Energetics of product formation during anaerobic degradation of phthalate isomers and benzoate

Article abstract of DOI:10.1016/S0168-6496(99)00022-7

Methanogenic enrichment cultures grown on phthalate, isophthalate and terephthalate were incubated with the corresponding phthalate isomer on which they were grown, and a mixture of benzoate and the phthalate isomer. All cultures were incubated with bromoethanosulfonate to inactivate the methanogens in the mixed culture. Thus, product formation during fermentation of the aromatic substrates could be studied. It was found that reduction equivalents generated during oxidation of the aromatic substrates to acetate were incorporated in benzoate under formation of carboxycyclohexane. During fermentation of the phthalate isomers, small amounts of benzoate were detected, suggesting that the initial step in the anaerobic degradation of the phthalate isomers is decarboxylation to benzoate. Gibbs free energy analyses indicated that during degradation of the phthalate isomers, benzoate, carboxycyclohexane, acetate and molecular hydrogen accumulated in such amounts that both the reduction and oxidation of benzoate yielded a constant and comparable amount of energy of approximately 30 kJ mol^-1. Based on these observations it is suggested that within narrow energetic limits, oxidation and reduction of benzoate may proceed simultaneously. Whether this is controlled by the Gibbs free energy change for carboxycyclohexane oxidation remains unclear. Copyright (C) 1999 Federation of European Microbiological Societies.

Full text of DOI:10.1016/S0168-6496(99)00022-7

FEMS Microbiology Ecology 29 (1999) 273^282
Energetics of product formation during anaerobic degradation of
phthalate isomers and benzoate
Robbert Kleerebezem *, Look W. Hulsho¡ Pol, Gatze Lettinga
Department of Agricultural, Environmental and Systems Technology, Sub-department of Environmental Technology,
Wageningen Agricultural University, `Biotechnion', Bomenweg 2, 6703 HD Wageningen, The Netherlands
Received 18 November 1998; revised 24 February 1999; accepted 17 March 1999
Abstract
Methanogenic enrichment cultures grown on phthalate, isophthalate and terephthalate were incubated with the
corresponding phthalate isomer on which they were grown, and a mixture of benzoate and the phthalate isomer. All cultures
were incubated with bromoethanosulfonate to inactivate the methanogens in the mixed culture. Thus, product formation
during fermentation of the aromatic substrates could be studied. It was found that reduction equivalents generated during
oxidation of the aromatic substrates to acetate were incorporated in benzoate under formation of carboxycyclohexane. During
fermentation of the phthalate isomers, small amounts of benzoate were detected, suggesting that the initial step in the anaerobic
degradation of the phthalate isomers is decarboxylation to benzoate. Gibbs free energy analyses indicated that during
degradation of the phthalate isomers, benzoate, carboxycyclohexane, acetate and molecular hydrogen accumulated in such
amounts that both the reduction and oxidation of benzoate yielded a constant and comparable amount of energy of
31
approximately 30 kJ mol . Based on these observations it is suggested that within narrow energetic limits, oxidation and
reduction of benzoate may proceed simultaneously. Whether this is controlled by the Gibbs free energy change for
carboxycyclohexane oxidation remains unclear. ß 1999 Federation of European Microbiological Societies. Published by
Elsevier Science B.V. All rights reserved.
Keywords: Phthalate; Benzoate; Anaerobic; Energetics; Gibbs free energy
1
. Introduction
can only sustain growth if the reaction is exergonic.
It further states that the amount of energy generated
during a biochemical conversion equals the Gibbs
free energy change of the chemical reaction. The
best known application of thermodynamic principles
for biotechnological processes is the observed corre-
lation between the microbial yield and Gibbs free
energy changes of microbial conversions [1^3]. Fur-
thermore, the mechanisms of formation of intermedi-
ate compounds can be analysed using thermodynam-
ic considerations [4^7]. A principal limitation of
The application of thermodynamic laws to bio-
chemical processes provides a theoretical basis for
analysis of experimental results. The second law of
thermodynamics states that microbial conversions
*
Corresponding author. Tel.: +31 (317) 483798;
Fax: +31 (317) 482108;
E-mail: robbert.kleerebezem@algemeen.mt.wau.nl
0
168-6496 / 99 / $20.00 ß 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 6 4 9 6 ( 9 9 ) 0 0 0 2 2 - 7

2
74
R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
thermodynamic laws is that they provide no infor-
mation about the rate of electron transport in com-
plex microbial conversion reactions. It has been sug-
gested, however, that Gibbs free energy dissipation
values can be correlated with reaction rates in com-
plex microbial conversions [8^10].
Microbial conversions in methanogenic environ-
ments proceed close to thermodynamic equilibrium.
Several anaerobic fermentation reactions are ender-
gonic under standard conditions. Examples of these
reactions are the fermentation of ethanol, propio-
nate, butyrate and benzoate to a mixture of acetate,
carbon dioxide and hydrogen (or formate). Com-
bined with the fact that methanogenic organisms
only utilise a limited range of simple substrates
benzoate (or benzoyl-CoA) as the central intermedi-
ate, analogous to the denitrifying organism Pseudo-
monas K136 [21,22]. All three cultures were found to
have the ability to degrade benzoate without a lag
phase, and they were only able to degrade the phtha-
late isomer on which they were cultivated. In this
paper we describe product formation of the three
cultures incubated with the phthalate isomer, or a
mixture of benzoate and the phthalate isomer, in
the presence of bromoethanosulfonate (BES), a spe-
ci¢c inhibitor of methanogenesis. The Gibbs free en-
ergy changes of the observed conversions were calcu-
lated, and the implications for the energetic limits of
the conversions are discussed.
(
e.g. CO2/H2, formate, methanol or acetate), the deg-
radation of substrates in methanogenic environments
becomes dependent on a mixture of fermenting and
methanogenic organisms. Only if the methanogenic
organisms keep the concentration of the fermenta-
tion products low, the fermentation is exergonic
and the reaction can be pulled to the product side.
Because of their mutual dependence these mixed cul-
tures are referred to as syntrophic cultures [11].
Concerning the minimum amount of Gibbs free
energy needed to sustain growth and/or conversion
of a substrate, some controversy exists. Values for
critical Gibbs free energy changes for the fermenta-
tion of e.g. ethanol, propionate, butyrate, lactate or
2. Materials and methods
2.1. Biomass
Three enrichment cultures were used with the abil-
ity to degrade either phthalate, isophthalate or tere-
phthalate. The cultures were obtained from digested
sewage sludge or granular sludge and were enriched
on one of the three phthalate isomers as described
previously [19]. The phthalate isomer-grown cultures
had the ability to degrade benzoate without a lag
phase, at rates comparable to the phthalate isomers'
degradation rates.
31
benzoate range from 32 to 330 kJ mol [4,5,8,12^
7]. Schink [11] postulated that the minimum
amount of Gibbs free energy should equal approxi-
1
2.2. Experimental procedure
31
mately 320 to 330 kJ mol , corresponding to 1/3
to 1/4 ATP. ATP can be synthesised in these small
quanta via ion translocation across the cytoplasmic
membrane. Considering, however, that anaerobic
fermentations have been observed at Gibbs free en-
Experiments were performed in 117-ml serum bot-
tles using a liquid volume of 25 or 40 ml. Medium,
substrate and BES were added to the bottles from
concentrated stock solutions. The composition of the
medium has been described elsewhere [19]. Either the
phthalate isomer or a mixture of the phthalate iso-
mer and benzoate was used as substrate at concen-
trations between 2 and 5 mM. Serum bottles were
sealed with butyl rubber stoppers and the headspace
was £ushed with an N2/CO2 mixture (70/30 v/v).
Sul¢de was dosed from a concentrated stock (¢nal
concentration 0.7 mM) to ensure anaerobic condi-
tions. Serum bottles were pre-incubated at 37³C in
an orbital shaker prior to inoculation with the
phthalate isomer-grown cultures by syringe. Liquid
samples (2 ml) were withdrawn from the bottles for
31
ergy changes much closer to 0 kJ mol , it may be
suggested that electron transport across the cytoplas-
mic membrane and ATP formation should be varia-
ble, implying that any exergonic reaction may sus-
tain growth [18].
In previous studies we described the kinetics and
related speci¢c characteristics of three methanogenic
enrichment cultures grown on phthalate, isophtha-
late and terephthalate [19,20]. It was postulated in
these papers that the phthalate isomers are fer-
mented to a mixture of acetate and hydrogen with

R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
275
component analysis. All data shown are average val-
ues from duplicate experiments.
study whether non-identi¢ed products accumulated
or analytical errors had occurred during the experi-
ments. These balances are based on the concept of
31
2.3. Analytical procedures
degree of reduction (Q, e-mol C-mol
or e-mol
31
mol for inorganic products) and the carbon chain
3
1
The concentration of aromatic acids was deter-
length (Cl, C-mol mol ) of all relevant compounds.
The degree of reduction of a compound is de¢ned as
the number of electrons liberated upon complete ox-
idation of 1 C-mol of organic material (or 1 mol of
inorganic material) to CO2 and H2O [1]. Thus these
balances are equivalent to chemical oxygen demand
(COD)-based balances. Mathematically these balan-
ces can be described using the following equation:
mined by high pressure liquid chromatography
HPLC). Intermediate formation of low concentra-
(
tions of benzoate could be detected by injection of
undiluted samples down to concentrations of 1 WM.
The concentrations of acetate and carboxycyclohex-
ane were measured by gas chromatography. The
concentrations of hydrogen and methane in the
headspace of the serum bottles were determined by
gas chromatography as well. A full account of these
analytical methods can be found elsewhere [19].
X
X
QSWClSWCSꢀt† ‡
QPWClP WCPꢀt† ˆ constant
ꢀ1†
In this equation, C(t) stands for time-dependent con-
centration (M), and subscripts S and P stand for
substrate(s) and product(s) respectively.
2.4. Energetic analyses
Standard Gibbs free energy changes for the ob-
Another approach is based on the fact that during
anaerobic fermentations, all electrons liberated dur-
ing oxidation of a substrate should be incorporated
in reduced products. Using the degree of reduction
of the substrate as a frame of reference, the following
electron balance can be derived:
served conversions during degradation of benzoate
and the phthalate isomers were calculated according
0
f
to Thauer et al. [23] (Table 1). vG values for the
phthalate isomers were calculated from benzoate, us-
ing the group contribution method described by
Dimroth [24]. The Gibbs free energy change for re-
duction of benzoate to carboxycyclohexane was cal-
culated from the reduction of benzene to cyclohex-
X
ꢀQP3 QS† WClPWCP ꢀt† ˆ 0
ꢀ2†
0
ane as suggested by Schink [11]. vGf values were
Using this equation, oxidation is de¢ned as negative
and reduction as positive. If the product concentra-
tions do not equal zero at the beginning of the ex-
periment, the initial concentrations should be sub-
tracted from the measured concentrations. From
the stoichiometry of the fermentations shown in Ta-
corrected for a temperature of 37³C using the Van
t Ho¡ equation [25].
'
2
.5. Mass and electron balances
3
3
Based on measured concentrations of all organic
ble 1, it can be seen that inorganic carbon (HCO ) is
substrates and products, balances were derived to
formed during the oxidation of benzoate and the
Table 1
Chemical reaction equations for the individual steps in mineralisation of phthalate isomers and benzoate, and standard Gibbs free energy
changes during the conversions, corrected for a temperature of 37³C
0
vG P (37³C) [kJ reaction31]
Equation no.
Reaction
Equation
2
3
3
‡
3
3
‡
3
38.9a
320.7
59.6
393.5
153.1
1
2
3
4
5
phthalate oxidation
phthalate decarboxylation
benzoate oxidation
benzoate reduction
carboxycyclohexane oxidation
C8H4O +8H2OC3C2H3O +3H +3H2+2HCO
4
2
2
3
3
3
C8H4O +H2OCC7H5O +HCO
4
2
C7H5O +7H2OC3C2H3O +3H +3H2+HCO33
3
3
2
2
3
3
2
C7H5O +3H2CC7H11O
2
3
3
2
‡
3
C7H11O +7H2OC3C2H3O +3H +6H2+HCO
2
3
a
0
Value represents calculated vG P for isophthalate and terephthalate fermentation, the value for ortho-phthalate fermentation is estimated
3
1
0
to be 34.9 kJ reaction . Applied vG P values for ortho-phthalate, isophthalate and terephthalate were 3548.6, 3552.6 and 3552.7, re-
spectively [24].
f

2
76
R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
phthalate isomers. Oxidation equivalents formed due
to HCO 33 formation were calculated based on the
measured acetate concentrations according to reac-
tion stoichiometries shown in Table 1. Taking into
account all conversions with benzoate shown in Ta-
hexane concentrations were slightly higher in the ex-
periment incubated with a mixture of benzoate and
isophthalate. Hydrogen concentrations were approx-
imately 10 times higher than under exponential
growth conditions [19].
ble 1, the equation for benzoate fermentation be-
It is evident from these data that part of the re-
ducing equivalents generated during benzoate and
isophthalate oxidation (reactions 1 and 3, Table 1)
were incorporated into benzoate resulting in the for-
mation of carboxycyclohexane (reaction 4, Table 1).
From the electron balance, calculated according to
Eq. 3 and shown in Fig. 1, it can be seen that the
amount of reduction equivalents formed during oxi-
dation of the aromatic substrates corresponds rea-
sonably well to the amount of reduction equivalents
incorporated in the reduced products. Furthermore,
it should be noted that molecular hydrogen corre-
sponds to less than 1% of the accumulating concen-
tration of reduced products and therefore plays a
minor role in the electron balance.
comes the following:
CC2
3
ꢀQC23QBA† WClC2WCC2 ‡ ꢀQHCO 3QBA† WClHCO W
‡
3
3
ꢀ
QCCH3QBA† WClCCHWCCCH ‡ QH WCH ˆ 0
ꢀ3†
2
2
Subscripts C2, BA, HCO3, CCH, and H2, stand
for acetate, benzoate, bicarbonate, carboxycyclohex-
ane and molecular hydrogen respectively. Because
decarboxylation of terephthalate to benzoate does
not involve net oxidation or reduction, the same
equation can be used for analyses of products
formed during conversion of the phthalate isomers.
Incorporation of electrons into biomass is ignored
during these calculations.
The actual Gibbs free energy changes of the di¡er-
ent conversions observed were calculated from the
measured substrate and product concentrations. De-
3
. Results
31
carboxylation of isophthalate (vGPw331 kJ mol
,
Phthalate-, isophthalate- and terephthalate-grown
reaction 2, Table 1) and oxidation of isophthalate to
31
methanogenic cultures were incubated with the cor-
responding phthalate isomers, or a mixture of ben-
zoate and the phthalate isomers in the presence of
BES (20 mM). BES is a speci¢c inhibitor of metha-
nogenesis. During the incubation, substrate depletion
and product formation were studied for analysis of
product formation from the phthalate isomers and
benzoate fermenting cultures.
It was found that during incubation of the iso-
phthalate-grown culture with isophthalate as the
sole carbon and energy source, benzoate, acetate,
carboxycyclohexane and molecular hydrogen accu-
mulated (Fig. 1). When the culture was incubated
with a mixture of benzoate and isophthalate, ben-
zoate was preferred over isophthalate, as was previ-
ously described for the terephthalate-grown enrich-
ment culture [20]. As in the culture incubated with
isophthalate, acetate, hydrogen and carboxycyclo-
hexane accumulated during incubation with a ben-
zoate/isophthalate mixture. In both experiments
methane formation was negligible. Molecular hydro-
gen accumulated to comparable concentrations in
both experiments, whereas acetate and carboxycyclo-
acetate and hydrogen (vGPw359 kJ mol , reaction
1, Table 1) remained exergonic throughout the peri-
od of the elevated hydrogen concentration. Gibbs
free energy changes for oxidation and reduction of
benzoate (reactions 3 and 4, Table 1) are shown in
Fig. 1. In the experiment with isophthalate as sole
substrate, both reduction and oxidation of benzoate
remained exergonic throughout the experiment, and
the Gibbs free energy change of approximately 330
kJ mol3 for both conversions are highly compara-
ble. Consequently, the Gibbs free energy change for
oxidation of carboxycyclohexane remained around
0 kJ mol3 , because the Gibbs free energy change
for this conversion equals the value for benzoate
oxidation minus the value for benzoate reduction.
Also during the experiment with a mixture of iso-
phthalate and benzoate, the Gibbs free energy
change for carboxycyclohexane oxidation was rela-
tively constant and close to 0 kJ mol3 . The initial
values for benzoate oxidation and reduction were,
however, more negative than in the experiment
with isophthalate as sole carbon source, due to the
higher benzoate concentrations.
1
1
1

R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
277
Fig. 1. Degradation of isophthalate (left graphs) and a mixture of isophthalate and benzoate (right graphs) by the isophthalate-grown en-
richment culture in the presence of 20 mM BES. From top to bottom the graphs show (i) the concentration of isophthalate (IF) and ben-
zoate (BA), (ii) the concentration of acetate (C2), carboxycyclohexane (CCH) and molecular hydrogen (H2), (iii) the actual Gibbs free en-
ergy change for benzoate oxidation (BAox, reaction 3, Table 1), benzoate reduction (BAred, reaction 4, Table 1) and carboxycyclohexane
oxidation (CCHox, reaction 5, Table 1), and (iv) the electron balance according to Eq. 2.

2
78
R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
serum bottles. Results of the experiments with a mix-
ture of benzoate and phthalate are shown in Fig. 2,
and the Gibbs free energy changes for the di¡erent
conversions involving benzoate are shown in Fig. 3.
In the absence of molecular hydrogen, benzoate and
phthalate were initially converted to carboxycyclo-
hexane and acetate. After approximately 3 days,
phthalate conversion stopped and only benzoate
was degraded further. As in the isophthalate experi-
ment, the Gibbs free energy change for carboxycy-
clohexane oxidation was approximately 0 kJ mol3
1
.
In the presence of hydrogen in the headspace, no
conversion of phthalate was observed, but benzoate
was degraded at a comparable rate compared to the
experiment without hydrogen. Initially, however, no
acetate was produced, despite the observation that
benzoate oxidation is exergonic, and all benzoate
converted was reduced to carboxycyclohexane with
molecular hydrogen. Acetate was only formed once
the hydrogen concentration in the headspace had
reached comparable values to those observed in ex-
periments without hydrogen dosage. As with the bot-
tles incubated with the benzoate/phthalate mixture,
an initial reduction of phthalate to carboxycyclohex-
ane was observed in serum bottles incubated with
phthalate and molecular hydrogen (data not shown).
Trace amounts of benzoate (approximately 50 WM)
accumulated in these bottles. Once the Gibbs free
energy change for carboxycyclohexane oxidation be-
came exergonic due to hydrogen uptake, acetate for-
mation was observed.
Fig. 2. Degradation of benzoate (BA, E/F) and phthalate (OF,
a/b, top graph) by the phthalate grown enrichment culture, and
the accumulation of carboxycyclohexane (CCH, middle graph)
and acetate (C2, bottom graph) in the absence (open markers,
solid lines) and presence (solid markers, dashed lines) of hydro-
gen in the headspace.
The phthalate-grown culture was incubated with
phthalate, and a mixture of phthalate and benzoate
as well. In this case, experiments were also per-
formed in the absence or presence of approximately
Product formation during degradation of phtha-
late and terephthalate by their corresponding enrich-
ment cultures (without hydrogen dosage) showed a
similar pattern as described for isophthalate. The
2.5% molecular hydrogen in the headspace of the
Fig. 3. Gibbs free energy changes for benzoate oxidation (BAox, a), benzoate reduction (BAred, F) and carboxycyclohexane oxidation
CCHox, O) in the absence (left graph) or presence (right graph) of molecular hydrogen in the headspace.
(

R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
279
average values for the Gibbs free energy changes of
the various conversions (Table 1) are presented in
Table 2. It is evident that the Gibbs free energy
changes of the di¡erent conversions were highly
comparable for the di¡erent cultures utilised. A mi-
nor di¡erence between the three cultures was that
isophthalate degradation was completely inhibited
by benzoate (Fig. 1), while limited conversion of
phthalate and terephthalate was observed during in-
cubation with benzoate (not shown). However, con-
version of phthalate and terephthalate stopped with-
in a few days when incubated with benzoate, even
though benzoate conversion continued. This obser-
vation indicates that the cultures rapidly lost their
capacity to convert the phthalate isomers in the pres-
ence of the preferred substrate benzoate, as described
previously for the terephthalate-grown enrichment
culture [20].
Because carboxycyclohexane was formed during
our experiments, we checked the cultures for their
ability to degrade carboxycyclohexane in the absence
of BES. All three phthalate isomer-grown enrich-
ment cultures were found to be capable of carboxy-
cyclohexane degradation (initial concentration 5
mM) at rates ranging from 5 to 25% of the degra-
dation rates of the phthalate isomers and without a
lag phase (data not shown).
ments part of the phthalate isomer or benzoate was
oxidised to acetate, reduction equivalents generated
during the oxidation was incorporated in benzoate
and carboxycyclohexane was formed. Small amounts
of benzoate accumulated in the experiments incu-
bated with the phthalate isomers as sole carbon
and energy source. A schematic representation of
the conversions observed is presented in Fig. 4.
Reduced product formation at elevated hydrogen
partial pressures, comparable to the carboxycyclo-
hexane formation in our experiments, has been ob-
served before. Smith and McCarty demonstrated
that ethanol perturbation of an ethanol- and propi-
onate-fed chemostat led to accumulation of hydro-
gen in the biogas and consequently the formation of
n-propanol and four to seven n-carboxylic acids [4,5].
Hickey and Switzenbaum suggested that hydrogen
accumulated only to slightly higher concentrations
during organic overloads of an anaerobic digester,
due to reduced product formation [12,26].
The electron balances showed that 85^95% of the
reduction equivalents generated during the oxidation
of the aromatic substrates was incorporated into car-
boxycyclohexane. The observation that in all our
experiments the concentration of reduced products
could not fully account for the oxidised products
detected suggests that an unidenti¢ed reduced prod-
uct may have accumulated. A possible reduced prod-
uct is formate. Formate has previously been identi-
¢ed as an alternative electron carrier for molecular
hydrogen in methanogenic systems [11,27,28].
Mainly at elevated bicarbonate concentrations, as
applied during our experiments, formate may play
an important role in electron transfer among species.
However, the calculated formate concentrations,
considering a thermodynamic equilibrium between
4. Discussion
The patterns of product formation during incuba-
tion with either the phthalate isomer or a mixture of
phthalate and benzoate were found to be highly
comparable for all three phthalate isomer-grown
methanogenic enrichment cultures. In all experi-
Table 2
Actual Gibbs free energy change (vGP, kJ reaction31) for the conversions shown in Table 1 in experiments incubated with the phthalate
isomers as sole carbon and energy sources in the presence of BES
Culture
Phthalate isomer oxidation
reaction 1)
Benzoate oxidation
(reaction 3)
Benzoate reduction
(reaction 4)
Carboxycyclohexane oxidation
(reaction 5)
(
Phthalate
Isophthalate
Terephthalate
360.5 (1.3)
359.6 (1.8)
362.2 (2.3)
332.5 (1.5)
328.2 (1.1)
329.7 (2.8)
331.0 (1.5)
328.7 (0.6)
327.4 (2.6)
31.5 (2.1)
0.5 (1.5)
32.3 (1.6)
Calculations were performed during the time-period values were more or less constant and an equilibrium appeared to exist. Reaction
numbers correspond to the chemical reaction equations shown in Table 1. Values between brackets represent standard deviations in the
calculated vGP values.

2
80
R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
Fig. 4. Schematic representation of the conversions observed in the BES-amended phthalate isomer-grown cultures.
formate and hydrogen/bicarbonate, could not ac-
count for the observed gap in the electron balance
[15]. These authors determined the threshold concen-
trations of benzoate and the corresponding critical
Gibbs free energy change as a function of the acetate
concentration in a de¢ned coculture consisting of the
syntrophic benzoate degrader, strain SB, and the De-
sulfovibrio sp. strain G-11.
(
calculations not shown).
4.1. Energetics of the conversions observed
The observation that the Gibbs free energy
changes for oxidation and reduction of benzoate
are highly comparable in all our experiments sug-
gests that the Gibbs free energy change for carbox-
ycyclohexane oxidation is close to 0 kJ mol3 (see
Table 2). Two lines of reasoning can be used to
describe the mechanistic basis for this observation:
(i) carboxycyclohexane may be a true intermediate in
the oxidation of benzoate, or (ii) it may be a side
product originating from a di¡erent non-aromatic
intermediate. The ¢rst possibility suggests that oxi-
dation of benzoate only proceeds as long as the
Gibbs free energy change for carboxycyclohexane
oxidation is negative. The second possibility suggests
that an equilibrium may develop if both oxidation
and reduction of benzoate proceed at a comparable
e¤ciency. In this case intermediates may accumulate
to such concentrations that the Gibbs free energy
changes for both oxidation and reduction of ben-
zoate remain su¤ciently negative to sustain growth
and/or conversion of the aromatic substrate.
From the data in Table 2 it appears that all phtha-
late isomer-grown cultures gave comparable Gibbs
free energy changes for all conversions observed.
The energetics of the di¡erent conversions will there-
fore be discussed regardless of the phthalate isomer-
grown culture utilised.
1
The Gibbs free energy change for oxidation of the
phthalate isomers reached a stable value of approx-
3
1
imately 360 kJ mol in all cultures incubated with
the phthalate isomer as sole carbon source. Even
though initially some conversion of the phthalate
isomers was observed in cultures incubated with a
mixture of the phthalate isomers and benzoate, ben-
zoate was the preferred substrate in all cases.
Both in cultures incubated with the phthalate iso-
mers and the phthalate/benzoate mixture, the Gibbs
free energy change for oxidation and reduction of
3
1
benzoate was found to be around 330 kJ mol
.
No benzoate conversion was observed at Gibbs
free energy changes for oxidation or reduction of
3
1
benzoate exceeding approximately 328 kJ mol
,
In summary, we suggest that within narrow ener-
getic limits, simultaneous oxidation and reduction of
benzoate may proceed. However, our experiments
provide no answer to the question if carboxycyclo-
hexane (or its CoA derivative) is an intermediate in
the oxidation of benzoate, or a byproduct originat-
suggesting that this value may represent a minimum
amount of energy required to sustain the conversion
of benzoate. This critical Gibbs free energy change
for benzoate conversion is remarkably close to the
31
value of 330 kJ mol , reported by Warikoo et al.

R. Kleerebezem et al. / FEMS Microbiology Ecology 29 (1999) 273^282
281
[
[
[
11] Schink, B. (1992) Syntrophism among prokaryotes. In: The
Prokaryotes (Balows, A., Tr u« per, H.G., Dworkin, M., Hard-
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fact that we worked with mixed cultures furthermore
keeps the question open whether one or more organ-
isms were involved in the conversions observed.
Acknowledgements
This project was supported through IOP Milieu-
biotechnologie (Innovative Research Program Envi-
ronmental Biotechnology, The Netherlands). R.K.
wishes to thank Alfons J.M. Stams for critical review
of the manuscript.
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