The little bacteria that can - diversity, genomics and ecophysiology of 'Dehalococcoides' spp. in contaminated environments.
Bottom Line: Recent genome sequencing projects revealed a large number of genes that are potentially involved in reductive dechlorination.Molecular approaches towards analysis of diversity and expression especially of reductive dehalogenase-encoding genes are providing a growing body of knowledge on biodegradative pathways active in defined pure and mixed cultures as well as directly in the environment.Moreover, several successful field cases of bioremediation strengthen the notion of dedicated degraders such as Dehalococcoides spp. as key players in the restoration of contaminated environments.
Affiliation: Laboratory of Microbiology, Wageningen University, Dreijenplein 10, 6703 HB, Wageningen, the Netherlands.Show MeSH
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Mentions: Dehalococcoides is a genus of strictly anaerobic Gram‐negative bacteria that to the best of our knowledge are restricted to gaining energy from the reduction of chlorinated compounds by organohalide respiration. Cultured Dehalococcoides spp. isolates have an irregular, spherical shape (approximately 0.5 µm) often referred to as coccoid. These mesophilic (25–40°C) bacteria prefer neutral pH environments. Their growth on alternative electron acceptors such as oxygen, nitrate or sulfate has never been reported (Kube et al., 2005; Seshadri et al., 2005). Reductive dechlorination by Dehalococcoides spp. occurs via the replacement of a chlorine atom in the chlorinated compound by hydrogen (reductive hydrogenolysis) and results in a net input of one proton and two electrons (Fig. 2) (Holliger et al., 1998). Gibbs free energy (ΔG0′) generated with reductive dechlorination of chlorinated compounds could range from −130 to −180 kJ mol−1 per chlorine removed. The redox potential thus generated is comparable to the redox potential of denitrification and higher than that generated by sulfate reduction. As a result it was suggested that reductively dechlorinating bacteria could out‐compete sulfate reducers and methanogens for reducing equivalents when the formation of reducing equivalents is rate limiting (Dolfing, 2003). Dehalococcoides spp. are also capable of degrading chlorinated aliphatic compounds, i.e. 1,2‐DCA, via so‐called dihaloelimination. In this process two neighbouring chlorine atoms are concurrently replaced via the formation of a double bond between the two carbon atoms. Dihaloelimination requires less H2 for the removal of chlorine atoms than reductive hydrogenolysis, thus its energy balance is more favourable under H2‐limited conditions (Smidt and de Vos, 2004).
Affiliation: Laboratory of Microbiology, Wageningen University, Dreijenplein 10, 6703 HB, Wageningen, the Netherlands.