Reductive dehalogenation as a respiratory process

Abstract

Anaerobic bacteria can reductively dehalogenate aliphatic and aromatic halogenated compounds in a respiratory process. Only a few of these bacteria have been isolated in pure cultures. However, long acclimation periods, substrate specificity, high dehalogenation rates, and the possibility to enrich for the dehalogenation activity by subcultivation in media containing an electron donor indicate that many of the reductive dehalogenations in the environment are catalyzed by specific bacteria. Molecular hydrogen or formate appear to be good electron donors for the enrichment of such organisms. Furthermore, systems have to be employed which supply the cultures with the halogenated compounds beyond their toxicity level. All bacteria that are presently available in pure culture and grow with a halogenated compound as electron acceptor are members of new genera. Based on experimental results with the membrane-impermeable electron mediator methyl viologen, a model of the respiration system of Dehalobacter restrictus, a tetrachloroethene-dechlorinating bacterium, is presented. Further studies of the biochemistry and energetics of respiratory-dehalogenating strains will help to understand the mechanisms involved and perhaps reveal the evolutionary origin of the dehalogenating enzyme systems.

The proton/electron ration of the menaquinone-dependent electron transport from dihydrogen to tetrachloroethene in "Dehalobacter restrictus"

“…''Dehalobacter restrictus'' was isolated from a fixed-bed column fed with lactate and PCE (10) and exclusively utilizes H 2 as an electron donor and PCE and TCE as electron acceptors (11). The reducing equivalents derived from oxidation of H 2 have been shown to be quantitatively recovered in dechlorination products and biomass (10).…”

“…Preliminary experiments suggested that the two enzymes in ''D. restrictus'' are arranged in the third way (11). The formation of two protons at the outside of the cell upon H 2 oxidation by a periplasmically oriented hydrogenase and the concomitant transmembrane transport of two electrons to a cytoplasmically oriented PCE reductase were suggested as the mechanism to generate a proton electrochemical potential (11).…”

“…The first indications of the use of PCE as an electron acceptor supporting bacterial growth were obtained with enrichment cultures on benzoate and PCE (23) or on methanol and PCE (4). It was only recently that two anaerobic bacterial strains were isolated (10,11,24) which conserve their energy for growth by the oxidation of H 2 coupled to the reductive dechlorination of PCE to cis-1,2-dichloroethene (cis-DCE) and chloride as end products:…”

Demonstration of ISCO Treatment of a DNAPL Source Zone at Launch Complex 34 in Cape Canaveral Air Station

“…strain PCEl (Gerritse et al, 1996), Desulfitobacterium sp. strain PCE-S (Miller et al, 1997;Miller et al, 1998), Desulfomonile tiedjei (Fathepure et al, 1987;DeWeerd et al, 1990), Dehalobacter restrictus (HoUiger and Schumacher, 1994;Holliger et al, 1998), strain TT4B (Krumholz et al, 1996), and the facultative organism strain MS-1 (Sharma and McCarty, 1996). With the exception oiDehalococcoides ethenogenes which dechlorinates PCE to ethene, and Desulfitobacterium sp.…”

“…Both laboratory studies and field observations suggest that the addition of electron donors for the enhancement of dechlorination can induce complex scenarios that are a function of the subsurface conditions (Carr and Hughes, 1998;Fennell and Gossett, 1997) and the indigenous microbial population (Gibson and Sewell, 1992). Although it is known that hydrogen serves as the specific electron donor for reductive dechlorination (HoUiger et al, 1993;HoUiger and Schumacher, 1994;Maymo-Gatell et al, 1995), different concentrations of hydrogen stimulate different groups of anaerobic microbial populations which may or may not be responsible for dechlorination, and may out compete the halorespirers, making the direct addition of hydrogen problematic. In fact, recent research has indicated that dechlorinating bacteria possess lower half-velocity coefficients for Ha utilization than methanogens, suggesting that dechlorinating bacteria should out compete methanogens at low Ha concentrations (Ballapragadaetal., 1997;SmatlaketaL, 1996).…”