DEHALOCOCCOIDES AND REDUCTIVE DECHLORINATION OF CHLORINATED SOLVENTS - Bioaugmentation for Groundwater Remediation - page 72

an early evolutionary development rather than a recent adaptation to the anthropogenic release
of large amounts of CAHs. This assertion also is consistent with the deep phylogenetic
branching of Dhc and related bacteria within the Chloroflexi (Figure 2.3 ).
As discussed in Section 2.7.1 above, the known Dhc strains possess up to 36 RDase genes
and dozens of distinct (putative) Dhc RDase genes have been identified (H¨lscher et al., 2004 ;
McMurdie et al., 2009 ). The complement of RDase genes is dynamic and appears to undergo
continuous exchange and recombination events, possibly mediated by phage activity, thus
allowing evolution of new dehalogenation phenotypes. The staggering diversity of potential
RDase genes indicates that the few characterized dehalogenation reactions are just the tip of
the iceberg and that the vast dehalogenation potential of Dhc has yet to be explored. Emerging
lines of evidence, such as the distinct codon usage in the VC RDase genes vcrA and bvcA,
suggest that at least some Dhc RDase genes originated from a non- Dhc source (i.e., were
acquired from a foreign host) (McMurdie et al., 2007 ) and that Dhc RDase genes are subject to
HGT events (Krajmalnik-Brown et al., 2007 ; McMurdie et al., 2009 , 2011 ). Genome sequencing
demonstrated that transposable elements flank RDase operons suggesting that intra-

chromosomal rearrangements and inter-chromosomal HGT events occur.
The mechanisms and rates (i.e., frequency) of these HGT events are unclear but obviously
of great importance for understanding Dhc adaptation and possible evolution of dechlorinating
ability in response to anthropogenic CAH exposure. For example, if the indigenous Dhc
population cannot respire VC because it lacks the genes encoding for VC RDases, is it possible
that over time a Dhc strain with the ability to respire VC will emerge? If so, what are the
evolutionary mechanisms promoting this evolution, and how long would it take to establish a
VC-respiring phenotype? Knowledge of the rates of RDase gene HGT, adaptation and evolu-
tion of new strains could enable researchers to predict the longevity and fate of chloroorganic
contaminants. An expression study performed with a Dhc -containing consortium revealed that
phage-related genes are up-regulated during active TCE dechlorination, but the reason and
consequences remain elusive (Johnson et al., 2008 ). Indeed, phage particles were detected in
growing cultures of Dhc strain BAV1 and genes located in strain BAV1 HPR regions were
identified on the phage genomes suggesting that phage are vehicles for HGT. The involvement
of “dehalophage” in HGT of Dhc RDase genes is an intriguing hypothesis and is being explored
in more detail.

2.12 DEHALOCOCCOIDES BIOGEOGRAPHY
Dhc isolates and mixed cultures have been obtained from sludge, contaminated and
uncontaminated river sediments, contaminated estuarine harbor sediments, and contaminated
aquifers from geographically distinct locations. Dhc environmental clone sequences have been
recovered from an even broader suite of environments, including the deep subsurface and
terrestrial habitats (Futagami et al., 2009 ; Krzmarzick et al., 2012 ). Dhc sensitivity to oxidizing
conditions implies that Dhc distribution is limited to habitats where reducing conditions prevail,
but anoxic microsites are common even in environments that are considered oxic (e.g., surface
soil). Dhc strains may respond differently to sulfide but both Dhc strain 195 and strain FL2 were
not inhibited by 1 mM sulfide (Adrian et al., 2007a ; He et al., 2005 ). However, no dechlorination
occurred in strain FL2 cultures at 5 mM sulfide indicating that Dhc dechlorination activity
cannot be expected in zones where very high dissolved sulfide concentrations exist.
The majority of Dhc cultures originated from freshwater environments although Dhc and
Dhc -related bacteria implicated in the dechlorination of PCBs and PCDDs have been detected in
and enriched from estuarine sediments (Ahn et al., 2007 ; Bedard, 2008 ; Cutter et al., 2001 ;
Fagervold et al., 2007 ; May et al., 2008 ). The PCB-dechlorinating Dhc relative “Dehalobium
chlorocoercia” was isolated from Charleston, South Carolina, harbor sediment but does not