279
8.3
Aaerobic Oxidation of Methane (AOM)
As minor sources, also methanol, formaldehyde, and some methylated compounds such as
methyl amines and methyl mercaptans may be
used for methanogenesis. The energy yields by
these terminal steps in organic degradation are
lower than by anaerobic respiration, e.g. with
nitrate, metal oxides or sulfate. Sulfate reducing
bacteria therefore compete effectively with the
methanogenic archaea for substrates which both
groups of organisms can utilize, primarily acetate
and H 2 . As a result, methane is not produced as
the main end product in marine sediments until
sulfate has been largely depleted by sulfate
reducing bacteria and electron acceptors for a
respiratory oxidation of the organic matter are
therefore no longer available (Martens and Berner
1974). Only small amounts of methane may still be
generated in the sulfate zone from non-competitive methylated substrates. This is a major difference from the degradation of organic matter in
freshwater sediments where sulfate is highly
limited and methane is the main end product of
anaerobic mineralization.
Methane is a chemically unreactive molecule
that requires microbiological catalysis for the
activation and oxidation to CO 2 at environmental
temperatures. Aerobic bacteria that oxidize methane
with O 2 are well known from diverse environments,
including sea water and marine sediments, and
have been studied in pure culture in the laboratory
(Bowman 2000). These earlier physiological and
biochemical studies indicated that biological
methane oxidation requires molecular oxygen.
Anaerobic oxidation of methane (AOM) was
recognized by geochemists already in the 1970’ies,
however, as a key process in marine sediments
(Reeburgh 1969, 1976; Martens and Berner 1974;
Barnes and Goldberg 1976). A large number of
studies based on reaction-transport modeling,
radiotracer experiments, inhibition techniques, and
stable isotope data have since then firmly
established that methane is oxidized biologically in
the absence of O 2 at the transition between sulfate
and methane. The microorganisms responsible for
the process remained elusive, however, and even
today it has not been possible to bring anaerobic
methane oxidizers into pure culture for detailed
laboratory study. In spite of this, the identity of the
organisms has now been discovered and research
on the biogeochemistry of anaerobic oxidation of
methane has made significant progress in recent
years (see reviews by Valentine and Reeburgh
2000; Hinrichs and Boetius 2002).
8.3.1
The AOM Zone in Marine Sediments
Most of the methane produced in the sulfate
depleted sub-surface sediment diffuses upwards
along a steep methane gradient until it reaches the
lower sulfate zone and becomes oxidized. Within
this reaction zone, referred to as the “sulfatemethane transition” (SMT), pore-water methane
and sulfate are both consumed to depletion. This
distinct zone is typically located one to several
meters below the sediment surface in continental
margin sediments and plays a key role in the
biogeochemistry of the sea bed. In this zone most
of the energy and reducing power of organic
carbon mineralized to CO 2 and CH 4 below the
sulfate zone comes into contact with an efficient
electron acceptor, sulfate, and is finally oxidized to
CO 2 . Since this methane flux comprises most of
the subsurface methanogenesis, the anaerobic
oxidation of methane integrates the degradation of
buried organic carbon from the lower boundary of
the sulfate zone to very deep sediment layers
which were deposited many thousands to millions
of years ago. The coupled sulfate-methane reaction has been proposed to proceed according to
the following net equation, assuming a one to one
stoichiometry between methane and sulfate (e.g.
Murray et al. 1978; Devol and Ahmed 1981):
CH 4 + SO 4
2→ HCO 3
-
+ HS
-
+ H 2 O
(8.8)
Figure 8.5 shows an example of the sulfatemethane transition zone in a continental shelf
sediment from Kattegat (Denmark) at the transition between the Baltic Sea and the North Sea.
The sediment was characterized by a high sedimentation rate (0.16 cm yr
-1
) and consisted of finegrained silt and clay with an average organic
matter content of 12% (Iversen and Jørgensen
1985). Sulfate diffused downwards to become
depleted by sulfate reduction at 160 cm sediment
depth while methane diffused upwards and
penetrated ca. 40 cm into the lower sulfate zone to
ca. 120 cm depth. In order to determine anaerobic
methane oxidation rates in the sulfate-methane
transition zone, Iversen and Jørgensen (1985)
carried out radiotracer measurements of sulfate
reduction using
35
SO 4
2and of methane oxidation
using
14
CH 4 . Fig. 8.5 (right frame) shows that the
rates of sulfate reduction were high, starting from
below the 10-15 cm deep suboxic sediment, and
gradually dropped with increasing depth and age
in the sediment. In the SMT, the rates increased
8.3
Aaerobic Oxidation of Methane (AOM)
As minor sources, also methanol, formaldehyde, and some methylated compounds such as
methyl amines and methyl mercaptans may be
used for methanogenesis. The energy yields by
these terminal steps in organic degradation are
lower than by anaerobic respiration, e.g. with
nitrate, metal oxides or sulfate. Sulfate reducing
bacteria therefore compete effectively with the
methanogenic archaea for substrates which both
groups of organisms can utilize, primarily acetate
and H 2 . As a result, methane is not produced as
the main end product in marine sediments until
sulfate has been largely depleted by sulfate
reducing bacteria and electron acceptors for a
respiratory oxidation of the organic matter are
therefore no longer available (Martens and Berner
1974). Only small amounts of methane may still be
generated in the sulfate zone from non-competitive methylated substrates. This is a major difference from the degradation of organic matter in
freshwater sediments where sulfate is highly
limited and methane is the main end product of
anaerobic mineralization.
Methane is a chemically unreactive molecule
that requires microbiological catalysis for the
activation and oxidation to CO 2 at environmental
temperatures. Aerobic bacteria that oxidize methane
with O 2 are well known from diverse environments,
including sea water and marine sediments, and
have been studied in pure culture in the laboratory
(Bowman 2000). These earlier physiological and
biochemical studies indicated that biological
methane oxidation requires molecular oxygen.
Anaerobic oxidation of methane (AOM) was
recognized by geochemists already in the 1970’ies,
however, as a key process in marine sediments
(Reeburgh 1969, 1976; Martens and Berner 1974;
Barnes and Goldberg 1976). A large number of
studies based on reaction-transport modeling,
radiotracer experiments, inhibition techniques, and
stable isotope data have since then firmly
established that methane is oxidized biologically in
the absence of O 2 at the transition between sulfate
and methane. The microorganisms responsible for
the process remained elusive, however, and even
today it has not been possible to bring anaerobic
methane oxidizers into pure culture for detailed
laboratory study. In spite of this, the identity of the
organisms has now been discovered and research
on the biogeochemistry of anaerobic oxidation of
methane has made significant progress in recent
years (see reviews by Valentine and Reeburgh
2000; Hinrichs and Boetius 2002).
8.3.1
The AOM Zone in Marine Sediments
Most of the methane produced in the sulfate
depleted sub-surface sediment diffuses upwards
along a steep methane gradient until it reaches the
lower sulfate zone and becomes oxidized. Within
this reaction zone, referred to as the “sulfatemethane transition” (SMT), pore-water methane
and sulfate are both consumed to depletion. This
distinct zone is typically located one to several
meters below the sediment surface in continental
margin sediments and plays a key role in the
biogeochemistry of the sea bed. In this zone most
of the energy and reducing power of organic
carbon mineralized to CO 2 and CH 4 below the
sulfate zone comes into contact with an efficient
electron acceptor, sulfate, and is finally oxidized to
CO 2 . Since this methane flux comprises most of
the subsurface methanogenesis, the anaerobic
oxidation of methane integrates the degradation of
buried organic carbon from the lower boundary of
the sulfate zone to very deep sediment layers
which were deposited many thousands to millions
of years ago. The coupled sulfate-methane reaction has been proposed to proceed according to
the following net equation, assuming a one to one
stoichiometry between methane and sulfate (e.g.
Murray et al. 1978; Devol and Ahmed 1981):
CH 4 + SO 4
2→ HCO 3
-
+ HS
-
+ H 2 O
(8.8)
Figure 8.5 shows an example of the sulfatemethane transition zone in a continental shelf
sediment from Kattegat (Denmark) at the transition between the Baltic Sea and the North Sea.
The sediment was characterized by a high sedimentation rate (0.16 cm yr
-1
) and consisted of finegrained silt and clay with an average organic
matter content of 12% (Iversen and Jørgensen
1985). Sulfate diffused downwards to become
depleted by sulfate reduction at 160 cm sediment
depth while methane diffused upwards and
penetrated ca. 40 cm into the lower sulfate zone to
ca. 120 cm depth. In order to determine anaerobic
methane oxidation rates in the sulfate-methane
transition zone, Iversen and Jørgensen (1985)
carried out radiotracer measurements of sulfate
reduction using
35
SO 4
2and of methane oxidation
using
14
CH 4 . Fig. 8.5 (right frame) shows that the
rates of sulfate reduction were high, starting from
below the 10-15 cm deep suboxic sediment, and
gradually dropped with increasing depth and age
in the sediment. In the SMT, the rates increased