Alternate electron acceptors

Price-Carter M, J Tingey, TA Bobik, JR Roth (2001) The alternative electron acceptor tetrathionate supports Bjj-dependent anaerobic growth of Salmonella enterica serovar typhimurium on ethanolamine or 1,2-propandiol. J Bacterial 183 2463-2475. 

Arsenite is also an intermediate in the fungal biomethylation of arsenic (Bentley and Chasteen 2002) and oxidation to the less toxic arsenate can be accomplished by heterotrophic bacteria including Alcaligenes faecalis. Exceptionally, arsenite can serve as electron donor for chemolithotrophic growth of an organism designated NT-26 (Santini et al. 2000), and both selenate and arsenate can be involved in dissimilation reactions as alternative electron acceptors. 

In the absence of molecular oxygen, a nnmber of alternative electron acceptors may be used these include nitrate, sulfate, selenate, carbonate, chlorate, Fe(III), Cr(VI), and U(VI), and have already been discussed in Chapter 3, Part 2. In Chapter 14, which deals with applications, attention is directed primarily to the role of nitrate, sulfate, and Fe(III)— with only parenthetical remarks on Cr(VI) and U(VI). The role of nitrate and sulfate as electron acceptors for the degradation of monocyclic aromatic compounds is discnssed, and the particularly broad metabolic versatility of sulfate-reducing bacteria is worthy of notice. 

Azo dye-containing wastewaters seems to be one of the most polluted wastewaters, which require efficient decolorization and subsequent aromatic amine metabolism. On the basis of the available literature, it can be concluded that anaerobic-aerobic SBR operations are quite convenient for the complete biodegradation of both azo dyes and their breakdown products. Nevertheless, like the other methods used for biological treatment, SBRs treating colored wastewaters have some limitations. Presence of forceful alternative electron acceptors such as nitrate and oxygen, availability of an electron donor, microorganisms, and cycle times of anaerobic and aerobic reaction phases can be evaluated as quite significant. 

Other thiophene-thiophene-5,5-dioxide copolymers were reported by Berlin et al. [544], who synthesized copolymers 443 and 444 with an alternating electron acceptor thiophene-5,5-dioxide unit and donor ethylenedioxythiophene (EDOT) units (Chart 2.107). The polymers absorbed at 535 nm (Eg = 2.3 eV) in chloroform solution and in films (which is consistent with their electrochemistry Eox 0.40-0.50 V, Emd -1.75-1.8 V AE 2.2-2.25 V) and emitted at 650 nm (<1> M (film) 1%). Such a high band gap (which exceeds that in PEDOT... 

While 02 serves as the electron acceptor in aerobic biodegradation processes forming H20 as the final product, degradation in anaerobic systems depends on alternative electron acceptors such as sulfate, nitrate or carbonate, which yield, ultimately, hydrogen sulfide (H2S), molecular nitrogen (N2) and/or ammonia (NH3) and methane (CH4), respectively. 

Anaerobic bacterial reactions are much slower and produce only a fraction of the energy produced by aerobic activity. Alternate electron acceptors are generally consumed in a stepwise process ... 

In settings that do not contain oxygen or alternative electron acceptors, methane production is favored. Methane production is the final step in anaerobic decomposition of organic matter. 

In some undisturbed subsurface systems, an equilibrium is established. Bacteria have acclimated to food sources, water availability, and electron acceptor types. The number and variety of microbial cells are balanced in this system. If the system is aerobic, the microbial activity continues at the rate of oxygen resupply. If the system is anaerobic, the rate of activity cannot exceed the accessibility of alternate electron acceptors. Generally, the subsurface (lower than the plant root zone) is relatively deficient in available carbon and electron acceptors. Under these normal semi-equilibrium conditions, a soil or aquifer system can consume organic materials within a reasonable range. When a chemical release is introduced into a well-established soil system, the system must change to react to this new energy source. The bacterial balance readjusts, in an effort to acclimate to the new carbon source. 

Each acetogen displays a different propensity relative to the use of alternative electron acceptors. However, a few generalizations can be made. 

Reduced of oxidized elements, particularly when an alternate electron acceptor to is required. However, some reduction reactions occur in which the oxidized species is not needed as an electron acceptor... 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

In soils, electrons are produced by the metabolic activity of soil biota. These electrons are usually accepted by O2 dissolved in the soil solution which is then replaced by O2 from the soil air. Oxygen may, however, become deficient if all pores are filled with water as in waterlogged or compacted soils. Fe in Fe oxides may then function as an alternative electron acceptor and Fe ions will be formed according to eq. (16.3). The electrons are transferred from the decomposing biomass to the Fe oxide by microbially produced enzymes. Other potential electron acceptors in soils are nitrate, Mn and sulphate. 

Reinhard, M. (1993). In situ bioremediation technologies for petroleum- derived hydrocarbons based on alternate electron acceptors (other than molecular oxygen). In In Situ Bioremediation of Ground Water and Geological Material A Review of Technologies, ed. R. D. Norris et al., section 7, pp. 7.1-7.7. EPA/600/R-93/124. NTIS Document No. PB93-215564, Washington, DC. 

Haggblom, M. M., Rivera, M. D. Young, L. Y. (1993). Influence of alternative electron acceptors on the anaerobic biodegradability of chlorinated phenols and benzoic acids. applied and Environmental Microbiology, 59, 1162-7. 

Table 3.2 Alternative Electron Acceptors (redox couple) Used by Aerobic, Facultatively Anaerobic, and Obligately Anaerobic Bacteria at Neutral pH and the Associated Microbial Processes...

Organisms with anaerobic mitochondria can be divided into two different types those which perform anaerobic respiration and use an alternative electron acceptor present in the environment, such as nitrate or nitrite, and those which perform fermentation reactions using an endogenously produced, organic electron acceptor, such as fumarate (Martin et al. 2001 Tielens et al. 2002). An example of the first type is the nitrate respiration that occurs in several ciliates (Finlay et al. 1983), and fungi (Kobayashi et al. 1996 Takaya et al. 2003), which use nitrate and/or nitrite as the terminal electron acceptor of their mitochondrial electron-transport chain, producing nitrous oxide as... 

Several heterofermentative LAB produce mannitol in large amounts, using fructose as an electron acceptor. Mannitol produced by heterofermentative bacteria is derived from the hexose phosphate pathway (Soetaert et al., 1999 Wisselink et al., 2002). The process makes use of the capability of the bacterium to utilize fructose as an alternative electron acceptor, thereby reducing it to mannitol with the enzyme mannitol dehydrogenase. In this process, the reducing equivalents are generated by conversion of one-third fructose to lactic acid and acetic acid. The enzyme reaction proceeds according to (theoretical) Equation 21.1 ... 

Various bacteria grow anaerobically using trimethylamine-N-oxide (TMAO) as an alternative electron acceptor of a respiratory transport chain (31, 32). The energy-yielding reaction involves the conversion of TMAO to tetramethylamine (TMA) catalyzed by a TMAO reductase (TMAOR). 

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