Biomedical Engineering Reference
In-Depth Information
2.1.3 Discovery of
Dehalococcoides
In 1989, Freedman and Gossett (Freedman and Gossett,
1989
) published a seminal paper
that demonstrated that microbes capable of reductive dechlorination past
cis
-DCE to ethene
exist. Subsequent studies by DiStefano and Gossett (DiStefano et al.,
1991
,
1992
) showed that
an anaerobic PCE-dechlorinating enrichment culture converted high concentrations of PCE to
ethene at unprecedented rates and that hydrogen (H
2
) served as the electron donor for
dechlorination. These results indicated that reductive dehalogenation in these cultures was
a catabolic process analogous to 3-chlorobenzoate dehalogenation by
Desulfomonile tiedjei
(Suflita et al.,
1982
), whereas the conventional wisdom at that time was that reductive
dehalogenation of chloroethenes was a slow and cometabolic process carried out by metha-
nogens and some other anaerobes. A few researchers, notably Perry McCarty from Stanford
University, were more visionary and encouraged colleagues to search for novel dechlorina-
tors, and subsequent studies by Gossett and Zinder led to the isolation of an unusual
bacterium that dechlorinated PCE to VC and ethene (Maym´ -Gatell et al.,
1997
)from
Freedman and Gossett's enrichment culture (Freedman and Gossett,
1989
). They called this
isolate “Dehalococcoides ethenogenes” strain 195 because individual cells were round and the
culture produced ethene from PCE. More detailed investigations revealed that strain 195 grew
with PCE, TCE,
cis
-DCE and 1,1-DCE but not with VC as electron acceptors. VC was slowly
and cometabolically dechlorinated to ethene after utilization of all polychlorinated ethenes
(Maym´ -Gatell et al.,
2001
).
Nevertheless, the discovery of strain 195 demonstrated the existence of microbes that can
overcome the “DCE stall”; however, the formation of VC remained a major concern for
bioremediation applications. Subsequent identification of PCE-to-ethene-dechlorinating enrich-
ment cultures that produced ethene without significant VC accumulation coupled with hydro-
gen consumption to very low concentrations provided strong evidence for VC-respiring
microbes (L¨ffler et al.,
1999
). Hydrogen consumption to certain threshold concentrations
serves as a measure of the free energy change associated with the hydrogen-consuming
oxidation reduction reaction (L¨ffler and Sanford,
2005
). The low consumption concentrations
measured in VC-dechlorinating cultures indicated that microbes present in these enrichment
cultures gained energy from VC-to-ethene reductive dechlorination (L¨ffler et al.,
1999
).
A milestone discovery was the isolation of
Dhc
sp. strain BAV1, the first isolate capable of
organohalide respiration of VC to ethene (He et al.,
2003b
). Additional
Dhc
isolates, strain GT
(Sung et al.,
2006b
), and strain VS (M¨ller et al.,
2004
), that both grew with VC as electron
acceptor were subsequently described, and several research groups obtained
Dhc
-containing
mixed cultures that dechlorinate chlorinated ethenes to ethene (e.g., Duhamel et al.,
2002
;
Richardson et al.,
2002
; Vainberg et al.,
2009
). Table
2.4
depicts the available
Dhc
isolates and
several consortia containing
Dhc
strains capable of chlorinated ethene reductive dechlorination.
The genus
Dehalococcoides
has recently been published, and all known
Dhc
strains are
classified as members of the same species,
Dehalococcoides mccartyi
(L¨ffler et al.,
2012
).
The dechlorination range of
Dhc
is not restricted to chlorinated alkenes and alkanes.
Dhc mccartyi
strain 195 has been demonstrated to dechlorinate several chlorinated aromatic
compounds including chlorinated benzenes, chlorinated phenols, polychlorinated-dibenzo-p-
dioxins (PCDDs) and polychlorinated biphenyls (PCBs) although growth has not been demon-
strated with all of these substrates (Adrian et al.,
2007a
; Fennell et al.,
2004
; Liu and Fennell,
2008
).
Dhc
strain CBDB1 was isolated with trichlorobenzenes as catabolic electron acceptors,
which were dechlorinated to dichlorobenzenes (Adrian et al.,
2000b
). In addition to polychlori-
nated chlorobenzenes, this isolate dechlorinates certain polychlorinated phenols and some
PCDD and PCB congeners (Adrian et al.,
2007a
,
2009
; Bunge et al.,
2003
; Jayachandran
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