Elizabeth L. Blunt-Harris

Quinone Moieties Act as Electron Acceptors in the Reduction of Humic Substances by Humics-Reducing Microorganisms

D U R E L L E T . S C O T T , * , † D I A N E M . M C K N I G H T , † E L I Z A B E T H L . B L U N T H A R R I S , ‡ S A R A H E . K O L E S A R , † , § A N D D E R E K R . L O V L E Y ‡ Institute of Arctic and Alpine Research, University of Colorado, Campus Box 450, Boulder, Colorado 80309-0450, Department of Microbiology, University of Massachusetts, Amherst, Massachusetts 01003, and Center for Environmental and Estuarine Studies, University of Maryland, Solomons, Maryland

Microbiological Evidence for Fe(III) Reduction on Early Earth

It is generally considered that sulphur reduction was one of the earliest forms of microbial respiration, because the known microorganisms that are most closely related to the last common ancestor of modern life are primarily anaerobic, sulphur-reducing hyperthermophiles. However, geochemical evidence indicates that Fe(III) is more likely than sulphur to have been the first external electron acceptor of global significance in microbial metabolism. Here we show that Archaea and Bacteria that are most closely related to the last common ancestor can reduce Fe(III) to Fe(II) and conserve energy to support growth from this respiration. Surprisingly, even Thermotoga maritima, previously considered to have only a fermentative metabolism, could grow as a respiratory organism when Fe(III) was provided as an electron acceptor. These results provide microbiological evidence that Fe(III) reduction could have been an important process on early Earth and suggest that microorganisms might contribute to Fe(III) reduction in modern hot biospheres. Furthermore, our discovery that hyperthermophiles that had previously been thought to require sulphur for cultivation can instead be grown without the production of toxic and corrosive sulphide, should aid biochemical investigations of these poorly understood organisms.

Humic substances as a mediator for microbially catalyzed metal reduction

The potential for humic substances to serve as a terminal electron acceptor in microbial respiration and to function as an electron shuttle between Fe(III)-reducing microorganisms and insoluble Fe(III) oxides was investigated. The Fe(III)-reducing microorganism Geobacter metallireducens conserved energy to support growth from electron transport to humics as evidenced by continued oxidation of acetate to carbon dioxide after as many as nine transfers in a medium with acetate as the electron donor and soil humic acids as the electron acceptor. Growth of G. metallireducens with poorly crystalline Fe(III) oxide as the electron acceptor was greatly stimulated by the addition of as little as 100 μM of the humics analog, anthraquinone-2,6-disulfonate. Other quinones investigated, including lawsone, menadione, and anthraquinone-2-sulfonate, also stimulated Fe(III) oxide reduction. A wide phylogenetic diversity of microorganisms capable of Fe(III) reduction were also able to transfer electrons to humics. Microorganisms which can not reduce Fe(III) could not reduce humics. Humics stimulated the reduction of structural Fe(III) in clay and the crystalline Fe(III) forms, goethite and hematite. These results demonstrate that electron shuttling between Fe(III)-reducing microorganisms and Fe(III) via humics not only accelerates the microbial reduction of poorly crystalline Fe(III) oxide, but also can facilitate the reduction of Fe(III) forms that are not typically reduced by microorganisms in the absence of humics. Addition of humic substances to enhance electron shuttling between Fe(III)-reducing microorganisms and Fe(III) oxides may be a useful strategy to stimulate the remediation of soils and sediments contaminated with organic or metal pollutants. n n n nHuminstoffe als Vermittler bei der mikrobiell katalysierten Metallreduktion n n n nEs wurde untersucht, inwieweit Huminstoffe als terminale Elektronenakzeptoren bei der mikrobiellen Atmung und als Vermittler bei der Elektronenubertragung zwischen Fe(III)-reduzierenden Mikroorganismen und unloslichen Fe(III)-oxiden fungieren konnen. Das Fe(III)-reduzierende Bakterium Geobacter metallireducens gewinnt Energie zum Wachstum aus der Elektronenubertragung auf Huminstoffe. Das wurde offensichtlich, als nach 9 aufeinanderfolgenden Transfers des Bakteriums auf frisches Medium mit Acetat als Elektronendonor und Boden-Huminstoff als Elektronenakzeptor seine Fahigkeit zur Oxidation von Acetat zu CO2 erhalten blieb. Das Wachstum von G. metallireducens mit niedrigkristallinem Fe(III)-oxid als Elektronenakzeptor konnte durch den Zusatz des Huminstoff-Analogen Anthrachinon-2,6-disulfonat bereits in Konzentrationen von 100 μmol/L deutlich stimuliert werden. Auch weitere untersuchte Chinone wie z.B. Lawson (2-Hydroxy-1,4-naphthochinon), Menadion (2-Methyl-1,4-naphthochinon) und Anthrachinon-2-sulfonat stimulierten die Fe(III)-oxid-Reduktion. Eine grose Anzahl phylogenetisch unterschiedlicher Mikroorganismen, die zur Fe(III)-Reduktion befahigt sind, zeigten gleichzeitig die Fahigkeit zum Elektronentransfer auf Huminstoffe. Zur Fe(III)-Reduktion nicht befahigte Mikroorganismen konnten auch Huminstoffe nicht reduzieren. Durch Huminstoffe konnte die Reduktion von Fe(III) stimuliert werden, das in die Struktur von Tonmineralen und in kristalline Formen des Fe(III)-oxids, Goethit und Hamatit, eingebaut ist. Diese Ergebnisse zeigen, das durch die vermittelnde Funktion der Huminstoffe bei der Elektronenubertragung zwischen Fe(III)-reduzierenden Mikroorganismen und Fe(III) nicht nur die mikrobielle Reduktion von niedrigkristallinem Fe(III)-oxid beschleunigt wird, sondern auch die Reduktion von solchen Formen des Fe(III) erleichtert wird, welche im allgemeinen in Abwesenheit von Huminstoffen durch Fe(III)-reduzierende Mikroorganismen nicht reduziert werden. Die Zugabe von Huminstoffen zur Verbesserung der Elektronenubertragung zwischen Fe(III)-reduzierenden Mikroorganismen und Fe(III)-oxiden konnte eine nutzliche Strategie zur Stimulierung der Sanierung von mit organischen oder metallischen Kontaminanten verunreinigten Boden und Sedimenten sein.

Reduction of humic substances and Fe III by hyperthermophilic microorganisms

Abstract The ability of hyperthermophilic microorganisms to transfer electrons to humic substances (humics) and other extracellular quinones was evaluated. When H 2 was provided as the electron donor, the hyperthermophile, Pyrobaculum islandicum , transferred electrons to highly purified humics and the humics analog, anthraquinone-2,6-disulfonate (AQDS). A diversity of other hyperthermophilic Archaea including: Pyrodictium abyssi , Pyrococcus furiosus , Archaeoglobus fulgidus , Thermococcus celer , Methanopyrus kandleri , as well as the thermophiles Methanococcus thermolithitrophicus and Methanobacterium thermoautotrophicum , exhibited H 2 -dependent AQDS reduction as did the hyperthermophilic bacterium Thermotoga maritima . AQDS acted as an electron shuttle between P. islandicum and poorly crystalline Fe(III) oxide and greatly accelerated rates of Fe(III) reduction. Electron shuttling by AQDS also promoted the reduction of the crystalline Fe(III) oxide forms, goethite and hematite. These results have implications for the potential mechanisms of Fe(III) reduction in various hot Fe(III)-containing environments such as near hydrothermal marine vents, terrestrial hot springs, and the deep terrestrial subsurface. The finding that the ability to reduce extracellular quinones is a characteristic of all of the hyperthermophiles evaluated and the fact that these hyperthermophiles are the organisms most closely related to the last common ancestor of extant organisms suggests that the last common ancestor had the ability to reduce humics. In combination with plausible geochemical scenarios, these results suggest that electron transfer to extracellular quinones and Fe(III) were initial steps in the eventual evolution of intracellular electron transport chains that employ quinones and iron-containing proteins.