@article {3074,
	title = {A Membrane-Bound Cytochrome Enables  To Conserve Energy from Extracellular Electron Transfer.},
	journal = {mBio},
	volume = {10},
	year = {2019},
	month = {2019 Aug 20},
	abstract = {<p>Extracellular electron exchange in  species and closely related  plays an important role in the global carbon cycle and enhances the speed and stability of anaerobic digestion by facilitating efficient syntrophic interactions. Here, we grew  with methanol provided as the electron donor and the humic analogue, anthraquione-2,6-disulfonate (AQDS), provided as the electron acceptor when methane production was inhibited with bromoethanesulfonate. AQDS was reduced with simultaneous methane production in the absence of bromoethanesulfonate. Transcriptomics revealed that expression of the gene for the transmembrane, multiheme, -type cytochrome MmcA was higher in AQDS-respiring cells than in cells performing methylotrophic methanogenesis. A strain in which the gene for MmcA was deleted failed to grow via AQDS reduction but grew with the conversion of methanol or acetate to methane, suggesting that MmcA has a specialized role as a conduit for extracellular electron transfer. Enhanced expression of genes for methanol conversion to methyl-coenzyme M and the Rnf complex suggested that methanol is oxidized to carbon dioxide in AQDS-respiring cells through a pathway that is similar to methyl-coenzyme M oxidation in methanogenic cells. However, during AQDS respiration the Rnf complex and reduced methanophenazine probably transfer electrons to MmcA, which functions as the terminal reductase for AQDS reduction. Extracellular electron transfer may enable the survival of methanogens in dynamic environments in which oxidized humic substances and Fe(III) oxides are intermittently available. The availability of tools for genetic manipulation of  makes it an excellent model microbe for evaluating -type cytochrome-dependent extracellular electron transfer in  The discovery of a methanogen that can conserve energy to support growth solely from the oxidation of organic carbon coupled to the reduction of an extracellular electron acceptor expands the possible environments in which methanogens might thrive. The potential importance of -type cytochromes for extracellular electron transfer to syntrophic bacterial partners and/or Fe(III) minerals in some  was previously proposed, but these studies with  provide the first genetic evidence for cytochrome-based extracellular electron transfer in  The results suggest parallels with Gram-negative bacteria, such as  and  species, in which multiheme outer-surface -type cytochromes are an essential component for electrical communication with the extracellular environment.  offers an unprecedented opportunity to study mechanisms for energy conservation from the anaerobic oxidation of one-carbon organic compounds coupled to extracellular electron transfer in  with implications not only for methanogens but possibly also for  that anaerobically oxidize methane.</p>},
	keywords = {Acetates, Anthraquinones, Cytochromes, Electron Transport, Electrons, Ferric Compounds, Gene Expression Regulation, Archaeal, Gram-Negative Bacteria, Membranes, Mesna, Methane, Methanol, Methanosarcina, Oxidation-Reduction, Oxidoreductases, Transcriptome},
	issn = {2150-7511},
	doi = {10.1128/mBio.00789-19},
	author = {Holmes, Dawn E and Ueki, Toshiyuki and Tang, Hai-Yan and Zhou, Jinjie and Smith, Jessica A and Chaput, Gina 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 {492,
	title = {Fluorescent properties of c-type cytochromes reveal their potential role as an extracytoplasmic electron sink in Geobacter sulfurreducens.},
	journal = {Environ Microbiol},
	volume = {10},
	year = {2008},
	month = {2008 Feb},
	pages = {497-505},
	abstract = {A novel fluorescence technique for monitoring the redox status of c-type cytochromes in Geobacter sulfurreducens was developed in order to evaluate the capacity of these extracytoplasmic cytochromes to store electrons during periods in which an external electron acceptor is not available. When intact cells in which the cytochromes were in a reduced state were excited at a wavelength of 350 nm, they fluoresced with maxima at 402 and 437 nm. Oxidation of the cytochromes resulted in a loss of fluorescence. This method was much more sensitive than the traditional approach of detecting c-type cytochromes via visible light absorbance. Furthermore, fluorescence of reduced cytochromes in individual cells could be detected via fluorescence microscopy, and the cytochromes in a G. sulfurreducens biofilm, remotely excited with an optical fibre, could be detected at distances as far as 5 cm. Fluorescence analysis of cytochrome oxidation and reduction of the external electron acceptor, anthraquinone-2,6-disulfonate, suggested that the extracytoplasmic cytochromes of G. sulfurreducens could store approximately 10(7) electrons per cell. Independent analysis of the haem content of the cells determined from analysis of incorporation of (55)Fe into cytochromes provided a similar estimate of cytochrome electron-storage capacity. This electron-storage capacity could, in the absence of an external electron acceptor, permit continued electron transfer across the inner membrane sufficient to supply the maintenance energy requirements for G. sulfurreducens for up to 8 min or enough proton motive force to power flagella motors for G. sulfurreducens motility. The fluorescence approach described here provides a sensitive method for evaluating the redox status of Geobacter species in culture and/or its environments. Furthermore, these results suggest that the periplasmic and outer-membrane cytochromes of Geobacter species act as capacitors, allowing continued electron transport, and thus viability and motility, for Geobacter species as they move between heterogeneously dispersed Fe(III) oxides during growth in the subsurface.},
	keywords = {Anthraquinones, Cytochromes c, Electron Transport, Ferric Compounds, Flagella, Fluorescence, Geobacter, Microscopy, Fluorescence, Oxidation-Reduction},
	issn = {1462-2920},
	doi = {10.1111/j.1462-2920.2007.01470.x},
	author = {Esteve-N{\'u}{\~n}ez, Abraham and Sosnik, Julian and Visconti, Pablo and Lovley, Derek R}
}
@article {555,
	title = {Identification of an uptake hydrogenase required for hydrogen-dependent reduction of Fe(III) and other electron acceptors by Geobacter sulfurreducens.},
	journal = {J Bacteriol},
	volume = {186},
	year = {2004},
	month = {2004 May},
	pages = {3022-8},
	abstract = {Geobacter sulfurreducens, a representative of the family Geobacteraceae that predominates in Fe(III)-reducing subsurface environments, can grow by coupling the oxidation of hydrogen to the reduction of a variety of electron acceptors, including Fe(III), fumarate, and quinones. An examination of the G. sulfurreducens genome revealed two operons, hya and hyb, which appeared to encode periplasmically oriented respiratory uptake hydrogenases. In order to assess the roles of these two enzymes in hydrogen-dependent growth, Hya- and Hyb-deficient mutants were generated by gene replacement. Hyb was found to be required for hydrogen-dependent reduction of Fe(III), anthraquinone-2,6-disulfonate, and fumarate by resting cell suspensions and to be essential for growth with hydrogen and these three electron acceptors. Hya, in contrast, was not. These findings suggest that Hyb is an essential respiratory hydrogenase in G. sulfurreducens.},
	keywords = {Anthraquinones, Ferric Compounds, Fumarates, Geobacter, Hydrogen, Operon, Oxidation-Reduction, Oxidoreductases, Phenotype},
	issn = {0021-9193},
	author = {Coppi, Maddalena V and O{\textquoteright}Neil, Regina A and Lovley, Derek R}
}
@article {588,
	title = {Electrode-reducing microorganisms that harvest energy from marine sediments.},
	journal = {Science},
	volume = {295},
	year = {2002},
	month = {2002 Jan 18},
	pages = {483-5},
	abstract = {Energy in the form of electricity can be harvested from marine sediments by placing a graphite electrode (the anode) in the anoxic zone and connecting it to a graphite cathode in the overlying aerobic water. We report a specific enrichment of microorganisms of the family Geobacteraceae on energy-harvesting anodes, and we show that these microorganisms can conserve energy to support their growth by oxidizing organic compounds with an electrode serving as the sole electron acceptor. This finding not only provides a method for extracting energy from organic matter, but also suggests a strategy for promoting the bioremediation of organic contaminants in subsurface environments.},
	keywords = {Aerobiosis, Anaerobiosis, Anthraquinones, Benzoates, Biodegradation, Environmental, Carbon Dioxide, Colony Count, Microbial, Deltaproteobacteria, DNA, Ribosomal, Electricity, Electrodes, Electrons, Energy Metabolism, Geologic Sediments, Oxidation-Reduction, RNA, Ribosomal, 16S, Seawater, Sodium Acetate},
	issn = {1095-9203},
	doi = {10.1126/science.1066771},
	author = {Bond, Daniel R and Holmes, Dawn E and Tender, Leonard M and Lovley, Derek R}
}
@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}
}
@article {605,
	title = {Humics as an electron donor for anaerobic respiration.},
	journal = {Environ Microbiol},
	volume = {1},
	year = {1999},
	month = {1999 Feb},
	pages = {89-98},
	abstract = {The possibility that microorganisms might use reduced humic substances (humics) as an electron donor for the reduction of electron acceptors with a more positive redox potential was investigated. All of the Fe(III)- and humics-reducing microorganisms evaluated were capable of oxidizing reduced humics and/or the reduced humics analogue anthrahydroquinone-2,6,-disulphonate (AHODS), with nitrate and/or fumarate as the electron acceptor. These included Geobacter metallireducens, Geobacter sulphurreducens, Geothrix fermentans, Shewanella alga, Wolinella succinogenes and {\textquoteright}S. barnesii{\textquoteright}. Several of the humics-oxidizing microorganisms grew in medium with AHQDS as the sole electron donor and fumarate as the electron acceptor. Even though it does not reduce Fe(III) or humics, Paracoccus denitrificans could use AHQDS and reduced humics as electron donors for denitrification. However, another denitrifier, Pseudomonas denitrificans, could not. AHODS could also serve as an electron donor for selenate and arsenate reduction by W. succinogenes. Electron spin resonance studies demonstrated that humics oxidation was associated with the oxidation of hydroquinone moieties in the humics. Studies with G. metallireducens and W. succinogenes demonstrated that the anthraquinone-2,6-disulphonate (AQDS)/AHQDS redox couple mediated an interspecies electron transfer between the two organisms. These results suggest that, as microbially reduced humics enter less reduced zones of soils and sediments, the reduced humics may serve as electron donors for microbial reduction of several environmentally significant electron acceptors.},
	keywords = {Aerobiosis, Anaerobiosis, Anthraquinones, Arsenates, Bacteria, Electron Transport, Fumarates, Humic Substances, Selenium Compounds, Tumor Cells, Cultured},
	issn = {1462-2912},
	author = {Lovley, D R and Fraga, J L and Coates, J D and Blunt-Harris, E L}
}
@article {625,
	title = {Recovery of humic-reducing bacteria from a diversity of environments.},
	journal = {Appl Environ Microbiol},
	volume = {64},
	year = {1998},
	month = {1998 Apr},
	pages = {1504-9},
	abstract = {To evaluate which microorganisms might be responsible for microbial reduction of humic substances in sedimentary environments, humic-reducing bacteria were isolated from a variety of sediment types. These included lake sediments, pristine and contaminated wetland sediments, and marine sediments. In each of the sediment types, all of the humic reducers recovered with acetate as the electron donor and the humic substance analog, 2,6-anthraquinone disulfonate (AQDS), as the electron acceptor were members of the family Geobacteraceae. This was true whether the AQDS-reducing bacteria were enriched prior to isolation on solid media or were recovered from the highest positive dilutions of sediments in liquid media. All of the isolates tested not only conserved energy to support growth from acetate oxidation coupled to AQDS reduction but also could oxidize acetate with highly purified soil humic acids as the sole electron acceptor. All of the isolates tested were also able to grow with Fe(III) serving as the sole electron acceptor. This is consistent with previous studies that have suggested that the capacity for Fe(III) reduction is a common feature of all members of the Geobacteraceae. These studies demonstrate that the potential for microbial humic substance reduction can be found in a wide variety of sediment types and suggest that Geobacteraceae species might be important humic-reducing organisms in sediments.},
	keywords = {Acetic Acid, Anthraquinones, Base Sequence, DNA Primers, Electron Transport, Fresh Water, Gram-Negative Anaerobic Bacteria, Humic Substances, Iron, Molecular Sequence Data, Phylogeny, Polymerase Chain Reaction, RNA, Bacterial, RNA, Ribosomal, 16S, Seawater, Sulfur-Reducing Bacteria, Water Microbiology},
	issn = {0099-2240},
	author = {Coates, J D and Ellis, D J and Blunt-Harris, E L and Gaw, C V and Roden, E E and Lovley, D R}
}
