Biomedical Engineering Reference
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consumes two electrons and two protons and releases H
+
and Cl
.Thetwoelectronsare
added to the organic substrate resulting in a reduced product. The oxidation state of the
chlorine substituent is
1 and does not change upon release as chloride (i.e., Cl
). A special
case of reductive dechlorination is vicinal reduction or dichloroelimination, which can occur
when aliphatic chloroorganic compounds have chlorine substituents located on adjacent
saturated carbon atoms. An example is the reductive dechlorination of 1,2-dichloropropane
to propene, which was observed in anaerobic mixed cultures containing
Dehalococcoides
(
Dhc
)(L¨ffler et al.,
1997a
; Ritalahti and L¨ffler,
2004a
)and
Dehalobacter
(Schl¨telburg
et al.,
2000
,
2002
).
As depicted in Figure
2.1
, PCE and TCE can be reductively dechlorinated stepwise to less
chlorinated ethenes. Cometabolic reductive dechlorination of PCE and TCE was first observed in
methanogenic cultures (Bouwer and McCarty,
1983
; Fathepure et al.,
1987
; Vogel and McCarty,
1985
). Under conditions of anaerobic growth, methanogens and other microbial groups harbor
abundant reduced transition-metal cofactors that fortuitously dechlorinate PCE and TCE.
Unfortunately, the cometabolic reductive dechlorination rates decrease by an order of magni-
tude with each chlorine substituent removed, leading to the accumulation of
cis
-DCE and VC
and detoxification (i.e., ethene formation) is not achieved (Gantzer and Wackett,
1991
). Reduc-
tive dechlorination reactions are thermodynamically favorable and are associated with a consid-
erable change in free energy under standard conditions at pH 7 (
G
0
) ranging from
D
approximately
172 kilojoules (kJ)/reaction (Dolfing and Janssen,
1994
; Dolfing,
2000
). A fundamental breakthrough was the discovery of
Desulfomonile tiedjei
, a Deltaproteo-
bacterium that derives energy from reductive dechlorination of 3-chlorobenzoate to benzoate
(DeWeerd et al.,
1991
; Suflita et al.,
1982
). In other words, this bacterium uses 3-chlorobenzoate
as a terminal electron acceptor (just like mammals breathe oxygen), captures the energy released
in replacing the chlorine substituent with a hydrogen atom and grows via reductive dechlorina-
tion as the sole energy source.
This process has been named chlororespiration, dechlororespiration, halorespiration, deha-
lorespiration, chloridogenesis, catabolic reductive dechlorination, metabolic reductive dechlo-
rination or respiratory reductive dechlorination. All these terms are justified in their own right
but this flurry of names is clearly confusing to experts and non-experts. The key information
relevant for bioremediation is that the organisms gain energy for maintenance or growth from
reductive dechlorination reactions (i.e., the organisms benefit from contaminant transforma-
tion) and the term
140 to
accurately describes this process.
The discovery of organohalide-respiring bacteria sparked new dialogue as to how dechlorinat-
ing microbes can be obtained. The new strategy provided chlorinated compounds like PCE and
TCE as electron acceptors supplied to microcosms with the hope that microbes respiring CAHs
would grow and could be enriched. This strategy proved fruitful, and
Dehalobacter restrictus
was
the first organism described that grew with PCE as electron acceptor (Holliger et al.,
1993
,
1998
).
Dehalobacter restrictus
dechlorinated PCE to
cis
-DCE as the end product, so detoxification was
not achieved.
Dehalobacter
isolates have a highly specialized metabolism and require PCE or TCE
as electron acceptors; strain PER23 and strain TEA require hydrogen as electron donor. Subse-
quently, several PCE-to-
cis
-DCE-dechlorinating bacteria belonging to the Firmicutes and the delta/
epsilon subdivisions of the Proteobacteria were isolated and characterized (Table
2.3
). The
substrate range of
Dehalobacter
spp. is not restricted to chlorinated alkenes and strain TCA1
gains energy from the reductive dechlorination of 1,1,1-trichloroethane to chloroethane (Sun et al.,
2002
). Recent work demonstrated that a
Dehalobacter
strain present in a 1,1,1-trichloroethane-
dechlorinating consortium coupled chloroform-to-dichloromethane reductive dechlorination with
growth, providing the first evidence for the existence of microbes capable of organohalide
respiration with chlorinated methanes (Grostern et al.,
2010
).
organohalide respiration
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