@article {3043, title = {Response to Wang et al.: evidence contradicting the cytochrome-only model.}, journal = {Trends Microbiol}, volume = {31}, year = {2023}, month = {2023 Jun}, pages = {548-549}, keywords = {Cytochromes, Electron Transport}, issn = {1878-4380}, doi = {10.1016/j.tim.2023.03.006}, author = {Lovley, Derek R} } @article {3096, title = {Reply to {\textquoteright}Measuring conductivity of living Geobacter sulfurreducens biofilms{\textquoteright}.}, journal = {Nat Nanotechnol}, volume = {11}, year = {2016}, month = {2016 Nov 08}, pages = {913-914}, keywords = {Biofilms, Electric Conductivity, Geobacter}, issn = {1748-3395}, doi = {10.1038/nnano.2016.191}, author = {Malvankar, Nikhil S and Rotello, Vincent M and Tuominen, Mark T and Lovley, Derek R} } @article {3112, title = {Real-time monitoring of subsurface microbial metabolism with graphite electrodes.}, journal = {Front Microbiol}, volume = {5}, year = {2014}, month = {2014}, pages = {621}, abstract = {

Monitoring in situ microbial activity in anoxic submerged soils and aquatic sediments can be labor intensive and technically difficult, especially in dynamic environments in which a record of changes in microbial activity over time is desired. Microbial fuel cell concepts have previously been adapted to detect changes in the availability of relatively high concentrations of organic compounds in waste water but, in most soils and sediments, rates of microbial activity are not linked to the concentrations of labile substrates, but rather to the turnover rates of the substrate pools with steady state concentrations in the nM-μM range. In order to determine whether levels of current produced at a graphite anode would correspond to the rates of microbial metabolism in anoxic sediments, small graphite anodes were inserted in sediment cores and connected to graphite brush cathodes in the overlying water. Currents produced were compared with the rates of [2-(14)C]-acetate metabolism. There was a direct correlation between current production and the rate that [2-(14)C]-acetate was metabolized to (14)CO2 and (14)CH4 in sediments in which Fe(III) reduction, sulfate reduction, or methane production was the predominant terminal electron-accepting process. At comparable acetate turnover rates, currents were higher in the sediments in which sulfate-reduction or Fe(III) reduction predominated than in methanogenic sediments. This was attributed to reduced products (Fe(II), sulfide) produced at distance from the anode contributing to current production in addition to the current that was produced from microbial oxidation of organic substrates with electron transfer to the anode surface in all three sediment types. The results demonstrate that inexpensive graphite electrodes may provide a simple strategy for real-time monitoring of microbial activity in a diversity of anoxic soils and sediments.

}, issn = {1664-302X}, doi = {10.3389/fmicb.2014.00621}, author = {Wardman, Colin and Nevin, Kelly P and Lovley, Derek R} } @article {408, title = {Real-time spatial gene expression analysis within current-producing biofilms.}, journal = {ChemSusChem}, volume = {5}, year = {2012}, month = {2012 Jun}, pages = {1092-8}, abstract = {

The expression of genes involved in central metabolism and extracellular electron transfer was examined in real-time in current-producing anode biofilms of Geobacter sulfurreducens. Strains of G. sulfurreducens were generated, in which the expression of the gene for a short half-life fluorescent protein was placed under control of the promoter of the genes of interest. Anode biofilms were grown in a chamber that permitted direct examination of the cell fluorescence with confocal scanning laser microscopy. Studies on nifD and citrate synthase expression in response to environmental changes demonstrated that the reporter system revealed initiation and termination of gene transcription. Uniform expression throughout the biofilms was noted for the genes for citrate synthase; PilA, the structural protein of the conductive pili; and OmcZ, a c-type cytochrome essential for optimal current production, which was localized at the anode-biofilm interface. These results demonstrate that even cells at great distance from the anode, or within expected low-pH zones, are metabolically active and likely to contribute to current production and that there are factors other than gene expression differences influencing the distribution of OmcZ. This real-time reporter approach is likely to be a useful tool in optimizing the design of technologies relying on microbe-electrode interactions.

}, keywords = {Bacterial Proteins, Bioelectric Energy Sources, Biofilms, Citrate (si)-Synthase, Cytochrome c Group, Fimbriae Proteins, Fimbriae, Bacterial, Gene Expression Regulation, Bacterial, Geobacter, Quaternary Ammonium Compounds}, issn = {1864-564X}, doi = {10.1002/cssc.201100714}, author = {Franks, Ashley E and Glaven, Richard H and Lovley, Derek R} } @article {3148, title = {The Rnf complex of Clostridium ljungdahlii is a proton-translocating ferredoxin:NAD+ oxidoreductase essential for autotrophic growth.}, journal = {mBio}, volume = {4}, year = {2012}, month = {2012 Dec 26}, pages = {e00406-12}, abstract = {

UNLABELLED: It has been predicted that the Rnf complex of Clostridium ljungdahlii is a proton-translocating ferredoxin:NAD(+) oxidoreductase which contributes to ATP synthesis by an H(+)-translocating ATPase under both autotrophic and heterotrophic growth conditions. The recent development of methods for genetic manipulation of C. ljungdahlii made it possible to evaluate the possible role of the Rnf complex in energy conservation. Disruption of the C. ljungdahlii rnf operon inhibited autotrophic growth. ATP synthesis, proton gradient, membrane potential, and proton motive force collapsed in the Rnf-deficient mutant with H(2) as the electron source and CO(2) as the electron acceptor. Heterotrophic growth was hindered in the absence of a functional Rnf complex, as ATP synthesis, proton gradient, and proton motive force were significantly reduced with fructose as the electron donor. Growth of the Rnf-deficient mutant was also inhibited when no source of fixed nitrogen was provided. These results demonstrate that the Rnf complex of C. ljungdahlii is responsible for translocation of protons across the membrane to elicit energy conservation during acetogenesis and is a multifunctional device also implicated in nitrogen fixation.

IMPORTANCE: Mechanisms for energy conservation in the acetogen Clostridium ljungdahlii are of interest because of its potential value as a chassis for the production of biocommodities with novel electron donors such as carbon monoxide, syngas, and electrons derived from electrodes. Characterizing the components implicated in the chemiosmotic ATP synthesis during acetogenesis by C. ljungdahlii is a prerequisite for the development of highly productive strains. The Rnf complex has been considered the prime candidate to be the pump responsible for the formation of an ion gradient coupled with ATP synthesis in multiple acetogens. However, experimental evidence for a proton-pumping Rnf complex has been lacking. This study establishes the C. ljungdahlii Rnf complex as a proton-translocating ferredoxin:NAD(+) oxidoreductase and demonstrates that C. ljungdahlii has the potential of becoming a model organism to study proton translocation, electron transport, and other functions of the Rnf complex in energy conservation or other processes.

}, keywords = {Adenosine Triphosphate, Autotrophic Processes, Clostridium, Energy Metabolism, Fructose, Gene Knockout Techniques, Genes, Essential, Nitrogen, Operon, Oxidoreductases, Proton-Motive Force}, issn = {2150-7511}, doi = {10.1128/mBio.00406-12}, author = {Tremblay, Pier-Luc and Zhang, Tian and Dar, Shabir A and Leang, Ching and Lovley, Derek R} } @article {411, title = {Role of the NiFe hydrogenase Hya in oxidative stress defense in Geobacter sulfurreducens.}, journal = {J Bacteriol}, volume = {194}, year = {2012}, month = {2012 May}, pages = {2248-53}, abstract = {Geobacter sulfurreducens, an Fe(III)-reducing deltaproteobacterium found in anoxic subsurface environments, contains 4 NiFe hydrogenases. Hyb, a periplasmically oriented membrane-bound NiFe hydrogenase, is essential for hydrogen-dependent growth. The functions of the three other hydrogenases are unknown. We show here that the other periplasmically oriented membrane-bound NiFe hydrogenase, Hya, is necessary for growth after exposure to oxidative stress when hydrogen or a highly limiting concentration of acetate is the electron source. The beneficial impact of Hya on growth was dependent on the presence of H(2) in the atmosphere. Moreover, the Hya-deficient strain was more sensitive to the presence of superoxide or hydrogen peroxide. Hya was also required to safeguard Hyb hydrogen oxidation activity after exposure to O(2). Overexpression studies demonstrated that Hya was more resistant to oxidative stress than Hyb. Overexpression of Hya also resulted in the creation of a recombinant strain better fitted for exposure to oxidative stress than wild-type G. sulfurreducens. These results demonstrate that one of the physiological roles of the O(2)-resistant Hya is to participate in the oxidative stress defense of G. sulfurreducens.}, keywords = {Bacterial Proteins, Gene Expression Regulation, Bacterial, Geobacter, Hydrogen Peroxide, Hydrogenase, Mutation, Oxidative Stress, Oxygen, Reactive Oxygen Species, Reverse Transcriptase Polymerase Chain Reaction, Xanthine Oxidase}, issn = {1098-5530}, doi = {10.1128/JB.00044-12}, author = {Tremblay, Pier-Luc and Lovley, Derek R} } @article {447, title = {Role of Geobacter sulfurreducens outer surface c-type cytochromes in reduction of soil humic acid and anthraquinone-2,6-disulfonate.}, journal = {Appl Environ Microbiol}, volume = {76}, year = {2010}, month = {2010 Apr}, pages = {2371-5}, abstract = {Deleting individual genes for outer surface c-type cytochromes in Geobacter sulfurreducens partially inhibited the reduction of humic substances and anthraquinone-2,6,-disulfonate. Complete inhibition was obtained only when five of these genes were simultaneously deleted, suggesting that diverse outer surface cytochromes can contribute to the reduction of humic substances and other extracellular quinones.}, keywords = {Anthraquinones, Bacterial Proteins, Cytochromes, Gene Deletion, Genes, Bacterial, Geobacter, Humic Substances, Oxidation-Reduction, Soil, Soil Microbiology}, issn = {1098-5336}, doi = {10.1128/AEM.02250-09}, author = {Voordeckers, James W and Kim, Byoung-Chan and Izallalen, Mounir and Lovley, Derek R} } @article {506, title = {Reclassification of Trichlorobacter thiogenes as Geobacter thiogenes comb. nov.}, journal = {Int J Syst Evol Microbiol}, volume = {57}, year = {2007}, month = {2007 Mar}, pages = {463-6}, abstract = {Reclassification of the species Trichlorobacter thiogenes as Geobacter thiogenes comb. nov. is proposed on the basis of physiological traits and phylogenetic position. Characteristics additional to those provided in the original description revealed that the type strain (strain K1(T)=ATCC BAA-34(T)=JCM 14045(T)) has the ability to use Fe(III) as an electron acceptor for acetate oxidation and has an electron donor and acceptor profile typical of a Geobacter species, contains abundant c-type cytochromes, and has a temperature optimum of 30 degrees C and a pH optimum near pH 7.0; traits typical of members of the genus Geobacter. Phylogenetic analysis of nifD, recA, gyrB, rpoB, fusA and 16S rRNA genes further indicated that T. thiogenes falls within the Geobacter cluster of the family Geobacteraceae. Based on extensive phylogenetic evidence and the fact that T. thiogenes has the hallmark physiological characteristics of a Geobacter species, Trichlorobacter thiogenes should be reclassified as a member of the genus Geobacter.}, keywords = {DNA, Bacterial, DNA, Ribosomal, Genes, Bacterial, Geobacter, Phylogeny, RNA, Ribosomal, 16S, Temperature}, issn = {1466-5026}, doi = {10.1099/ijs.0.63408-0}, author = {Nevin, Kelly P and Holmes, Dawn E and Woodard, Trevor L and Covalla, Sean F and Lovley, Derek R} } @article {511, title = {Role of RelGsu in stress response and Fe(III) reduction in Geobacter sulfurreducens.}, journal = {J Bacteriol}, volume = {188}, year = {2006}, month = {2006 Dec}, pages = {8469-78}, abstract = {Geobacter species are key members of the microbial community in many subsurface environments in which dissimilatory metal reduction is an important process. The genome of Geobacter sulfurreducens contains a gene designated rel(Gsu), which encodes a RelA homolog predicted to catalyze both the synthesis and the degradation of guanosine 3{\textquoteright},5{\textquoteright}-bispyrophosphate (ppGpp), a regulatory molecule that signals slow growth in response to nutrient limitation in bacteria. To evaluate the physiological role of Rel(Gsu) in G. sulfurreducens, a rel(Gsu) mutant was constructed and characterized, and ppGpp levels were monitored under various conditions in both the wild-type and rel(Gsu) mutant strains. In the wild-type strain, ppGpp and ppGp were produced in response to acetate and nitrogen deprivation, whereas exposure to oxygen resulted in an accumulation of ppGpp alone. Neither ppGpp nor ppGp could be detected in the rel(Gsu) mutant. The rel(Gsu) mutant consistently grew to a higher cell density than the wild type in acetate-fumarate medium and was less tolerant of oxidative stress than the wild type. The capacity for Fe(III) reduction was substantially diminished in the mutant. Microarray and quantitative reverse transcription-PCR analyses indicated that during stationary-phase growth, protein synthesis genes were up-regulated in the rel(Gsu) mutant and genes involved in stress responses and electron transport, including several implicated in Fe(III) reduction, were down-regulated in the mutant. The results are consistent with a role for Rel(Gsu) in regulating growth, stress responses, and Fe(III) reduction in G. sulfurreducens under conditions likely to be prevalent in subsurface environments.}, keywords = {Bacterial Proteins, Culture Media, Ferric Compounds, Gene Expression Regulation, Bacterial, Geobacter, Guanosine Tetraphosphate, Heat-Shock Response, Ligases, Mutation, Oligonucleotide Array Sequence Analysis, Oxidation-Reduction, Reverse Transcriptase Polymerase Chain Reaction, Sulfur}, issn = {0021-9193}, doi = {10.1128/JB.01278-06}, author = {DiDonato, Laurie N and Sullivan, Sara A and Meth{\'e}, Barbara A and Nevin, Kelly P and England, Reg and Lovley, Derek R} } @article {537, title = {Regulation of two highly similar genes, omcB and omcC, in a 10 kb chromosomal duplication in Geobacter sulfurreducens.}, journal = {Microbiology}, volume = {151}, year = {2005}, month = {2005 Jun}, pages = {1761-7}, abstract = {The Fe(III)-reducing micro-organism Geobacter sulfurreducens requires an outer-membrane c-type cytochrome, OmcB, for Fe(III) reduction, but a related cytochrome, OmcC, which is 73 \% identical to OmcB, is not required. The omcB and omcC genes are part of a tandem chromosomal duplication consisting of two repeated clusters of four genes. The 2.7 kb sequences preceding omcB and omcC are identical with the exception of a single base pair change. Studies that combined genetic, Northern blotting and primer extension analyses demonstrated that both omcB and omcC are transcribed as monocistronic and polycistronic (orf1-orf2-omcB/omcC) transcripts. All of the promoters for the various transcripts were found to be located within the 2.7 kb identical region upstream of omcB and omcC. The sequences of the promoter regions for the two monocistronic transcripts are identical and equidistant from the omcB or omcC start codons. The promoters for the two polycistronic transcripts, in contrast, are distinct. One is specific for transcription of orf1-orf2-omcB and the other is associated with transcription of orf1-orf2-omcC. Studies with an RpoS-deficient mutant suggested that transcription from all four promoters is RpoS dependent under one or more growth conditions. Deletion of orfR, a gene immediately upstream of orf1-orf2-omcB that encodes a putative transcriptional regulator, significantly lowered the omcB transcription when Fe(III) was the electron acceptor and partially inhibited Fe(III) reduction. In contrast, levels of omcC transcripts were unaffected in the orfR mutant. These results indicate that omcB and omcC operons represent a rare instance in which duplicated operons, located in tandem on the chromosome, have different transcriptional regulation.}, keywords = {Bacterial Outer Membrane Proteins, Bacterial Proteins, Blotting, Northern, DNA, Bacterial, Gene Deletion, Gene Expression Regulation, Bacterial, Geobacter, Molecular Sequence Data, Operon, Promoter Regions, Genetic, RNA, Bacterial, RNA, Messenger, Sequence Analysis, DNA, Sigma Factor, Transcription, Genetic}, issn = {1350-0872}, doi = {10.1099/mic.0.27870-0}, author = {Leang, Ching and Lovley, Derek R} } @article {528, title = {Remediation and recovery of uranium from contaminated subsurface environments with electrodes.}, journal = {Environ Sci Technol}, volume = {39}, year = {2005}, month = {2005 Nov 15}, pages = {8943-7}, abstract = {Previous studies have demonstrated that Geobacter species can effectively remove uranium from contaminated groundwater by reducing soluble U(VI) to the relatively insoluble U(IV) with organic compounds serving as the electron donor. Studies were conducted to determine whether electrodes might serve as an alternative electron donor for U(VI) reduction by a pure culture of Geobacter sulfurreducens and microorganisms in uranium-contaminated sediments. Electrodes poised at -500 mV (vs a Ag/AgCl reference) rapidly removed U(VI) from solution in the absence of cells. However, when the poise at the electrode was removed, all of the U(VI) returned to solution, demonstrating that the electrode did not reduce U(VI). If G. sulfurreducens was present on the electrode, U(VI) did not return to solution until the electrode was exposed to dissolved oxygen. This suggeststhat G. sulfurreducens on the electrode reduced U(VI) to U(IV) which was stably precipitated until reoxidized in the presence of oxygen. When an electrode was placed in uranium-contaminated subsurface sediments, U(VI) was removed and recovered from groundwater using poised electrodes. Electrodes emplaced in flow-through columns of uranium-contaminated sediments readily removed U(VI) from the groundwater, and 87\% of the uranium that had been removed was recovered from the electrode surface after the electrode was pulled from the sediments. These results suggest that microorganisms can use electrons derived from electrodes to reduce U(VI) and that it may be possible to remove and recover uranium from contaminated groundwater with poised electrodes.}, keywords = {Biodegradation, Environmental, Electrodes, Geobacter, Graphite, Soil Pollutants, Uranium, Water Pollutants, Chemical}, issn = {0013-936X}, author = {Gregory, Kelvin B and Lovley, Derek R} } @article {542, title = {Resistance of solid-phase U(VI) to microbial reduction during in situ bioremediation of uranium-contaminated groundwater.}, journal = {Appl Environ Microbiol}, volume = {70}, year = {2004}, month = {2004 Dec}, pages = {7558-60}, abstract = {Speciation of solid-phase uranium in uranium-contaminated subsurface sediments undergoing uranium bioremediation demonstrated that although microbial reduction of soluble U(VI) readily immobilized uranium as U(IV), a substantial portion of the U(VI) in the aquifer was strongly associated with the sediments and was not microbially reducible. These results have important implications for in situ uranium bioremediation strategies.}, keywords = {Acetates, Biodegradation, Environmental, Deltaproteobacteria, Fresh Water, Geologic Sediments, Oxidation-Reduction, Solubility, Uranium, Water Pollutants, Radioactive}, issn = {0099-2240}, doi = {10.1128/AEM.70.12.7558-7560.2004}, author = {Ortiz-Bernad, Irene and Anderson, Robert T and Vrionis, Helen A and Lovley, Derek R} } @article {549, title = {The RpoS sigma factor in the dissimilatory Fe(III)-reducing bacterium Geobacter sulfurreducens.}, journal = {J Bacteriol}, volume = {186}, year = {2004}, month = {2004 Aug}, pages = {5543-6}, abstract = {Geobacter sulfurreducens RpoS sigma factor was shown to contribute to survival in stationary phase and upon oxygen exposure. Furthermore, a mutation in rpoS decreased the rate of reduction of insoluble Fe(III) but not of soluble forms of iron. This study suggests that RpoS plays a role in regulating metabolism of Geobacter under suboptimal conditions in subsurface environments.}, keywords = {Adaptation, Physiological, Bacterial Proteins, Base Sequence, Ferric Compounds, Gene Expression Regulation, Bacterial, Genes, Bacterial, Geobacter, Molecular Sequence Data, Mutation, Oxidation-Reduction, Oxidative Stress, Sigma Factor, Transcription Initiation Site}, issn = {0021-9193}, doi = {10.1128/JB.186.16.5543-5546.2004}, author = {N{\'u}{\~n}ez, Cinthia and Adams, Lorrie and Childers, Susan and Lovley, Derek R} } @article {567, title = {Rhodoferax ferrireducens sp. nov., a psychrotolerant, facultatively anaerobic bacterium that oxidizes acetate with the reduction of Fe(III).}, journal = {Int J Syst Evol Microbiol}, volume = {53}, year = {2003}, month = {2003 May}, pages = {669-73}, abstract = {To further investigate the diversity of micro-organisms capable of conserving energy to support growth from dissimilatory Fe(III) reduction, Fe(III)-reducing micro-organisms were enriched and isolated from subsurface sediments collected in Oyster Bay, VA, USA. A novel isolate, designated T118(T), was recovered in a medium with lactate as the sole electron donor and Fe(III) as the sole electron acceptor. Cells of T1 18(T) were Gram-negative, motile, short rods with a single polar flagellum. Strain T1 18(T) grew between pH 6.7 and 7.1, with a temperature range of 4-30 degrees C. The optimal growth temperature was 25 degrees C. Electron donors utilized by strain T1 18(T) with Fe(III) as the sole electron acceptor included acetate, lactate, malate, propionate, pyruvate, succinate and benzoate. None of the compounds tested was fermented. Electron acceptors utilized with either acetate or lactate as the electron donor included Fe(III)-NTA (nitrilotriacetic acid), Mn(IV) oxide, nitrate, fumarate and oxygen. Phylogenetic analysis demonstrated that strain T1 18(T) is most closely related to the genus Rhodoferax. Unlike other species in this genus, strain T1 18(T) is not a phototroph and does not ferment fructose. However, phototrophic genes may be present but not expressed under the experimental conditions tested. No Rhodoferax species have been reported to grow via dissimilatory Fe(III) reduction. Based on these physiological and phylogenetic differences, strain T1 18(T) (=ATCC BAA-621(T) = DSM 15236(T)) is proposed as a novel species, Rhodoferax ferrireducens sp. nov.}, keywords = {Acetates, Anaerobiosis, Bacterial Typing Techniques, Betaproteobacteria, Cold Temperature, Culture Media, DNA, Ribosomal, Electron Transport, Ferric Compounds, Fresh Water, Molecular Sequence Data, Oxidation-Reduction, Phylogeny, RNA, Ribosomal, 16S, Sequence Analysis, DNA}, issn = {1466-5026}, author = {Finneran, Kevin T and Johnsen, Claudia V and Lovley, Derek R} } @article {579, title = {Rapid evolution of redox processes in a petroleum hydrocarbon-contaminated aquifer.}, journal = {Ground Water}, volume = {40}, year = {2002}, month = {2002 Jul-Aug}, pages = {353-60}, abstract = {Ground water chemistry data collected over a six-year period show that the distribution of contaminants and redox processes in a shallow petroleum hydrocarbon-contaminated aquifer has changed rapidly over time. Shortly after a gasoline release occurred in 1990, high concentrations of benzene were present near the contaminant source area. In this contaminated zone, dissolved oxygen in ground water was depleted, and by 1994 Fe(III) reduction and sulfate reduction were the predominant terminal electron accepting processes. Significantly, dissolved methane was below measurable levels in 1994, indicating the absence of significant methanogenesis. By 1996, however, depletion of solid-phase Fe(III)-oxyhydrox ides in aquifer sediments and depletion of dissolved sulfate in ground water resulted in the onset of methanogenesis. Between 1996 and 2000, water-chemistry data indicated that methanogenic metabolism became increasingly prevalent. Molecular analysis of 16S-rDNA extracted from sediments shows the presence of a more diverse methanogenic community inside as opposed to outside the plume core, and is consistent with water-chemistry data indicating a shift toward methanogenesis over time. This rapid evolution of redox processes reflects several factors including the large amounts of contaminants, relatively rapid ground water flow (approximately 0.3 m/day [approximately foot/day]), and low concentrations of microbially reducible Fe(III) oxyhydroxides ( approximately 1 micromol/g) initially present in aquifer sediments. These results illustrate that, under certain hydrologic conditions, redox conditions in petroleum hydrocarbon-contaminated aquifers can change rapidly in time and space, and that the availability of solid-phase Fe(III)-oxyhydroxides affects this rate of change.}, keywords = {Hydrocarbons, Iron, Methane, Oxidation-Reduction, Petroleum, Soil Pollutants, Solubility, Sulfates, Water Pollutants, Water Supply}, issn = {0017-467X}, author = {Chapelle, Francis H and Bradley, Paul M and Lovley, Derek R and O{\textquoteright}Neill, Kathleen and Landmeyer, James E} } @article {585, title = {Reduction of Fe(III) oxide by methanogens in the presence and absence of extracellular quinones.}, journal = {Environ Microbiol}, volume = {4}, year = {2002}, month = {2002 Feb}, pages = {115-24}, abstract = {Five methanogens (Methanosarcina barkeri MS, Methanosphaera cuniculi 1R7, Methanobacterium palustre F, Methanococcus voltaei A3 and Methanolobus vulcani PL-12/M) were investigated for their ability to reduce Fe(III) oxide and the soluble quinone anthraquinone-2,6-disulphonate (AQDS). Two species (M. barkeri and M. voltaei) reduced significant amounts of Fe(III) oxide using hydrogen as the electron donor, and 0.1 mM AQDS greatly accelerated Fe(III) reduction by these organisms. Although Fe(III) appeared to inhibit growth and methanogenesis of some strains, hydrogen partial pressures under donor-limited conditions were much lower (<0.5 Pa) in the presence of Fe(III) than in normal media (1-10 Pa) for all species except for M. vulcani. These results demonstrate that electrons were transferred to Fe(III) by hydrogen-utilizing methanogens even when growth and methanogenesis were inhibited. All species except the obligate methylotroph M. vulcani were able to reduce AQDS when their growth substrates were present as electron donors, and rates were highest when organisms used hydrogen as the electron donor. Purified soil humic acids could also be reduced by the AQDS-reducing methanogens. The ability of methanogens to interact with extracellular quinones, humic acids and Fe(III) oxides raises the possibility that this functional group of organ-isms contributes to Fe(III) and humic acid reduction under certain conditions in the environment and provides an alternative explanation for the inhibition of methanogenesis in some Fe(III)-containing ecosystems.}, keywords = {Anthraquinones, Euryarchaeota, Ferric Compounds, Humic Substances, Oxidation-Reduction, Species Specificity}, issn = {1462-2912}, author = {Bond, Daniel R and Lovley, Derek R} }