to the group of gammaproteobacteria, based on the analysis
of the gene encoding RNA16S. Among type II
methanotrophs (Methylosinus, Methylocystis), the inner
membranes are arranged at the cell periphery, and formaldehyde is incorporated into an amino acid, serine (cf. Sect. 3.4.
2). They often contain a soluble monooxygenase, in addition
to a particulate monooxygenase, and belong to the group of
alphaproteobacteria. The third type of methanotrophs
belongs to the genus Methylococcus (deltaproteobacteria)
and combines the features of both former types: disc-shaped
flattened vesicles and ribulose monophosphate pathway, but
elements of the serine cycle and a soluble monooxygenase
are also present.
14.2.7.7 Anaerobic Degradation of Methane
CH 4 can also be oxidized anaerobically by anaerobic
prokaryotes (Box 14.9).
Box 14.9: Anaerobic Oxidation of Methane (AOM)
Vincent Grossi
It has long been considered that the degradation of
methane in natural environments could only be driven
by aerobic microbial processes. During the last
decades, however, diverse studies have demonstrated
that methane can be oxidized in the absence of oxygen
and that anaerobic oxidation of methane (AOM)
essentially controls the biogeochemical cycle of this
potent greenhouse gas.
Following different geochemical observations
indicating the removal of methane within marine anoxic
sediments and seawater (e.g., Barnes and Goldberg
1976), field and laboratory experiments led to the
hypothesis that, in the marine environment, AOM is
carried out by a consortium of methanogenic archaea
and sulfate-reducing bacteria (SRB), despite the low
thermodynamic energy yield from this reaction (Hoeler
et al. 1994). In this model of obligate syntrophic interaction, SRB use the hydrogen produced during the oxidation of methane by methanogenic Archaea, which
perform the reverse reaction of the reduction of CO 2 to
methane (referred to as “reverse methanogenesis”).
Under such conditions, the final reaction becomes thermodynamically favorable (Box Scheme 14.1).
The principle of “reverse methanogenesis” has been
extensively discussed and generally adopted, but the
complete mechanism of sulfate-dependent AOM remains
V. Grossi (*)
Laboratoire de Ge ´ologie de Lyon: Terre, Plane `tes,
Environnement, UMR CNRS 5276, Universite ´ Claude Bernard
Lyon 1, 69622 Villeurbanne Cedex, France
(continued)
Box 14.9 (continued)
incompletely understood. A key issue is the nature of
the intermediate produced by the methanotrophic
archaea and scavenged by the sulfate-reducing bacterial
partner. Various alternatives have been proposed, often
with typical methanogenic growth substrates serving as
intermediates (such as H 2 , formate, acetate, or methanol)
that could eventually feed several SRB populations
(see example in Box Scheme 14.2 showing the use of
acetate and hydrogen by two different populations of
SRB; Hoeler et al. 1994; Valentine and Reeburgh
2000). Other studies have suggested that the SRB partner
uses (in addition to H 2 ) environmental CO 2 as a carbon
source, or methyl sulfides produced by the archaea
(e.g., CH 3 SH) as electron donor (Moran et al. 2008).
Confirmation of the existence of anaerobic
methanotrophs (ANME) at AOM sites came from the
measurement of stable carbon isotopes in lipid
biomarkers specific to archaea (see examples of
structures in Box Scheme 14.3a), which appeared
strongly depleted in
13 C compared with the source
methane (À130‰ < δ
13 C archaeal lipids <À50‰),
indicating that methane was the carbon source for this
archaeal community (Hinrichs et al. 1999; Michaelis
et al. 2002). Phylogenetic (16S rRNA) analysis of the
same samples further revealed the presence of novel
archaeal phylotypes likely linked to methane oxidation, whereas FISH microscopy and ion microprobe
mass spectrometry were elegantly used to confirm the
occurrence of consortia of ANME and SRB mediating
the AOM process (Boetius et al. 2000; Orphan et al.
2001; Strous and Jetten 2004).
The stable carbon isotope composition of other lipid
biomarkers proposed to derive from the SRB partners
involved in AOM (see examples of structures in
Box Scheme 14.3b) further supports a syntrophy
between ANME and SRB. These lipids are usually
less depleted (À90‰ < δ
13
C < À40‰) than the
archaeal lipids. Moreover, differences in biomarker
profiles from one AOM site to the other typify distinct
anaerobic microbial consortia involved in AOM
(Hinrichs et al. 2000; Michaelis et al. 2002; Elvert
et al. 2003; Blumenberg et al. 2004). In such settings,
the diversity of
13
C-depleted biomarkers of prokaryotic
origin are indicative of methane carbon transferring via
ANME to several SRB populations (see example in
Box Scheme 14.2) and then, eventually, to other microbial populations (Pancost and Sinninghe Damste ´ 2003).
Investigations based on 16S rRNA or mcrA (the key
gene of methanotrophic and methanogenic archaea)
gene phylogeny have shown that three distinct clusters
(continued)
562
J.-C. Bertrand et al.
of the gene encoding RNA16S. Among type II
methanotrophs (Methylosinus, Methylocystis), the inner
membranes are arranged at the cell periphery, and formaldehyde is incorporated into an amino acid, serine (cf. Sect. 3.4.
2). They often contain a soluble monooxygenase, in addition
to a particulate monooxygenase, and belong to the group of
alphaproteobacteria. The third type of methanotrophs
belongs to the genus Methylococcus (deltaproteobacteria)
and combines the features of both former types: disc-shaped
flattened vesicles and ribulose monophosphate pathway, but
elements of the serine cycle and a soluble monooxygenase
are also present.
14.2.7.7 Anaerobic Degradation of Methane
CH 4 can also be oxidized anaerobically by anaerobic
prokaryotes (Box 14.9).
Box 14.9: Anaerobic Oxidation of Methane (AOM)
Vincent Grossi
It has long been considered that the degradation of
methane in natural environments could only be driven
by aerobic microbial processes. During the last
decades, however, diverse studies have demonstrated
that methane can be oxidized in the absence of oxygen
and that anaerobic oxidation of methane (AOM)
essentially controls the biogeochemical cycle of this
potent greenhouse gas.
Following different geochemical observations
indicating the removal of methane within marine anoxic
sediments and seawater (e.g., Barnes and Goldberg
1976), field and laboratory experiments led to the
hypothesis that, in the marine environment, AOM is
carried out by a consortium of methanogenic archaea
and sulfate-reducing bacteria (SRB), despite the low
thermodynamic energy yield from this reaction (Hoeler
et al. 1994). In this model of obligate syntrophic interaction, SRB use the hydrogen produced during the oxidation of methane by methanogenic Archaea, which
perform the reverse reaction of the reduction of CO 2 to
methane (referred to as “reverse methanogenesis”).
Under such conditions, the final reaction becomes thermodynamically favorable (Box Scheme 14.1).
The principle of “reverse methanogenesis” has been
extensively discussed and generally adopted, but the
complete mechanism of sulfate-dependent AOM remains
V. Grossi (*)
Laboratoire de Ge ´ologie de Lyon: Terre, Plane `tes,
Environnement, UMR CNRS 5276, Universite ´ Claude Bernard
Lyon 1, 69622 Villeurbanne Cedex, France
(continued)
Box 14.9 (continued)
incompletely understood. A key issue is the nature of
the intermediate produced by the methanotrophic
archaea and scavenged by the sulfate-reducing bacterial
partner. Various alternatives have been proposed, often
with typical methanogenic growth substrates serving as
intermediates (such as H 2 , formate, acetate, or methanol)
that could eventually feed several SRB populations
(see example in Box Scheme 14.2 showing the use of
acetate and hydrogen by two different populations of
SRB; Hoeler et al. 1994; Valentine and Reeburgh
2000). Other studies have suggested that the SRB partner
uses (in addition to H 2 ) environmental CO 2 as a carbon
source, or methyl sulfides produced by the archaea
(e.g., CH 3 SH) as electron donor (Moran et al. 2008).
Confirmation of the existence of anaerobic
methanotrophs (ANME) at AOM sites came from the
measurement of stable carbon isotopes in lipid
biomarkers specific to archaea (see examples of
structures in Box Scheme 14.3a), which appeared
strongly depleted in
13 C compared with the source
methane (À130‰ < δ
13 C archaeal lipids <À50‰),
indicating that methane was the carbon source for this
archaeal community (Hinrichs et al. 1999; Michaelis
et al. 2002). Phylogenetic (16S rRNA) analysis of the
same samples further revealed the presence of novel
archaeal phylotypes likely linked to methane oxidation, whereas FISH microscopy and ion microprobe
mass spectrometry were elegantly used to confirm the
occurrence of consortia of ANME and SRB mediating
the AOM process (Boetius et al. 2000; Orphan et al.
2001; Strous and Jetten 2004).
The stable carbon isotope composition of other lipid
biomarkers proposed to derive from the SRB partners
involved in AOM (see examples of structures in
Box Scheme 14.3b) further supports a syntrophy
between ANME and SRB. These lipids are usually
less depleted (À90‰ < δ
13
C < À40‰) than the
archaeal lipids. Moreover, differences in biomarker
profiles from one AOM site to the other typify distinct
anaerobic microbial consortia involved in AOM
(Hinrichs et al. 2000; Michaelis et al. 2002; Elvert
et al. 2003; Blumenberg et al. 2004). In such settings,
the diversity of
13
C-depleted biomarkers of prokaryotic
origin are indicative of methane carbon transferring via
ANME to several SRB populations (see example in
Box Scheme 14.2) and then, eventually, to other microbial populations (Pancost and Sinninghe Damste ´ 2003).
Investigations based on 16S rRNA or mcrA (the key
gene of methanotrophic and methanogenic archaea)
gene phylogeny have shown that three distinct clusters
(continued)
562
J.-C. Bertrand et al.