production that constitutes the bulk of the biomass, i.e., the
basis of all aquatic food webs. However, there are substantial
differences between lacustrian and marine ecosystems,
related primarily to:
1. Element distribution and concentration: concentrations of
sodium chloride and sulfates are high in marine
environments and low in lacustrian ones.
2. The involved organisms. Indeed, microbial communities
are depending on the nature of electron acceptors available in the environment. Hence, under anoxic conditions,
the high sulfate concentration in marine environment
favors the development of sulfate-reducing bacteria,
whereas in lacustrian sediments, methanogenic Archaea
predominate.
3. Environment forcing variables (hydrology, climatology,
geology, etc.) that differently vary in space and time in
both ecosystem types. In particular, the ratio (R) between
the food zone and the water body surface is much higher
in lakes (R is generally comprised between 10 and 20)
than in marine environment (R ¼ 0.32). As a result, the
importance of sediments and allochthonous organic matter
is much greater in lacustrian than marine environments.
Likewise, continental climate conditions have a larger
impact on lacustrian ecosystems.
14.2.7 Methane Production and Consumption
14.2.7.1 What Is Methane?
Methane is an odorless gas whose chemical formula is CH 4 .
This is a very simple hydrocarbon molecule that represents
the most reduced form of carbon. It was discovered as an
inflammable gas by the Italian physicist Alessandro Volta in
1776. It is responsible for the phenomena of will-o’-the-wisp
and firedamp explosion. CH 4 played a major role in the
carbon cycle throughout the Earth history and still is a
major component in all anoxic ecosystems: Part of the carbon fixed by primary producers is regenerated as methane
by methanogenic Archaea. Moreover, as a greenhouse gas,
CH 4 is involved in global warming.
14.2.7.2 Methane Sources
It is very difficult to know the exact amount of CH 4 released
into the atmosphere; proposed values are orders of magnitude. There is likely that between 500 (Fung et al. 1991) and
600 million tons of CH 4 (Lelieveld et al. 1998) are released
each year in the atmosphere.
The sources are both biological (natural) and abiotic
(human activities). The values for these CH 4 sources may
vary with their authors; those considered by Kvenvolden and
Rogers (2005) are listed below:
1. The main biological sources (methanogenic habitats)
expressed in Tg.year
À1 (Tg ¼ 10
12 g) are from ruminants
such as cows, sheep (115), termites (20), rice fields (110),
natural wetlands (125), waste discharge areas (55),
oceans (10), and freshwater (5). Plants that cover a large
part of the Earth, would also represent a significant source
of CH 4 , but the opinions differ on the amount of CH 4
emitted by plants.
2. The main abiotic sources expressed in Tg.year
À1 are coal
mines (40), losses of natural gas (70), combustion of
biomass (40), and methane hydrates (10) (Box 14.7).
It appears, however, that many sources of fossil methane
from volcanoes, geothermal areas, and ocean ridges have
been underestimated at 45–48 Tg.year
À1 (Kvenvolden and
Rogers 2005).
Box 14.7: Methane Hydrates
Danielle Marty
Definition and Structure (Buffet 2000;
Beauchamp 2004)
“Methane hydrates” are the most abundant naturallyoccurring “clathrates”. “Clathrates” (from the Greek
word Khlatron, barrier, lattice) are crystalline
compounds in which small guest atoms or molecules
are physically sequestered within cage-like structures
formed by a host molecule or a large complex network
of host molecules. These “inclusion compounds” are
called “clathrate hydrates” when they contain water
molecules, and “gas hydrates” when the enclosed
molecules are natural gases. Within natural gas
hydrates, methane is the most common encountered
guest molecule; however, other gases, such as carbon
dioxide or hydrogen sulfide, and less frequently,
hydrocarbons, may also be present, they stabilize the
aqueous structure, and form gas hydrates.
About 90 % of natural gas hydrates contain methane; these methane hydrates compose a unique class of
solid ice-like substances in which a rigid cage structure of hydrogen-bonded water molecules enclose
methane molecules, without chemical bonding
between the gas molecules and the aqueous structure.
The presence of a sufficient number of gas molecules
trapped within the network of host water molecules
acts to stabilize thermodynamically the crystalline
structure through physical bonding; in addition, very
D. Marty (*)
Institut Me ´diterrane ´en d’Oce ´anologie (MIO), UM 110, CNRS
7294 IRD 235, Universite ´ de Toulon, Aix-Marseille Universite ´,
Campus de Luminy, 13288 Marseille Cedex 9, France
(continued)
554
J.-C. Bertrand et al.
basis of all aquatic food webs. However, there are substantial
differences between lacustrian and marine ecosystems,
related primarily to:
1. Element distribution and concentration: concentrations of
sodium chloride and sulfates are high in marine
environments and low in lacustrian ones.
2. The involved organisms. Indeed, microbial communities
are depending on the nature of electron acceptors available in the environment. Hence, under anoxic conditions,
the high sulfate concentration in marine environment
favors the development of sulfate-reducing bacteria,
whereas in lacustrian sediments, methanogenic Archaea
predominate.
3. Environment forcing variables (hydrology, climatology,
geology, etc.) that differently vary in space and time in
both ecosystem types. In particular, the ratio (R) between
the food zone and the water body surface is much higher
in lakes (R is generally comprised between 10 and 20)
than in marine environment (R ¼ 0.32). As a result, the
importance of sediments and allochthonous organic matter
is much greater in lacustrian than marine environments.
Likewise, continental climate conditions have a larger
impact on lacustrian ecosystems.
14.2.7 Methane Production and Consumption
14.2.7.1 What Is Methane?
Methane is an odorless gas whose chemical formula is CH 4 .
This is a very simple hydrocarbon molecule that represents
the most reduced form of carbon. It was discovered as an
inflammable gas by the Italian physicist Alessandro Volta in
1776. It is responsible for the phenomena of will-o’-the-wisp
and firedamp explosion. CH 4 played a major role in the
carbon cycle throughout the Earth history and still is a
major component in all anoxic ecosystems: Part of the carbon fixed by primary producers is regenerated as methane
by methanogenic Archaea. Moreover, as a greenhouse gas,
CH 4 is involved in global warming.
14.2.7.2 Methane Sources
It is very difficult to know the exact amount of CH 4 released
into the atmosphere; proposed values are orders of magnitude. There is likely that between 500 (Fung et al. 1991) and
600 million tons of CH 4 (Lelieveld et al. 1998) are released
each year in the atmosphere.
The sources are both biological (natural) and abiotic
(human activities). The values for these CH 4 sources may
vary with their authors; those considered by Kvenvolden and
Rogers (2005) are listed below:
1. The main biological sources (methanogenic habitats)
expressed in Tg.year
À1 (Tg ¼ 10
12 g) are from ruminants
such as cows, sheep (115), termites (20), rice fields (110),
natural wetlands (125), waste discharge areas (55),
oceans (10), and freshwater (5). Plants that cover a large
part of the Earth, would also represent a significant source
of CH 4 , but the opinions differ on the amount of CH 4
emitted by plants.
2. The main abiotic sources expressed in Tg.year
À1 are coal
mines (40), losses of natural gas (70), combustion of
biomass (40), and methane hydrates (10) (Box 14.7).
It appears, however, that many sources of fossil methane
from volcanoes, geothermal areas, and ocean ridges have
been underestimated at 45–48 Tg.year
À1 (Kvenvolden and
Rogers 2005).
Box 14.7: Methane Hydrates
Danielle Marty
Definition and Structure (Buffet 2000;
Beauchamp 2004)
“Methane hydrates” are the most abundant naturallyoccurring “clathrates”. “Clathrates” (from the Greek
word Khlatron, barrier, lattice) are crystalline
compounds in which small guest atoms or molecules
are physically sequestered within cage-like structures
formed by a host molecule or a large complex network
of host molecules. These “inclusion compounds” are
called “clathrate hydrates” when they contain water
molecules, and “gas hydrates” when the enclosed
molecules are natural gases. Within natural gas
hydrates, methane is the most common encountered
guest molecule; however, other gases, such as carbon
dioxide or hydrogen sulfide, and less frequently,
hydrocarbons, may also be present, they stabilize the
aqueous structure, and form gas hydrates.
About 90 % of natural gas hydrates contain methane; these methane hydrates compose a unique class of
solid ice-like substances in which a rigid cage structure of hydrogen-bonded water molecules enclose
methane molecules, without chemical bonding
between the gas molecules and the aqueous structure.
The presence of a sufficient number of gas molecules
trapped within the network of host water molecules
acts to stabilize thermodynamically the crystalline
structure through physical bonding; in addition, very
D. Marty (*)
Institut Me ´diterrane ´en d’Oce ´anologie (MIO), UM 110, CNRS
7294 IRD 235, Universite ´ de Toulon, Aix-Marseille Universite ´,
Campus de Luminy, 13288 Marseille Cedex 9, France
(continued)
554
J.-C. Bertrand et al.