Electron-accepting processes

The predominant electron-accepting process can affect the rate and extent of Dicamba degradation in anaerobic environments. 

In contrast, Meckenstock et al. [280] reported larger isotopic enrichments in residual toluene, 3-6%o and up to 10%o during anaerobic and aerobic biodegradation experiments, respectively. These results indicated that isotopic fractionation effects may be different for different compounds, terminal electron-accepting processes (TEAP), degradative metabolic pathways, or microbial populations. 

Table 3. Terminal electron accepting processes and typical related hydrogen concentrations.

Terminal electron accepting process Hydrogen (H2) concentration (nmol/1)... 

Second, the availability of CO2 can influence the ability of an acetogen to engage an alternative terminal electron-accepting process. However,... 

There are relatively few field hydrocarbon biodegradation studies in which methanogenesis was reported to be the dominant electron-accepting process. Wilson et al. (1990) have reported the loss of all three xylene isomers at a rate of 0.03/wk. However, the lack of tracers in this study make it more difficult to interpret. Therefore, the prospects for the methanogenic biodegradation of both met a and para xylene must be considered with caution. 

Chapelle F. H., McMahon P. B., Dubrovsky N. M., Fujii R. F., Oaksford E. T., and Vroblesky D. A. (1995) Deducing the distribution of terminal electron-accepting processes in hydrologically diverse groundwater systems. Water Resour. 

Vroblesky D. A. and ChapeUe F. H. (1994) Temporal and spatial changes of terminal electron-accepting processes in a petroleum hydrocarbon-contaminated aquifer and the significance for contaminant biodegradation. Water Resour. Res. 30, 1561-1570. 

Figure 27 Steady-state H2 concentrations in sediments with different dominant terminal electron accepting processes (Lovely and Goodwin, 1988) (reproduced by permission of Elsevier from Geochim. Cosmochim. Acta. 1988, 52, 2993-3003).

The order of competing terminal electron accepting processes can vary with any number of factors that influence the thermodynamics of the system. One factor that must be considered in ecosystems with mineral sediments or soils is the composition of Fe(III) and Mn(IV) minerals. The typical sequence of Fe(III) reduction before SOl reduction can be reversed with a change in the abundance of labile Fe(III) minerals such as ferrihydrite (Postma and Jakobsen, 1996). This is one explanation for the common observation that the zones of Fe(III) reduction and SOl reduction overlap in marine sediments (Boesen and Postma, 1988 Canfield, 1989 Canfield et al., 1993b Goldhaber et al., 1977 Jakobsen and Postma, 1994). Postma and Jakobsen (1996) predicted that the overlap between Fe(III) reduction and SOl reduction should increase as Fe(III) oxide stability (or surface area) increases. 

McGuire J. T., Smith E. W., Long D. T., Hyndman D. W., Haack S. K., Klug M. J., and Velbel M. A. (2000) Temporal variations in parameters reflecting terminal electron accepting processes in an aquifer contaminated with waste fuel and chlorinated solvents. Chem. Geol. 169, 471-485. 

Figure 8 Some examples of electron-transfer reactions of pesticide compounds in relation to dominant terminal electron accepting processes (TEAPs) in natural waters. TEAP sequence based on Christensen et al. (2000). Manganese, iron, sulfur, or carbon may be present in either the dissolved or soUd phase. Location of each compound indicates the condition(s) under which it has been found to be relatively stable in natural waters. Incomplete lists of transformation products denoted by ellipsis (...) compounds shown are those inferred to have been derived directly from the parent compound. One-way arrows denote essentially irreversible reactions two-way arrow denotes a reversible reaction. The various references cited are a, Egli et al. 1988 b, Picardal et al., 1995 c, Milligan and Haggblom, 1999 d, Tesoriero et al., 2001 e, Klecka et al., 1990 f, Gibson and Suflita, 1986 g, Adrian and Suflita, 1990 h. Miles and Delfino, 1985 i, Lightfoot era/., 1987 j, Nairand Schnoor, 1992 k, Papiemik and Spalding, 1998 ...

Harris S. H., Ulrich G. A., and Suflita J. M. (1999) Dominant Terminal Electron Accepting Processes Occuring at a Landfill Leachate-impacted Site as Indicated by Field and Laboratory Measurements. US Geological Survey Water Resources Investigations Report 99-4018C, Reston, VA. 

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... 

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