DEHALOCOCCOIDES AND REDUCTIVE DECHLORINATION OF CHLORINATED SOLVENTS - Bioaugmentation for Groundwater Remediation - page 59

rRNA gene sequences but exhibit distinct dechlorination activities. For example, Dhc strains
BAV1 and KS1 share an identical 16S rRNA gene sequence, but strain BAV1 does not
dechlorinate 1,2-dichloropropane and strain KS1 cannot grow with chlorinated ethenes. This
incongruence between 16S rRNA gene sequence and dechlorination activity indicates that 16S
rRNA gene-based analysis is insufficient to infer dechlorination activity, and this fact has
triggered a search for process-specific biomarker genes (see below).
Knowledge of Dhc nutritional requirements and appropriate cultivation techniques enhances
the ability of researchers to grow and enrich for these fastidious dechlorinators. These efforts
generated a suite of 16S rRNA gene sequences of Dhc relatives within the Chloroflexi ,andthe
corresponding organisms also have been associated with reductive dehalogenation (Kittelmann
and Friedrich, 2008a , b ). Recently, a few novel dechlorinating isolates have been obtained that are
only distantly related to the Dhc ( > 10% 16S rRNA gene sequence divergence). These isolates
share features with the Dhc but represent novel lineages of dechlorinating bacteria within the
Chloroflexi . A recently described, tiny, disc-shaped organism with 89% 16S rRNA gene sequence
similarity to Dhc dehalogenates 1,2,3-trichloropropane to allyl chloride (3-chloro-1-propene), which
abiotically reacts with water and sulfide (the reducing agent in the medium) to form allyl alcohol
and allyl sulfide (Figure 2.3 , Table 2.4 ) (Yan et al., 2008 ). This organism was named Dehalogen-
imonas lykanthroporepellens , with the species name reminiscent of the alleged ability of garlic,
which contains allyl sulfide, to repel werewolves (Moe et al., 2009 ).
Another distant Dhc relative is strain DF-1, tentatively named “Dehalobium chlorocoercia,”
also a tiny, disc-shaped bacterium that removes doubly-flanked chlorine substituents from
PCBs provided as single congeners or as Aroclor 1260 (a commercial PCB mixture) (May et al.,
2008 ). Strain DF-1 gains energy from PCB reductive dechlorination and also dechlorinates
highly chlorinated benzenes (Wu et al., 2002 ) and PCE (Miller et al., 2005 ). Remarkably, PCE is
dechlorinated to predominantly trans -DCE and some cis -DCE as end products. A similar
observation was made in microcosms and enrichment cultures in which Dhc populations
were implicated in PCE and TCE dechlorination to predominantly trans -DCE (Griffin et al.,
2004 ). Strain o- 17, a bacterium similar to strain DF-1, was identified in a culture that removes
ortho chlorine substituents from PCBs (Cutter et al., 2001 ). A considerable diversity of 16S
rRNA gene sequences related to the sequences of these PCB-dechlorinating Chloroflexi has
been detected at anoxic, PCB-contaminated sites (Watts et al., 2005 ). As mentioned above,
PCBs also are dechlorinated by Dhc, and quantitative monitoring of cell growth demonstrated
that some Dhc strains belonging to the Pinellas subgroup grow at the expense of Aroclor 1260
dechlorination (Bedard et al., 2007 ). Recently, Dhc strain CBDB1, a member of the Pinellas
subgroup, was shown to dechlorinate a number of congeners present in Aroclor 1260 (Adrian
et al., 2009 ). These findings suggest that the ability to dechlorinate PCBs is not uncommon
among members of the Dhc group.
Environmental clone sequences related to Dhc have been recovered from a variety of
environments including sediments (Adrian et al., 2000a ; He et al., 2005 ), aquifers (Sung et al.,
2006b ), the deep subsurface (Chandler et al., 1998 ; Inagaki et al., 2003 , 2006 ; Teske, 2006 ), acid
mine drainage biofilms (GenBank # AY082458), anaerobic digestors (Godon et al., 1997 ) and
the open ocean (Morris et al., 2004 ). Although cultivation and isolation are prerequisites to shed
light on the physiology of these elusive bacteria, their apparent diversity suggests a vast
untapped reservoir of novel organisms, genes and enzymes for biotechnological applications
including bioremediation. Apparently, the ability to perform reductive dechlorination is not
uncommon in deeply branching groups within the Chloroflexi phylum and the available isolates
merely represent the tip of the iceberg while a huge diversity of dechlorinators awaits discovery.
It is interesting that the next most closely related 16S rRNA gene sequences from cultured
organisms or environmental clone sequences are only about 90% similar to those of Dhc .