Download Physiological meaning and potential for application of reductive

FEMS MicrobiologyReviews 15 (1994) 297-305
© 1994 Federation of European Microbiological Societies 0168-6445/94/$15.00
Published by Elsevier
297
FEMSRE 00437
Physiological meaning and potential for application
of reductive dechlorination by anaerobic bacteria
C h r i s t o f H o l l i g e r a,, a n d G o s s e S c h r a a b
a Limnological Research Center, EAWAG, CH-6047 Kastanienbaum, Switzerland, and b Department of Microbiology, Wageningen
Agricultural Uni~'ersity, Hesselink L'an Suchtelenweg 4, NL-6703 CT Wageningen, the Netherlands
Abstract: The physiological meaning of reductive dechlorination reactions catalyzed by anaerobic bacteria can be explained as a
co-metabolic activity or as a novel type of respiration. Co-metabolic activities have been found mainly with alkyl halides. They are
non-specific reactions catalyzed by various enzyme systems of facultative as well as obligate anaerobic bacteria. In contrast, the
reductive dechlorinations involved in metabolic respiration processes are very specific reactions. Only a limited number of alkyl and
aryl chlorinated compounds is presently known to function as a terminal electron acceptor in a few, recently isolated bacteria.
Metabolic dechlorination rates are in general several orders of magnitude higher than co-metabolic ones. Both reaction types are
suitable for the anaerobic treatment of waste streams.
Key words: Reductive dechlorination; Anaerobic bacteria; Co-metabolic activity; Respiration; Substrate specificity
Introduction
B i o t r a n s f o r m a t i o n s of c h l o r i n a t e d c o m p o u n d s
via reductive d e c h l o r i n a t i o n have b e e n s t u d i e d by
m a n y researchers a n d different aspects of the
process have b e e n reviewed extensively (e.g. [ 1 4]). I n the m e t a b o l i s m of ( a n a e r o b i c ) m i c r o o r g a n isms, organic c o m p o u n d s can serve as e l e c t r o n
d o n o r a n d / o r c a r b o n source, as t e r m i n a l electron acceptor in a n a n a e r o b i c r e s p i r a t i o n (e.g.
f u m a r a t e ) , or they can b e t r a n s f o r m e d via a com e t a b o l i c reaction. I n the first two f u n c t i o n s energy from exergonic reactions is used for growth
of the m i c r o o r g a n i s m s . I n contrast, a co-metabolic r e a c t i o n is m e r e l y a fortuitous m o d i f i c a t i o n
* Corresponding author.
SSDI 0168-6445(94)00039-2
of a c o m p o u n d by enzymes or co-factors, which
n o r m a l l y catalyze o t h e r reactions. M i c r o o r g a n isms do not b e n e f i t from c o - m e t a b o l i c transformations.
Methyl chloride is the only c h l o r i n a t e d comp o u n d k n o w n to serve as c a r b o n a n d energy
source for an a n a e r o b i c b a c t e r i u m [5]. T h e methyl
chloride-utilizing o r g a n i s m is a h o m o a c e t o g e n ,
f e r m e n t i n g methyl chloride plus c a r b o n dioxide
to acetate a n d chloride.
In general, a n a e r o b i c b a c t e r i a t r a n s f o r m chlor i n a t e d c o m p o u n d s via r e d u c t i o n reactions. In
these reactions c h l o r i n e s u b s t i t u e n t s are r e m o v e d
a n d electrons are a d d e d to the c o m p o u n d . F r o m
the different reductive d e c h l o r i n a t i o n activities
described a n d from the results of o u r own research the c o n c l u s i o n is that the physiological
m e a n i n g of the reductive d e c h l o r i n a t i o n by
a n a e r o b i c b a c t e r i a c a n be two-fold: (i) a co-
298
metabolic activity; and (ii) a novel type of anaerobic respiration [6]. In the following, an overview
of the known co-metabolic and metabolic reac-
tions will be given and the difference in potential
of application of the two types of reductive
dechlorination will be discussed.
Table 1
Reductive dehalogenation reactions catalyzed by pure cultures of bacteria in a metabolic or co-metabolic process
ttalogenated
compound ~
Aliphatic
CT
CF
CI3CNO 2
1,2-DCA
1,1,I-TCA
BA
1,2-DBA
PCE
1.2-DBE
",/-HCH
Aromatic
3-CB
PCP •
1,2,4-TCB
Bacteria
Products :'
Metabolic ( m e ) /
co-metabolic (co)
References
Methanobacterium thermoautotrophicum
Methanosarcina barkeri
DesulJobacterium autotrophicum
Acetobacterium woodii
Clostridium thermoaceticum
67ostridium sp.
Escherichia coli
Shewanella putrefaciens
two Methanosarcina sp.
Pseudomonasputida PgG-786
Several methanogens
Methanobacterium thermoautotrophicum
Desulfobacterium autotrophicum
Acetobacterium woodii
(7ostridium sp.
Several methanogens
Several methanogens
Several methanogens
Desu(fi)monih" tiedjei
Acetobacterium woodii
culture PER-K23
several methanogens
Several Clostridium spp.
Several Bacillus spp.
Citrobacter freundii
Escherichia coil
Enterobacter aerogenes
Enterobacter cloacae
Serratia marcescens
Proteus rnirabilis
Clostridium rectum
CF, DCM, CO:
CF, DCM
CF, DCM, CO ~
CF, DCM, CM, CO 2
CF, DCM, CM, CO 2
CF, DCM, unidentified
CF
CF
DCM, CM
CI2CHNO 2, CICH2NO e, CH3NO 2
CA, ethene
1,1-DCA
1,1-DCA
1,1-DCA
1,1-DCA, acetate, unidentified
ethane
ethene
TCE
TCE
TCE
cis-I,2-DCET
acetylene
y-TCH, MCB, benzene
y-TCH, MCB, benzene
y-TCH
y-TCH
y-TCH
y-TCH
y-TCH
y-TCH
-/-TCH, MCB
co
co
co
co
co
co
co
co
co
co
co
co
co
co
co
co
co
co
co
co
me
co
co
co
co
co
co
co
co
co
(me) b
[16,59]
[13]
[ 16,17,59J
[ 16,17]
[17]
[28]
[60]
[25]
[56]
[10]
[59,61,62]
[59]
[59]
[16]
[28]
[6 l]
[61 ]
[50,55,59]
[50]
[17]
[36]
[61]
[7,8,21,63,64]
[8,21]
[21 ]
[21 ]
[21 ]
[21]
[21]
[21 ]
[31,32]
Desulfomonile tiedjei
Desulfomonile tiedjei
Staphylococcus epidermidis
benzoate
2,4,6-TCPA
DCBs, MCB
me
co
co
[42,43]
[49]
[51 ]
CT, tetrachloromethane; CF, trichloromethane; DCM, dichloromethane; CM, chloromethane; C13CNO2, trichloronitromethane;
TCA, trichloroethane; DCA, dichloroethane; CA, chloroethane: DBA, dibromoethane; BA, bromoethane; PCE, tetrachloroethene;
TCE, trichloroethene; DCE, dichloroethene; DBE, dibromoethene; y-HCH, y-hexachlorocyclohexane; y-TCH, y-tetrachlorocyclohexene; CB, chlorobenzoate; PCP, pentachlorophenol; TCP, trichlorophenol; TCB, trichlorobenzene; DCB, dichlorobenzene;
MCB, monochlorobenzene.
b ATP formation can be coupled to reductive dechlorination of T-TCH: however, it is not known whether Clostridium rectum can
grow on this process.
c Desulfomonile tiedjei also partly dechlorinates several tetra-, tri- and dichlorinated phenols.
299
Co-metabolic and metabolic reductive dechlorination
Alkyl reductit,e dechlorination
A number of studies reports on alkyl reductive
dehalogenation by pure cultures of bacteria (Table 1). The bacteria involved range from strict
anaerobic organisms such as methanogenic, sulfate-reducing, and fermenting bacteria to facultative anaerobic ones such as Escherichia coil or
Pseudomonas putida. The first investigations
which showed pure culture catalyzed reductive
dechlorinations, were carried out with the pesticide lindane [7,8]. In most studies, pure cultures
have not been isolated for their ability to dehalogenate, but culture collection strains were
screened on dehalogenating activity. The broad
variety of bacteria with the property to reductively dehalogenate aliphatic hydrocarbons indicates that alkyl reductive dehalogenation is common for many bacteria. Several enzyme systems
have been suggested to be involved in the alkyl
dehalogenation reactions. They include proteinbound tetra-pyrrole cofactors (iron(lI) porphyrins
[9,10], corrinoids [11-17], or factor F430 [13,15,18]),
flavoprotein-flavin complexes [19,20], and ferredoxins [21].
Ps. putida produces high concentrations of the
heme protein cytochrome P-450 . . . . if grown on
camphor as energy and carbon source [10]. This
protein was purified and found to be involved in
the reductive dechlorination of trichloronitromethane [10]. Earlier reports on reductive dehalogenating activity of iron(II) porphyrins [22],
heine proteins [23], and reduced liver microsomes
[24] already indicated that protein-bound iron(II)
porphyrins were the catalysts of the observed
dehalogenations by whole cells. Reductive
dechlorination of tetrachloromethane by the
iron-reducing bacterium Shewanella putrefaciens
appears to be catalyzed by cytochromes produced
upon microaerophilic growth [25].
Dechlorinations catalyzed by corrinoids in
buffer with excess of a reducing agent suggest
that this cofactor is the catalyst of the dehalogenations by methanogens, sulfate reducers, or
homoacetogens [13-15,26]. The fact that only
bacteria with the acetyl-CoA pathway, where a
c o r r i n o i d / F e - S protein is one of the central enzymes, had dechlorinating activity [13-15,26] supports this hypothesis. Attempts to lay a direct link
between corrinoid content and dechlorinating activity of cell-free in vitro systems of Acetobacterium woodii have failed [12]. Apparently also
other enzyme systems are involved in the reductive dechlorination reactions catalyzed by whole
cells of this bacterium.
Methanogens contain in addition to corrinoids
also high amounts of factor 1=430, a nickel porphinoid cofactor that is only found in this group of
bacteria [27]. Reductive dechlorination of tetrachloromethane, 1,2-dichloroethane, and chloroethenes by free factor F430 was the first evidence
for the involvement of this cofactor in in vivo
catalyzed dechlorinations [13,15,26]. Mcthylcoenzyme M reductase, the factor F4~ containing
enzyme catalyzing the last step in methanogencsis, was also shown to reductively dechlorinate
1,2-dichloroethane [18]. The involvement of the
corrinoid containing methyl-tetrahydromethanopterin:coenzyme M methyltransferase could
be excluded by the use of specific inhibitors [18].
The dehalogenation by flavoprotein-flavin
complexes was shown with E. coli [20] and Ps.
putida [19]. Membrane fractions of E. coil cells
reductively dechlorinated 1,1,l-trichloro-2,2bis(p-chlorophenyl)ethane (DDT) under anaerobic conditions to 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (DDD) in the presence of reduced
FAD. In anaerobic incubations of a 6000-10000
Da flavoprotein of Ps. putida, D D T was dechlorinated when flavin cofactors such as FAD, riboflavins, or FMN were added to the reaction
mixture. The finding that only those bacteria that
have Fe-S protein dependent fermentative H~
evolution actively dechlorinated y-hexachlorocyclohexane provided evidence for ferredoxin catalyzed reductive dehalogenations [21]. Further,
only substrates that are electron donors for ferredoxin-dependent hydrogen evolution supported
the dehalogenation by these bacteria.
The production of carbon dioxide from tetrachloromethane [16,17] and acetate from 1,1,1-trichloroethane [28] are overall substitutive reac-
300
tions and seem to be exceptions to the rule that
anaerobic bacteria transform halogenated compounds predominantly via reduction reactions.
However, these transformations could involve a
two-electron reduction to a carbenoid which
would be hydrolysed to form carbon monoxide
and acetaldehyde [29]. These products could then
be oxidized to carbon dioxide and acetate. The
formation of carbon monoxide from tetrachloromethane by reduced corrinoids supports
this hypothesis [30].
The above-mentioned alkyl-dechlorinating
strains co-metabolically transform the chlorinated
compounds and thus do not benefit from the
exergonic reaction which they catalyse. Ha[ogenated aliphatic compounds are quite strong
oxidants [4] and can in principle serve as terminal
electron acceptors in an anaerobic respiration.
The first indications that alkyl dehalogenating
bacteria could indeed benefit from the dechlorination reaction were obtained in studies with
Clostridium rectum [31,32]. Cell suspensions of
this strain formed about equal amounts of ATP if
incubated with pyruvate as electron donor together with proline or 7-hexachlorocyclohexene
as electron acceptor [31]. These results suggested
a strong link between the Stickland reaction and
the dechlorination.
A further example of a metabolic reaction has
been found for the degradation of tetrachloroethene. In an anaerobic packed-bed column, tetrachloroethene was completely dechlorinated to
ethene at high rates [33]. In enrichment studies
with methanol [34] or benzoate [35] plus tetrachloroethene, large amounts of electrons were
recovered in dechlorination products (up to 31%)
showing that tetrachloroethene served as electron
acceptor. Recently, a highly purified enrichment
culture, designated PER-K23, was isolated from
material of the tetrachloroethene-dechlorinating
packed-bed column. This culture couples the reductive dechlorination of tetrachloroethene to
cis-l,2-dichloroethene to growth [36]. Molecular
hydrogen and formate were the only energy
sources that supported growth. Tetra- and trichloroethene were the only electron acceptors
utilized. Neither an inorganic nor an organic
electron acceptor could replace tetrachloro-
ethene. All electrons derived from hydrogen and
formate consumption were recovered in dechlorination products and biomass. Apparently, the
bacterium that dominates the enrichment culture
has a very restricted substrate spectrum. It is not
yet known what its physiological properties were
before tetrachloroethene was present as an environmental pollutant. However, this finding indicates that a physiologically new type of bacterium
has evolved within a considerably short time.
Aryl reductice dechlorination
Despite numerous observations of the involvement of biological processes in the reductive dehalogenation of aryl-chlorinated compounds in
soil, sediment and ground water environments,
there are only few reports on the catalysis of aryl
reductive dechlorination reactions by bacterial
pure cultures. One bacterium, strain DCB-1, has
been isolated which is able to dehalogenate
meta-substituted benzoates. After extensive physiological characterization [37-40] and 16S rRNA
sequence analysis, the Gram-negative, non-motile,
non-sporeforming large rod, with an unusual
morphological feature resembling a collar, was
named Desulfomonile tiedjei [41]. This sulfate-reducing bacterium was the first organism isolated
that couples growth to a reductive dechlorination
reaction [42,43]. Evidence for a chemiosmotic
coupling of the reductive dechlorination and ATP
synthesis was obtained in experiments with respiratory inhibitors and imposed pH gradients [44].
The inhibition of sulfite reduction and dechlorination by the same respiratory inhibitors, together with the inhibition of the 3-chlorobenzoate
dechlorination by sulfite and thiosulfate, suggest
that the dechlorination in D. tiedjei is a novel
type of anaerobic respiration [45]. The enzymatic
character of the aryl reductive dechlorination was
shown in cell extracts of D. tiedjei [46]. Dechlorination by cell extracts depended on the presence
of reduced Methyl viologen, was membrane-associated and inducible. In a 3-chlorobenzoate-degrading consortium of three bacteria, D. tiedjei
had a symbiotic relationship with the benzoateoxidizing bacterium [47]. Strain DCB-1 depended
301
on
the
h y d r o g e n p r o d u c e d by t h e b e n z o a t e w h e r e a s the b e n z o a t e - o x i d i z i n g
o r g a n i s m d e p e n d e d on the b e n z o a t e p r o d u c e d by
strain DCB-1, a n d on a low p a r t i a l p r e s s u r e o f
h y d r o g e n m a i n t a i n e d by strain DCB-1 a n d the
m e t h a n o g e n . D e c h l o r i n a t i o n c o u p l e d to g r o w t h
on 3 - c h l o r o b e n z o a t e as sole s u b s t r a t e was also
possible in a d e f i n e d b i c u l t u r e w i t h o u t t h e
m e t h a n o g e n [48]. D. tiedjei also p a r t l y d e c h l o r i nares highly c h l o r i n a t e d p h e n o l s [49] a n d t e t r a c h l o r o e t h e n e to t r i c h l o r o e t h e n e [50]. H o w e v e r ,
t h e s e d e c h l o r i n a t i o n s a r e t h o u g h t to be f o r t u i t o u s
activities. C h l o r i n a t e d p h e n o l s w e r e u n a b l e to
oxidizing strain,
i n d u c e d e c h l o r i n a t i o n activities in D. tiedjei, while
the r a t e o f d e c h l o r i n a t i o n of t e t r a c h l o r o e t h e n e
was e x t r e m e l y low (1.6 p m o l P C E / m i n p e r mg
p r o t e i n ) . B o t h d e c h l o r i n a t i o n s could n o t be coup l e d to g r o w t h o f D. tiedjei.
A Staphylococcus epidermidis strain, i s o l a t e d
f r o m intestinal c o n t e n t s o f rats, d e c h l o r i n a t e d
1 , 2 , 4 - t r i c h l o r o b e n z e n e to d i c h l o r o b e n z e n e s a n d
c h l o r o b e n z e n e [51]. This r e a c t i o n o c c u r r e d only
with h y d r o g e n in t h e gas p h a s e . In i n c u b a t i o n s
with cell-free extracts of S. epidermidis, N A D P H
s t i m u l a t e d the d e c h l o r i n a t i o n of 1,2,4-trichlorob e n z e n e . O n l y t r a c e a m o u n t s of d e c h l o r i n a t i o n
Table 2
Reductive dechlorination reactions catalyzed by hematin, cobalamin, and factor F430
Catalyst
Substrate "
Products a
Reference(s)
Hematin
CT
CF
CI3CNO 2
HCA
PCA
1,1,1,2-TeCA
1,1,2,2-TeCA
1,1,1-TCA
1,2-DBA
PCE
y-HCH
HCB
CF
DCM
CI2CHNO 2, CICH 2NO 2, CH3NO 2
PCE, TCE
TCE, cis-+ trans-l,2-DCE
1,1-DCE, VC
cis- + trans-l,2-DCE, 1,1,2-TCA, TCE
1,1-DCA, CA, ethane
Ethene
TCE, cis-l,2-DCE
T-TCH, CB
QCB
[ 10,26,65]
[65]
[10]
[22,66]
[66]
[66]
[66]
[65,66]
[22]
[26]
[67]
[26]
Cobalamin
CT
CFCI 3
HCA
PCA
I,I,I,2-TeCA
1,1,2,2-TeCA
I,I,I-TCA
1,2-DCA
PCE
T-HCH
HCB
PCP
2,3,4,5,6-PCB
CF, DCM CM, CH 4, CO, CO 2
CHFCI 2, CO, formate
PCE, TCE
TCE, cis-+ trans-l,2-DCE, VC
1,1-DCE, VC
cis-+ trans-l,2-DCE, 1,1,2-TCA, VC, ethene
1,I-DCA, CA, ethane
CA, ethene
TCE, cis-+ trans-l,2-DCE, 1,1-DCE, VC, ethene
y-TCH, CB
QCB, 1,2,3,5-TeCB, 1,2,4,5-TeCB
2,3,4,6-TeCP. 2,3,5,6-TeCP
2,3,5,6-TCB,
2,3,4,6-TCB
[11,12,14,26,30]
[30]
[66]
[66]
[66]
[66]
[66]
[15]
[26]
[67]
[26,68]
[26]
[68]
Factor F430
CT
1,2-DCA
PCE
CF, DCM, CM, CH 4
CA, ethene
TCE, cis-+ trans-l,2-DCE, VC, ethene
[13]
[15]
[26]
~ CT, tetrachloromethane; CF, trichloromethane; DCM, dichloromethane; CM, chloromethane; C13CNO2, trichloronitromethane;
CH3NO2, nitromethane; CFCI3, trichlorofluoromethane; CHFCI 2, dichlorofluoromethane; HCA, hexachloroethane; PCA, pentachloroethane; TeCA, tetrachloroethane; TCA, trichloroethane; DCA, dichloroethane; CA, chloroethane; DBA, dibromoethane;
PCE, tetrachloroethene; TCE, trichloroethene; DCE, dichloroethene; VC, chloroethene; y-HCH, y-hexachlorocyclohexane;
y-TCH, y-tetrachlorocyclohexene; HCB, hexachlorobenzene; QCB, pentachlorobenzene; TeCB, tetrachlorobenzene; CB,
ehlorobenzene; PCP, pentachlorophenol; TeCP, tetrachlorophenol; PCB, pentachlorobiphenyl; TCB, tetrachlorobiphenyl.
302
products were formed by whole ceils, and it is not
known how these findings relate to aryl reductive
dehalogenation reactions observed in investigations with other environmental samples. Reductive dechlorination of aromatic chlorinated compounds can be catalyzed by free corrinoids and
iron(II) porphyrins [26]. However, no report has
been published as yet showing co-metabolic
dechlorinating activity of bacteria containing such
cofactors in high amounts. This, together with
very specific dechlorinations of chlorinated benzenes observed in enrichment cultures [52,69] and
the exclusion so far of methanogens to be involved in aryl dechlorinations, indicates that
aryl-reductive dechlorinations are catalyzed by
bacteria which may use the aromatic chlorinated
compound as terminal electron acceptor in an
anaerobic respiration [52].
Potential for application in treatment processes
Reductive dechlorination reactions in treatment processes for chlorinated compounds containing hazardous waste, soil, and waste- and
groundwater have a large potential. The benefits
of the reactions, in which highly chlorinated compounds are transformed into non- or less-chlorinated products, are two-fold. The products are
in general less toxic (an exception is the formation of vinylchloride in the dechlorination of tctrachloroethene), and they are often more susceptible to mineralization by aerobic a n d / o r
anaerobic bacteria. For a number of chlorinated
compounds, e.g. for polychlorinated biphenyls,
aromatics and aliphatics, reductive dechlorination
reactions have been found to be necessary before
mineralization can occur.
Two aspects are important for an application
of the process. The degree of specificity of the
known dechlorination reactions is the first aspect
to be considered. Metabolic dechlorination reactions have been found to be very substratespecific. Culture PER-K23, which originates from
a mixed culture in which tetrachloroethene is
transformed to ethane, dechlorinates only tetraand trichloroethene to cis-l,2-dichloroethene.
Other bacteria carry out the remaining transtk~rmation steps. This was demonstrated in our laboratory with an enrichment culture which dechlorinates cis-l,2-dichloroethene to ethene (de Bruin,
unpublished results). Also D. tiedjei is limited in
its ability to metabolically dehalogenatc chlorinated compounds. Only with 3-chlorobenzoate
and 3,5-dichlorobenzoate energy was conscrved
for growth [44]. Co-mctabolic dechlorination reactions have a broader substrate range. The possible catalysts hematin, cobalamin and factor ~.~
in bacteria such as methanogens, sulfate-reducers
and acetogens display a wide variety of substrates
that can be dehalogenated (Table 2). Immobilized porphyrins and corrins have been even been
considered to be used in treatment systems [53].
In laboratory experiments, y-hexachlorohexane
and dichloromethane were dechlorinated in
flow-through systems with efficiencies of up to
98% for more than 90 days. Although the reactivity of the involved cofactors may be different for
Table 3
T e t r a c h l o r o e t h e n e ( P C E ) d e c h l o r i n a t i o n r a t e s by d i f f e r e n t a n a e r o b i c bacteria
Bacterium
Metabolic (me)/
c o - m e t a b o l i c (co)
P r o d u c t ~'
Dechlorination rate
(nmol PCE/min per mg
protein)
Reference
Culture PER-K23A
me
co
co
co
co
cis-l,2-DCE
3.3 × 1[) a
<6
×10 ~
5 . 8 × 10 4
3.3 × 10 4
[361
[171
150]
[5o]
[501
Acetobacterium woodii
Methanosarcina sp.
Methanosarcina mazei
Desulfornonile tiedjei
;' T C E , t r i c h l o r o e t h e n e ; D C E , d i c h l o r o e t h e n e .
TCE
TCE
TCE
TCE
1.6×10
~
3(13
enzyme-bound cofactors or whole-cell systems
compared with the free forms [2,54], the substrate
range for co-metabolically dechlorinating bacteria
is certainly broader (Table 1) than the one for
bacteria using a chlorinated compound as terminal electron acceptor.
The second aspect to be considered is the
difference in dechlorination rate. The metabolic
dechlorination rate of tetrachloroethene has been
found to be several orders of magnitude higher
than the co-metabolic one (Table 3). Only the
co-metabolic dechlorination of tetrachloromethane by Acetobacteriurn woodii, with a rate of 21
n m o l / m i n per mg protein [17], occurs at a similar
rate as the metabolic dechlorination of tetrachloroethene by culture PER-K23 [36]. This difference in rate is partly caused by the nonspecificity of the reaction. The flow of electrons is
not only directed towards the chlorinated compounds. In methanogenic bacteria, only 0.0051.6% of the electrons generated from substrate
consumption was used for the dechlorination of
chloroform and tetrachloroethene compared with
the amount of methane produced [55,56]. In the
case of tetrachloroethene dechlorination by culture PER-K23, approximately 90% of the electrons derived from hydrogen consumption were
recovered in dechlorination products [36].
The narrow substrate spectrum of specific bacteria with metabolic dechlorination capabilities
may make them suitable to treat single-compound-containing waste streams. Experiments
with D. tiedjei have shown that this bacterium
can be immobilized in sludge granules in an upflow anaerobic sludge blanket reactor [57]. This
resulted in granules with the ability to degrade
3-chlorobenzoate.
The broader substrate spectrum of the bacteria with co-metabolic activities makes them more
suitable to treat waste streams with mixtures of
chlorinated compounds. Because of the low
dechlorination rates, the latter treatment will become more feasible, when large quantities of the
desired biomass can be retained in the reactor.
The use of sugar-grown granular U A S B sludge,
containing high concentrations of acetogenic and
methanogenic bacteria, has been successful in the
dechlorination of pentachlorophenol [58].
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