Electron acceptors chlorate

Chlorate can serve as electron acceptor under anaerobic conditions (Thorell et al. 2003 Coates et al. 1999), and chlorate reductase has been found both in organisms such as Proteus mirabilis that can reduce chlorate but is unable to use to couple this to growth, and in true chlorate-respiring organisms. 

Malmqvist A, T Welander, L Gunnarsson (1991) Anaerobic growth of microorganisms with chlorate as an electron acceptor. Appl Environ Microbiol 57 2229-2232. 

Malmqvist A, T Wellander, E Moore, A Temstrom, G Molin, I-M Stenstrom (1994) Ideonella dechlorans gen. nov., sp. nov., a new bacterium capable of growing anaerobically with chlorate as electron acceptor. System Appl Microbiol 17 58-64. 

The selenate reductase from Enterobacter cloacae SLDla-1 functions only under aerobic conditions, and is not able to serve as an electron acceptor for anaerobic growth, in contrast to the periplasmic enzyme from Thauera selenatis (Schroder et al. 1997). In E. cloacae there are separate nitrate and selenate reductases, both of which are membrane-bound. The selenate reductase is able to reduce chlorate and bromate though not nitrate, contains Mo, heme and nonheme iron, and consists of three subunits in an a3p3y3 configuration. 

The chlorate reductase has been characterized in strain GR-1 where it was found in the periplasm, is oxygen-sensitive, and contains molybdenum, and both [3Fe-4S] and [4Fe-4S] clusters (Kengen et al. 1999). The arsenate reductase from Chrysiogenes arsenatis contains Mo, Fe, and acid-labile S (Krafft and Macy 1998), and the reductase from Thauera selenatis that is specific for selenate, is located in the periplasmic space, and contains Mo, Fe, acid-labile S, and cytochrome b (Schroeder et al. 1997). In contrast, the membrane-bound selenate reductase from Enterobacter cloacae SLDla-1 that cannot function as an electron acceptor under anaerobic conditions contains Mo and Fe and is distinct from nitrate reductase (Ridley et al. 2006). 

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. 

Nitrite oxidoreductase has been purified also from Nitrobacter hamburgensis (Sundermeyer-Klinger et al., 1984) and Nitrospira moscoviensis (Spieck et al., 1998). The N. hamburgensis enzyme has been reported to have heme C but not to have heme A. It does not catalyze the reduction of horse ferricytochrome c with nitrite although it catalyzes the reduction of ferricyanide. The N. moscoviensis enzyme has been reported to have heme B but not to have either heme A or heme C. Although the N. moscoviensis enzyme catalyzes the oxidation of nitrite with chlorate as the electron acceptor, it has not been reported whether cytochrome c or ferricyanide acts as the electron acceptor for the enzyme. The enzyme of this bacterium is known to be located in the periplasmic space. Table 3.3 lists some properties of nitrite oxidoreductase purified from three species of the nitrite-oxidizing bacteria. 

The MBfR is another process variation and typically referred to a membrane biofilm reactor where hydrogen is the electron acceptor. A large number of bacteria can use hydrogen as an electron acceptor, and the delivery of hydrogen via a membrane offers an efficient and safe solution. A number of studies have been undertaken for the hydrogenotrophic reduction of pollutants such as perchlorate, chlorate, chlorite, bromate, chromate, selenate, selenite, arsenate, and dichlo-romethane. A recent review of the technology is provided by Martin and Nerenberg [67]. 

Nitrate ion (NOp is reduced to N2O, NHsOH", or NH4 by two-electron donors, Zn, Sn(II), and so forth, and to NO2 or NO by one-electron donors such as Cu, Fe(II), Ti(III), and VO +. Similarly, chlorate ion (C10 ) is reduced to C102(gas) by one-electron donors and to Cl by two-electron donors. In each case it appears that two-electron donors can bypass stable odd-electron molecules by donating electrons in pairs to the acceptor, even when several successive donations are required to reach the final product. Hydrazine is oxidized by one-electron acceptors by the following mechanism ... 

Apart from metallic salts, simultaneous chemical synthesis and doping of polypyrrole has been achieved by an halogenic electron acceptor, as bromine or iodine, in several solvents [27-31]. Both PPy-l2 and PPy-Br2 complexes have conductivities in the order of 1 to 30 S cm [30]. PPy-Cl2 prepared polymers have conductivities from 10 to 0.5 S cm [27]. The loss of conductivity with respect to PPy-Br2 or PPy-h has been associated with a partial chloration of the pyrrole ring, with some loss of conjugation. Both PPy-l2 and PPy-Br2 complexes show a good stability upon repeated redox cycling in both aqueous and organic electrolytes [31]. 

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