Anaerobic respiration electron donors

Coates JD, KA Cole, R Chakraborty, SM O Connor, LA Achenbach (2002) Diversity and ubiquity of bacteria capable of utilizing humic substances as electron donors for anaerobic respiration. Appl Environ Microbiol 68 2445-2452. 

Lovley, D. R., J. L. Fraga, J. D. Coates, and E. L. Blunt-Harris. 1999. Humics as an electron donor for anaerobic respiration. Environmental Microbiology 1 89—98. 

SQR (respiratory complex II) is involved in aerobic metabolism as part of the citric acid cycle and of the aerobic respiratory chain (Saraste, 1999). QFR participates in anaerobic respiration with fumarate as the terminal electron acceptor (Kroger, 1978 Kroger etal., 2002) and is part of the electron transport chain catalyzing the oxidation ofvarious donor substrates (e.g., H2 or formate) by fumarate. These reactions are coupled via an electrochemical proton potential (Ap) to ADP phosphorylation with inorganic phosphate by ATP synthase (Mitchell, 1979). 

Siderite (FeC03) has been found to accumulate extracellularly in some instances of anaerobic bacterial Fe(III) reduction (Fye, 1984 Pye et al., 1990 Coleman eta/., 1993 Ehrlich Wickert, 1997). The exact conditions under which the biogenic siderite forms in nature remain to be elucidated. A major or exclusive source of the carbonate in biogenic siderite is assumed to be the CO2 formed from organic electron donors used in the anaerobic respiration on Fe(III). 

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). 

A clearer picture has emerged for Complex II and crystal sfructures are available for the E. coli succinate quinone oxidoreductase (SQR) and closely related bacterial quinol frnnarate oxidoreductases (QFR) that catalyze the reverse reaction in order to use frnnarate as a terminal electron donor in anaerobic respiration. The sfructure of the SQR monomer showing the location of the redox cofactors is shown in Figure 7. The [2Fe-2S] + + (Tim = 4-10 mV), [4Fe-ASf+ + ( = -175 mV) and [3Fe S]+ ° Em = +65 mV) clusters in the SdhB subunit provide a linear electron transport chain for transferring electrons from the... 

The outcome of competition between microorganisms for electron donors can be predicted from thermodynamic theory (Section 8.08.1.2 Table 1 Zehnder and Stumm, 1988), and these predictions are generally consistent with empirical data. Temporal succession of the microbial metabolic pathways that dominate respiration occurs upon the flooding of an oxidized soil or sediment (Figure 1). Not surprisingly, most examples of temporal succession in anaerobic respiration processes have come from wetland soils, which are subject to cycles of flooding and exposure (Turner and Patrick, 1968 Ponnamperuma, 1972 Achtnich et al., 1995a Yao et al., 1999). However, the same pattern is observed in sediments and even upland soils (Peters and Conrad, 1996). 

This has been attributed to the anaerobic respiration by microorganisms like Desulfovibrio in seep sediments (Aharon, 2000) they use the abundant reduced carbon forms as electron donors and seawater-derived S042 as an electron acceptor. In addition to H2S, this metabolism can also produce carbonate species and ammonia whose concentration and type depend on the nature of the reduced carbon substrates and on the buffering capacity of the environment. Sulphate and H2S often show a linear, inverse relationship in profiles of seep sediment pore fluids, further indicating the link between sulphate reduction and H2S production (Aharon, 2000). 

It could be that the break between respiration and photosynthesis in these bacteria is more recent than we think. Cytochrome Ca has been suggested to have a respiratory as well as a photosynthetic role in R. spheroides (S72) and R. capsulata (372a-c) and no alternative respiratory chain has yet been identified in any of the Athiorhodaceae. In some of these organisms a situation may exist as in Fig. 46 with electrons flowing to both from light-excited bacteriochlorophyll and from external donors, and then from c either to an electron-depleted bacteriochlorophyll or to an oxidase molecule. This would account for the observed control mechanism in the purple nonsulfur bacteria. Under aerobic conditions in the dark, bacteriochlorophyll would not be electron-defi.cient, whereas the oxidase would be in its oxidized state and capable of accepting electrons from c. Under anaerobic conditions, electrons would reduce the oxidase, and further electron transfer down that path would be blocked. Light then would promote electrons away from bacteriochlorophyll and set cyclic photophosphorylation in motion. 

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