MICROBIOLOGY AND MOLECULAR BIOLOGY REVIEWS,
1092-2172/97/$04.0010
June 1997, p. 262–280 Vol. 61, No. 2
Copyright © 1997, American Society for Microbiology
Energetics of Syntrophic Cooperation in
Methanogenic Degradation
BERNHARD SCHINK*
Fakulta¨t fu¨r Biologie der Universita¨t Konstanz, D-78434 Konstanz, Germany
INTRODUCTION.......................................................................................................................................................262
ENERGETICS OF ATP FORMATION...................................................................................................................263
COOPERATION IN METHANOGENIC COMMUNITIES.................................................................................263
TYPES OF SYNTROPHIC ASSOCIATIONS.........................................................................................................264
ENERGETICS AND BIOCHEMISTRY OF SYNTROPHIC BUTYRATE OXIDATION.................................266
ENERGETICS AND BIOCHEMISTRY OF PROPIONATE OXIDATION .......................................................269
ENERGETICS AND BIOCHEMISTRY OF GLYCOLATE OXIDATION .........................................................270
ENERGETICS AND BIOCHEMISTRY OF ETHANOL OXIDATION ..............................................................271
OXIDATION OF AROMATIC COMPOUNDS......................................................................................................271
ACETATE OXIDATION............................................................................................................................................272
OXIDATION OF BRANCHED-CHAIN FATTY ACIDS .......................................................................................273
ANAEROBIC METHANE OXIDATION .................................................................................................................274
INTERSPECIES FORMATE VERSUS HYDROGEN TRANSFER.....................................................................274
INTERSPECIES ACETATE TRANSFER................................................................................................................275
HOMOACETOGENIC CONVERSIONS AND THE EFFECT OF TEMPERATURE......................................275
METABOLITE TRANSFER AND THE ADVANTAGE OF AGGREGATION...................................................276
METHANOGENS AS ENDOSYMBIOTIC PARTNERS OF PROTOZOA........................................................276
MODELLING OF SYNTROPHIC OXIDATION PROCESSES..........................................................................277
CONCLUSIONS .........................................................................................................................................................277
ACKNOWLEDGMENTS ...........................................................................................................................................278
REFERENCES ............................................................................................................................................................278
INTRODUCTION
The degradation of complex organic matter to methane and
CO
2
is a widespread process in anoxic environments which
receive only a limited supply of oxygen, nitrate, sulfate, or
oxidized iron or manganese species. It is the typical terminal
electron-accepting process in freshwater sediments rich in or-
ganic matter, in swamps and waterlogged soils such as rice
paddies, and in sewage treatment plants. Methanogenesis is
also an important process in fermentations occurring in the
intestinal tract of animals, especially ruminants. Methanogenic
degradation is the least exergonic process when compared to
aerobic degradation or the alternative anaerobic respirations.
Conversion of hexose to methane and carbon dioxide releases
only 15% of the energy that would be available in aerobic
degradation,
C
6
H
12
O
6
1 6O
2
3 6CO
2
1 6H
2
O(DG°9522,870 kJ z mol
21
)
C
6
H
12
O
6
3 3CO
2
1 3CH
4
(DG°952390 kJ z mol
21
)
The low energy yield of methanogenic degradation as com-
pared to that of the alternative oxidative processes may be the
reason why methanogenesis is the last step to occur, after the
other electron acceptors have been reduced. As a consequence
of this small energy gain, the reaction product, methane, stores
a major part of the energy available in aerobic biomass con-
version. This energy can be exploited subsequently for energy
in the presence of oxygen by other organisms, e.g., by aerobic
methane oxidizers or by humans in heating and other physical
processes.
The small amount of energy available in methanogenic con-
version forces the microorganisms involved into a very efficient
cooperation. The mutual dependence of partner bacteria with
respect to energy limitation can go so far that neither partner
can operate without the other and that together they exhibit a
metabolic activity that neither one could accomplish on its
own. Such cooperations are called syntrophic relationships.
Syntrophism is a special case of symbiotic cooperation be-
tween two metabolically different types of bacteria which de-
pend on each other for degradation of a certain substrate,
typically for energetic reasons. The term was coined to de-
scribe the close cooperation of fatty acid-oxidizing, fermenting
bacteria with hydrogen-oxidizing methanogens (77) or of pho-
totrophic green sulfur bacteria with chemotrophic sulfur-re-
ducing bacteria (11). In both cases, the pool size of the shut-
tling intermediate has to be kept small to allow efficient
cooperation of both partner organisms.
The term “syntrophy” should be restricted to cooperations
in which both partners depend on each other to perform the
metabolic activity observed and in which the mutual depen-
dence cannot be overcome by simply adding a cosubstrate or
any type of nutrient. A classical example is the “Methanobaci-
lus omelianskii” culture (6), which was later shown to be a cocul-
ture of two partner organisms, strain S and strain M.o.H. (18).
The two strains cooperate in the conversion of ethanol to acetate
and methane by interspecies hydrogen transfer, as follows:
Strain S: 2CH
3
CH
2
OH 1 2H
2
O 3 2CH
3
COO
2
1
2H
1
1 4H
2
(DG°95119 kJ per 2 mol of ethanol)
Strain M.o.H.: 4H
2
1 CO
2
3 CH
4
1 2H
2
O
* Mailing address: Fakulta¨t fu¨r Biologie der Universita¨t Konstanz,
Postfach 5560, D-78434 Konstanz, Germany. Phone: 49-7531-882140.
Fax: 49-7531-882966. E-mail: bernhard.schink@uni-konstanz.de.
262
First publ. in: Microbiology and Molecular Biology Reviews 61 (1997), 2, pp. 262-280
Konstanzer Online-Publikations-System (KOPS)
URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2700/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-27009
(DG°952131 kJ per mol of methane)
Coculture: 2CH
3
CH
2
OH 1 CO
2
3 2CH
3
COO
2
1
2H
1
1 CH
4
(DG°952112 kJ per mol of methane)
In this case, the fermenting bacterium cannot be grown with
ethanol in the absence of the hydrogen-scavenging partner
organism because it carries out a reaction which is endergonic
under standard conditions. The first reaction can occur and
provide energy for strain S only if the hydrogen partial pres-
sure is kept low enough (,100 Pa) by the methanogen. There-
fore, neither partner can grow with ethanol alone, and the
degradation of ethanol depends on the cooperation of the two
strains.
The dependence of the partner bacteria on each other has
caused severe problems in the cultivation of such bacteria, and
defined cocultures have been obtained only recently. For iso-
lation, pure cultures of known methanogenic or sulfate-reduc-
ing partner bacteria are usually provided in excess as a back-
ground “lawn” during the cultivation and dilution process to
isolate the syntrophically fermenting bacterium in defined bi-
nary or ternary mixed culture. Today, all well-described syn-
trophically fermenting bacteria can also be cultivated in pure
culture with different substrate combinations (see below).
Hence, we should no longer talk about “obligately syntrophic
bacteria” (because they are not obligately syntrophic) but only
about syntrophic relationships or syntrophic conversion pro-
cesses.
I avoid in this article the term “consortium,” which is quite
often used to describe any kind of cooperating enrichment
cultures. This term was originally coined for the structured
phototrophic aggregates Pelochromatium and Chlorochroma-
tium, etc., and is preferably used for such spatially well-orga-
nized systems (83, 123).
It is obvious from this introduction that energetic aspects are
of special importance for an understanding of syntrophic co-
operations in general. Earlier review articles have stressed this
view quite convincingly (17, 30, 74, 75, 95, 97, 109, 135, 136,
142, 144). The present overview focuses on the degradation of
fatty acids, alcohols, and benzoate and will try to reexamine
some of the earlier energetic concepts used in this field (118)
in the context of recent advances in the microbiology and
biochemistry of syntrophically fermenting bacteria. Details of
the biology, cultivation, and isolation of these bacteria have
been treated elsewhere (95, 109) or may be found in the orig-
inal references given in these review articles.
ENERGETICS OF ATP FORMATION
Anaerobes operate with small amounts of energy, and syn-
trophically cooperating anaerobes have been found to be “ex-
perts” in the exploitation of minimal energy. The synthesis of
ATP as the general currency of metabolic energy in living cells
requires 132 kJ per mol at equilibrium under standard condi-
tions; under the conditions assumed to prevail in an actively
growing cell ([ATP] 5 10 mM; [ADP] 5 1 mM, [P
i
] 5 10 mM),
about 150 kJ per mol is required (119). In addition, part of the
total energy budget is always lost in irreversible reaction steps
as heat, thus rendering the overall metabolic process irrevers-
ible. This heat loss (about 20 kJ per mol of ATP) has to be
added to the above value, which gives a total of about 70 kJ per
mol of ATP synthesized irreversibly in the living cell. This is
the minimum amount of energy required for the synthesis of 1
mol of ATP by all known metabolic systems (94). One may
argue that especially under conditions of energy limitation, an
organism may waste less energy in heat production or that it
may operate at a considerably lower energy charge than that
quoted above for well-growing Escherichia coli cells. Nonethe-
less, one cannot expect that the energy requirement for irre-
versible ATP synthesis would be substantially lower than about
160 kJ per mol.
The essential postulate of the Mitchell hypothesis of respi-
ratory ATP synthesis is that ATP formation is coupled to a
vectorial transport of charged groups, typically protons, across
a semipermeable membrane (79). Today, it has been widely
accepted that three protons cross the membrane per molecule
of ATP hydrolyzed, no matter whether bacterial (71) or mito-
chondrial (41, 127) membranes are studied. As a consequence,
the equivalent of one ATP unit is no longer regarded as the
smallest quantum of energy a living cell can make use of.
Actually, the smallest quantum of metabolically convertible
energy is that of an ion transported across the cytoplasmic
membrane, equivalent to one-third of an ATP unit. Combined
with the above calculations, this means that a bacterium needs
a minimum of about 220 kJ per mol to exploit the free energy
change in a reaction (94, 99). We will see that this is the
amount of energy with which bacteria, cooperating in syntro-
phic fermentations, have to make their living.
COOPERATION IN METHANOGENIC COMMUNITIES
The conversion of complex organic matter, e.g., cellulose, to
methane and carbon dioxide in a natural habitat is possible
only by the concerted action of at least four different groups of
bacteria, including primary fermenting bacteria, secondary fer-
menting bacteria, and two types of methanogens (17, 75, 95,
142, 144, 146). The degree of mutual dependence among these
different bacterial types varies considerably; whereas the later
members of the food chain always depend on the earlier ones
for their substrates, they may also exert a significant influence
on the earlier members in the chain by removing metabolic
products. Polymers (polysaccharides, proteins, nucleic acids,
and lipids) are first converted to oligomers and monomers
(sugars, amino acids, purines, pyrimidines, fatty acids, and glyc-
erol), typically through the action of extracellular hydrolytic
enzymes. These enzymes are produced by the “classical” pri-
mary fermenting bacteria, which ferment the resulting mono-
mers further to fatty acids, succinate, lactate, alcohols, etc.
(Fig. 1, group 1). Some of these fermentation products, espe-
cially acetate, H
2
,CO
2
and other one-carbon compounds, can
be converted directly by methanogenic bacteria into methane
and carbon dioxide (Fig. 1, groups 2 and 3). For degradation of
other fermentation products, e.g., fatty acids longer than two
carbon atoms, alcohols longer than one carbon atom, and
branched-chain and aromatic fatty acids, a further group of
fermenting bacteria, the so-called secondary fermenters or ob-
ligate proton reducers (Fig. 1, group 4), is needed. These
bacteria convert their substrates to acetate, carbon dioxide,
hydrogen, and perhaps formate, which are subsequently used
by the methanogens.
The situation is slightly different in sulfate-rich anoxic hab-
itats such as marine sediments. Also here, the primary pro-
cesses of polymer degradation are carried out by primary fer-
menting bacteria which form the fermentation products
mentioned above (Fig. 2, group 1). Different from methano-
genic bacteria, sulfate-reducing bacteria are metabolically ver-
satile, and a broad community of sulfate reducers can use all
products of primary fermentations and oxidize them to carbon
dioxide, simultaneously reducing sulfate to sulfide (133) (Fig.
2, groups 2 to 4). As a consequence, complete oxidation of
complex organic matter to carbon dioxide with simultaneous
VOL. 61, 1997 SYNTROPHIC COOPERATION IN METHANOGENIC DEGRADATION 263
sulfate reduction is a two-step process and does not depend on
syntrophic fermentations.
In methanogenic and sulfate-rich environments, the primary
fermenting bacteria (group 1) profit from the activities of the
hydrogen-oxidizing partners at the end of the degradation
chain as well. A low hydrogen partial pressure (,10 Pa) allows
electrons at the redox potential of NADH (2320 mV) to be
released as molecular hydrogen, and the fermentation patterns
can shift to more acetate, CO
2
, and hydrogen production
rather than ethanol or butyrate formation, thus allowing addi-
tional ATP synthesis. For example, Clostridium butyricum fer-
ments hexose in pure culture roughly according to the follow-
ing equation (119):
Glucose 1 2H
2
O 3 0.7 butyrate 1 0.6 acetate 1
1.3 H
1
1 2CO
2
1 2.6H
2
(DG°952233 kJ per mol)
yielding 3.3 ATP units per glucose.
At [H
2
] 5 10 Pa, the overall reaction changes to
Glucose 1 2H
2
O 3 2 acetate 1 2H
1
1 2CO
2
1 4H
2
(DG952280 kJ per mol)
yielding four ATP units per molecule of glucose, two in glyco-
lysis and two in the acetate kinase reaction. Obviously, the
bacteria balance the system to optimal energy exploitation
because exactly 70 kJ is used for the synthesis of one ATP unit
in both cases.
In a well-balanced anoxic sediment in which an active hy-
drogen-utilizing population maintains a low hydrogen partial
pressure, the flux of carbon and electrons goes nearly exclu-
sively through the “outer” paths of the flow schemes (Fig. 1
and 2), and reduced fermentation intermediates therefore play
only a minor role. Nonetheless, the flux through the “central”
paths will never become zero, because fatty acids, etc., are
always produced in the fermentation of lipids and amino acids
as well. The “central” reduced intermediates become more
important if the hydrogen pool increases for any reason, e.g.,
excess supply of fermentable substrate and inhibition of hy-
drogenotrophic methanogens due to a drop in pH (,6.0) or to
the presence of toxic compounds. Under such conditions, the
pools of fatty acids increase and might even shift the pH fur-
ther downward, thus inhibiting the hydrogenotrophic meth-
anogens even further. The consequence may be that the whole
system “turns over,” meaning that methanogenesis ceases en-
tirely and the fermentation stops with the accumulation of
huge amounts of ill-smelling fatty acids, as is frequently en-
countered in ill-balanced anaerobic sewage digestors. Obvi-
ously, the hydrogen- and formate-utilizing methanogens act as
the primary regulators in the total methanogenic conversion
process (17, 142, 144, 146) and the syntrophically fatty acid-
oxidizing bacteria are affected most severely by a failure in
methanogenic hydrogen or formate removal.
The function of homoacetogenic bacteria (Fig. 1, group 5) in
the overall process is less well understood. They connect the
pool of one-carbon compounds and hydrogen to that of ace-
tate. Due to their metabolic versatility, they can also partici-
pate in sugar fermentation and degradation of special sub-
strates such as N-methyl compounds or methoxylated phenols
(96). In certain environments, e.g., at lower pH or low tem-
perature, they may even successfully compete with hydrog-
enotrophic methanogens and take over their function to vari-
ous extents (see below).
TYPES OF SYNTROPHIC ASSOCIATIONS
The above-mentioned case of “Methanobacillus omelianskii”
is the classical example of interspecies hydrogen transfer. Both
partners operate in an overall reaction process which is exer-
gonic but becomes exergonic for the first partner only through
maintenance of a low hydrogen partial pressure by the second
partner. After description of the cooperative nature of this
process, the original S strain was lost, but other syntrophically
ethanol-oxidizing bacteria, such as Thermoanaerobium brockii
(10) and various Pelobacter strains (39, 91, 92), have been
isolated. Also, certain ethanol-oxidizing sulfate reducers such
as Desulfovibrio vulgaris have proven to be able to oxidize
ethanol in the absence of sulfate by hydrogen transfer to a
hydrogen-oxidizing methanogenic partner bacterium (19).
Similar cooperations have been described with syntrophic
FIG. 1. Carbon and electron flow through the various trophic groups of
microorganisms involved in the methanogenic degradation of complex organic
matter in an anoxic freshwater habitat. Groups of bacteria involved: 1, primary
fermenting bacteria; 2, hydrogen-oxidizing methanogens; 3, acetate-cleaving
methanogens; 4, secondary-fermenting (“syntrophic”) bacteria; 5, homoaceto-
genic bacteria. I, II, and III, steps in degradation. Based on reference 144;
modified from reference 95.
FIG. 2. Carbon and electron flow through the various trophic groups of
microorganisms involved in sulfate-dependent degradation of complex organic
matter in, e.g., a marine sediment. Groups of bacteria involved: 1, primary
fermenting bacteria; 2 to 4, sulfate-reducing bacteria. I and II, steps in degrada-
tion. Based on reference 133.
264 SCHINK MICROBIOL.MOL.BIOL.REV.
cultures degrading fatty acids. An overview of the reactions
catalyzed is presented in Table 1; a list of strains of syntrophi-
cally fermenting bacteria follows in Table 2. In general, deg-
radation of fatty acids to acetate and hydrogen or, in the case
of propionate, to acetate, hydrogen, and CO
2
is far more en-
dergonic under standard conditions than is the ethanol oxida-
tion described above. Consequently, for fatty acid degradation,
the hydrogen partial pressure has to be decreased to substan-
tially lower values (,10 Pa) than with ethanol (,100 Pa).
A special case is the syntrophic conversion of acetate to
2CO
2
and 4H
2
, which is catalyzed by a moderately thermo-
philic (58°C) bacterium, strain AOR (149). This is a homoace-
togenic bacterium which can either oxidize or synthesize ace-
tate, depending on the external hydrogen concentration (see
below).
Syntrophic oxidation of glycolate to 2CO
2
was discovered
only recently. Its energetic situation is comparable to that of
ethanol oxidation. Also, aromatic compounds and amino acids
can be oxidatively converted to acetate, CO
2
(and NH
4
1
) with
concomitant interspecies hydrogen transfer to methanogenic
partner bacteria.
On the side of hydrogen-consuming reactions, the function
of methanogens can also be taken over by homoacetogenic,
sulfur-reducing, sulfate-reducing, glycine-reducing, or fumar-
ate-reducing bacteria (Table 1). Thus, the classical Stickland
fermentation of pairs of amino acids can also be uncoupled
and be carried out by two partner bacteria cooperating in
interspecies hydrogen transfer. One partner oxidizes, e.g., ala-
nine to acetate, CO
2
,NH
4
1
, and hydrogen, and the other one
uses hydrogen for the reduction of glycine to acetate:
CH
3
CH(NH
3
1
)COO
2
1 2H
2
O 3 CH
3
COO
2
1
CO
2
1 NH
4
1
1 2H
2
(DG°9512.7 kJ per mol)
CH
2
(NH
3
1
)COO
2
1 H
2
3 CH
3
COO
2
1 NH
4
1
(DG°95278 kJ per mol)
Combined, the equations give
CH
3
CH(NH
3
1
)COO
2
1 2CH
2
(NH
3
1
)COO
2
1
2H
2
O 3 3CH
3
COO
2
1 CO
2
1 3NH
4
1
(DG°952151 kJ per mol)
Thus, the electrons derived in amino acid degradation by a
fermenting bacterium can be used in glycine reduction, as
shown, but can also be transferred to sulfate-reducing, ho-
moacetogenic, or methanogenic partner bacteria, depending
on the availability of such partner bacteria and their respective
electron acceptors. That amino acid oxidation and glycine re-
duction can be uncoupled from each other has been shown in
detail with Eubacterium acidaminophilum (148). This bacte-
rium can perform either of the first two reactions separately or
combine them on its own, according to the third reaction,
depending on the partner bacteria which act as hydrogen
sources or sinks and on the availability in the medium of
selenium, which is required for activity of the glycine reductase
enzyme complex.
Efforts have been made to grow these syntrophically fer-
menting bacteria without partner bacteria. Removal of hydro-
gen by nonbiological procedures (low pressure or gas diffusion
through thin membranes) had only little success with ethanol
oxidation and no success at all with fatty acid oxidation. In
other cases, hydrogen removal by palladium catalysts spread
TABLE 1. Changes of Gibbs free energies under standard conditions in hydrogen-releasing and hydrogen-consuming
reactions and corresponding redox potentials
a
Reaction
DG°9
(kJ per mol)
No. of
electron pairs
E°9 of electron-releasing
redox reactions (mV)
Hydrogen-releasing reactions
Primary alcohols
CH
3
CH
2
OH 1 H
2
O 3 CH
3
COO
2
1 H
1
1 2H
2
19.6 2 2190, 2375
Fatty acids
CH
3
CH
2
CH
2
COO
2
1 2H
2
O 3 2CH
3
COO
2
1 2H
1
1 2H
2
148.3 2 2125, 2250
CH
3
CH
2
COO
2
1 2H
2
O 3 CH
3
COO
2
1 CO
2
1 3H
2
176.0 3 130, 2176, 2470
CH
3
COO
2
1 H
1
1 2H
2
O 3 2CO
2
1 4H
2
194.9 4 2200, 2300, 2430, 2520
CH
3
CH(CH
3
)CH
2
COO
2
1 CO
2
1 2H
2
O 3 3CH
3
COO
2
1 2H
1
1 H
2
125.2 1
Glycolic acid
CH
2
OHCOO
2
1 H
1
1 H
2
O 3 2CO
2
1 3H
2
119.3 3 292, 2331, 2470
Aromatic compounds
C
6
H
5
COO
2
1 6H
2
O 3 3CH
3
COO
2
1 2H
1
1 CO
2
1 3H
2
149.5 3
C
6
H
5
OH 1 5H
2
O 3 3CH
3
COO
2
1 3H
1
1 2H
2
110.2 2
Amino acids
CH
3
CH(NH
3
1
)COO
2
1 2H
2
O 3 CH
3
COO
2
1 NH
4
1
1 CO
2
1 2H
2
12.7 2 2115, 2375
Hydrogen-consuming reactions
4H
2
1 2CO
2
3 CH
3
COO
2
1 H
1
1 2H
2
O 294.9
4H
2
1 CO
2
3 CH
4
1 2H
2
O 2131.0
H
2
1 S° 3 H
2
S 233.9
4H
2
1 SO
4
22
1 H
1
3 HS 1 4H
2
O 2151.0
H
2
C(NH
3
1
)COO
2
1 H
2
3 CH
3
COO
2
1 NH
4
1
278.0
Fumarate 1 H
2
3 succinate 286.0
a
All calculations are based on published tables (28, 119). For H
2
S and CO
2
, values for the gaseous state were used.
VOL. 61, 1997 SYNTROPHIC COOPERATION IN METHANOGENIC DEGRADATION 265
onto either charcoal or CaCO
3
surfaces, with alkenes or
alkines as oxidant, has shown some success (80), as have efforts
to couple hydrogen release to reoxidation by electrochemically
controlled platinum electrodes. More successful was the use of
fumarate as an external electron acceptor in the cultivation of
syntrophic propionate degraders (111). Today, pure cultures of
syntrophically fermenting bacteria of all known metabolic
types have been isolated. Typically, this has been accomplished
with substrates that are more oxidized than the original one
and can be fermented by dismutation. For example, ethanol-
oxidizing syntrophs can be grown in pure culture with acetal-
dehyde analogs such as acetoin or acetylene (39, 92), butyrate-
or benzoate-degrading syntrophs can be grown with crotonate
(7, 131, 147) or with pentenoate as an external electron accep-
tor (34), and syntrophically propionate-degrading bacteria can
be grown with pyruvate (129) or propionate plus fumarate
(111). Beyond that, all syntrophic propionate degraders have
also been shown to be able to reduce sulfate and can be
isolated in pure culture with propionate plus sulfate, although
they are very slow growers with this substrate combination (49,
50, 129, 130).
Biochemical studies with defined cocultures of syntrophi-
cally fermenting bacteria have been carried out successfully
with cell extracts prepared by, e.g., lysozyme (134) or mutano-
lysin (128) treatment, which lyses selectively only the ferment-
ing bacterium and leaves the methanogenic partner intact, due
to its archaeal cell wall chemistry. In another approach, the
partner organisms were separated by centrifugation in Percoll
gradients before being subjected to cell disruption and enzyme
assays (9).
ENERGETICS AND BIOCHEMISTRY OF SYNTROPHIC
BUTYRATE OXIDATION
The energetic situation of the partner bacteria involved in
butyrate conversion to methane and CO
2
is illustrated in Fig. 3.
The overall reaction
2CH
3
CH
2
CH
2
COO
2
1 2H
1
1 2H
2
O 3 5CH
4
1 3CO
2
yields, under standard conditions, a DG°9 of 2177 kJ per 2 mol
of butyrate. With concentrations more comparable to those
prevailing in natural habitats, e.g., a freshwater sediment or a
sewage sludge digestor (butyrate at 10 mM, CH
4
at 0.7 3 10
5
Pa; CO
2
at 0.3 3 10
5
Pa), the free energy of this process
changes to 2140 kJ per 2 mol of butyrate. This reaction is
catalyzed by a community of three different bacteria which
cooperate in seven independent partial reactions: butyrate
conversion to acetate and hydrogen runs twice, CO
2
reduction
to methane runs once, and acetate cleavage to methane and
CO
2
runs four times. All these seven partial reactions have to
yield ATP for the bacteria catalyzing them; they are specialists
TABLE 2. Pure or defined cultures of bacteria catalyzing syntrophic substrate oxidations via interspecies hydrogen transfer,
organized on the basis of the usual substrate
Isolate Substrate range Gram type Reference(s)
Oxidation of primary alcohols 2
Strain S Ethanol 2 18
Desulfovibrio vulgaris Ethanol 1 sulfate 2 19
Thermoanaerobium brockii Ethanol, sugars, etc. 1 10
Pelobacter venetianus Ethanol, propanol 98
Pelobacter acetylenicus Ethanol, acetylene 2 92
Pelobacter carbinolicus Ethanol, 2,3-butanediol 91
Oxidation of butyrate and higher homologs
Syntrophomonas wolfei C
4
–C
8
2 77, 78
Syntrophomonas sapovorans C
4
–C
18
2 89
Syntrophospora bryantii C
4
–C
11
, 2-methylvalerate 1 114, 147
Strains SF-1, NSF-2 C
4
–C
6
1 108
“Thermophilic coculture” C
4
?53
“Thermophilic coculture” C
4
?1
Oxidation of propionate
Syntrophobacter wolinii Propionate 2 14
Syntrophobacter pfennigii Pyruvate 2 130
Thermophilic coculture Propionate 1 fumarate 2 113
Strain MPOB Propionate 1 fumarate 2 33
Oxidation of acetate
Strain AOR Acetate, ethanol, ethylene glycol 1 66
Clostridium ultunense Acetate, formate, cysteine 1 103
Oxidation of isovalerate
Strain GraIva1 Isovalerate only 1 115
Oxidation of glycolate
Syntrophobotulus glycolicus Glycolate, glyoxylate 1 45, 46
Oxidation of aromatic compounds
Syntrophus buswellii Benzoate, crotonate 2 81
Syntrophus gentianae Benzoate, gentisate,
hydroquinone
2 131
Strain SB Benzoate, crotonate 2 56
266 SCHINK M
ICROBIOL.MOL.BIOL.REV.
