Elizabeth J. P. Phillips

Geobacter metallireducens gen. nov. sp. nov., a microorganism capable of coupling the complete oxidation of organic compounds to the reduction of iron and other metals

The gram-negative metal-reducing microorganism, previously known as strain GS-15, was further characterized. This strict anaerobe oxidizes several short-chain fatty acids, alcohols, and monoaromatic compounds with Fe(III) as the sole electron acceptor. Furthermore, acetate is also oxidized with the reduction of Mn (IV), U (VI), and nitrate. In whole cell suspensions, the c-type cytochrome(s) of this organism was oxidized by physiological electron acceptors and also by gold, silver, mercury, and chromate. Menaquinone was recovered in concentrations comparable to those previously found in gram-negative sulfate reducers. Profiles of the phospholipid ester-linked fatty acids indicated that both the anaerobic desaturase and the branched pathways for fatty acid biosynthesis were operative. The organism contained three lipopolysaccharide hydroxy fatty acids which have not been previously reported in microorganisms, but have been observed in anaerobic freshwater sediments. The 16S rRNA sequence indicated that this organism belongs in the delta proteobacteria. Its closest known relative is Desulfuromonas acetoxidans. The name Geobacter metallireducens is proposed.

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. Huminstoffe als Vermittler bei der mikrobiell katalysierten Metallreduktion Es 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.

Remediation of Uranium Contaminated Soils with Bicarbonate Extraction and Microbial U(VI) Reduction

SummaryA process for concentrating uranium from contaminated soils in which the uranium is first extracted with bicarbonate and then the extracted uranium is precipitated with U(VI)-reducing microorganisms was evaluated for a variety of uranuum-contaminated soils. Bicarbonate (100 mM) extracted 20–94% of the uranium that was extracted with nitric acid. The U(VI)-reducing microorganism,Desulfovibrio desulfuricans reduced the U(VI) to U(IV) in the bicarbonate extracts. In some instances unidentified dissolved extracted components, presumably organics, gave the extract a yellow color and inhibited U(VI) reduction and/or the precipitation of U(IV). Removal of the dissolved yellow material with the addition of hydrogen peroxide alleviated this inhibition. These results demonstrate that bicarbonate extraction of uranium from soil followed by microbial U(VI) reduction might be an effective mechanism for concentrating uranium from some contaminated soils.

Manganese inhibition of microbial iron reduction in anaerobic sediments

Abstract Potential mechanisms for the lack of Fe(II) accumulation in Mn(IV)‐con‐taining anaerobic sediments were investigated. The addition of Mn(IV) to sediments in which Fe(III) reduction was the terminal electron‐accepting process removed all the pore‐water Fe(II), completely inhibited net Fe(III) reduction, and stimulated Mn(IV) reduction. In a solution buffered at pH 7, Mn(IV) oxidized Fe(II) to amorphic Fe(III) oxide. Mn(IV) naturally present in oxic freshwater sediments also rapidly oxidized Fe(II). A pure culture of a dissimilatory FE(III)‐ and Mn(FV)‐reducing organism isolated from the sediments reduced Fe(III) to Fe(II) in the presence of Mn(IV) when ferrozine was present to trap Fe(II) before Mn(IV) oxidized it. Depth profiles of dissolved iron and manganese reported in previous studies suggest that Fe(II) diffusing up from the zone of Fe(III) reduction is consumed within the Mn(IV)‐reducing zone. These results demonstrate that preferential reduction of Mn(IV) by Fe(III)‐reducing bacteria canno...

Fe(III)-reducing bacteria in deeply buried sediments of the Atlantic Coastal Plain

The possibility that microorganisms are catalyzing the ongoing reduction of Fe(III) in the sediments of deep (20-250 m) aquifers was investigated. Acetate-oxidizing, Fe(III)-reducing bacteria were recovered from deep subsurface sediments, but only from sediments in which it appeared that Fe(III) reduction was the terminal electron-accepting process for oxidation of organic matter. The Fe(III)-reducing microorganisms were capable of reducing ferric oxides present in deep subsurface sediments. Although Fe(III) reduction in subsurface sediments is frequently regarded as an abiological reaction, the enzymatic reduction of Fe(III) by microorganisms reported here is the first mechanism of any kind actually shown to have the potential to couple the oxidation of organic matter to carbon dioxide with the reduction of Fe(III) under the environmental conditions typically found in deep aquifers. We propose that microbially catalyzed Fe(III) reduction is responsible for such late postdepositional phenomena as the formation of variegated red beds and the release of high concentrations of dissolved iron into anaerobic ground waters.

Release of226Ra from uranium mill tailings by microbial Fe(III) reduction

Uranium mill tailings were anaerobically incubated in the presence of H2 with Alteromonas putrefaciens, a bacterium known to couple the oxidation of H2 and organic compounds to the reduction of Fe(III) oxides. There was a direct correlation between the extent of Fe(III) reduction and the accumulation of dissolved226Ra. In sterile tailings in which Fe(III) was not reduced, there was negligible leaching of226Ra. The behavior of Ba was similar to that of Ra in inoculated and sterile systems. These results demonstrate that under anaerobic conditions, microbial reduction of Fe(III) may result in the release of dissolved226Ra from uranium mill tailings.