14.2.7.3 Consequences of Methane Emissions:
Greenhouse Effect
CH 4 is a major greenhouse gas that plays a significant role in
climate change. Indeed, for the same mass as CO 2 , CH 4
absorbs 24–30 times more energy; 1 kg CH 4 heats the planet
24–30 times more than 1 kg CO 2 . The CH 4 contribution to
the greenhouse effect is at about 12 %.
Through the analysis of gases trapped in tiny gas bubbles
preserved in ice cores from Greenland and Antarctica, it was
observed that an increase in the amounts of CO 2 and CH 4
released into the atmosphere since the beginning of the
preindustrial era. This increase continues at about the same
rate as the intensification of human activities: development
of natural gas exploitation, proliferation of waste discharges
and garbage, increase of flooded agricultural areas (especially rice fields), increase in livestock.
The atmosphere CH 4 content that was estimated between
0.5 and 0.7 ppmv (parts per million by volume) 15,000 years
ago, was at 715 ppmv before the industrial era and steadily
increased since then: 1,720 ppmv in 1990, 1,745 ppmv in
2001, and 1,770 ppmv in 2005.
14.2.7.4 Methanogenesis
Microorganisms : Methanogenic Archaea
The CH 4 biological production is due to the activity of strictly
anaerobic Archaea: methanoarchaea. Methanoarchaea are all
included in the phylum Euryarchaeota that is described in
Sect. 6.6.1. The main CH 4 production pathways are presented
in Sect. 3.3.2 and in the book of Vandecasteele (2008).
All methanogenic microorganisms belong to the
phylum Euryarchaeota that includes four classes and six
orders (cf. Sect. 6.6.1). It is only recently that Methanomassiliicoccus luminyensis has been isolated (Dridi et al 2012).
The latter belongs to a seventh order which is still under
consideration.
Most of methanogens grow at pH 6–8. They are able to
grow in a wide range of temperature (4–110
C), with
psychrophilic, mesophilic, and thermophilic species. For
example, during a field campaign carried out by the submersible Alvin of the coast of Mexico, one hyperthermophilic species of the genus Methanopyrus, whose optimum
growth temperature is 98
C, was isolated at 2,000 m depth.
Halophilic and hyperhalophilic species were also described.
Methanogens Habitats
Methanogens live in all wet ecosystems, highly reductive
(redox potential <À300 mV), organic matter rich, and
completely free of oxygen. They are very sensitive to oxygen;
low dioxygen partial pressures (5–10 ppm) cause lethal and
irreversible dissociation of some enzymatic complexes (e.g.,
F 420 -hydrogenase) involved in methanogenesis. Thus, their
isolation, study and conservation require the implementation
of highly stringent deoxygenation techniques: anaerobic jars,
Hungate tubes, anaerobic chambers maintained reduced
atmosphere (5 % H 2 in the presence of palladium, cf. Sect.
17.8.2). They are also inhibited by some pollutants (carbon
tretrachloride, chloroform), light and 2-bromoethanesulfonic
acid (structural analogue of coenzyme M).
Fully anoxic environments, favorable to methanogens
development, are abundant and very diverse:
1. Sediments: wetlands, ponds, lakes, oceans, rice fields,
flooded anoxic zones.
2. Rumen of ruminants (cows, sheep, deer, elk, camel).
3. Cecum of many herbivores (horse, rabbit, etc.).
4. Human intestines (about one third of healthy adult
humans excrete methane), most mammals, some insects
(digestive tract of insects), etc.
5. Hydrothermal vents along ocean ridges.
6. Hot springs (e.g., those of Yellowstone National Park).
Processes of industrial interest, using bioreactors, rely
on the activity of methanogenic Archaea (methanation,
cf. Sect. 16.5.4); the same happens for the anaerobic treatment
of wastewaters and related sludge, and for the treatment of
solid wastes in waste discharges as well.
Role of Methanogenesis During Anaerobic
Biodegradation of Organic Matter
The first step of organic material degradation starts with
hydrolysis of complex polymers leading to the formation
of monomers which, by fermentations, provide substrates
to be used by methanogenic Archaea (H 2 , formate, acetate,
alcohols, methylated compounds) in the absence of sulfate.
Indeed, it is known that in the presence of sulfate (with the
exception of methylated compounds), sulfate-reducing bacteria may outcompete methanogens. The heterotrophic
methanogenic archaea develop most often in close association with other organisms that supply hydrogen. The
consumption of hydrogen, and to a lesser extent of acetate
by methanogenic archaea, is a crucial step in the
complete oxidation of organic matter under anaerobic
conditions (cf. Sect. 3.3.3).
The Different Methane Production Pathways
There are three pathways for CH 4 formation (Fig. 14.21):
1. Through anaerobic respiration: most methanogens use CO 2
as terminal electron acceptor (e.g., Methanobacterium,
Methanococcus). Electron donors are hydrogen or formate.
Changes in free energy, ΔG
0 , expressed in kJ.mol
À1 CH 4 ,
are given in brackets:
CO 2 þ 4H 2 ! CH 4 þ 2H 2 O ‐131
ð
Þ
Some methanoarchaea are able to oxidize hydrogen
using methanol as terminal electron acceptor (e.g.,
Methanosphaera, Methanomassiliicoccus):
CH 3 OH þ H 2 ! CH 4 þ H 2 O ‐113
ð
Þ
14 Biogeochemical Cycles
557
Greenhouse Effect
CH 4 is a major greenhouse gas that plays a significant role in
climate change. Indeed, for the same mass as CO 2 , CH 4
absorbs 24–30 times more energy; 1 kg CH 4 heats the planet
24–30 times more than 1 kg CO 2 . The CH 4 contribution to
the greenhouse effect is at about 12 %.
Through the analysis of gases trapped in tiny gas bubbles
preserved in ice cores from Greenland and Antarctica, it was
observed that an increase in the amounts of CO 2 and CH 4
released into the atmosphere since the beginning of the
preindustrial era. This increase continues at about the same
rate as the intensification of human activities: development
of natural gas exploitation, proliferation of waste discharges
and garbage, increase of flooded agricultural areas (especially rice fields), increase in livestock.
The atmosphere CH 4 content that was estimated between
0.5 and 0.7 ppmv (parts per million by volume) 15,000 years
ago, was at 715 ppmv before the industrial era and steadily
increased since then: 1,720 ppmv in 1990, 1,745 ppmv in
2001, and 1,770 ppmv in 2005.
14.2.7.4 Methanogenesis
Microorganisms : Methanogenic Archaea
The CH 4 biological production is due to the activity of strictly
anaerobic Archaea: methanoarchaea. Methanoarchaea are all
included in the phylum Euryarchaeota that is described in
Sect. 6.6.1. The main CH 4 production pathways are presented
in Sect. 3.3.2 and in the book of Vandecasteele (2008).
All methanogenic microorganisms belong to the
phylum Euryarchaeota that includes four classes and six
orders (cf. Sect. 6.6.1). It is only recently that Methanomassiliicoccus luminyensis has been isolated (Dridi et al 2012).
The latter belongs to a seventh order which is still under
consideration.
Most of methanogens grow at pH 6–8. They are able to
grow in a wide range of temperature (4–110
C), with
psychrophilic, mesophilic, and thermophilic species. For
example, during a field campaign carried out by the submersible Alvin of the coast of Mexico, one hyperthermophilic species of the genus Methanopyrus, whose optimum
growth temperature is 98
C, was isolated at 2,000 m depth.
Halophilic and hyperhalophilic species were also described.
Methanogens Habitats
Methanogens live in all wet ecosystems, highly reductive
(redox potential <À300 mV), organic matter rich, and
completely free of oxygen. They are very sensitive to oxygen;
low dioxygen partial pressures (5–10 ppm) cause lethal and
irreversible dissociation of some enzymatic complexes (e.g.,
F 420 -hydrogenase) involved in methanogenesis. Thus, their
isolation, study and conservation require the implementation
of highly stringent deoxygenation techniques: anaerobic jars,
Hungate tubes, anaerobic chambers maintained reduced
atmosphere (5 % H 2 in the presence of palladium, cf. Sect.
17.8.2). They are also inhibited by some pollutants (carbon
tretrachloride, chloroform), light and 2-bromoethanesulfonic
acid (structural analogue of coenzyme M).
Fully anoxic environments, favorable to methanogens
development, are abundant and very diverse:
1. Sediments: wetlands, ponds, lakes, oceans, rice fields,
flooded anoxic zones.
2. Rumen of ruminants (cows, sheep, deer, elk, camel).
3. Cecum of many herbivores (horse, rabbit, etc.).
4. Human intestines (about one third of healthy adult
humans excrete methane), most mammals, some insects
(digestive tract of insects), etc.
5. Hydrothermal vents along ocean ridges.
6. Hot springs (e.g., those of Yellowstone National Park).
Processes of industrial interest, using bioreactors, rely
on the activity of methanogenic Archaea (methanation,
cf. Sect. 16.5.4); the same happens for the anaerobic treatment
of wastewaters and related sludge, and for the treatment of
solid wastes in waste discharges as well.
Role of Methanogenesis During Anaerobic
Biodegradation of Organic Matter
The first step of organic material degradation starts with
hydrolysis of complex polymers leading to the formation
of monomers which, by fermentations, provide substrates
to be used by methanogenic Archaea (H 2 , formate, acetate,
alcohols, methylated compounds) in the absence of sulfate.
Indeed, it is known that in the presence of sulfate (with the
exception of methylated compounds), sulfate-reducing bacteria may outcompete methanogens. The heterotrophic
methanogenic archaea develop most often in close association with other organisms that supply hydrogen. The
consumption of hydrogen, and to a lesser extent of acetate
by methanogenic archaea, is a crucial step in the
complete oxidation of organic matter under anaerobic
conditions (cf. Sect. 3.3.3).
The Different Methane Production Pathways
There are three pathways for CH 4 formation (Fig. 14.21):
1. Through anaerobic respiration: most methanogens use CO 2
as terminal electron acceptor (e.g., Methanobacterium,
Methanococcus). Electron donors are hydrogen or formate.
Changes in free energy, ΔG
0 , expressed in kJ.mol
À1 CH 4 ,
are given in brackets:
CO 2 þ 4H 2 ! CH 4 þ 2H 2 O ‐131
ð
Þ
Some methanoarchaea are able to oxidize hydrogen
using methanol as terminal electron acceptor (e.g.,
Methanosphaera, Methanomassiliicoccus):
CH 3 OH þ H 2 ! CH 4 þ H 2 O ‐113
ð
Þ
14 Biogeochemical Cycles
557