Physiology of Organotrophic and Lithotrophic Growth of the Thermophilic Iron-Reducing Bacteria Thermoterrabacterium ferrireducens and Thermoanaerobacter siderophilus

Growth physiology of the iron-reducing bacteria Thermoterrabacterium ferrireducens and Thermoanaerobacter siderophilus was investigated. The stimulation of the organotrophic growth of T. ferrireducens and T. siderophilusin the presence of Fe(III) was shown to be due to the utilization of ferric iron as an electron acceptor in catabolic processes and not to the effect exerted on the metabolism by Fe(II) or by changes in the redox potential. It was established that Fe(III) reduction in T. ferrireducens is not a detoxication strategy. In T. siderophilus, this process is carried out to alleviate the inhibitory effect of hydrogen. T. ferrireducens was shown to be capable of lithoautotrophic growth with molecular hydrogen as an electron donor and amorphous ferric oxide as an electron acceptor, in the absence of any organic substances. The minimum threshold of H₂ consumption was 3 × 10^–5 vol % of H₂. The presence of CO dehydrogenase activity in T. ferrireducens suggests that CO₂ fixation in this organism involves the anaerobic acetyl-CoA pathway. T. siderophilus failed to grow under lithoautotrophic conditions. The fact that T. ferrireducens contains c-type cytochromes and T. siderophilus lacks them confirms the operation of different mechanisms of ferric iron reduction in these species.     

Novel physiological features of Carboxydothermus hydrogenoformans and Thermoterrabacterium ferrireducens

Carboxydothermus hydrogenoformans is able to grow by conversion of CO to H2 and CO2. Besides CO, only pyruvate was described as serving as an energy source. Based on 16S rRNA gene sequence similarity, C. hydrogenoformans is closely related to Thermoterrabacterium ferrireducens. T. ferrireducens is like C. hydrogenoformans a gram-positive, thermophilic, strict anaerobic bacterium. However, it is capable of using various electron donors and acceptors for growth. Growth of C. hydrogenoformans with multiple electron donors and acceptors was tested. C. hydrogenoformans oxidized formate, lactate, glycerol, CO, and H2 with 9,10-anthraquinone-2,6-disulfonate as an electron acceptor. Sulfite, thiosulfate, sulfur, nitrate, and fumarate were reduced with lactate as an electron donor. T. ferrireducens oxidized CO with 9,10-anthraquinone-2,6-disulfonate as an electron acceptor but did not produce H2 from CO. In contrast to what was published before, T. ferrireducens was able to grow on lactate with sulfite, sulfur, and nitrate as electron acceptors.     

Melanin production and use as a soluble electron shuttle for Fe(III) oxide reduction and as a terminal electron acceptor by Shewanella algae BrY

Dissimilatory metal-reducing bacteria (DMRB) utilize numerous compounds as terminal electron acceptors, including insoluble iron oxides. The mechanism(s) of insoluble-mineral reduction by DMRB is not well understood. Here we report that extracellular melanin is produced by Shewanella algae BrY. The extracted melanin served as the sole terminal electron acceptor. Upon reduction the reduced, soluble melanin reduced insoluble hydrous ferric oxide in the absence of bacteria, thus demonstrating that melanin produced by S. algae BrY is a soluble Fe(III)-reducing compound. In the presence of bacteria, melanin acted as an electron conduit to Fe(III) minerals and increased Fe(III) mineral reduction rates. Growth of S. algae BrY occurred in anaerobic minimal medium supplemented with melanin extracted from previously grown aerobic cultures of S. algae BrY. Melanin produced by S. algae BrY imparts increased versatility to this organism as a soluble Fe(III) reductant, an electron conduit for iron mineral reduction, and a sole terminal electron acceptor that supports growth.     

Escherichia coli ferredoxin-NADP+ reductase and oxygen-insensitive nitroreductase are capable of functioning as ferric reductase and of driving the Fenton reaction

Two free flavin-independent enzymes were purified by detecting the NAD(P)H oxidation in the presence of Fe(III)-EDTA and t-butyl hydroperoxide from E. coli. The enzyme that requires NADH or NADPH as an electron donor was a 28 kDa protein, and N-terminal sequencing revealed it to be oxygen-insensitive nitroreductase (NfnB). The second enzyme that requires NADPH as an electron donor was a 30 kDa protein, and N-terminal sequencing revealed it to be ferredoxin-NADP⁺ reductase (Fpr). The chemical stoichiometry of the Fenton activities of both NfnB and Fpr in the presence of Fe(III)-EDTA, NAD(P)H and hydrogen peroxide was investigated. Both enzymes showed a one-electron reduction in the reaction forming hydroxyl radical from hydrogen peroxide. Also, the observed Fenton activities of both enzymes in the presence of synthetic chelate iron compounds were higher than their activities in the presence of natural chelate iron compounds. When the Fenton reaction occurs, the ferric iron must be reduced to ferrous iron. The ferric reductase activities of both NfnB and Fpr occurred with synthetic chelate iron compounds. Unlike NfnB, Fpr also showed the ferric reductase activity on an iron storage protein, ferritin, and various natural iron chelate compounds including siderophore. The Fenton and ferric reductase reactions of both NfnB and Fpr occurred in the absence of free flavin. Although the k _cat/K _m value of NfnB for Fe(III)-EDTA was not affected by free flavin, the k _cat/K _m value of Fpr for Fe(III)-EDTA was 12-times greater in the presence of free FAD than in the absence of free FAD.     

Corrosion-enhancing potential of Shewanella putrefaciens isolated from industrial cooling waters

Microbially influenced corrosion (MIC) is catalysed by a series of metabolic activities of selected micro-organisms, notably by oxidation of cathodic hydrogen by hydrogenase, by hydrogen sulphide and by reduction of ferric iron. The sulphate-reducing bacteria are considered to be the most common catalyst of MIC, whereas the role of other bacteria has been neglected. This study examined the corrosive potential of the facultative sulphide producer, Shewanella putrefaciens , isolated from an industrial cooling water system. Shewanella putrefaciens was shown to reduce ferric iron and sulphite under anaerobic conditions and with ferric iron being the preferred electron acceptor. The isolate could utilize cathodic hydrogen as an energy source, especially when using sulphite as a terminal electron acceptor. In pure culture corrosion experiments, the highest mass loss of mild steel was observed in the presence of sulphite as sole electron acceptor, although mass loss was also detected where ferric iron was the sole electron acceptor. Our data indicate that S. putefaciens plays a role in MIC as it was able to catalyse a variety of corrosion-promoting reactions and to corrode mild steel under pure culture conditions.     

Melanin Production and Use as a Soluble Electron Shuttle for Fe(III) Oxide Reduction and as a Terminal Electron Acceptor by Shewanella algae BrY

Dissimilatory metal-reducing bacteria (DMRB) utilize numerous compounds as terminal electron acceptors, including insoluble iron oxides. The mechanism(s) of insoluble-mineral reduction by DMRB is not well understood. Here we report that extracellular melanin is produced by Shewanella algae BrY. The extracted melanin served as the sole terminal electron acceptor. Upon reduction the reduced, soluble melanin reduced insoluble hydrous ferric oxide in the absence of bacteria, thus demonstrating that melanin produced by S. algae BrY is a soluble Fe(III)-reducing compound. In the presence of bacteria, melanin acted as an electron conduit to Fe(III) minerals and increased Fe(III) mineral reduction rates. Growth of S. algae BrY occurred in anaerobic minimal medium supplemented with melanin extracted from previously grown aerobic cultures of S. algae BrY. Melanin produced by S. algae BrY imparts increased versatility to this organism as a soluble Fe(III) reductant, an electron conduit for iron mineral reduction, and a sole terminal electron acceptor that supports growth.     

Effect of Competing Electron Acceptors on the Reduction of U(VI) by Desulfotomaculum reducens

The biological reduction of soluble U(VI) to the less soluble U(IV) has been proposed as a strategy to remediate uranium-contaminated sites. However, the majority of the contaminated sites contain, in addition to U(VI), competing electron acceptors (CEAs) that can either enhance or inhibit U(VI) reduction. Desulfotomaculum reducens MI-1 is a sulfate-reducing bacterium able to reduce a variety of electron acceptors including U(VI). We characterized U(VI) reduction by D. reducens in the presence of pyruvate and three CEAs: sulfate, nitrate or soluble ferric iron. In the presence of sulfate or ferric iron and U(VI), cell growth was driven by respiration of the CEA. Nitrate was not used as an electron acceptor for growth and vegetative cells grew instead by fermenting pyruvate. Sulfate remaining after sulfate reduction has ceased or the presence of nitrate did not affect U(VI) reduction. However, in the case of sulfate, the addition of H₂ after the depletion of pyruvate greatly enhanced U(VI) reduction. Contrary to sulfate and nitrate, the presence of Fe(II), the product of Fe(III) reduction, abolished U(VI) reduction. The results from this investigation suggest that this microorganism and others with similar characteristics may play a role in U(VI) bioremediation efforts but only after the soluble Fe(II) produced by Fe(III) reduction has been advected away.     

海洋沉积物中铁还原细菌组成及异化铁还原与产氢性质分析

【背景】一些铁还原细菌具有异化铁还原与产氢的能力,该类细菌在环境污染修复的同时能够解决能源问题。【目的】从海洋沉积物中富集获得异化铁还原菌群,明确混合菌群组成、异化铁还原及产氢性质。获得海洋沉积物中异化铁还原混合菌群组成,分析菌群异化铁还原和产氢性质。【方法】利用高通量测序技术分析异化铁还原菌群的优势菌组成,在此基础上,分析异化铁还原混合菌群在不同电子供体培养条件下异化铁还原能力和产氢性质。【结果】高通量数据表明,在不溶性氢氧化铁为电子受体和葡萄糖为电子供体厌氧培养条件下,混合菌群的优势菌属主要是梭菌(Clostridium),属于发酵型异化铁还原细菌。混合菌群能够利用电子供体蔗糖、葡萄糖以及丙酮酸钠进行异化铁还原及发酵产氢。葡萄糖为电子供体时,菌群累积产生Fe(Ⅱ)浓度和产氢量最高,分别是59.34±6.73 mg/L和629.70±11.42 mL/L。【结论】异化铁还原混合菌群同时具有异化铁还原和产氢能力,拓宽了发酵型异化铁还原细菌的种质资源,探索异化铁还原细菌在生物能源方面的应用。     

Characterization of Fe(III) reduction by chlororespiring Anaeromyxobacter dehalogenans

Anaeromyxobacter dehalogenans strain 2CP-C has been shown to grow by coupling the oxidation of acetate to the reduction of ortho-substituted halophenols, oxygen, nitrate, nitrite, or fumarate. In this study, strain 2CP-C was also found to grow by coupling Fe(III) reduction to the oxidation of acetate, making it one of the few isolates capable of growth by both metal reduction and chlororespiration. Doubling times for growth of 9.2 and 10.2 h were determined for Fe(III) and 2-chlorophenol reduction, respectively. These were determined by using the rate of [(14)C]acetate uptake into biomass. Fe(III) compounds used by strain 2CP-C include ferric citrate, ferric pyrophosphate, and amorphous ferric oxyhydroxide. The addition of the humic acid analog anthraquinone 2,6-disulfonate (AQDS) increased the reduction rate of amorphous ferric iron oxide, suggesting AQDS was used as an electron shuttle by strain 2CP-C. The addition of chloramphenicol to fumarate-grown cells did not inhibit Fe(III) reduction, indicating that the latter activity is constitutive. In contrast, the addition of chloramphenicol inhibited dechlorination activity, indicating that chlororespiration is inducible. The presence of insoluble Fe(III) oxyhydroxide did not significantly affect dechlorination, whereas the presence of soluble ferric pyrophosphate inhibited dechlorination. With its ability to respire chlorinated organic compounds and metals such as Fe(III), strain 2CP-C is a promising model organism for the study of the interaction of these potentially competing processes in contaminated environments.     

Dissimilatory reduction of Fe(III) and other electron acceptors by a Thermus isolate

A thermophilic bacterium that can use O2, NO3-, Fe(III), and S0 as terminal electron acceptors for growth was isolated from groundwater sampled at a 3.2-km depth in a South African gold mine. This organism, designated SA-01, clustered most closely with members of the genus Thermus, as determined by 16S rRNA gene (rDNA) sequence analysis. The 16S rDNA sequence of SA-01 was >98% similar to that of Thermus strain NMX2 A.1, which was previously isolated by other investigators from a thermal spring in New Mexico. Strain NMX2 A.1 was also able to reduce Fe(III) and other electron acceptors. Neither SA-01 nor NMX2 A.1 grew fermentatively, i.e., addition of an external electron acceptor was required for anaerobic growth. Thermus strain SA-01 reduced soluble Fe(III) complexed with citrate or nitrilotriacetic acid (NTA); however, it could reduce only relatively small quantities (0.5 mM) of hydrous ferric oxide except when the humic acid analog 2,6-anthraquinone disulfonate was added as an electron shuttle, in which case 10 mM Fe(III) was reduced. Fe(III)-NTA was reduced quantitatively to Fe(II); reduction of Fe(III)-NTA was coupled to the oxidation of lactate and supported growth through three consecutive transfers. Suspensions of Thermus strain SA-01 cells also reduced Mn(IV), Co(III)-EDTA, Cr(VI), and U(VI). Mn(IV)-oxide was reduced in the presence of either lactate or H2. Both strains were also able to mineralize NTA to CO2 and to couple its oxidation to Fe(III) reduction and growth. The optimum temperature for growth and Fe(III) reduction by Thermus strains SA-01 and NMX2 A.1 is approximately 65 degrees C; their optimum pH is 6.5 to 7.0. This is the first report of a Thermus sp. being able to couple the oxidation of organic compounds to the reduction of Fe, Mn, or S.     

Bioenergetic challenges of microbial iron metabolisms

Before cyanobacteria invented oxygenic photosynthesis and O(2) and H(2)O began to cycle between respiration and photosynthesis, redox cycles between other elements were used to sustain microbial metabolism on a global scale. Today these cycles continue to occur in more specialized niches. In this review we focus on the bioenergetic aspects of one of these cycles - the iron cycle - because iron presents unique and fascinating challenges for cells that use it for energy. Although iron is an important nutrient for nearly all life forms, we restrict our discussion to energy-yielding pathways that use ferrous iron [Fe(II)] as an electron donor or ferric iron [Fe(III)] as an electron acceptor. We briefly review general concepts in bioenergetics, focusing on what is known about the mechanisms of electron transfer in Fe(II)-oxidizing and Fe(III)-reducing bacteria, and highlight aspects of their bioenergetic pathways that are poorly understood.     

Influence of electron donor/acceptor concentrations on hydrous ferric oxide (HFO) bioreduction

Dissimilatory metal-reducing bacteria (DMRB) facilitate the reduction of Feand Mn oxides in anoxic soils and sediments and play an important role inthe cycling of these metals and other elements such as carbon in aqueousenvironments. Previous studies investigating the reduction of Fe(III) oxidesby DMRB focused on reactions under constant initial electron donor (lactate)and electron acceptor (Fe oxide) concentrations. Because the concentrationsof these reactants can vary greatly in the environment and would be expectedto influence the rate and extent of oxide reduction, the influence of variableelectron acceptor and donor concentrations on hydrous ferric oxide (HFO)bioreduction was investigated. Batch experiments were conducted in pH 7 HCO₃– buffered media using Shewanella putrefaciens strain CN32. In general, the rate of Fe(III) reduction decreased with increasing HFO:lactateratios, resulting in a relatively greater proportion of crystalline Fe(III) oxidesof relatively low availability for DMRB. HFO was transformed to a variety ofcrystalline minerals including goethite, lepidocrocite, and siderite but was almostcompletely dissolved at high lactate to HFO ratios. These results indicate thatelectron donor and acceptor concentrations can greatly impact the bioreductionof HFO and the suite of Fe minerals formed as a result of reduction. The respirationdriven rate of Fe(II) formation from HFO is believed to be a primary factor governingthe array of ferrous and ferric iron phases formed during reduction.     

Periplasmic cytochrome c3 of Desulfovibrio vulgaris is directly involved in H2-mediated metal but not sulfate reduction

Kinetic parameters and the role of cytochrome c(3) in sulfate, Fe(III), and U(VI) reduction were investigated in Desulfovibrio vulgaris Hildenborough. While sulfate reduction followed Michaelis-Menten kinetics (K(m) = 220 micro M), loss of Fe(III) and U(VI) was first-order at all concentrations tested. Initial reduction rates of all electron acceptors were similar for cells grown with H(2) and sulfate, while cultures grown using lactate and sulfate had similar rates of metal loss but lower sulfate reduction activities. The similarities in metal, but not sulfate, reduction with H(2) and lactate suggest divergent pathways. Respiration assays and reduced minus oxidized spectra were carried out to determine c-type cytochrome involvement in electron acceptor reduction. c-type cytochrome oxidation was immediate with Fe(III) and U(VI) in the presence of H(2), lactate, or pyruvate. Sulfidogenesis occurred with all three electron donors and effectively oxidized the c-type cytochrome in lactate- or pyruvate-reduced, but not H(2)-reduced cells. Correspondingly, electron acceptor competition assays with lactate or pyruvate as electron donors showed that Fe(III) inhibited U(VI) reduction, and U(VI) inhibited sulfate loss. However, sulfate reduction was slowed but not halted when H(2) was the electron donor in the presence of Fe(III) or U(VI). U(VI) loss was still impeded by Fe(III) when H(2) was used. Hence, we propose a modified pathway for the reduction of sulfate, Fe(III), and U(VI) which helps explain why these bacteria cannot grow using these metals. We further propose that cytochrome c(3) is an electron carrier involved in lactate and pyruvate oxidation and is the reductase for alternate electron acceptors with higher redox potentials than sulfate.     

Reduction of Fe(III), Mn(IV), and toxic metals at 100 degrees C by Pyrobaculum islandicum

It has recently been noted that a diversity of hyperthermophilic microorganisms have the ability to reduce Fe(III) with hydrogen as the electron donor, but the reduction of Fe(III) or other metals by these organisms has not been previously examined in detail. When Pyrobaculum islandicum was grown at 100 degrees C in a medium with hydrogen as the electron donor and Fe(III)-citrate as the electron acceptor, the increase in cell numbers of P. islandicum per mole of Fe(III) reduced was found to be ca. 10-fold higher than previously reported. Poorly crystalline Fe(III) oxide could also serve as the electron acceptor for growth on hydrogen. The stoichiometry of hydrogen uptake and Fe(III) oxide reduction was consistent with the oxidation of 1 mol of hydrogen resulting in the reduction of 2 mol of Fe(III). The poorly crystalline Fe(III) oxide was reduced to extracellular magnetite. P. islandicum could not effectively reduce the crystalline Fe(III) oxide minerals goethite and hematite. In addition to using hydrogen as an electron donor for Fe(III) reduction, P. islandicum grew via Fe(III) reduction in media in which peptone and yeast extract served as potential electron donors. The closely related species P. aerophilum grew via Fe(III) reduction in a similar complex medium. Cell suspensions of P. islandicum reduced the following metals with hydrogen as the electron donor: U(VI), Tc(VII), Cr(VI), Co(III), and Mn(IV). The reduction of these metals was dependent upon the presence of cells and hydrogen. The metalloids arsenate and selenate were not reduced. U(VI) was reduced to the insoluble U(IV) mineral uraninite, which was extracellular. Tc(VII) was reduced to insoluble Tc(IV) or Tc(V). Cr(VI) was reduced to the less toxic, less soluble Cr(III). Co(III) was reduced to Co(II). Mn(IV) was reduced to Mn(II) with the formation of manganese carbonate. These results demonstrate that biological reduction may contribute to the speciation of metals in hydrothermal environments and could account for such phenomena as magnetite accumulation and the formation of uranium deposits at ca. 100 degrees C. Reduction of toxic metals with hyperthermophilic microorganisms or their enzymes might be applied to the remediation of metal-contaminated waters or waste streams.     

A Hydrogen-Oxidizing, Fe(III)-Reducing Microorganism from the Great Bay Estuary, New Hampshire

A dissimilatory Fe(III)- and Mn(IV)-reducing bacterium was isolated from bottom sediments of the Great Bay estuary, New Hampshire. The isolate was a facultatively anaerobic gram-negative rod which did not appear to fit into any previously described genus. It was temporarily designated strain BrY. BrY grew anaerobically in a defined medium with hydrogen or lactate as the electron donor and Fe(III) as the electron acceptor. BrY required citrate, fumarate, or malate as a carbon source for growth on H₂ and Fe(III). With Fe(III) as the sole electron acceptor, BrY metabolized hydrogen to a minimum threshold at least 60-fold lower than the threshold reported for pure cultures of sulfate reducers. This finding supports the hypothesis that when Fe(III) is available, Fe(III) reducers can outcompete sulfate reducers for electron donors. Lactate was incompletely oxidized to acetate and carbon dioxide with Fe(III) as the electron acceptor. Lactate oxidation was also coupled to the reduction of Mn(IV), U(VI), fumarate, thiosulfate, or trimethylamine n-oxide under anaerobic conditions. BrY provides a model for how enzymatic metal reduction by respiratory metal-reducing microorganisms has the potential to contribute to the mobilization of iron and trace metals and to the immobilization of uranium in sediments of Great Bay Estuary.     

Novel Mode of Microbial Energy Metabolism: Organic Carbon Oxidation Coupled to Dissimilatory Reduction of Iron or Manganese

A dissimilatory Fe(III)- and Mn(IV)-reducing microorganism was isolated from freshwater sediments of the Potomac River, Maryland. The isolate, designated GS-15, grew in defined anaerobic medium with acetate as the sole electron donor and Fe(III), Mn(IV), or nitrate as the sole electron acceptor. GS-15 oxidized acetate to carbon dioxide with the concomitant reduction of amorphic Fe(III) oxide to magnetite (Fe₃O₄). When Fe(III) citrate replaced amorphic Fe(III) oxide as the electron acceptor, GS-15 grew faster and reduced all of the added Fe(III) to Fe(II). GS-15 reduced a natural amorphic Fe(III) oxide but did not significantly reduce highly crystalline Fe(III) forms. Fe(III) was reduced optimally at pH 6.7 to 7 and at 30 to 35°C. Ethanol, butyrate, and propionate could also serve as electron donors for Fe(III) reduction. A variety of other organic compounds and hydrogen could not. MnO₂ was completely reduced to Mn(II), which precipitated as rhodochrosite (MnCO₃). Nitrate was reduced to ammonia. Oxygen could not serve as an electron acceptor, and it inhibited growth with the other electron acceptors. This is the first demonstration that microorganisms can completely oxidize organic compounds with Fe(III) or Mn(IV) as the sole electron acceptor and that oxidation of organic matter coupled to dissimilatory Fe(III) or Mn(IV) reduction can yield energy for microbial growth. GS-15 provides a model for how enzymatically catalyzed reactions can be quantitatively significant mechanisms for the reduction of iron and manganese in anaerobic environments.     

Fe(III) and S0 reduction by Pelobacter carbinolicus.

There is a close phylogenetic relationship between Pelobacter species and members of the genera Desulfuromonas and Geobacter, and yet there has been a perplexing lack of physiological similarities. Pelobacter species have been considered to have a fermentative metabolism. In contrast, Desulfuromonas and Geobacter species have a respiratory metabolism with Fe(III) serving as the common terminal electron acceptor in all species. However, the ability of Pelobacter species to reduce Fe(III) had not been previously evaluated. When a culture of Pelobacter carbinolicus that had grown by fermentation of 2,3-butanediol was inoculated into the same medium supplemented with Fe(III), the Fe(III) was reduced. There was less accumulation of ethanol and more production of acetate in the presence of Fe(III). P. carbinolicus grew with ethanol as the sole electron donor and Fe(III) as the sole electron acceptor. Ethanol was metabolized to acetate. Growth was also possible on Fe(III) with the oxidation of propanol to propionate or butanol to butyrate if acetate was provided as a carbon source. P. carbinolicus appears capable of conserving energy to support growth from Fe(III) respiration as it also grew with H2 or formate as the electron donor and Fe(III) as the electron acceptor. Once adapted to Fe(III) reduction, P. carbinolicus could also grow on ethanol or H2 with S0 as the electron acceptor. P. carbinolicus did not contain detectable concentrations of the c-type cytochromes that previous studies have suggested are involved in electron transport to Fe(III) in other organisms that conserve energy to support growth from Fe(III) reduction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Use of Fe(III) as an electron acceptor to recover previously uncultured hyperthermophiles: isolation and characterization of Geothermobacterium ferrireducens gen. nov., sp. nov

It has recently been recognized that the ability to use Fe(III) as a terminal electron acceptor is a highly conserved characteristic in hyperthermophilic microorganisms. This suggests that it may be possible to recover as-yet-uncultured hyperthermophiles in pure culture if Fe(III) is used as an electron acceptor. As part of a study of the microbial diversity of the Obsidian Pool area in Yellowstone National Park, Wyo., hot sediment samples were used as the inoculum for enrichment cultures in media containing hydrogen as the sole electron donor and poorly crystalline Fe(III) oxide as the electron acceptor. A pure culture was recovered on solidified, Fe(III) oxide medium. The isolate, designated FW-1a, is a hyperthermophilic anaerobe that grows exclusively by coupling hydrogen oxidation to the reduction of poorly crystalline Fe(III) oxide. Organic carbon is not required for growth. Magnetite is the end product of Fe(III) oxide reduction under the culture conditions evaluated. The cells are rod shaped, about 0.5 microm by 1.0 to 1.2 microm, and motile and have a single flagellum. Strain FW-1a grows at circumneutral pH, at freshwater salinities, and at temperatures of between 65 and 100 degrees C with an optimum of 85 to 90 degrees C. To our knowledge this is the highest temperature optimum of any organism in the BACTERIA: Analysis of the 16S ribosomal DNA (rDNA) sequence of strain FW-1a places it within the Bacteria, most closely related to abundant but uncultured microorganisms whose 16S rDNA sequences have been previously recovered from Obsidian Pool and a terrestrial hot spring in Iceland. While previous studies inferred that the uncultured microorganisms with these 16S rDNA sequences were sulfate-reducing organisms, the physiology of the strain FW-1a, which does not reduce sulfate, indicates that these organisms are just as likely to be Fe(III) reducers. These results further demonstrate that Fe(III) may be helpful for recovering as-yet-uncultured microorganisms from hydrothermal environments and illustrate that caution must be used in inferring the physiological characteristics of at least some thermophilic microorganisms solely from 16S rDNA sequences. Based on both its 16S rDNA sequence and physiological characteristics, strain FW-1a represents a new genus among the Bacteria. The name Geothermobacterium ferrireducens gen. nov., sp. nov., is proposed (ATCC BAA-426).     

Characterization of Fe(III) Reduction by Chlororespiring Anaeromxyobacter dehalogenans

Anaeromyxobacter dehalogenans strain 2CP-C has been shown to grow by coupling the oxidation of acetate to the reduction of ortho-substituted halophenols, oxygen, nitrate, nitrite, or fumarate. In this study, strain 2CP-C was also found to grow by coupling Fe(III) reduction to the oxidation of acetate, making it one of the few isolates capable of growth by both metal reduction and chlororespiration. Doubling times for growth of 9.2 and 10.2 h were determined for Fe(III) and 2-chlorophenol reduction, respectively. These were determined by using the rate of [¹⁴C]acetate uptake into biomass. Fe(III) compounds used by strain 2CP-C include ferric citrate, ferric pyrophosphate, and amorphous ferric oxyhydroxide. The addition of the humic acid analog anthraquinone 2,6-disulfonate (AQDS) increased the reduction rate of amorphous ferric iron oxide, suggesting AQDS was used as an electron shuttle by strain 2CP-C. The addition of chloramphenicol to fumarate-grown cells did not inhibit Fe(III) reduction, indicating that the latter activity is constitutive. In contrast, the addition of chloramphenicol inhibited dechlorination activity, indicating that chlororespiration is inducible. The presence of insoluble Fe(III) oxyhydroxide did not significantly affect dechlorination, whereas the presence of soluble ferric pyrophosphate inhibited dechlorination. With its ability to respire chlorinated organic compounds and metals such as Fe(III), strain 2CP-C is a promising model organism for the study of the interaction of these potentially competing processes in contaminated environments.