Bacteria reductive dechlorinators

Chlorinated hydrocarbons Organohalide respiring bacteria Reductive dechlorination... 

Certain anaerobic bacteria can reductively dechlorinate PCBs in sediments (EHC 140). Higher chlorinated PCBs are degraded more rapidly than lower chlorinated ones, which is in contrast to the trend for oxidative metabolism described earlier. Genetically engineered strains of bacteria have been developed to degrade PCBs in bioremediation programs. 

Holliger C, G Wohlfarth, G Diekert (1999) Reductive dechlorination in the energy metabolism of anaerobic bacteria. EEMS Microbiol Rev 22 383-398. 

Loffler FE, JE Champine, KM Ritalahti, SJ Sprague, JM Tiedje (1997) Complete reductive dechlorination of 1,2-dichloropropane by anaerobic bacteria. Appl Environ Microbiol 63 2870-2873. 

Numata M, N Nakamura, H Koshikawa, Y Terashima (2002) Chlorine isotope fractionation during reductive dechlorination of chlorinated ethenes by anaerobic bacteria. Environ Sci Technol 36 4389-4394. 

Tetrachoroethylene (perchloroethylene, PCE) is the only chlorinated ethene that resists aerobic biodegradation. This compound can be dechlorinated to less- or nonchlorinated ethenes only under anaerobic conditions. This process, known as reductive dehalogenation, was initially thought to be a co-metabolic activity. Recently, however, it was shown that some bacteria species can use PCE as terminal electron acceptor in their basic metabolism i.e., they couple their growth with the reductive dechlorination of PCE.35 Reductive dehalogenation is a promising method for the remediation of PCE-contaminated sites, provided that the process is well controlled to prevent the buildup of even more toxic intermediates, such as the vinyl chloride, a proven carcinogen. 

Newton R, Aranovich L (1996) Simple granulite melting in concentrated NaCl-KCl solutions at deep crustal conditions. Geol Soc Am Annu Meet Abstracts with Programs 158 Numata M, Nakamura N, Koshikawa H, Terashima Y (2002) Chlorine isotope fractionation during reductive dechlorination of chlorinated ethenes by anaerobic bacteria. Env Sci Tech 36(20) 4389-4394 Numata M, Nakamura N, Gamo T (2001) Precise measurement of chlorine stable isotopic ratios by thermal ionization mass spectrometry. Geochem J 35(2) 89-100 Owen HR, Schaeffer OA (1995) The isotope abundances of chlorine from various sources. J Am Chem Soc 77 898-899... 

Table 8.3. Bacteria that reductively dechlorinate chlorophenols...

In the presence of a strong reductant, abiotic reductive dechlorination of CPs is catalyzed by the reduced form of Vitamin B12 (Gantzer Wackett, 1991Smith Woods, 1994). These abiotic dechlorinations favor removal of m- and /(-chlorines, which differs substantially from reductive dechlorinations by anaerobic microbial consortia or by the CP-dehalogenating bacteria isolated to date. This indicates that abiotic dechlorinations of CPs are not central reactions in environmental or engineered anaerobic systems. 

Holliger C. and Schraa G. (1994) Physiological meaning and potential for apphcation of reductive dechlorination by anaerobic bacteria. FEMS Microbiol. Rev. 15, 297—305. 

Fig. 7.18 Reductive dechlorination of PCDDs by anaerobic bacteria (broken arrows show minor routes after Bunge et al. 2003). ( By convention, substituent positions bear the lowest possible numbers, so the loss of the Cl atom from C-l of 1,3-dichlorodibenzodioxin yields 2- rather than 3-chlorodibenzodioxin, because the structure can be rotated to bring the chlorine atom into the C-2 position, with reference to the numbering scheme in Fig. 7.17.)...

Another promising approach for the detoxification of PCBs is the finding that anaerobic bacteria dechlorinate PCBs reductively [79, 80]. The authors used anaerobic microorganisms from Hudson River sediment and report that, at PCB concentrations of 700 ppm Aroclor, 63 per cent of the total chlorine was removed in 16 weeks, and the proportion of mono- and dichlorobiphenyls increased from 9 to 88 per cent. Dechlorination occurred primarily from the meta and para positions. These results indicate that reductive dechlorination may be an important environmental fate of PCBs, and suggest that a sequential anaerobic-aerobic biological treatment system for PCBs may be feasible. The proton source for the microbial reductive dechlorination of 2,3,4,5,6-pentachlorobiphenyl has been identified by Nies and Vogel [81]. Tlie authors report that the exact mechanism of the electron transfer for the dechlorination of PCBs is imknown however, they could show that the sotirce of tiie hydrogen atom is the proton from water, and that chloride is released from the PCB. 

Hartmans et al. and Hartmans and de Bont show that vinyl chloride can be used as a primary substrate under aerobic conditions, with vinyl chloride being directly mineralized to carbon dioxide and water. Direct vinyl chloride oxidation has also been reported by Davis and Carpenter, McCarty and Semprini, and Bradley and Chapelle. Aerobic oxidation is rapid relative to reductive dechlorination of dichloroethene and vinyl chloride. Although direct DCE oxidation has not been verified, a recent study has suggested that DCE isomers may be used as primary substrates. Of the chlorinated ethanes, only 1,2-dichloroethane has been shown to be aerobically oxidized. Stucki et al. and Janssen et al. show that 1,2-DCA can be used as a primary substrate under aerobic conditions. In this case, the bacteria transform 1,2-DCA to chloroethanol, which is then mineralized to carbon dioxide. McCarty and Semprini describe investigations in which 1,2-dichloroethane (DCA) was shown to serve as primary substrates under aerobic conditions. 

T-RFLP has been used to identify bacteria hving in consortia. An example of this is the identification of an aerobic microbial consortium that reductively dechlorinates trichloroethene completely to ethane [82]. T-RFLP patterns suggested that the consortium was dominated by populations belonging to three phylogenetic groups Dehalococcoid.es, Desulfovibrio and members of the Clostridiaceae. This composition was further confirmed by cloning. 

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