Microbes Environ. Vol. 23, No. 1, 1–12, 2008 http://wwwsoc.nii.ac.jp/jsme2/ doi:10.1264/jsme2.23.1 Minireview Biodiversity of Dehalorespiring Bacteria with Special Emphasis on Polychlorinated Biphenyl/Dioxin Dechlorinators AKIRA HIRAISHI1* 1Department of Ecological Engineering, Toyohashi University of Technology, Toyohashi 441–8580, Japan (Received December 30, 2007—Accepted January 18, 2008) A wide variety of haloorganic compounds undergo reductive dehalogenation by certain anaerobic microorganisms. Metabolic reductive dehalogenation is coupled with energy-conserving respiratory electron transport in which a halo- genated compound is used as the terminal electron acceptor, the biological process called dehalorespiration or halores- piration. Dehalorespiring bacteria may play important roles in the geochemical cycle with organohalogens in nature and have great promise in their application to the bioremediation of haloorganic contaminants derived from anthropo- genic sources. During the past decade, a number of dehalorespiring microorganisms, including a unique group of strictly dehalorespiring bacteria, “Dehalococcoides”, have been isolated and characterized at phylogenetic, physio- logic, and genetic levels. Also, new perspectives of dehalorespiring bacteria have emerged based on information about genomics and molecular microbial ecology. This review article focuses on up-to-date knowledge of the biodiversity of dehalorespiring bacteria and reductively dehalogenating microbial consortia with special emphasis on those capable of transforming polychlorinated biphenyls and dioxins. Key words: dehalorespiring bacteria, reductive dehalogenation, polychlorinated aromatics, Dehalococcoides, bioremediation 10) Introduction these unique microorganisms . As it has been estimated that there are more than 3,800 species of organohalogens pro- Large amounts of structurally diverse haloorganic com- duced by living organisms and via natural abiogenic pounds have been and still are released into the environment processes53), much more attention should be paid toward the as a consequence of human and industrial activities. These geochemical cycle with halogenated compounds in which halogenated compounds are recognized as persistent pollut- anaerobic dehalogenating microorganisms may play impor- ants in general but possibly undergo microbial breakdown in tant ecological roles. nature. One of the most important biological processes Another important feature of reductive dehalogena- for this is reductive dehalogenation, which provides new tion is relevant to its application to environmental bio- insights into our understanding of the significance of remediation128). Most congeners of polychlorinated biphe- haloorganic compounds in microbial physiology and eco- nyls (PCBs), polychlorinated dibenzo-p-dioxins/furans logy21,67,93,113). The most important feature of microbial (PCDD/Fs), and other haloaromatics, as well as aliphatic reductive dehalogenation is the utilization of halogenated haloorganic compounds, are highly toxic and recalcitrant compounds as terminal electron acceptors for energy-con- contaminants whose potential risk to human health and wild serving anaerobic respiratory electron transport, i.e., the met- life should be taken into account31,55,104). Thus, the problem of abolic process called dehalorespiration, halorespiration, or how to remedy organohalogen pollution is central to environ- chlororespiration. It has long been believed that most haloor- mental science and technology. Harnessing microbial reduc- ganic compounds are xenobiotics, and the occurrence of tive dehalogenation may offer scientifically sound and cost- these compounds in nature is due to contamination from effective bioremediation procedures, because this anaerobic anthropogenic sources. However, accumulated scientific process may work more efficiently than aerobic biodegrada- knowledge of reductive dehalogenation and dehalorespiring tion in removing halogen atoms from (poly)halogenated bacteria implies the importance of organohalides potentially compounds54,61,80,96,109). produced in natural environments, as well as anthropogenic During the past decade, a number of dehalorespiring ones, as physiological substrates for growth and survival of microorganisms, including a unique group of dehalorespir- ers, “Dehalococcoides”, have been isolated and character- + * Corresponding author. E-mail: [email protected]; Tel: 81– ized from phylogenetic, physiologic, and genetic points of + 532–44–6913; Fax: 81–532–44–6929. view81,113). Also, new perspectives of dehalorespiring bacteria Abbreviations: DCDD, dichlorodibenzo-p-dioxin; DCE, dichloroet- hene; DLG, Dehalococcoides-like group; MCDD, monochlorod- have emerged based on information about genomics and ibenzo-p-dioxin; PBDs, polychlorinated biphenyls/dioxins; PCB molecular microbial ecology. However, while microbial (s), polychlorinated biphenyl(s); PCE, tetrachloroethene, PCDD/ reductive dehalogenation of haloaliphatic compounds has F(s), polychlorinated dibenzo-p-dioxin(s)/furan(s); PeCDDs, pen- been studied extensively, less information is available on tachlorodibenzo-p-dioxins; RDase, reductive dehalogenase; TCE, haloaromatic-dehalogenating microorganisms. There have trichloroethene, TCDD, tetrachlorodibenzo-p-dioxin; TrCDD, in recent years been excellent review articles on the pro- trichlorodibenzo-p-dioxin; VC, vinyl chloride. cesses of microbial reductive dehalogenation of halo- 2 HIRAISHI aromatics4,44,45,49,132). The present review article focuses nated- or dichlorinated congeners are the end products in on up-to-date knowledge of the biodiversity of dehalorespir- general, and no complete dechlorination has so far been ing bacteria and reductively dehalogenating microbial con- reported. sortia with particular emphasis given to those capable of The redox potential for haloorganic compounds including transforming polychlorinated biphenyls/dioxins (PBDs). PBDs and chloroethenes (tetrachloroethene [PCE], trichloro- ethene [TCE], dichloroethene [DCE], and vinyl chloride 37) Biological significance of reductive dehalogenation [VC]) is relatively high, ranging from 260 to 570 mV . For example, if the dehalogenation of 1,2,3,4-tetachlorodibenzo- Reductive dehalogenation is defined as the removal of a p-dioxin (TCDD) to 2-monochlorodibenzo-p-dioxin halogen substituent from a molecule with the concomitant (MCDD) takes place with H2 as the electron donor via a addition of two electrons (Fig. 1)37,93). There are two modes pathway forming 1,2,3-trichlorodibenzo-p-dioxin (TrCDD) of reductive dehalogenation, called hydrogenolysis and and 2,3-dichlorodibenzo-p-dioxin (DCDD) as the intermedi- dichloroelimination, but the biological process takes place ates (see Fig. 1), the total reaction and ∆G°’ for this is given mostly as the hydrogenolytic reaction. Co-metabolic reduc- by: tive dehalogenation, which is of no benefit to the catalyzing + − 1,2,3,4-TCDD (C12H4Cl4O2)+6H +6e → 2-MCDD microorganisms, happens with enzymes that normally cata- (C12H7ClO2)+3HCl lyze other reactions. On the other hand, in metabolic reduc- tive dehalogenation, i.e., dehalorespiration, energy is con- ∆G°’=−469 kJ mol−1 served via an anaerobic respiratory process with a halogenated compound as the terminal electron acceptor and As described above and previously37,71), it is clear that the reductive dehalogenase (RDase) as the key enzyme involved. hydrogenotrophic redox process with PBDs as terminal elec- Dehalogenated end products from polyhalogenated com- tron acceptors can provide energy sufficient for the growth pounds during dehalorespiration differ among species and and survival of dehalogenating microorganisms. strains, as can be seen in the reductive dechlorination of tet- Although microorganisms capable of metabolic or co-met- rachloroethene (PCE). In the case of dehalorepiration with abolic reductive dehalogenation have been described as both PBDs and other polychlorinated aromatics, their monochlori- obligate and facultative anaerobes, all of the dehalorespiring Fig. 1. Examples of reductive dehalogenation: hydrogenolytic dechlorination of chloroethenes (A) and 1,2,3,4-TCDD (B) found in “Dehalococ- coides” species. Biodiversity of Dehalorespiring Bacteria 3 bacteria so far described are strictly anaerobic, with the proximity to aerobic environments in wetland systems73). exception of Anaeromyxobacter (see below). As mentioned above, however, the redox potentials for organohalides are 2− Phylogeny of dehalogenating microorganisms much higher than that for the SO4 /H2S redox couple (E’0= − − −217 mV) and are comparable to the value for the NO3 /NO2 Anaerobic dehalogenating bacteria from different organo- couple (E’0=433 mV). Therefore, the reductive dehalogena- halogen-contaminated environments have been isolated as tion process itself may not always require strictly anoxic, pure cultures or highly enriched consortia and affiliated to low-potential conditions37). In fact, populations of “Dehalo- diverse phylogenetic groups (Fig. 2). Among these dehaloge- coccoides” and its phylogenetic relatives within the phylum nating bacteria, dehalorespiring species belong to three Chloroflexi were detected together with ubiquinone-contain- major phyla, Chloroflexi, Firmicutes, and Proteobacteria. ing aerobic bacteria at the surface of lake sediment at a broad There seems no correlation between the specificity for the 63) Eh gradient of 5 to −75 mV . In semi-aerobic fed-batch organohalides used by and the phylogenetic affiliations of composting reactors