Organohalide Respiring Bacteria and Reductive Dehalogenases: Key Tools in Organohalide Bioremediation

Abstract

Organohalides are recalcitrant pollutants that have been responsible for substantial contamination of soils and groundwater. Organohalide-respiring bacteria (ORB) provide a potential solution to remediate contaminated sites, through their ability to use organohalides as terminal electron acceptors to yield energy for growth (i.e., organohalide respiration). Ideally, this process results in non- or lesser-halogenated compounds that are mostly less toxic to the environment or more easily degraded. At the heart of these processes are reductive dehalogenases (RDases), which are membrane bound enzymes coupled with other components that facilitate dehalogenation of organohalides to generate cellular energy. This review focuses on RDases, concentrating on those which have been purified (partially or wholly) and functionally characterized. Further, the paper reviews the major bacteria involved in organohalide breakdown and the evidence for microbial evolution of RDases. Finally, the capacity for using ORB in a bioremediation and bioaugmentation capacity are discussed.

Keywords: Dehalobacter; Dehalococcoides; bioremediation; organohalide respiration; reductive dehalogenase.

Figure 1

Dechlorination of chlorinated ethenes and ethanes via anaerobic biotic and abiotic pathways. Examples of enzymes catalyzing the biotic reactions are given: PceA (Miller et al., 1998), TceA (Magnuson et al., ; Fung et al., 2007), VcrA (Parthasarathy et al., 2015), CtrA (Zhao et al., 2015), BvcA (Tang et al., 2013), DcaA (Marzorati et al., 2007), CfrA (Tang and Edwards, 2013), CtrA (Ding et al., 2014), and DcrA (Tang and Edwards, 2013).

Figure 2

Putative representation of electron transfer chain with H₂ as electron donor, and organohalide as electron acceptor in S. multivorans (derived from Bommer et al., and Goris et al., 2014). Rdh A, reductive dehalogenase catalytic subunit; Rdh B, reductive dehalogenase membrane anchor protein; MBH, membrane-bound uptake hydrogenase; Cyt b, cytochrome b subunit of the MBH; MQ, menaquinone; MQH₂, dihydromenaquinone; R-Cl, organohalide.

Figure 3

Maximum Likelihood phylogenetic analysis of reductive dehalogenases characterized to date. The evolutionary history was inferred by using the Maximum Likelihood method based on the JTT matrix-based model (Jones et al., 1992). The tree with the highest log likelihood (−7407.3363) is shown. The percentage of trees in which the associated taxa clustered together is shown next to the branches. Initial tree(s) for the heuristic search were obtained by applying the Neighbor-Joining method to a matrix of pairwise distances estimated using a JTT model. The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. The analysis involved 30 amino acid sequences. All positions containing gaps and missing data were eliminated. There were a total of 201 positions in the final dataset. Evolutionary analyses were conducted in MEGA6 (Tamura et al., 2013).