Antonie van Leeuwenhoek 81: 271–282, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Biogeochemistry and microbial ecology of methane oxidation in anoxic
environments: a review
David L. Valentine
Scripps Institution of Oceanography-0202, University of California at San Diego, 9500 Gilman Drive, San Diego
CA, 92093-0202, USA and Department of Geological Sciences, University of California at Santa Barbara, Santa
Barbara, CA 93106, USA (E-mail: firstname.lastname@example.org)
Key words: acetate, anaerobic methane oxidation, anoxic environments, archaea, interspecies hydrogen transfer,
methane hydrates, subsurface biosphere, syntrophy, sulfate reduction
Evidencesupportinga key role for anaerobicmethane oxidationin the global methanecycle is reviewed. Emphasis
is on recent microbiological advances. The driving force for research on this process continues to be the fact that
microbial communities intercept and consume methane from anoxic environments, methane that would otherwise
enter the atmosphere. Anaerobic methane oxidation is biogeochemically important because methane is a potent
greenhouse gas in the atmosphere and is abundant in anoxic environments. Geochemical evidence for this process
has been observed in numerous marine sediments along the continental margins, in methane seeps and vents,
around methane hydrate deposits, and in anoxic waters. The anaerobic oxidation of methane is performed by at
least two phylogenetically distinct groups of archaea, the ANME-1 and ANME-2. These archaea are frequently
observed as consortia with sulfate-reducing bacteria, and the metabolism of these consortia presumably involves
a syntrophic association based on interspecies electron transfer. The archaeal member of a consortium apparently
oxidizes methane and shuttles reduced compounds to the sulfate-reducing bacteria. Despite recent advances in
understanding anaerobic methane oxidation, uncertainties still remain regarding the nature and necessity of the
syntrophic association, the biochemical pathway of methane oxidation, and the interaction of the process with the
local chemical and physical environment.This review will consider the microbial ecology and biogeochemistry of
anaerobic methane oxidation with a special emphasis on the interactions between the responsible organisms and
More than 25 years have elapsed since geochemical
studies first revealed the anaerobic oxidation of meth-
ane (AOM) in anoxic marine sediments and waters
(Barnes & Goldberg 1976; Reeburgh 1976; Martens
& Berner 1977). Subsequent geochemical, micro-
biological and biogeochemical studies have contrib-
uted to understanding the importance of this process
to the global methane (CH4) cycle. Recent studies
employing modern tools of molecular biology and
biogeochemistry have provided further insight into
the microbial ecology of this process (Hinrichs et al.
1999; Boetius et al. 2000; Orphan et al. 2001b).
There now exists compelling evidence that AOM is
performed by a consortium of CH4-oxidizing archaea
and sulfate-reducing bacteria (SRB) in some environ-
ments. There is also now compelling evidence that
AOM is mediated by more than one species of archaea
with the possibility that some archaea oxidize CH4
without the need for a syntrophic partner bacterium.
This review integrates previous studies of AOM and
considers the process in the context of the surround-
ing environment. Particular emphasis is placed on the
environmental prevalence of AOM, the nature of the
syntrophic association, the chemical and physical in-
teractions between this process and the environment,
and the relevance of this process to the subsurface
Environmental prevalence and biogeochemistry
Depth distributions of CH4 concentration in anoxic
marinesediments providedthe first evidencefor AOM
(Barnes & Goldberg 1976; Reeburgh 1976; Martens
& Berner 1977). Subsequent geochemical studies in-
cluding radio-tracer studies (Panganiban et al. 1979;
Reeburgh 1980; Zehnder & Brock 1980; Devol &
Ahmed1981; Iversen& Blackburn 1981; Devol 1983;
Alperin & Reeburgh 1985; Iversen & Jørgensen 1985;
Iversen et al. 1987; Ward et al. 1987; Alperin 1989;
Ward et al. 1989; Reeburgh et al. 1991; Hoehler et al.
1994; Hansen et al. 1998; Joye et al. 1999; Boetius et
al. 2000; Fossing et al. 2000; Jørgensen et al. 2001;
Thomsen et al. 2001), stable isotope distributions
(DesMarais 1983; Whiticar & Faber 1986; Alperin et
al. 1988; Alperin 1989; Reeburgh et al. 1991; Blair &
car 1999; Borowski et al. 2000; Oremland & Paull et
al. 2000; Valentine & Reeburgh 2000), and the ap-
plication of diagenetic (advection-reaction-diffusion)
models (Barnes & Goldberg 1976; Reeburgh 1976;
Martens & Berner 1977; Reeburgh & Heggie 1977;
Berner 1980; Devol & Ahmed 1981; Alperin & Ree-
burgh 1984; Scranton 1988; Alperin 1989; Hoehler
et al. 1994; Blair & Aller 1995; Borowski et al.
1996; Borowski et al. 1997; Niewohner et al. 1998;
Borowski et al. 1999; Martens et al. 1999; Borowski
et al. 2000; Fossing et al. 2000; Jørgensen et al. 2001;
Thomsen et al. 2001) provided compelling evidence
for this process in marine sediments. Several previ-
ous works have reviewed the geochemical evidence
supporting AOM (Alperin & Reeburgh 1984; Hoehler
et al. 1994; Hoehler & Alperin 1996; Valentine &
Reeburgh 2000). The net chemical reaction associated
with AOM is given in Equation (1).
Marine sediments cover approximately 70% of the
Earth’s solid surface, and sediments display signific-
ant physical and chemical diversity. One important
factor pertaining to AOM is the organic content of
the sediment. Sediments with high organic content
tend to deplete their supply of oxidants closer to the
sediment–water interface than sediments with low or-
ganic content. Oxidants (O2, NO−
interfaceand are used by microbesfor the oxidationof
organic material in a thermodynamically-determined
3+ HS−+ H2O (1)
3, Fe(III), Mn(IV),
4) enter the sediment at the sediment-water
order, with SO2−
ply of oxidants becomes depleted, CO2becomes the
most powerful oxidant, and decomposition of organic
material is coupled to CH4 production. The depth
(zone) in the sediment where sulfate reduction gives
way to methanogenesis is referred to as the sulfate to
methane transition. This depth also corresponds to the
zone of AOM, where CH4 from depth first encoun-
not to deplete their supply of oxidants, and such sedi-
ments do not give way to CH4production. Sediments
along the worlds continents tend to contain signific-
antly moreorganicmaterial thansediments in the deep
surface waters. Thus, methanogenesis (and AOM) is
more prevalent in sediments along the continents than
in the deep ocean.
Within CH4-containing sediments there is a di-
versity of conditions that impact AOM including or-
ganic content, CH4 supply rate, sulfate penetration,
temperature, and pressure. In marine sediments that
are characterized by diffusion as the dominant mixing
process (the majority of sediments), organic content
and CH4 supply significantly impact AOM. For ex-
ample, the sediments of Skan Bay, AK contain large
quantities of organic material, and the CH4-sulfate
transition is located between 25 and 35 cm depth
below the seafloor (Alperin 1989). Conversely, sedi-
ments of the Blake Ridge have a low organic content,
and the CH4-sulfate transition is located around 20
m below the seafloor in some areas (Borowski et
al. 2000). Because of differences in organic content
between sediments, and the corresponding CH4flux,
the process of AOM can be spread out spatially with
corresponding changes in the rate.
Several recent advances in understanding AOM
have come from studies of CH4 seeps and vents in
geologically active areas (Rusanov et al. 1994; Pi-
menov et al. 1997; Elvert et al. 1999; Hinrichs et al.
1999; Peckmann et al. 1999; Suess et al. 1999; Thiel
et al. 1999; Boetius et al. 2000; Elvert et al. 2000;
Hinrichs et al. 2000; Pancost et al. 2000; Pancost et
al. 2001; Thiel et al. 2001; Orphan et al. 2001a,b)
In such sediments advection is the dominant mixing
process, and porewaters are forced through the sed-
iment along cracks and faults by pressure gradients.
Porewaters travelling upward are often rich in CH4
due to the decomposition of organic material below.
The combination of high CH4concentration and ad-
vective flow provides abundant CH4 for AOM, and
leads to very high metabolic rates and dense microbial
being consumed last. When the sup-
4. Sediments with low organic content tend
Figure 1. The influence of CH4partial pressure (atm) on the Gibbs
Free Energy yield (kJ mol−1) for AOM (Equation (1)). Calcula-
tions assume the following conditions: Temperature 4◦C; pH 7.2;
of anoxic waters, diffusion-dominated sediments, and seeps/vents
are given for reference.
320 mM; HS−2 mM; SO2−
10 mM. Methane levels typical
communities (Suess et al. 1999; Boetius et al. 2000;
Tryon & Brown 2001). The rates of AOM in such
high-CH4environments can be orders of magnitude
higher than in typical diffusion-dominated sediments.
However, vents and seeps are not widely distributed,
and the rapid rates are not representative of AOM in
most marine sediments.
Some anoxic sediments contain CH4 hydrates,
from CH4and water under low temperature and high
pressure. Hydrates occur naturally in some high-CH4
environmentsincluding tectonically active continental
margin sediments, as well as in sediments or per-
mafrost areas overlying oil and gas deposits. The
anaerobic oxidation of CH4occurs in the vicinity of
hydrate deposits. Evidence indicates that CH4 dis-
solved in pore fluids around the hydrates seems to
drive AOM, though hydrates themselves do not har-
bor many microbes (Lanoil et al. 2001). Factors such
as the presence of oxygen (in the case of surficial
hydrates), the availability of sulfate, physical inter-
actions between microbes and hydrates, the presence
of non-CH4hydrocarbons, hydrate stability, and het-
erogeneity within the hydrate/sediment system further
complicates our understanding of AOM in hydrate-
bearing environments; these topics are ripe for future
Modeling and tracer experiments indicate that AOM
occursin a varietyofanoxic, sulfate-containingwaters
including the Black Sea, Cariaco Basin, Mono Lake,
CA, and Big Soda Lake, NV (Panganiban et al. 1979;
Oremland & DesMarais 1983; Iversen et al. 1987;
Oremland et al. 1987; Ward et al. 1987; Scranton
1988; Ward et al. 1989; Reeburgh et al. 1991; Joye et
al. 1999). In these environments AOM is an important
sink for CH4, though the rates of AOM vary depend-
ing on environmental conditions. Less is known about
AOM in anoxic waters than is known about AOM
in sediments. One major difference between AOM
in the water column compared to sediments is that
the CH4concentration is generally much lower in an-
oxic waters, and the sulfate concentration is generally
higher. The Gibbs Free Energy available to perform
AOM in anoxic waters is poor compared to typical
sediments (Figure 1), and it is not clear that organ-
isms in the anoxic waters can utilize CH4as their sole
carbon and energy source. However, recent observa-
tions of isotopically-depleted lipid biomarkers in the
anoxic waters of the Black Sea indicate that archaea
in such environments may be capable of assimilating
CH4(Schouten et al. 2001).
AOM in other environments
Available evidence indicates that AOM is coupled to
sulfate reduction. Theoretically, other terminal elec-
tron acceptors (NO−
to oxidize CH4under anoxic conditions. Such reac-
tions would provide a greater free energy yield than
sulfate-dependent CH4oxidation. Furthermore, CH4
does comeintocontactwith oxidantsincludingabund-
(Smemo & Yavitt 2000). Despite theoretical argu-
ments for the existence of AOM coupled to alternative
electron acceptors, no compelling evidence has been
3, Fe(III), Mn(IV)), could also act
Relevance to the global CH4cycle
In most CH4-containing marine sediments, AOM oc-
curs at the base of the sulfate reducing zone. Methane
travelling upward meets the sulfate travelling down-
ward and AOM ensues. Nearly all of the CH4 is
oxidized in this situation, inhibiting CH4 transport
upward from the subsurface. Assuming that AOM
consumes all CH4produced in marine sediments, and
that the system is near steady state (i.e., the CH4reser-
voir in marine sediments remains constant over time),
the net production rate of CH4in marine sediments is
approximately equal to the rate of AOM plus the rate
of methane escape into the water column. The net rate
of AOM in marine sediments has been estimated from
70 Tg of CH4per year (Reeburgh et al. 1993; Ree-
burgh 1996) to 300 Tg of CH4per year (Hinrichs &
thatescapes thesediments, thenet rateof CH4produc-
tion in marine sediments can be estimated between 75
and320Tgof CH4per year. The discrepancyin estim-
ates presented by Reeburgh et al. (1991, 70 Tg CH4
per year) and Hinrichs & Boetius (2002, 300 Tg CH4
per year) arises because of the AOM rates and spatial
extents used in their calculations. Both estimates rely
on averaging previously published rate calculations
to estimate global AOM rates. One potential bias in
both calculations comes from the choice of represent-
ative environments used in the rate calculations. The
primaryreason many of these environmentshave been
studied is precisely because they exhibit high rates
of AOM, much higher than surrounding areas. Both
estimates assume that these environments are repres-
entative of the entire depth interval worldwide, which
is likely an overestimation of the average rate. One
further complication is that estimates made by Ree-
burgh(1991)consider continentalshelf sediments, but
not deeper sediments along the margins. This likely
leads to an underestimation in the spatial extent where
AOM occurs. Hinrichs & Boetius (2002) consider a
larger depth interval, thoughthey assume high rates of
AOM. Their estimate (300 Tg CH4per year) is likely
an upper estimate of the global rate of AOM. Ree-
burgh et al. (1991) also use high rates for calculations,
though they consider only shelf sediments. The extent
to which these factors offset is not clear, and their
estimate (70 Tg CH4per year) is still viable. These
estimates for net CH4production/oxidation in marine
sediments may prove useful in considering rates of
organic matter remineralization and in calculating the
magnitude of the marine CH4reservoir. Experimental
and modeling studies are needed to provide further
constraints on these values, and such studies should
account for sampling biases.
Phylogenetic, isotopic, and visual evidence clearly
indicate that a consortium of archaea and bacteria
oxidize CH4in some seep environments (Hoehler &
Alperin 1996; Hinrichs et al. 1999; Boetius et al.
2000; Valentine & Reeburgh 2000; Orphan et al.
2001b). The close physical association of ANME-2
archaea and bacteria indicates a syntrophic associ-
ation (Valentine 2001). However, there is only indirect
evidence pertaining to the nature of the syntrophic
coupling, and limited evidence indicates that ANME-
1 archaea may oxidize CH4without a tightly coupled
syntrophic partner (Orphan 2002). Relevant evidence
can be considered in five distinct categories: phylo-
genetic inferences, pure culture studies, mesocosm
studies, observational studies, and theoretical studies.
Each of these categories is considered below.
a powerful tool to identify organisms in the natural
environment, but is of only limited use in determin-
ing microbial activity. In the case of AOM, 16SrDNA
sequencing has revealed the phylogenetic placement
of both archaea and bacteria involved in the process
(Hinrichs et al. 1999; Boetius et al. 2000; Hin-
richs et al. 2000; Thomsen et al. 2001; Orphan et
al. 2001a,b). The ANME-2 archaea are closely re-
lated to the Methanosarcinales, a group of largely
methylotrophic methanogens, including all known
aceticlastic methanogens. A few species within the
Methanosarcinalesare capable of performingH2/CO2
methanogenesis, thoughnone are capable of methano-
genesis using formate. The Methanosarcinales have
the broadest substrate range among known methano-
genic orders, and many species run an oxidative
metabolism (from methyl to CO2) during the dis-
mutation of methylated compounds. Based on in-
formation about the Methanosarcinales it is possible
to infer details about AOM. For example, the ar-
chaea involved in the CH4-oxidizing consortium are
most likely the CH4 oxidizers, as they fall within
a group that exclusively metabolizes CH4. It is also
likely that the organisms employ many of the oxidat-
ive steps used in methylotrophic methanogenesis, and
that they produce reduced intermediates such as H2,
acetate, or other methylated compounds (Zehnder &
Brock 1980; Hoehler et al. 1994; Hoehler & Alperin
1996; Valentine & Reeburgh 2000). The bacteria in-
reducing bacteria. These groups of sulfate-reducing
bacteria are generally complete oxidizers of organic
acids, and are often involved in hydrocarbon degrada-
tion. Although less is known about these bacteria than
the archaea, it is possible to infer that bacteria are
directly reducing sulfate, and receiving some sort of
reduced intermediate from the archaea.
Pure culture studies
No organisms have been isolated capable of perform-
ing AOM in a manner consistent with environmental
observations. However, there have been several pure
culture studies that provide insight into the process.
Early studies by Zehnder & Brock (1979, 1980)
showed that methanogens were capable of oxidizing
CH4to CO2, but the rate of oxidation was only a frac-
tion of the simultaneous CH4-production rate. These
studies showed that methanogens are capable of activ-
ating the CH4molecule. Studies of stable (13C) iso-
tope fractionation during methanogenesis (Summons
et al. 1998) have shown that some methylotrophic
methanogens produce lipids that are highly isotopic-
ally depleted compared to both substrate and product,
aiding in the interpretation of lipid biomarker13C.
Studies performed in our laboratory have shown that
low H2is not a simple trigger to reverse methanogen-
esis, indicating that reverse methanogenesis is not a
general ability of methanogens(Valentine et al. 2000).
The term mesocosm study is used here to include all
studies in which natural samples were collected and
manipulated to measure biological activity. Examples
include radiotracer additions, inhibition studies, and
incubations. Rate studies employing the addition of
radioactive tracers provided evidence that CH4oxida-
tionwas coupledto sulfate reduction(Devol &Ahmed
1981; Devol 1983); inhibition studies employing bro-
moethanesulfonic acid (BES – a specific inhibitor of
reducing bacteria to the process (Alperin & Reeburgh
1985; Hoehler et al. 1994; Hansen et al. 1998); incub-
ation studies have provided further evidence linking
methanogens and sulfate-reducing bacteria to AOM
(Lidstrom 1983; Hoehler et al. 1994).
Recent mesocosm studies performed by Nauhaus
et al. (2002) provide additional evidence for the link
between CH4oxidation and sulfate reduction, as well
as insights into the microbiologyof the process. These
studies utilized sediment samples taken from CH4
ance (∼1010cells per gram of dry sediment) of the
(ANME-2, Desulfosarcina/Desulfococcus)AOM con-
sortia. Sulfate reduction was observed to be tightly
coupled to CH4oxidation in all incubations. A broad
temperature optimum was observed from 4 to 16◦C,
with significantly lower metabolic rates at temperat-
ures greater than 20◦C. This temperature behavior
indicates either that one or both of the organisms in-
volved are adapted to low temperature, or that the
syntrophic association only functions at low temper-
ature. This work is also the first to clearly demonstrate
the predicted influence of CH4concentration on rates
of AOM and sulfate reduction. A ten-fold increase in
CH4 concentration led to a 3–4 fold increase in the
rate of sulfate reduction. The authors also amended
incubations with potential intermediates including hy-
drogen, formate, acetate and methanol, and observed
that none of these compounds inhibited or facilitated
sulfate reduction. These results can be used to argue
against any of these compounds as important inter-
mediates in AOM, though Nauhaus et al. (2002) stop
short of making this argument.
Observational studies include the variety of phys-
ical, geochemical, biogeochemical, and microbiolo-
gical studies which attempt to quantify or observe the
natural condition within the environment. Examples
include pore water chemical distributions, isotopic
abundance of biomarkers, and visual (microscopic)
analysis of microbial communities. The similarities
in sediment pore water chemical distributions across
a variety of marine sediments have led to the gen-
eral consensus that CH4 oxidation is dependent on
sulfate. Because CH4 is depleted in13C relative to
other compounds, natural isotopic abundance can be
used to track CH4-derived carbon. Observations of
13C-depleted archaeal and bacterial lipid biomarkers
provide evidence for the assimilation of CH4-derived
carbon by the organisms producing the lipids (Elvert
et al. 1999, 2000; Hinrichs et al. 1999, 2000; Peck-
mann et al. 1999; Thiel et al. 1999, 2001; Boetius
et al. 2000; Pancost et al. 2000, 2001; Bian et al.
2001; Orphan et al. 2001a,b). Consistent isotopic dif-
ferences between13C-depleted bacterial and archaeal
lipids has been interpreted to indicate that archaea are
directly oxidizing CH4, and that bacteria are acting
as syntrophic partners. Microscopic studies employ-
ing florescence in situ hybridization (FISH), coupled
to isotopic analysis of individual microbial aggregates
(using secondary ion mass spectrometry, or SIMS),
Table 1. Potential reactions performed by archaea and bac-
teria involved in AOM
Previously proposed reaction mechanisms for the
Associated reactions catalyzed by SRB (syntrophic
have provided compelling evidence that a consortium
of CH4-oxidizing archaea and sulfate-reducing bac-
teria oxidize CH4 in seep environments (Orphan et
al. 2001b). The combination of FISH with SIMS
provides the first direct evidence for CH4metabolism
by a consortium of archaea and bacteria, and confirms
previous observations linking phylogenetic data and
lipid isotope abundanceto the archaea and the bacteria
In a morerecent study Orphanet al. (2002)applied
the FISH-SIMS approach to probe other archaea and
bacteria involved in AOM. The primary result of this
work is the discovery that ANME-1 archaea are iso-
topically depleted in13C and are thus likely involved
as active partners in AOM. Another important obser-
vation from this work is that the ANME-1 archaea are
not usually found as a close consortium with bacteria,
though they are often found in a loose association. In
addition to enhancing our knowledge of the archaea
involved in AOM, the Orphan et al. (2002) study also
indicates additional diversity among the bacteria as-
sociated with AOM. Bacterial groups distinct from
the Desulfosarcina-related organisms (Boetius et al.
2000) were found in close associations with ANME-
2 archaea. These results provide clear evidence for
the biological complexity of AOM, and indicate that
spatially-constrained consortia are not required for
AOM. Future research on these topics is likely to re-
veal even greater diversity in AOM communities than
Theoretical considerations of the syntrophic associ-
ation driving AOM have focussed on the bioenerget-
ics of the process (Zehnder & Brock 1979; Zehnder
& Brock 1980; Hoehler et al. 1994; Harder 1997;
Valentine & Reeburgh 2000). Proposed mechanisms
presume that the oxidation of CH4 by an archaea
is coupled to the generation of reduced intermedi-
ates that are subsequently oxidized by SRB. Zehnder
& Brock (1979) proposed that a reversal of H2/CO2
methanogenesiscouldleadto the net oxidationofCH4
under appropriate environmental conditions (Table 1,
Equation (2)). Hoehler et al. (1994) provided evid-
ence supportingthis hypothesis,and furthercalculated
the bioenergetic constraints on the process. Studies
in our lab indicate that acetate production from two
CH4 molecules (Table 1, Equation (5)) is also con-
sistent with available evidence, and could provide
greater Gibbs Free Energy yields for the organisms
involved (Valentine & Reeburgh 2000). A direct re-
versal of aceticlastic methanogenesis has also been
considered (Table 1, Equation (4)), but the possibility
discarded because of unfavorablekinetics and thermo-
dynamics (Hoehler et al. 1994; Valentine & Reeburgh
2000). However, given the more favorable thermo-
dynamic conditions found in high-CH4environments
(Figure 1), the phylogenetic placement of the ANME-
2 archaea (Hinrichs et al. 1999), the possibility of
multiple mechanisms, and favorable kinetics (Søren-
son et al. 2001), interspecies acetate transfer (Table 1,
Equation (4)) remains a viable mechanisms for AOM
in high-CH4 environments. Other mechanisms such
as interspecies formate transfer (Table 1, Equation
(3)) are also possible, though less likely than those
considered above. The two most viable mechanisms
(Table 1, Equations (2) and (5)) both represent novel
catabolism in the CH4fixation step, but are otherwise
feasible with knownenzymaticpathwaysfoundwithin
the Methanosarcinales. The metabolism of the syn-
trophic sulfate reducer (Table 1, Equations (6)–(8))
need not be different from standard metabolism for
any of these potential mechanisms.
A recent study by Sørenson et al. (2001) couples
thermodynamic considerations into a kinetic model
of interspecies electron transfer. By considering the
impact of intercellular chemical gradients and dif-
fusion on the Gibbs Free Energy yield of catabol-
ism, Sørenson et al. (2001) conclude that it is not
possible for a methane-oxidizing consortium to be
based on interspecies transfer of H2, acetate, or
methanol. This study also concludes that formate
is a possible shuttle, though revised calculations at
environmentally-relevant temperatures indicate it is
not. This work does not consider alternative mechan-
isms including the simultaneous transfer of hydrogen
andacetate (Valentine& Reeburgh2000).The conclu-
sions reached by Sørenson et al. (2001) assume low
CH4and low sulfate concentrations, as are found at
the sulfate to methane transition in many sediments.
The generalization of these results to methane seep
and vent areas may not be valid as the Gibbs Free En-
ergy available in these settings is much higher (Figure
1) than for the conditions assumed by Sørenson et al.
(2001). Calculations assuming high-CH4 conditions
indicate that interspecies acetate transfer is favorable,
while transfer of hydrogen, formate, or methanol are
Observations indicate that the process of AOM inter-
acts extensively with the local physical and chemical
environment. The responsible communities can be
controlled by environmental factors, and the process
supply, sediment organic content, sediment porosity,
and sediment mineralogy all affect AOM, and lead to
complications in studying this process.
Organic rich coastal sediments
Seasonal changes tend to impact AOM in shallow
marine environments, but are unlikely to impact deep
environments as conditions there tend to remain rel-
atively constant. In some cases, seasonal changes in
climate significantly alter the rates of sulfate reduc-
tion and methanogenesis, and also change the depth
of the CH4-sulfate transition. For example, shallow
sediments of Cape Lookout Bight, NC change from
net CH4consumption in the winter to net CH4pro-
duction in the summer (Hoehler et al. 1994). The
process of AOM influences the sediments primarily
through impacts on pore water chemical distributions
and remineralization rates.
CH4seeps and hydrate-bearing sediments
The chemical reaction associated with AOM involves
consumption of sulfate and CH4to produce carbonate
and hydrogen sulfide (Equation (9)). This chemical
change has the net effect of increasing the alkalinity
of the porewater, which facilitates the precipitation
of carbonate minerals. Massive authigenic carbonate
deposits are frequently associated with marine CH4
seeps, and are found to be depleted in the heavy
isotope of carbon,13C (Suess et al. 1999); lipid bio-
markers have further linked these formations to AOM
(Thiel et al. 1999). Unlike AOM which tends to pre-
cipitate carbonate minerals, aerobic CH4 oxidation
(Equation (10)) tends to dissolve carbonates as CO2
is a weak acid. Carbonate mineral deposits associated
with AOMcanpersist formillionsofyears(Thielet al.
2001), and provide some evidence for the persistence
of this process through time.
+ H2S + H2O
?ALK = 2
CH4+ 2O2→ CO2+ 2H2O
Marine CH4 seeps and vents give rise to vent
communities at the sediment–water interface. Com-
munities generally fall into one of two categories:
microbial mats or clam beds. Recent evidence indic-
ates that hydrologic flow patterns dictate the nature
of the community (Tryon & Brown 2001). Microbial
mats dominate at sites with a consistent net outflow of
porewater, while clam beds dominate at sites with a
transient flow direction. The surface communities are
linked to AOM by sulfide, one of the waste products
of AOM (Equation (1)). The microbial mats around
CH4seeps are composed largely of sulfide oxidizing
bacteria, and the mats form over areas of net outflow
where sediments exhibit high rates of AOM and high
sulfide concentrations (up to 15 mM sulfide). Clam
communities tend to die under elevated sulfide and
apparently prefer environments with slower rates of
AOM and periodic inflow of seawater into the sedi-
ments. Additional diversity within the microbial mats
may also be related to subsurface AOM, but such a
hypothesis is unsubstantiated.
The relation between AOM and CH4hydrates is
complex and involves physical, chemical, and biolo-
gical interactions. Prior studies have considered the
physical interaction between microbes and hydrates
(Lanoil et al. 2001), as well as the chemical influ-
ence of high CH4 on AOM (Boetius et al. 2001).
Basic physical and chemical principles are used here
to constrain the interactions between hydrates and
AOM within marine sediments, and to consider the
question: do microbes consume CH4hydrates? While
the answer to this question is not currently known,
the following considerations may apply. In order for
a sediment-bound hydrate to be stable it must be
?ALK = 0 (10)
in chemical equilibrium with the surrounding envir-
onment, which requires a significant methane con-
centration (∼10–20 mM) in the pore waters. Given
to consume CH4directly from the hydrate. Further-
more, CH4bound in the hydrate lattice is presumably
unavailable to microbes unless the hydrate lattice is
broken. Because the rate of hydrate dissociation is
likely rapid relative to the rates of AOM there would
be little advantage to mining CH4 from the hydrate
directly. Effectively, microbes mediating AOM would
facilitate the decomposition of hydrates by creating a
disequilibrium between hydrate-bound CH4and CH4
dissolved in the pore water. The closer the AOM com-
munityresides to the hydrate,the greaterthe mass flux
equate heat flow into the hydrate). However, the AOM
community also requires sulfate, which must be trans-
portedfromthe water columnto theAOM community.
In the case of diffusion limitation, the optimal loca-
tion for AOM communities to maximize metabolism
lies between the hydrate and the sediment–water in-
terface. Future field, laboratory, and modeling studies
are needed to determine these complex relationships.
The deep subsurface biosphere
Below Earth’s solid surface exists a microbially-
dominated biosphere (Whitman et al. 1998). While
there is active debate about the nature and magnitude
of this biosphere, much of the microbial activity is
undoubtedly due to the remineralization of buried or-
ganic material. Nearly all of the organic material
present in the subsurface is ultimately derived from
face biosphere is ultimately driven by solar radiation
at the surface. In marine sedimentary environments
(comprising ∼70% of Earth’s solid surface) the dom-
inant microbial processes are sulfate reduction and
organic material to CO2as the primary product, while
methanogenesis acts to remineralize organic material
to near-equal amounts of CO2and CH4; AOM links
these two processes by oxidizing CH4to CO2at the
expense of sulfate. While AOM occurs in subsurface
environments and is presumably a chemoautotrophic
process, the primary substrates, CH4and SO2−
exist because of the Earth’s photosynthetically-driven
Like all biological processes AOM is constrained
by environmental extremes including temperature,
redox conditions, and pH. However, AOM seems well
adapted to some extreme conditions including high
sulfide levels, high pressure, near-freezing temper-
atures, and low energy conditions. Sulfide levels in
CH4seeps frequently exceed 15 mM as sulfide is a
waste product of AOM. It is not known how hydro-
static pressure influences AOM, though the process
occurs at moderate depths in the seafloor (Elvert et
al. 2000). AOM occurs at two different energetic
extremes, starvation and energy conservation. Geo-
chemical evidence from sediment cores collected by
the ocean drilling program indicate that AOM pro-
ceeds very slowly, with activity proportional to the
CH4flux (Borowski et al. 1999). While available data
does not allow for the calculation of metabolic rates
for individual cells, the population likely lives at the
edge of starvation with respect to substrate supply
(Harder 1997). Bioenergetic calculations also indicate
that AOM occurs with a minimal Gibbs Free Energy
yield, and that this process occurs near the biolo-
gical energy quantum (Hoehler et al. 1994; Hoehler
& Alperin 1996; Valentine & Reeburgh 2000). Future
studies, including ocean drilling program leg 201, are
likely to tighten the link between AOM and the deep
Community complexity: arguments for a
The combination of 16SrDNA gene surveys and iso-
tope analysis of lipid biomarkers applied to meth-
anotrophic communities in CH4 seeps has provided
evidence for a complex community structure. Based
primarilyon lipidisotopeevidence,some authorshave
proposed that a variety of organisms and mechan-
isms may be active at any given site. While available
evidence is consistent with a variety of organisms per-
forming AOM, evidence is also consistent with the
presence of a secondary microbial community living
from the waste products and remains of the primary
Highrates ofAOM will supplysignificantamounts
of organic carbon to the sediment in the form of mi-
crobial biomass and waste products. With high rates
of metabolism in CH4 seeps (Boetius et al. 2000),
and correspondingly high cell densities, a signific-
ant buildup of CH4-derived carbon is expected. Lipid
biomarkersare but one exampleof such a buildup. As-
suming a constant CH4supply, the population should
eventually reach a steady state in which cell growth
and cell death occur at the same rate. The event of
cell death will provide labile organic material to the
porewater, and such CH4-derived carbon would be
available to the remainder of the sediment microbial
community. If certain heterotrophic microbes special-
ize in consuming a particular class of compounds
produced by the primary methanotrophic community,
they couldbe expectedto maintain the isotopic abund-
anceofthe primarycommunityas heterotrophstendto
retainthe isotopicsignatureoftheirfoodsource. Other
microbes might acquire organic material from mixed
sources, and could be expected to maintain an iso-
topic content intermediate between their food sources.
Still other microbes may grow autotrophically and ac-
quire their cellular carbon from CO2. However, most
of the CO2in CH4seeps is generated from CH4, and
many of these organisms may show moderate isotopic
The relative importance of endogenous sediment
organic material and CH4-derived organic material to
the secondary microbial community need not be pro-
portional to the pool size. Organic material deposited
with the sediment is likely to be more refractory than
many of the products from the methanotrophic com-
munity, which receives a continual supply of carbon.
The relatively rapid turnoverof carbon from the meth-
anotrophic community could continually supply the
secondary community at a higher rate than the larger
pool of endogenous sedimentary organic material. In
such a scenario, the secondary microbial community
community. The relative isotopic influences of other
sedimentary organic material and CO2would depend
on the organiccontent of the sediment, the availability
of CH4and sulfate, and the flow history of the seep.
Additional evidence indicates that remineralization of
sedimentary organicmaterial is retarded in seep envir-
onments (Hinrichs et al. 2000), providing additional
evidence that much of the metabolic activity is likely
based on CH4.
Advective CH4 flow through marine sediments
generally occurs along faults and fractures and exhib-
its transience. Both the rates and direction of seepage
can change on short time scales (Tryon et al. 1999;
Tryon & Brown 2001). The methanotrophic com-
munity living within such sediments undoubtedly un-
dergoessignificant changesconcurrentlywith changes
in substrate supply. In addition to the general trends
in flow, there is extensive heterogeneity around the
seeps themselves. Seep environments are neither con-
sistent nor homogenous, and the associated microbial
communities likely share these traits. Lipid biomarker
evidence seems to differentiate highly active com-
munities from less active, dying, and dead communit-
ies (Hinrichs et al. 1999; Peckmann et al. 1999; Boet-
ius et al. 2000; Elvert et al. 2000; Hinrichs et al. 2000;
Pancost et al. 2000, 2001; Bian et al. 2001; Orphan
et al. 2001a; Thiel et al. 2001). However, this tool
indicates little about the past history of the seep and
associated microbial community, and the past history
likely confuses the interpretation of biomarker evid-
ence. In the event of complete substrate limitation, it
is likely the primary microbial community will die off
over time, and release much of their organic carbon to
the sediment porewaters. Accompanyingthis situation
is likely an enhancement of the secondary microbial
community, a dispersment of13C-depleted carbon to
the secondary community, a decrease in the absolute
quantity of13C-depleted biomarkers, and an increase
in the abundance of13C-depleted lipid breakdown
products (i.e., crocetane; Elvert et al. 2000).
Given recent observations of the ANME-1 and
ANME-2 archaea involved in AOM, it seems likely
that these organisms will eventually be isolated either
in pure culture or in coculture. The isolation of these
organisms will allow for a variety of novel physiolo-
gical, biochemical, and genomic studies of AOM.
Such studies will also provide a unique opportunity
to compare the well-characterized ecology of AOM to
the behavior of axenic cultures. Environmental gen-
into AOM. Large quantities of genetic sequence data
libraries (Beja et al. 2000) or using other methods. It
may also be possible to sequence the entire genome of
one or more key organism. Additional environmental
studies focussing on mRNA transcripts, coenzymes or
cofactors also have the potential to yield insight into
AOM. Future biogeochemical studies also hold the
potential to further our understanding of this process.
Funding for this work was provided by the National
in Microbial Biology (DBI-0074368), and through
the Life in Extreme Environments special competition
(OCE-0085607). I would like to thank Bill Reeburgh
and Ketil Sørensen for detailed information about
their previously-published calculations, Antje Boet-
ius and Vicki Orphan for sharing their unpublished
work, as well as Kai Hinrichs and Ryan Mueller for
constructive comments on the manuscript.
Alperin MJ (1989) The carbon cycle in an anoxic marine sediment:
concentrations, rates, isotope ratios, and diagenetic models.
Ph.D. Thesis, University of Alaska, Fairbanks.
Alperin MJ & Reeburgh WS (1984) Geochemical observations sup-
porting anaerobic methane oxidation. In: Crawford R & Hanson
R (Eds), Microbial Growth on C-1 Compounds ( pp 282–289).
American Society for Microbiology.
Alperin MJ & Reeburgh WS (1985) Inhibition experiments on
anaerobic methane oxidation. Appl. Environ. Microbiol. 50:
Alperin MJ, Reeburgh WS & Whiticar MJ (1988) Carbon and hy-
drogen isotope fractionation resulting from anaerobic methane
oxidation. Global Biogeochem. Cycles 2: 279–288.
Barnes RO & Goldberg ED (1976) Methane production and con-
sumption in anaerobic marine sediments. Geology 4: 297–300.
Beja O, Suzuki MT, Koonin EV, Aravind L, Hadd A, Nguyen LP,
Villacorta R, Amjadi M, Garrigues C, Jovanovich SB, Feldman
RA & DeLong EF (2000). Construction and analysis of bac-
terial artificial chromosome libraries from a marine microbial
assemblage. Environ. Microbiol. 2: 516–529.
Berner RA (1980) Early Diagenesis: A Theoretical Approach.
Princeton University Press.
Bian L, Hinrichs K-U, Xie T, Brassell SC, Iversen N, Fossing H,
Jørgensen BB & Hayes JM (2001) Algal and archaeal poly-
isoprenoids in a recent marine sediment: Molecular isotopic
evidence for anaerobic oxidation of methane. Geochemistry,
Geophysics, Geosystems 2: paper number 2000GC000112.
Blair NE & Aller RC (1995) Anaerobic methane oxidation on the
Amazon shelf. Geochim. Cosmochim. Acta 59: 3707–3715.
Boetius A, Ravenschlag K, Schubert CJ, Rickert D, Widdel F,
Gieseke A, Amann R, Jørgensen BB, Witte U & Pfannkuche
O (2000) A marine microbial consortium apparently mediating
anaerobic oxidation of methane. Nature 407: 623–626.
Borowski WS, Hoehler TM, Alperin MJ, Rodriguez NM & Paull
CK (2000) Significance of anaerobic methane oxidation in
methane-rich sediments overlying the Blake Ridge gas hydrates.
In: Paull CK, Matsumoto R, Wallace PJ & Dillon WP (Eds),
Proceedings of the Ocean Drilling Program, Scientific Results,
Vol. 164 (pp 87–99). (Ocean Drilling Program).
Borowski WS, Paull CK & Ussler W (1996) Marine pore-water
sulfate profiles indicate in situ methane flux from underlying gas
hydrate. Geology 24: 655–658.
Borowski WS, Paull CK & Ussler W (1997) Carbon cycling within
the upper methanogenic zone of continental rise sediments: An
example from the methane-rich sediments overlying the Blake
Ridge gas hydrate deposits. Marine Chem. 57: 299–311.
Borowski WS, Paull CK & Ussler W (1999) Global and local vari-
ations of interstitial sulfate gradients in deep-water, continental
margin sediments: Sensitivity to underlying methane and gas
hydrates. Marine Geol. 159: 131–154.
Devol AH (1983) Methane oxidation rates in the anaerobic sedi-
ments of Saanich Inlet. Limnol. Oceanogr. 28: 738–742.
Devol AH & Ahmed SL (1981) Are high rates of sulfate reduction
associated with anaerobic oxidation of methane? Nature 291:
Elvert M, Suess E, Greinert J & Whiticar MJ (2000) Archaea medi-
ating anaerobic methane oxidation in deep-sea sediments at cold
seeps of the eastern Aleutian subduction zone. Org. Geochem.
Elvert M, Suess E and & MJ (1999) Anaerobic methane oxidation
associated with marine gas hydrates: superlight C-isotopes from
saturated and unsaturated C-20 and C-25 irregular isoprenoids.
Naturwissenschaften 86: 295–300.
Fossing H, Ferdelman TG & Berg P (2000) Sulfate reduction
and methane oxidation in continental margin sediments influ-
enced by irrigation (South-East Atlantic off Namibia). Geochim.
Cosmochim. Acta 64: 897–910.
Hansen LB, Finster K, Fossing H & Iversen N (1998) Anaer-
obic methane oxidation in sulfate depleted sediments: effects
of sulfate and molybdate additions. Aquatic Microbial Ecol. 14:
Harder J (1997) Species-independent maintenance energy and nat-
ural population sizes. FEMS Microbiol. Ecol. 23: 39–44.
Hinrichs K-U & Boetius A (in press) The anaerobic oxidation of
methane: new insights in microbial ecology and biogeochem-
istry. In: Wefer G, Billet D, Hebbeln D, Jørgensen BB, Schlueter
M & Weering TV(Eds)Ocean Margin Systems, Springer Verlag,
Hinrichs K-U, Hayes JM, Sylva SP, Brewer PG & DeLong EF
(1999) Methane-consuming archaebacteria in marine sediments.
Nature 398: 802–805.
Hinrichs K-U, Summons RE, Orphan V, Sylva SP & Hayes JM
(2000) Molecular and isotopic analysis of anaerobic methane-
oxidizing communities in marine sediments. Org. Geochem. 31:
Hoehler TM & Alperin MJ (1996) Anaerobic methane oxidation by
a methanogen-sulfate reducer consortium: geochemical evidence
and biochemical considerations. In: Microbial Growth on C-1
Compounds (pp 326–333).
Hoehler TM, Alperin MJ, Albert DB & Martens CS (1994) Field
and laboratory studies of methane oxidation in an anoxic marine
sediment – evidence for a methanogen-sulfate reducer consor-
tium. Global Biogeochem. Cycles 8: 451–463.
Iversen N & Blackburn HT (1981) Seasonal rates of methane oxid-
ation in anoxic marine sediments. Appl. Environ. Microbiol. 41:
Iversen N & Jørgensen BB (1985) Anaerobic methane oxida-
tion rates at the sulfate-methane transition in marine sediments
from Kattegat and Skagerrak (Denmark). Limnol. Oceanogr. 30:
Iversen N, Oremland R & Klug MJ (1987) Big Soda Lake (Nevada).
3. Pelagic methanogenesis and anaerobic methane oxidation.
Limnol. Oceanogr. 32: 804–814.
Jørgensen BB, Weber A & Zopfi J (2001) Sulfate reduction and
anaerobic methane oxidation in Black Sea sediments. Deep-Sea
Research Part I – Oceanographic Research Papers 48: 2097–
Joye SB, Connell TL, Miller LG, Oremland RS & Jellison RS
(1999) Oxidation of ammonia and methane in an alkaline, saline
lake. Limnol. Oceanogr. 44: 178–188.
Lanoil BD, Sassen R, La Duc MT, Sweet ST & Nealson KH (2001)
Bacteria and Archaea physically associated with Gulf of Mexico
gas hydrates. Appl. Environ. Microbiol. 67: 5143–5153.
Lidstrom ME (1983) Methane consumption inFramvaren, ananoxic
marine fjord. Limnol. Oceanogr. 28: 1247–1251.
Martens CS, Albert DB & Alperin MJ (1999) Stable isotope tra-
cing of anaerobic methane oxidation in the gassy sediments of
Eckernforde Bay, German Baltic Sea. Am. J. Sci. 299: 589–610.
Martens CS & Berner RA (1977) Interstitial water chemistry of
LongIsland Soundsediments, I, dissolved gases. Limnol. Ocean-
ogr. 22: 10–25.
Nauhaus K, Boetius A, Kruker M & Widdel F (2002) In vitro
demonstration of anaerobic oxidation of methane coupled to
sulfate reduction in sediment from a marine gas hydrate area.
Niewohner C, Hensen C, Kasten S, Zabel M & Schulz HD (1998)
Deep sulfate reduction completely mediated by anaerobic meth-
ane oxidation in sediments of the upwelling area off Namibia.
Geochim. Cosmochim. Acta 62: 455–464.
Oremland RS & DesMarais DJ (1983) Distribution, abundance
and carbon isotope composition of gaseous hydrocarbons in Big
Soda Lake, Nevada: an alkaline meromictic lake. Geochim.
Cosmochim. Acta 47: 2107–2114.
Oremland RS, Miller LG & Whiticar MJ (1987) Sources and
fluxes of natural gases from Mono Lake, California. Geochim.
Cosmochim. Acta 51: 2915–2929.
Orphan VJ, Hinrichs K-U, Ussler W, Paull CK, Taylor LT, Sylva
SP, Hayes JM & DeLong EF (2001a) Comparative analysis
of methane-oxidizing archaea and sulfate-reducing bacteria in
anoxic marine sediments. Appl. Environ. Microbiol. 67: 1922–
Orphan VJ, House CH, Hinrichs K-U, McKeegan KD & DeLong
EF (2001b) Methane-consuming archaea revealed by directly
coupled isotopic and phylogenetic analysis. Science 293: 484–
Orphan VJ,House CH, Hinrichs K-U, McKeegan KD& DeLongEF
Multiple microbial groups mediate methane oxidation in anoxic
marine sediments. Proc. Nat. Acad. Sci.
Pancost RD, Hopmans EC & Sinninghe Damste JSS (2001) Ar-
chaeal lipids in Mediterranean cold seeps: Molecular proxies for
anaerobic methane oxidation. Geochim. Cosmochim. Acta 65:
Pancost RD, Sinninghe Damaste JS, DE Lint S, Van Der Maarel
MJEC, Gottschal JC & Science Party (2000) Biomarker evid-
ence for widespread anaerobic methane oxidation in Mediter-
ranean sediments by a consortium of methanogenic Archaea and
Bacteria. Appl. Environ. Microbiol. 66: 1126–1132.
Panganiban AT, Patt TE, Hart W & Hanson RS (1979) Oxidation of
methane in the absence of oxygen in lake water samples. Appl.
Environ. Microbiol. 37: 303–309.
Paull CK, Lorenson TD, Borowski WS, Ussler III W, Olsen K
& Rodriguez NM (2000) Isotopic composition of CH4, CO2
species, and sedimentary organic matter within samples from
the Blake Ridge: gas source implications. In: Paull CK, Mat-
sumoto R, Wallace PJ & Dillon WP (Eds) Proceedings of the
Ocean Drilling Program, Scientific Results, Vol. 164 (pp 67–78).
(Ocean Drilling Program).
Peckmann J, Thiel V, Michaelis W, Clari P, Gaillard C, Martire L &
Reitner J (1999) Cold seep deposits of Beauvoisin (Oxfordian;
southeastern France) and Marmorito (Miocene; northern Italy):
microbially induced authigenic carbonates. Int. J. Earth Sci. 88:
Pimenov NV, Rusanov, II, Poglazova MN, Mityushina LL, Sor-
okin DY, Khmelenina VN & Trotsenko YA (1997) Bacterial
mats on coral-like structures at methane seeps in the Black Sea.
Microbiology 66: 354–360.
Reeburgh WS (1976) Methane consumption in Cariaco Trench
waters and sediments. Earth Planetary Sci. Lett. 28: 337–344.
Reeburgh WS (1980) Anaerobic methane oxidation: rate depth dis-
tributions in Skan Bay sediments. Earth Planetary Sci. Lett. 47:
Reeburgh WS (1996) ‘Soft Spots’ in the Global Methane Budget.
In: Lidstrom ME & Tabita FR (Eds) Microbial Growth on C-1
Compounds (pp 335–342). Kluwer Academic Publishers.
Reeburgh WS & Heggie DT (1977) Microbial methane consump-
tion reactions and their effect on methane distributions in fresh-
water and marine environments. Limnol. Oceanogr. 22: 1–9.
Reeburgh WS,WardB, Whalen SC,Sandbeck KA,Kilpatrick KA&
Kerkhof LJ (1991) Black Sea methane geochemistry. Deep-Sea
Research Part a – Oceanographic Research Papers 38: S1189-
Reeburgh WS, Whalen SC & Alperin MJ (1993) The role of
methylotrophy in the global methane budget. In: Murrell JC
& Kelley DP (Eds) Microbial Growth on C-1 Compounds (pp
1–14). Kluwer Academic Publishers.
Rusanov II, Galchenko VF, Pimenov NV & Ivanov MV (1994)
Microbiology of carbon cycling in the Black Sea methane seep
region. Microbiology 63: 499–502.
Schink B (1997) Energetics of syntrophic cooperation in methano-
genic degradation. Microbiol. Mol. Biol. Rev. 61: 262–280.
Schouten S, Wakeham SG & Damste JSS (2001) Evidence for an-
aerobic methane oxidation by archaea in euxinic waters of the
Black Sea. Org. Geochem. 32: 1277–1281.
Scranton MI (1988) Temporal variation in the methane content of
the Cariaco Trench. Deep Sea Research part A 35: 1511–1523.
Smemo K, & Yavitt JB (2000) Evidence for anaerobic methane ox-
idation in freshwater peatlands. Eos Trans. AGU, 81(48), Fall
Meet. Suppl., B71C-02.
Sørensen KB, Finster K & Ramsing NB (2001) Thermodynamic
and kinetic requirements in anaerobic methane oxidizing consor-
tia exclude hydrogen, acetate, and methanol as possible electron
shuttles. Microbial Ecol. 42: 1–10.
Suess E, Torres ME, Bohrmann G, Collier RW, Greinert J, Linke
P, Rehder G, Trehu A, Wallmann K, Winckler G & Zuleger
E (1999) Gas hydrate destabilization: enhanced dewatering,
benthic material turnover and large methane plumes at the Cas-
cadia convergent margin. Earth Planetary Sci. Lett. 170: 1–15.
SummonsRE,Franzmann PD&Nichols PD(1998) Carbon isotopic
fractionation associated with methylotrophic methanogenesis.
Org. Geochem. 28: 465–475.
Thiel V, Peckmann J, Richnow HH, Luth U, Reitner J & Michaelis
W (2001) Molecular signals for anaerobic methane oxidation in
Black Sea seep carbonates and a microbial mat. Marine Chem.
Thiel V, Peckmann J, Seifert R, Wehrung P, Reitner J & Michaelis
W (1999) Highly isotopically depleted isoprenoids: Molecular
markers for ancient methane venting. Geochim. Cosmochim.
Acta 63: 3959–3966.
Thomsen TR, Finster K & Ramsing NB (2001) Biogeochemical and
molecular signatures of anaerobic methane oxidation in a marine
sediment. Appl. Environ. Microbiol. 67: 1646–1656.
Tryon MD & Brown KM (2001) Complex flow patterns through
Hydrate Ridge and their impact on seep biota. Geophys. Res.
Lett. 28: 2863–2866.
Tryon MD, Brown KM, Torres ME, Trehu AM, McManus J & Col-
lier RW (1999) Measurements of transience and downward fluid
flow near episodic methane gas vents, Hydrate Ridge, Cascadia.
Geology 27: 1075–1078.
Valentine DL (2001) Thermodynamic ecology of hydrogen-based
syntrophy. In: Seckbach J (Ed) Symbiosis:
and Model Systems, 147–161 Kluwer Academic Publishers,
Valentine DL, Blanton DC & Reeburgh WS (2000) Hydrogen pro-
duction by methanogens under low-hydrogen conditions. Arch.
Microbiol. 174: 415–421.
Valentine DL & Reeburgh WS (2000) New perspectives on anaer-
obic methane oxidation. Environ. Microbiol. 2: 477–484.
Ward BB, Kilpatrick KA, Novelli PC & Scranton MI (1987) Meth-
ane oxidation and methane fluxes in the ocean surface layer and
deep anoxic waters. Nature 327: 226–229.
Ward BB, Kilpatrick KA, Wopat AE, Minnich EC & Lidstrom
ME (1989) Methane oxidation in Saanich Inlet during summer
stratification. Continental Shelf Res. 9: 65–75.
Whiticar MJ (1996) Isotope tracking of microbial methane forma-
tion and oxidation. Mitt. Internat. Verein. Limnol. 25: 39–54.
Whiticar MJ (1999) Carbon and hydrogen isotope systematics of
bacterial formation and oxidation of methane. Chem. Geol. 161:
Whiticar MJ & Faber E (1986) Methane oxidation in sediment
and water column environments-Isotope evidence. Adv. Org.
Geochem. 10: 759–768.
Whitman WB, Coleman DC & Wiebe WJ (1998) Prokaryotes: The
unseen majority. Proc. Natl. Acad. Sci. USA 95: 6578–6583.
Zehnder AJ & Brock TD (1979) Methane formation and methane
oxidation by methanogenic bacteria. J. Bacteriol. 137: 420–432.
Zehnder AJ & Brock TD (1980) Anaerobic methane oxidation:
occurrence and ecology. Appl. Environ. Microbiol. 39: 194–204.