Sediment electron shuttle

A proposed electron shuttle In which the oxidative power of O2 can be conveyed below the oxic zone In marine sediments. 

Biological reductive dissolution by Shewanella putrifaciens of Fe oxides in material from four Atlantic pleistocene sediments (ca. 1.5-41 g/kg Fe oxides) was compared with that of the synthetic analogues (ferrihydrite, goethite, hematite) (Zachara et al. 1998). In the presence of AQDS as an electron shuttle, the percentage of bio-reduc-tion of the three oxides was increased from 13.3 %(fh) 9.2%(gt) and 0.6%(hm) to 94.6% 32.8% and 9.9% with part of the Fe formed being precipitated as vivianite and siderite, but not as magnetite. The quinone was reduced to hydroquinone which in turn, and in agreement with thermodynamics, reduced the Fe as it had much better access to the oxide surface than did the bacteria themselves. 

Nevin, K.P. Lovley, D.R. (2000) Potential for nonenzymatic reduction of Fe(III) via electron shuttling in subsurface sediments. Environ. Sci. Techn. 34 2472-2479 Nickel, E.H. Mandarino, J.A. (1977) Mineral nomenclature. Am. Min. 62 188 Nightingale, E.R. Benck, R.F. (1960) Precipitation of crystalline iron(IIl) oxide from homogeneous solution. Anal. Chem. 32 566-567... 

Variability in the effectiveness of humic substances as electron shuttling compounds is expected due to differences in their chemical structure. For example, the electron-accepting capacity of humic substances from three distinct sources varied nearly 700-fold in the following order soils > sediments > dissolved river-borne (Scott et al., 1998 Figure 19). Microorganisms are a source of quinone moities and other electron shuttling compounds. For example, S. oneidensis... 

Organic compounds have the potential to abiotically reduce Fe(III) and Mn(IV) (Luther et al., 1992 Stone, 1987). Phenols and a variety of other aromatic compounds reduce Fe(III) rapidly at acidic pH, but slowly at circum neutral pH (LaKind and Stone, 1989). Humics can reduce Fe(III) effectively at circumneutral pH and they are abundant in soils and sediments. Because humics and other organic compounds often serve as electron shuttles between metal-reducing bacteria and metal oxides (Lovley et al., 1996a), it may be difficult to separate microbial and nonmicrobial sources of electrons. Finally, aerobic photoreduction of Fe(III) has been observed in freshwater and marine environments (Barbeau et al., 2001 Emmenegger et al., 2001), but it is unknown to what degree this process... 

Nevin K. P. and Lovley D. R. (2000) Potential for none-nzymatic reduction of Fe(lll) via electron shuttling in subsurface sediments. Environ. Sci. Technol. 34, 2472-2478. 

Kappler, A., M. Benz, B. Schink, and A. Brune. 2004. Electron shuttling via humic acids in microbial iron(III) reduction in a freshwater sediment. Microb. Ecol. 47 85-92. 

These compounds are common in soils and sediments as the products of anaerobic fermentation reactions. Metabolism of these compounds can directly consume O2 (through monooxygenase enzymes) or indirectly, through formation of NADH which is shuttled into oxidative phosphorylation and the electron transport chain. Oxidative metabolism of Ci compounds is an important microbial process in soils and sediments, consuming the methane produced by methanogenesis. 

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