Methane as Energy Source
Fermentation processes of organic matter from animal or
plant (from food industries, domestic wastes, urban wastewater) origin, yields a mixture of combustible gases, known
as “biogas.”
14.2.7.5 Photochemical Methane Destruction
Much of CH 4 emissions in the atmosphere is photo-oxidized
(Vandecasteele 2008), mainly in the troposphere (first
10–12 km altitude) where CH 4 is oxidized by hydroxyl
radicals (420 Æ 80 Mt year
À1 ) into carbon monoxide,
ozone, and water vapor. These reactions correspond to an
average lifetime of CH 4 at 8–12 years. In the stratosphere
(12–15 km altitude), CH 4 consumption amounts to 10 Æ 5
Mt year
À1 .
14.2.7.6 Methane Removal by Methanotrophic
Bacteria in the Presence of Oxygen
Most of the produced CH 4 is not released into the atmosphere but is oxidized near the production areas. For example, for CH 4 produced in rice fields, the percentage of gas
released into the atmosphere with respect to the overall
production would not exceed 17 %. Similarly, CH 4
emissions from marine sediments are very low compared
with the total production (anaerobic and aerobic biodegradation in sediment and water column, respectively).
Biological CH 4 removal is carried out by methanotrophic
bacteria for which CH 4 is a carbon and energy source
(Hanson and Hanson 1996). Methanotrophs, that also use
methanol, are called methylotrophs, i.e., organisms that use
methylated compounds.
Methanotrophs are Gram-negative and strict aerobes,
omnipresent wherever there is a source of CH 4 and
dioxygen. They grow on the surface of soil and sediment,
in water, around roots of plants in the rhizosphere. Some of
these microorganisms form exospores that are resistant to
heat and desiccation (Methylosinus, Methylocystis), while
others form cysts which are resistant to desiccation
(Methylomonas, Methylobacter).
CH 4 biodegradation in the presence of dioxygen results
from four successive oxidations:
CH4 ! CH 3 OH
Methanol
ð
Þ
! HCHO
formaldehyde
ð
Þ
! HCOOH
formate
ð
Þ
The first step is the action of a monooxygenase that
converts CH 4 to methanol; this enzyme may be linked to
the cell membrane or soluble in the cytosol. The lack of
specificity of the soluble methane-oxidase enables it to oxidize more than 150 different compounds: saturated or unsaturated hydrocarbons (ethylene, propylene), aromatics
(naphthalene), chloroform, dichloroethane, tri- and
dichloroethylene, etc. These oxidative capacities, especially
xenobiotic oxidation, have been used in the decontamination
of polluted biotopes, e.g., soils contaminated with
organochlorines (cf. Sect. 16.7.3).
Other steps of methanol oxidation supply electrons to an
electron transport chain for ATP synthesis. The assimilation
of compounds with a single carbon atom (C1 compound)
occurs at the level of formaldehyde through two distinct
pathways.
There are three types of methanotrophic bacteria that
essentially differ by their cell membrane organization, their
mode of using formaldehyde, their taxonomic position
(Chistoserdova et al. 2005). In type I methanotrophs
(Methylococcus, Methylobacter, Methylomonas), membrane
system consists of vesicles with a disc shape distributed
throughout the cell. Formaldehyde is assimilated by a pathway known as ribulose monophosphate (cf. Sect. 3.4.2).
Taxonomically, type I methanotrophs have been affiliated
Box 14.8 (continued)
adaptations for persistence in the intestine such as the
production of surface glycans analogous to mucus
structures, adhesin-like proteins and an enzymatic repertoire to efficiently use nitrogen sources and products
of saccharolytic prokaryotes.
Although they only come into the ecological framework at the end of the trophic chain, hydrogenotrophic
microorganisms play an essential role in favoring a
more complete oxidation of substrates and thereby
maximizing the total ATP harvest of the microbiota
(Wolin 1974). Hydrogen is nonetheless always detectable in the colon (Bond et al. 1971) and concentrations
of short chain fatty acids are not significantly different
whatever the status of the individual. This could come
from the co-existence of different metabolisms in the
proximal colon (acetogenesis) and the distal colon
(methanogenesis) conditioned by substrate availability
but also by differences in pH and osmolarity. In turn,
being methano-excretor or not, and hence the
enterotype of an individual could have an incidence
on the prevalence of different cellulolytic species present in the human digestive tract.
The production of methane by methanogenic
archaea is only one of the three major pathways of
H 2 metabolism in the colon. In non-methaneexcretors, alternative hydrogenotrophic pathways
were evidenced among which reductive acetogenesis
and sulfate reduction (Bernalier et al. 1996).
Finally, no causal link was ever established
between the presence of dominant methanogenic
archaea and intestinal pathologies. Cancer patients
repeatedly showed a higher prevalence of methanoexcretors, but these proportions return to normal after
surgical resection of tumors.
14 Biogeochemical Cycles
561
Fermentation processes of organic matter from animal or
plant (from food industries, domestic wastes, urban wastewater) origin, yields a mixture of combustible gases, known
as “biogas.”
14.2.7.5 Photochemical Methane Destruction
Much of CH 4 emissions in the atmosphere is photo-oxidized
(Vandecasteele 2008), mainly in the troposphere (first
10–12 km altitude) where CH 4 is oxidized by hydroxyl
radicals (420 Æ 80 Mt year
À1 ) into carbon monoxide,
ozone, and water vapor. These reactions correspond to an
average lifetime of CH 4 at 8–12 years. In the stratosphere
(12–15 km altitude), CH 4 consumption amounts to 10 Æ 5
Mt year
À1 .
14.2.7.6 Methane Removal by Methanotrophic
Bacteria in the Presence of Oxygen
Most of the produced CH 4 is not released into the atmosphere but is oxidized near the production areas. For example, for CH 4 produced in rice fields, the percentage of gas
released into the atmosphere with respect to the overall
production would not exceed 17 %. Similarly, CH 4
emissions from marine sediments are very low compared
with the total production (anaerobic and aerobic biodegradation in sediment and water column, respectively).
Biological CH 4 removal is carried out by methanotrophic
bacteria for which CH 4 is a carbon and energy source
(Hanson and Hanson 1996). Methanotrophs, that also use
methanol, are called methylotrophs, i.e., organisms that use
methylated compounds.
Methanotrophs are Gram-negative and strict aerobes,
omnipresent wherever there is a source of CH 4 and
dioxygen. They grow on the surface of soil and sediment,
in water, around roots of plants in the rhizosphere. Some of
these microorganisms form exospores that are resistant to
heat and desiccation (Methylosinus, Methylocystis), while
others form cysts which are resistant to desiccation
(Methylomonas, Methylobacter).
CH 4 biodegradation in the presence of dioxygen results
from four successive oxidations:
CH4 ! CH 3 OH
Methanol
ð
Þ
! HCHO
formaldehyde
ð
Þ
! HCOOH
formate
ð
Þ
The first step is the action of a monooxygenase that
converts CH 4 to methanol; this enzyme may be linked to
the cell membrane or soluble in the cytosol. The lack of
specificity of the soluble methane-oxidase enables it to oxidize more than 150 different compounds: saturated or unsaturated hydrocarbons (ethylene, propylene), aromatics
(naphthalene), chloroform, dichloroethane, tri- and
dichloroethylene, etc. These oxidative capacities, especially
xenobiotic oxidation, have been used in the decontamination
of polluted biotopes, e.g., soils contaminated with
organochlorines (cf. Sect. 16.7.3).
Other steps of methanol oxidation supply electrons to an
electron transport chain for ATP synthesis. The assimilation
of compounds with a single carbon atom (C1 compound)
occurs at the level of formaldehyde through two distinct
pathways.
There are three types of methanotrophic bacteria that
essentially differ by their cell membrane organization, their
mode of using formaldehyde, their taxonomic position
(Chistoserdova et al. 2005). In type I methanotrophs
(Methylococcus, Methylobacter, Methylomonas), membrane
system consists of vesicles with a disc shape distributed
throughout the cell. Formaldehyde is assimilated by a pathway known as ribulose monophosphate (cf. Sect. 3.4.2).
Taxonomically, type I methanotrophs have been affiliated
Box 14.8 (continued)
adaptations for persistence in the intestine such as the
production of surface glycans analogous to mucus
structures, adhesin-like proteins and an enzymatic repertoire to efficiently use nitrogen sources and products
of saccharolytic prokaryotes.
Although they only come into the ecological framework at the end of the trophic chain, hydrogenotrophic
microorganisms play an essential role in favoring a
more complete oxidation of substrates and thereby
maximizing the total ATP harvest of the microbiota
(Wolin 1974). Hydrogen is nonetheless always detectable in the colon (Bond et al. 1971) and concentrations
of short chain fatty acids are not significantly different
whatever the status of the individual. This could come
from the co-existence of different metabolisms in the
proximal colon (acetogenesis) and the distal colon
(methanogenesis) conditioned by substrate availability
but also by differences in pH and osmolarity. In turn,
being methano-excretor or not, and hence the
enterotype of an individual could have an incidence
on the prevalence of different cellulolytic species present in the human digestive tract.
The production of methane by methanogenic
archaea is only one of the three major pathways of
H 2 metabolism in the colon. In non-methaneexcretors, alternative hydrogenotrophic pathways
were evidenced among which reductive acetogenesis
and sulfate reduction (Bernalier et al. 1996).
Finally, no causal link was ever established
between the presence of dominant methanogenic
archaea and intestinal pathologies. Cancer patients
repeatedly showed a higher prevalence of methanoexcretors, but these proportions return to normal after
surgical resection of tumors.
14 Biogeochemical Cycles
561