Geobacter metallireducens

Reduction of monocyclic aromatic nitro compounds has been demonstrated (a) with reduced sulfur compounds mediated by a naphthoquinone or an iron porphyrin (Schwarzenbach et al. 1990), and (b) by Fe(II) and magnetite produced by the action of the anaerobic bacterium Geobacter metallireducens (Heijman et al. 1993). Quinone-mediated reduction of monocyclic aromatic nitro compounds by the supernatant monocyclic aromatic nitro compounds has been noted (Glaus et al. 1992), and these reactions may be signihcant in determining the fate of aromatic nitro compounds in reducing environments (Dunnivant et al. 1992). 

Geobacter metallireducens is also able to grow by vanadate respiration supported by acetate, and it has been suggested that this could provide a new strategy for removing vanadate from groundwater (Ortiz-Bernad et al. 2004b). 

The microbial degradation of contaminants under anaerobic conditions using humic acids as electron acceptors has been demonstrated. These included the oxidations (a) chloroethene and 1,2-dichloroethene to CO2 that was confirmed using C-labeled substrates (Bradley et al. 1998) and (b) toluene to CO2 with AQDS or humic acid as electron acceptors (Cervantes et al. 2001). The transformation of l,3,5-trinitro-l,3,5-triazine was accomplished using Geobacter metallireducens and humic material with AQDS as electron shuttle (Kwon and Finneran 2006). 

The Function of Humic Acids in Reactions Catalyzed by Geobacter metallireducens... 

Nevin KP, Lovley DR (2000) Lack of production of electron-shuttling compounds or solubilization of Fe(III) during reduction of insoluble Fe(III) oxide by Geobacter metallireducens. App Environ Microbio 66 2248-2251... 

Since both Geobacter metallireducens and Geobacter sulfurreducens have triheme cyt C7 proteins (Champine et al. 2000, Afkar and Fukumori 1999 Seeliger et al. 1998), it is appropriate to consider that these electron carriers function in metal reduction in a manner similar to that reported for the cyt C7 of D. acetooxidans. The role of other cytochromes in Geobacter has not been explored with respect to metal reduction. 

Afkar E, Fukumori Y. 1999. Purification and characterization of triheme cytochrome c from the metal-redncing bacterinm, Geobacter metallireducens. FEMS Microbiol Lett 175 205-10. 

Champine JE, Underhill B, Johnson JM, et al. 2000. Electron transfer in the dissimilatory iron-reducing bacterium Geobacter metallireducens. Anaerobe 6 187-96. 

Lovley DR, Giovannoni SJ, White DC, et al. 1993a. 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. Arch Microbiol 159 336-14. 

Hence, these Qc values are a quantitative measure for the relative affinities of the various NACs to the reactive sites. Figs. 14.10e and/show plots of log Qc versus h(AtN02)/0.059 V of the 10 monosubstituted benzenes. A virtually identical picture was obtained for the log Qc values derived from an aquifer solid column and from a column containing FeOOH-coated sand and a culture of the iron-reducing bacterium, Geobacter metallireducens (GS15). Furthermore, a similar pattern (Fig. 14.10c) was found when correlating relative initial pseudo-first-order rate constants determined for NAC reduction by Fe(II) species adsorbed to iron oxide surfaces (Fig. 14.12) or pseudo-first-order reaction constants for reaction with an iron porphyrin (data not shown see Schwarzenbach et al., 1990). Fig. 14.12 shows that Fe(II) species adsorbed to iron oxide surfaces are very potent reductants, at least for NACs tv2 of a few minutes in the experimental system considered). 

Ring and aryl methyl group oxidation were the initial toluene-degradation routes speculated on for the nitrate-reducing enrichment obtained by Kuhn etal. (1988) and the metabolically diverse iron-reducing bacterium Geobacter metallireducens (Lovley Lonergan, 1990). The speculation was consistent with the fact that both of these cultures could metabolize the appropriate suite of putative intermediates. However, conclusive evidence as to which pathway was actually involved was not obtained. 

As previously mentioned (p. 7), bacteria like S. oneidensis and G. metallireducens have the capacity to reduce insoluble Mn02 to soluble Mn + enzymatically by anaerobic respiration with a suitable electron donor in a direct process in which the respective organisms attach to the surface of the oxide. In the case of S. oneidensis, the electron donor may be lactate, pyruvate, formate or H2, but not acetate. The lactate and pyruvate are oxidized to acetate as end-product. Geobacter metallireducens can use butyrate, propionate, lactate and acetate as electron donors, but not H2 or formate, and oxidizes the organic electron donors completely to CO2 and H2O. Geobacter sulfurreducens can use H2 as electron donor in Mn02 reduction (see Lovley, 2000). 

Childers S. E., Ciufo S., and Lovley D. R. (2002) Geobacter metallireducens accesses insoluble Fe(III) oxide by chemo-taxis. Nature 416, 767-769. 

Senko J. M. and Stolz J. F. (2001) Evidence for iron-dependent nitrate respiration in the dissimulatory iron-reducing bacterium Geobacter metallireducens. Appl. Environ. Microbiol. 67, 3750-3752. 

Geobacter metallireducens (soluble green vanadylphosphate precipitate) Acetate 140... 

Geobacter metallireducens is able to oxidize acetate to C02 under anaerobic conditions in the presence of Fe(III) (Section 4.3.3). A study of the intermediate role of humic and fulvic acids used ESR to detect and quantify free radicals produced from oxidized humic acids by cells of G. metallireducens in the presence of acetate. There was a substantial increase in the radical concentration after incubation with the cells, and it was plausibly suggested that these were semiquinones produced from quinone entities in the humic and fulvic structures (Scott et al. 1998). 

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