate as an electron donor. Metal reduction rates were quantified using Monod kinetics. In addition, Cr(VI) reduction experiments were carried out in the presence of Fe(III) to evaluate the possible enhancement of Cr(VI) reduction rates through electron shuttling mechanisms. While Fe(III) reduction rates were slow compared to previously reported rates, Cr(VI) reduction rates fell within range of previously reported rates.

Fe(III)-enhanced Azo Reduction by Shewanella decolorationis S12

Shewanella decolorationis S12 is capable of high rates of azo dye decolorization and dissimilatory Fe(III) reduction. Under anaerobic conditions, when Fe(III) and azo dye were copresent in S12 cultures, dissimilatory Fe(III) reduction and azo dye biodecolorization occurred simultaneously. Furthermore, the dye decolorization was enhanced by the presence of Fe(III). When 1 mM Fe(III) was added, the methyl red decolorizing efficiency was 72.1% after cultivation for 3 h, whereas the decolorizing efficiency was only 60.5% in Fe(III)-free medium. The decolorizing efficiencies increased as the concentration of Fe(III) was increased from 0 to 6 mM. Enzyme activities, which mediate the dye decolorization and Fe(III) reduction, were not affected by preadaption of cells to Fe(III) and azo dye nor by the addition of chloramphenicol. Both the Fe(III) reductase and the azo reductase were membrane associated. The respiratory electron transport chain inhibitors metyrapone, dicumarol, and stigmatellin showed significantly different effects on Fe(III) reduction than on azo dye decolorization.

Occurrence and Rates of Terminal Electron‐Accepting Processes and Recharge Processes in Petroleum Hydrocarbon‐Contaminated Subsurface

The occurrence and rates of terminal electron acceptor processes, and recharge processes in the unsaturated zone of a boreal site contaminated with petroleum hydrocarbons in the range C(10) to C(40) were examined. Soil microcosms were used to determine the rates of denitrification, iron (Fe) reduction, sulfate (SO(4)) reduction, and methanogenesis in two vertical soil profiles contaminated with oil, and in a noncontaminated reference sample. Furthermore, the abundances of the 16S rRNA genes belonging to Geobacteracaea in the samples were determined by real-time quantitative polymerase chain reaction (PCR). Analyses of ground water chemistry and soil gas composition were also performed together with continuous in situ monitoring of soil water and ground water chemistry. Several lines of evidence were obtained to demonstrate that both Fe reduction and methanogenesis played significant roles in the vertical profiles: Fe reduction rates up to 3.7 nmol h(-1) g(-1) were recorded and they correlated with the abundances of the Geobacteracaea 16S rRNA genes (range: 2.3 x 10(5) to 4.9 x 10(7) copies g(-1)). In the ground water, ferrous iron (Fe(2+)) concentration up to 55 mg L(-1) was measured. Methane production rates up to 2.5 nmol h(-1) g(-1) were obtained together with methane content up to 15% (vol/vol) in the soil gas. The continuous monitoring of soil water and ground water chemistry, microcosm experiments, and soil gas monitoring together demonstrated that the high microbial activity in the unsaturated zone resulted in rapid removal of oxygen from the infiltrating recharge thus leaving the anaerobic microbial processes dominant below 1.5 m depth both in the unsaturated and the saturated zones of the subsurface.

Towards determining details of anaerobic growth coupled to ferric iron reduction by the acidophilic archaeon ‘Ferroplasma acidarmanus’ Fer1

Elucidation of the different growth states of Ferroplasma species is crucial in understanding the cycling of iron in acid leaching sites. Therefore, a proteomic and biochemical study of anaerobic growth in 'Ferroplasma acidarmanus' Fer1 has been carried out. Anaerobic growth in Ferroplasma spp. occurred by coupling oxidation of organic carbon with the reduction of Fe(3+); but sulfate, nitrate, sulfite, thiosulfate, and arsenate were not utilized as electron acceptors. Rates of Fe(3+) reduction were similar to other acidophilic chemoorganotrophs. Analysis of the 'F. acidarmanus' Fer1 proteome by 2-dimensional polyacrylamide gel electrophoresis revealed ten key proteins linked with central metabolic pathways > or =4 fold up-regulated during anaerobic growth. These included proteins putatively identified as associated with the reductive tricarboxylic acid pathway used for anaerobic energy production, and others including a putative flavoprotein involved in electron transport. Inhibition of anaerobic growth and Fe(3+) reduction by inhibitors suggests the involvement of electron transport in Fe(3+)reduction. This study has increased the knowledge of anaerobic growth in this biotechnologically and environmentally important acidophilic archaeon.

Reduction of Fe(III) colloids by Shewanella putrefaciens: A kinetic model

A kinetic model for the microbial reduction of Fe(III) oxyhydroxide colloids in the presence of excess electron donor is presented. The model assumes a two-step mechanism: (1) attachment of Fe(III) colloids to the cell surface and (2) reduction of Fe(III) centers at the surface of attached colloids. The validity of the model is tested using Shewanella putrefaciens and nanohematite as model dissimilatory iron reducing bacteria and Fe(III) colloidal particles, respectively. Attachment of nanohematite to the bacteria is formally described by a Langmuir isotherm. Initial iron reduction rates are shown to correlate linearly with the relative coverage of the cell surface by nanohematite particles, hence supporting a direct electron transfer from membrane-bound reductases to mineral particles attached to the cells. Using internally consistent parameter values for the maximum attachment capacity of Fe(III) colloids to the cells, M max, the attachment constant, K P, and the first-order Fe(III) reduction rate constant, k, the model reproduces the initial reduction rates of a variety of fine-grained Fe(III) oxyhydroxides by S. putrefaciens. The model explains the observed dependency of the apparent Fe(III) half-saturation constant, K m ∗ , on the solid to cell ratio, and it predicts that initial iron reduction rates exhibit saturation with respect to both the cell density and the abundance of the Fe(III) oxyhydroxide substrate.

SEASONAL PATTERNS AND PLANT-MEDIATED CONTROLS OF SUBSURFACE WETLAND BIOGEOCHEMISTRY

In tidal marshes, spatial and temporal variability in the importance of mi- crobial metabolic pathways influences ecosystem-level processes such as soil carbon stor- age, the regeneration of inorganic nutrients, and the production of atmospherically important trace gases. We measured seasonal changes in rates of microbial Fe(III) reduction, sulfate reduction, and methanogenesis in tidal freshwater and brackish marshes on the Patuxent River, Maryland, USA, and assessed the ability of plant roots to influence these processes by regenerating electron acceptors and supplying electron donors. In both marshes, the importance of microbial Fe(III) reduction was greatest early in the summer and decreased through the study period. Coincident with the seasonal decline in Fe(III) reduction, me- thanogenesis (freshwater marsh) or sulfate reduction (brackish site) increased in importance. At the brackish marsh, the partitioning of anaerobic carbon metabolism between Fe(III) reduction and sulfate reduction was similar within and below the root zone, suggesting that rhizosphere processes did not control anaerobic metabolism at this site. Instead, seasonal biogeochemical patterns at the brackish marsh were affected by factors such as water table depth and iron-sulfur interactions. At the tidal freshwater site, our results suggest that changes in rates of Fe(III) reduction and methanogenesis were directly affected by plant- mediated processes. In midsummer, Fe(III) reduction accounted for a greater fraction of total anaerobic metabolism in rhizosphere-influenced surface soils than in soils below the root zone. High rates of Fe(III) reduction occurred at the expense of methanogenesis. This study documented strong temporal variations in the outcome of microbial competition for electron donors that ultimately affected the balance between Fe(III) reduction and me- thanogenesis within tidal freshwater marsh soils. Our data suggested that variations in microbial metabolic pathways were regulated by physiochemical factors at the brackish site and plant activity at the freshwater site. Plant regulation of Fe(III) reduction is a largely unstudied mechanism by which plants influence wetland carbon cycling and greenhouse gas production.

Temperature dependence of Fe(III) and sulfate reduction rates and its effect on growth and composition of bacterial enrichments from an acidic pit lake neutralization experiment

ABSTRACTMicrobial Fe(III) and sulfate reduction are important electron transport processes in acidic pit lakes and stimulation by the addition of organic substrates is a strategy to remove acidity, iron and sulfate. This principle was applied in a pilot‐scale enclosure in pit lake 111 (Brandenburg, Germany). Because seasonal and spatial variation of temperature may affect the performance of in situ experiments considerably, the influence of temperature on Fe(III) and sulfate reduction was investigated in surface sediments from the enclosure in the range of 4–28 °C. Potential Fe(III) reduction and sulfate reduction rates increased exponentially with temperature, and the effect was quantified in terms of the apparent activation energy Ea measuring 42–46 kJ mol−1 and 52 kJ mol−1, respectively. Relatively high respiration rates at 4 °C and relatively low Q10 values (∼2) indicated that microbial communities were well adapted to low temperatures. In order to evaluate the effect of temperature on growth and enrichment of iron and sulfate‐reducing bacterial populations, MPN (Most Probable Number) dilution series were performed in media selecting for the different bacterial groups. While the temperature response of specific growth rates of acidophilic iron reducers showed mesophilic characteristics, the relatively high specific growth rates of sulfate reducers at the lowest incubation temperature indicated the presence of moderate psychrophilic bacteria. In contrast, the low cell numbers and low specific growth rates of neutrophilic iron reducers obtained in dilution cultures suggest that these populations play a less significant role in Fe and S cycling in these sediments. SSCP (Single‐Strand Conformation Polymorphism) or DGGE (Denaturing Gradient Gel Electrophoresis) fingerprinting based on 16S rRNA genes of Bacteria indicated different bacterial populations in the MPN dilution series exhibiting different temperature ranges for growth.

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced 56Fe/ 54Fe ratios for Fe(II) aq that are 2–3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II) aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II) aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO 3) and ca. +0.9‰ for Ca-substituted siderite (Ca 0.15Fe 0.85CO 3) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have 56Fe/ 54Fe ratios that are up to 1‰ lower than Fe(II) aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II) aq. The relative order of δ 56Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II) aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ 56Fe >0‰, the calculated δ 56Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ 56Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ 56Fe values. Based on this experimental study, formation of low-δ 56Fe Fe(II) aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria.

Quantification of Ammonia-Oxidizing Bacteria and Factors Controlling Nitrification in Salt Marsh Sediments

To elucidate the geomicrobiological factors controlling nitrification in salt marsh sediments, a comprehensive approach involving sediment geochemistry, process rate measurements, and quantification of the genetic potential for nitrification was applied to three contrasting salt marsh habitats: areas colonized by the tall (TS) or short (SS) form of Spartina alterniflora and unvegetated creek banks (CBs). Nitrification and denitrification potential rates were strongly correlated with one another and with macrofaunal burrow abundance, indicating that coupled nitrification-denitrification was enhanced by macrofaunal burrowing activity. Ammonia monooxygenase (amoA) gene copy numbers were used to estimate the ammonia-oxidizing bacterial population size (5.6 x 10(4) to 1.3 x 10(6) g of wet sediment(-1)), which correlated with nitrification potentials and was 1 order of magnitude higher for TS and CB than for SS. TS and CB sediments also had higher Fe(III) content, higher Fe(III)-to-total reduced sulfur ratios, higher Fe(III) reduction rates, and lower dissolved sulfides than SS sediments. Iron(III) content and reduction rates were positively correlated with nitrification and denitrification potential and amoA gene copy number. Laboratory slurry incubations supported field data, confirming that increased amounts of Fe(III) relieved sulfide inhibition of nitrification. We propose that macrofaunal burrowing and high concentrations of Fe(III) stimulate nitrifying bacterial populations, and thus may increase nitrogen removal through coupled nitrification-denitrification in salt marsh sediments.

In Situ Expression of nifD in Geobacteraceae in Subsurface Sediments

In order to determine whether the metabolic state of Geobacteraceae involved in bioremediation of subsurface sediments might be inferred from levels of mRNA for key genes, in situ expression of nifD, a highly conserved gene involved in nitrogen fixation, was investigated. When Geobacter sulfurreducens was grown without a source of fixed nitrogen in chemostats with acetate provided as the limiting electron donor and Fe(III) as the electron acceptor, levels of nifD transcripts were 4 to 5 orders of magnitude higher than in chemostat cultures provided with ammonium. In contrast, the number of transcripts of recA and the 16S rRNA gene were slightly lower in the absence of ammonium. The addition of acetate to organic- and nitrogen-poor subsurface sediments stimulated the growth of Geobacteraceae and Fe(III) reduction, as well as the expression of nifD in Geobacteraceae. Levels of nifD transcripts in Geobacteraceae decreased more than 100-fold within 2 days after the addition of 100 microM ammonium, while levels of recA and total bacterial 16S rRNA in Geobacteraceae remained relatively constant. Ammonium amendments had no effect on rates of Fe(III) reduction in acetate-amended sediments or toluene degradation in petroleum-contaminated sediments, suggesting that other factors, such as the rate that Geobacteraceae could access Fe(III) oxides, limited Fe(III) reduction. These results demonstrate that it is possible to monitor one aspect of the in situ metabolic state of Geobacteraceae species in subsurface sediments via analysis of mRNA levels, which is the first step toward a more global analysis of in situ gene expression related to nutrient status and stress response during bioremediation by Geobacteraceae.

Microbial reduction of iron(III) oxyhydroxides: effects of mineral solubility and availability

The rate of Fe(III) reductive dissolution by ascorbate has been shown by [Larsen, O. and Postma, D., 2001. Kinetics of reductive bulk dissolution of lepidocrocite, ferrihydrite and goethite. Geochim. Cosmochim. Acta, 65: 1367–1379.] to decrease in the order ferrihydrite>lepidocrocite>goethite>hematite. The abiotic rate of reductive dissolution thus correlates with the solubility of the iron oxyhydroxides. We investigated whether this also holds for the microbial reduction kinetics of Fe(III) iron oxyhydroxides. The solubilities of nanoparticulate hematite, lepidocrocite, ferrihydrite and amorphous Fe(III) oxyhydroxide were obtained from pe–pH titrations of oxyhydroxide/Fe 2+( aq) suspensions, in pH range 4 to 7. The solubility of low surface area (LSA) hematite was estimated from redox potential measurements at pH 2. The same solid phases and soluble Fe(III)-citrate were then reduced by the iron reducing bacterium Shewanella putrefaciens using lactate as the electron donor. In all cases, the microbial Fe(III) reduction rates exhibited saturation behavior with respect to the initial Fe(III) concentration. The maximum specific rate of reduction ( v max, in μmol cell −1 h −1) correlated positively with the solubility of the Fe(III) oxyhydroxides, with the highest values of v max for ferrihydrite and amorphous Fe(III) oxide, the lowest for LSA hematite. Hence, the solubility appears to be a rate-controlling parameter in both the abiotic and enzymatic reduction of Fe(III) oxyhydroxides.

Structural constraints of ferric (hydr)oxides on dissimilatory iron reduction and the fate of Fe(II)

Due to the strong reducing capacity of ferrous Fe, the fate of Fe(II) following dissimilatory iron reduction will have a profound bearing on biogeochemical cycles. We have previously observed the rapid and near complete conversion of 2-line ferrihydrite to goethite (minor phase) and magnetite (major phase) under advective flow in an organic carbon-rich artificial groundwater medium. Yet, in many mineralogically mature environments, well-ordered iron (hydr)oxide phases dominate and may therefore control the extent and rate of Fe(III) reduction. Accordingly, here we compare the reducing capacity and Fe(II) sequestration mechanisms of goethite and hematite to 2-line ferrihydrite under advective flow within a medium mimicking that of natural groundwater supplemented with organic carbon. Introduction of dissolved organic carbon upon flow initiation results in the onset of dissimilatory iron reduction of all three Fe phases (2-line ferrihydrite, goethite, and hematite). While the initial surface area normalized rates are similar (∼10 −11 mol Fe(II) m −2 g −1), the total amount of Fe(III) reduced over time along with the mechanisms and extent of Fe(II) sequestration differ among the three iron (hydr)oxide substrates. Following 16 d of reaction, the amount of Fe(III) reduced within the ferrihydrite, goethite, and hematite columns is 25, 5, and 1%, respectively. While 83% of the Fe(II) produced in the ferrihydrite system is retained within the solid-phase, merely 17% is retained within both the goethite and hematite columns. Magnetite precipitation is responsible for the majority of Fe(II) sequestration within ferrihydrite, yet magnetite was not detected in either the goethite or hematite systems. Instead, Fe(II) may be sequestered as localized spinel-like (magnetite) domains within surface hydrated layers (ca. 1 nm thick) on goethite and hematite or by electron delocalization within the bulk phase. The decreased solubility of goethite and hematite relative to ferrihydrite, resulting in lower Fe(III) aq and bacterially-generated Fe(II) aq concentrations, may hinder magnetite precipitation beyond mere surface reorganization into nanometer-sized, spinel-like domains. Nevertheless, following an initial, more rapid reduction period, the three Fe (hydr)oxides support similar aqueous ferrous iron concentrations, bacterial populations, and microbial Fe(III) reduction rates. A decline in microbial reduction rates and further Fe(II) retention in the solid-phase correlates with the initial degree of phase disorder (high energy sites). As such, sustained microbial reduction of 2-line ferrihydrite, goethite, and hematite appears to be controlled, in large part, by changes in surface reactivity (energy), which is influenced by microbial reduction and secondary Fe(II) sequestration processes regardless of structural order (crystallinity) and surface area.