Methanogenesis electron donors

Complete dechlorination of high concentrations of tetrachloroethene in the absence of methanogenesis has been achieved nsing methanol as electron donor (DiStefano et al. 1991). 

The final step in methanogenesis is the reductive demethylation of CH3-S-CoM to CH4. This reduction involves two reactions CH3-S-C0M is reduced with jV-7-mercaptoheptanoylthreonine phosphate (H-S-HTP) (Fig. 2B) as electron donor to yield CH4 and a heterodisulfide of H-S-CoM and H-S-HTP (CoM-S-S-HTP) (Reaction 7, Table 2). This reaction is catalyzed by CH3-S-C0M reductase [70-72] which contains a nickel porphinoid, factorF430, as prosthetic group (Fig. 2C) (for a recent review see Friedmann et al. [73]). The subsequent reduction of the heterodisulfide with H2 to yield H-S-HTP and H-S—CoM (Reaction 8, Table 2) is catalyzed by CoM-S-S-HTP-dependent heterodisulfide reductase. The enzyme is an iron-sulfur protein containing FAD as prosthetic group [74]. The physiological electron donor for the heterodisulfide reductase is not known. 

Competition for fermentation products produces a succession of dominant metabolic pathways as distance from the source of electron donors and acceptors increases. Aerobic metabolism dominates the surface of sediments, and methanogenesis the deeper depths, as suggested by their free energy yield (Table 1). There is often very little overlap between each zone, suggesting nearly complete exclusion of one group by another. The same pattern is observed with distance from the surface of a root or burrow, or with distance downstream from an organic pollutant source in rivers and aquifers. 

Spatial variation in the abundance of electron donors and acceptors explains large-scale and small-scale patterns of anaerobic metabolism. Sulfate reduction dominates anaerobic carbon metabohsm on about two-thirds of the planet because of the high abundance of SO4 in seawater (Capone and Kiene, 1988). Fe(III) reduction is important in aU anaerobic ecosystems with mineral-dominated soils or sediments, regardless of whether they are marine or freshwater (Thamdrup, 2000). Methanogenesis is important in freshwater environments generally, and it dominates the anaerobic carbon metabolism of bogs, fens, and other wetlands that exist on organic (i.e., peat) soils. 

In the absence of O2, competition among anaerobic microbes for electron donors sets up a series of alternative terminal electron-accepting processes in the order NO reduction, Mn(lV) reduction, Fe(III) reduction, SO -- reduction, and methanogenesis (Figure 9.2 Ponnamperuma, 1972 Megonigal et al., 2004). To a lirsi approximation, a single terminal electron-accepting process dominates... 

Hydrogenotrophic methanogenesis is associated with the reduction of COj carbon. In this case the substrate is formic acid HCOOH or CO and the electron donor is H ... 

Hydrogenotrophic methanogenesis are archaea, which, using H as electron donor, are reducing carbon of CO, HCOj or formiat HCOO and forming CH ... 

Internal Sources and Atmospheric Exchange of Methane. Methane is produced by specialized groups of obligate anaerobic bacteria (22, 23). The formation of methane as a metabolic product results either from the microbial reduction of CO2 with molecular H2, or via the fermentation of acetic acid. More structurally complex substrates may also serve as electron acceptors/donors, but the end result of methanogenesis is to produce methane and CO2 as end products (23). 

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