Microbial electron donors

Reductive reactions typically occur in anaerobic environments where there is an abundant supply of electron donors. Electron donors are typically of microbial origin, eg, porphyrins or cysteine, which sometimes leads to confusion regarding the nature, ie, chemical vs enzymatic, of the reductive reaction. By definition, all reductive reactions which are not enzymatically catalyzed are chemical. The most significant chemical reductive reaction is reductive dechlorination. 

The primary metabolism of an organic compound uses a substrate as a source of carbon and energy. For the microorganism, this substrate serves as an electron donor, which results in the growth of the microbial cell. The application of co-metabolism for bioremediation of a xenobiotic is necessary because the compound cannot serve as a source of carbon and energy due to the nature of the molecular structure, which does not induce the required catabolic enzymes. Co-metabolism has been defined as the metabolism of a compound that does not serve as a source of carbon and energy or as an essential nutrient, and can be achieved only in the presence of a primary (enzyme-inducing) substrate. 

The microbial catabolic processes, which proceed in wastewater, provide the biomass with energy. These processes include two process steps oxidation of organic matter and reduction of an electron acceptor. The entire oxidation-reduction process, or redox process, consists basically of transfer of electrons from the electron donor (the organic matter) to the relevant electron acceptor, i.e., from the oxidation step to the reduction step. 

The following exemplifies how the total balance of a redox reaction is completed taking an electron donor and an electron acceptor (cf. the outline of the total redox reaction in Figure 2.3). Example 2.2 is, in this respect, used as an example of an (microbial) oxidation of an electron donor (organic matter) under anoxic conditions, i.e., with reduction of NO3 as electron acceptor (cf. Example 2.4). 

The microbial processes in the sewer interact across the boundaries of the subsystems. Exchange of substrate (electron donors as well as electron acceptors) and biomass between these subsystems proceeds. 

In a system defined by wastewater in a sewer network, the heterotrophic bacteria dominate the microbial community, i.e., organic compounds are required as a carbon source. Furthermore, the energy source (electron donor) for the heterotrophs is primarily also organic compounds, i.e., the heterotrophs that dominate wastewater in sewers are chemoheterotrophic (chemoorganotrophic) microorganisms. 

The microbial transformations of the wastewater described in the concept shown in Figure 5.5 deal with the COD components defined in Section 3.2.6. The figure also depicts the major processes that include the transformations of the organic matter (the electron donors) in the two subsystems of the sewer the suspended wastewater phase and the sewer biofilm. The air-water oxygen transfer (the reaeration) provides the aerobic microbial processes with the electron acceptor (cf. Section 4.4). Sediment processes are omitted in the concept but are indirectly taken into account in terms of a biofilm at the sediment surface. Water phase/biofilm exchange of electron donors and dissolved oxygen is included in the description. 

The OUR is an activity-related quantitative measure of the aerobic biomass influence on the relationship between the electron donor (organic substrate) and the electron acceptor (dissolved oxygen, DO). It is a measure of the flow of electrons through the entire process system under aerobic conditions (Figure 2.2). The OUR versus time relationship of wastewater samples from sewers becomes a backbone for analysis of the microbial system. This relationship is crucial for characterization of the suspended wastewater phase in terms of COD components and corresponding kinetic and stoichiometric parameters of in-sewer processes. 

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

Ghiorse WC (1989) Manganese and iron as physiological electron donors and acceptors in aerobic-anaerobic transition zones. In Microbial mats. Cohen Y, Rosenberg E (eds) ASM Press, Washington DC, p 163-179... 

Ktlsel K, Dorsch T. 2000. Effect of supplemental electron donors on the microbial reduction of Fe(III), sulfate, and CO2 in coal mining-impacted freshwater lake sediments. Microb Ecol 40 238 9. 

Recent research has identified some other microbial routes for denitrification that are not heterotrophic. One, called the anammox reaction, involves the oxidation of ammonium to N2 using either nitrite or nitrate as the electron donor. The second has bacteria using Mn " to reduce nitrate to N2. As noted earlier, N2 is generated by the oxidation of ammonium using Mn02 as the electron acceptor. [Denitrification may also be supported by Fe " (aq) oxidation.] These reactions are summarized in Table 12.2. The overall consequence of these reactions is that ammonium does not accumulate in the pore waters where Mn respiration and denitrification are occurring. 

Holm KA. Automated determination of microbial peroxidase activity in fermentation samples using hydrogen peroxide as the substrate and 2,2 -azino-bis(3-ethylbenzothiazoUne-6-sulfo-nate) as the electron donor in a flow-injection system. Analyst 1995 120 2101-2105. 

Various hydroquinones have been used as model electron donors to study both abiotic degradation pathways (22-25) and microbial respiration (26). However, since quinones rather than hydroquinones are stable under aerobic conditions, the common form of contaminants is quinonoid and the pathway of primary environmental interest is reduction of quinones to the hydroquinone (i.e., the reverse of Equation 3). 

Nedwell, D.B., and Abram, J.W. (1979) Relative influence of temperature and electron donor and electron acceptor concentrations on bacterial sulfate reduction in saltmarsh sediment. Microbial. Ecol. 5, 67-72. 

The first reaction is catalyzed by a mixed microbial population in which Bacillus azotoformans is the main denitrifying bacteria, and ethanol is the electron donor. Fe(II) is also regenerated from Fe(III) by means of reducing bacteria Dcfcrri-bacteres, using ethanol as the electron donor. These bacteria are found in the Veendam and Eerbeek sludges, as described in a previous section. 

Transformation by a single microbial species Some geomicrobial transformations in nature involve a single microbial species. An example of such a transformation is the anaerobic reduction of a Mn(IV) oxide to Mn " " by S. oneidensis or G. metallireducens in an environment with a plentiful supply of an appropriate electron donor like lactate for S. oneidensis or acetate for G. metallireducens. Because each of these two microbial species can perform the reduction by themselves, and because electron donors like lactate and acetate are formed as major end-products in the energy metabolism of a variety of microbes present in the same environment that harbours S. oneidensis or G. metallireducens, the latter do not need to form specific microbial associations to bring about Mn(IV) oxide reduction. 

In various species of bacteria several different types of non-assimilatory nitrite reductases are found. Escherichia coli has a cytoplasmic NAD(P)H-dependent enzyme whose role seems to be detoxification of nitrite. This type of enzyme, coded for by the nirB gene, also contains siroheme as the redox active catalytic center (Cole, 1988). Additionally in E. coli, and expressed under different conditions to the cytoplasmic enzyme, is a periplasmic nitrite reductase that catalyses formation of ammonia from nitrite (Cole, 1988). This enzyme has five c-type (Figure 1) hemes per polypeptide chain one of these hemes, the catalytic site, has the unique CXXCK sequence as its attachment site (Einsle et al., 1999). Electrons reach this type of nitrite reductase, which is fairly widely distributed amongst the microbial world, from the cytoplasmic membrane electron transfer chain. The exact electron donor partner from such chains for this type of nitrite reductase is unknown (Berks et al., 1995). 

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