acetate is the sole energy substrate. In contrast, Methanosarcinaceae encompass nine genera (Halomethanococcus,
Methanimicrococcus, Methanococcoides, Methanohalobium,
Methanohalophilus, Methanolobus, Methanomethylovorans,
Methanosalsum, and Methanosarcina (Fig. 6.10a)) of
coccoidal or pseudosarcinal bacteria. All representatives of
this family can dismutate methyl compounds. Some are able
to reduce acetate or CO 2 with H 2 but none catabolize formate.
Methermicoccaceae contain a single genus Methermicoccus
of small, thermophilic cocci, able to use methanol, methylamine, and trimethylamine as substrates for methanogenesis.
From an evolutionary point of view, Methanosarcinales
belong to class II methanogens and are closely related to
Methanomicrobiales and Methanocellales and to
Halobacteriales (Fig. 6.9). For additional information on
Methanosarcinales, see Dworkin and collaborators (2006).
The Methanocellales order has been proposed by Sanae
Sakai and colleagues in 2008. It corresponds to the taxon
formerly designated RC-I (Rice Cluster I). This order contains
a single family Methanocellaceae and a single genus
Methanocella. Most cells are rod-shaped and occur singly;
however, coccoid shaped appear in late-exponential culture.
Described strains to date are nonmotile. They produce CH 4
from H 2 and formate. Optimal growth occurs at 35–37
C, at
neutral pH. Based on 16S rRNA environmental survey,
Methanocellales appear to be widely distributed, especially in
rice paddies that are one of the major sources of CH 4 on Earth,
contributing about 10–25% of global CH 4 emission. From an
evolutionary point of view, Methanocellales belong to class II
methanogens and are closely related to Methanomicrobiales
and Methanosarcinales and to Halobacteriales (Fig. 6.9).
The order Methanobacteriales has been proposed by
William E. Balch and Ralph S. Wolfe in 1981. Methanobacteriales are strict anaerobic mesophilic, thermophilic, or
hyperthermophilic microorganisms. They grow at temperature ranging from 15 to 97
C. Optimal growth of most
members of this order occurs at nearly neutral pH even if
alkaliphilic (e.g., Methanobacterium alcaliphilum) or moderately acidophilic (e.g., Methanobacterium espanolense)
strains have been characterized. They are found in anoxic
habitats (e.g., freshwater and marine sediments, hot springs,
groundwater, oil fields, peat bogs, rice paddies, terrestrial
subsurface environments, anaerobic sewages, sludge, and
gastrointestinal tracts of animals including humans, etc.).
Methanobacteriales are generally free-living, but some species are endosymbionts of anaerobic H 2 producer protists,
such as ciliates (e.g., Nyctotherus ovalis) thriving in the intestinal tracts of cockroaches, millipedes, and frogs. Cells are
either rod-shaped or coccoid, often forming chains or
aggregates. They can also form long filaments (up to
120 μm in length). Similarly to Methanopyrales but in contrast to other methanogens, the cell wall of Methanobacteriales contains pseudomurein. Most Methanobacteriales
use H 2 as electron donor to reduce CO 2 leading to CH 4
formation. Some representatives of this order can also use
formate, CO, or secondary alcohols as electron donors for
CO 2 reduction, whereas members of a single genus (e.g.,
Methanosphaera) can form CH 4 by using H 2 to reduce methanol. Methanobacteriales are divided into two families: the
Methanobacteriaceae and the Methanothermaceae. Methanobacteriaceae encompass rod-shaped mesophiles and
thermophiles. Most of them are nonflagellated. This family
is divided into four genera: Methanobacterium, Methanobrevibacter, Methanosphaera, and Methanothermobacter. They
are H 2 oxidizers, although some species can also oxidize
formate, CO, and/or secondary alcohols. Methanosphaera
reduce methanol, whereas other Methanobacteriaceae use
CO 2 as an electron acceptor. Methanobrevibacter smithii
and Methanosphaera stadtmanae are two main methanogens
found in human gut. The Methanothermaceae is represented
by a single genus, Methanothermus, which grows by reducing
CO 2 with H 2 . Members of this family are hyperthermophilic,
living at optimal temperature ranging from 80 to 85
C, at
pH
6.5.
In
contrast
to
Methanobacteriaceae,
Methanothermaceae are flagellates. From an evolutionary
point of view, Methanobacteriales belong to class I
methanogens and are closely related to Methanococcales
and Methanopyrales (Fig. 6.9). For additional information
on Methanobacteriales, see Dworkin and collaborators
(2006).
The order Methanopyrales has been proposed by Harald
Huber and Karl Setter in 2001. It is represented by a single
family Methanopyraceae and a single genus Methanopyrus,
a single species Methanopyrus kandleri that was isolated
from
hydrothermally
heated
deep-sea
sediment.
Methanopyrus are the only methanogens known to date
growing optimally at temperatures greater than 100
C.
Methanopyrus cells are rod-shaped and flagellated. Similarly
to Methanobacteriales but in contrast to other methanogens,
the cell wall of Methanopyrales contains pseudomurein.
Methanopyrus kandleri uses H 2 as electron donor to reduce
CO 2 leading to CH 4 formation. It is an obligate
chemolithoautotroph that uses CO 2 as sole carbon source.
From an evolutionary point of view, Methanopyrales belong
to class I methanogens and are closely related to
Methanococcales and Methanobacteriales (Fig. 6.9).
The order Methanococcales has been proposed by William
E. Balch and Ralph S. Wolfe in 1981. Representatives of this
order have been isolated from various anaerobic habitats (e.g.,
shores, estuary sediments, salt-marsh, coastal geothermally
heated marine sediments, reservoir water, deep-sea hydrothermal vents, high-temperature oil reservoirs, etc.).
Methanococcales have been divided into two families, each
being represented by two genera. Methanocaldococcaceae
gather Methanocaldococcus and Methanotorris, whereas
Methanococcaceae
encompass
Methanococcus
and
170
P. Caumette et al.
Methanimicrococcus, Methanococcoides, Methanohalobium,
Methanohalophilus, Methanolobus, Methanomethylovorans,
Methanosalsum, and Methanosarcina (Fig. 6.10a)) of
coccoidal or pseudosarcinal bacteria. All representatives of
this family can dismutate methyl compounds. Some are able
to reduce acetate or CO 2 with H 2 but none catabolize formate.
Methermicoccaceae contain a single genus Methermicoccus
of small, thermophilic cocci, able to use methanol, methylamine, and trimethylamine as substrates for methanogenesis.
From an evolutionary point of view, Methanosarcinales
belong to class II methanogens and are closely related to
Methanomicrobiales and Methanocellales and to
Halobacteriales (Fig. 6.9). For additional information on
Methanosarcinales, see Dworkin and collaborators (2006).
The Methanocellales order has been proposed by Sanae
Sakai and colleagues in 2008. It corresponds to the taxon
formerly designated RC-I (Rice Cluster I). This order contains
a single family Methanocellaceae and a single genus
Methanocella. Most cells are rod-shaped and occur singly;
however, coccoid shaped appear in late-exponential culture.
Described strains to date are nonmotile. They produce CH 4
from H 2 and formate. Optimal growth occurs at 35–37
C, at
neutral pH. Based on 16S rRNA environmental survey,
Methanocellales appear to be widely distributed, especially in
rice paddies that are one of the major sources of CH 4 on Earth,
contributing about 10–25% of global CH 4 emission. From an
evolutionary point of view, Methanocellales belong to class II
methanogens and are closely related to Methanomicrobiales
and Methanosarcinales and to Halobacteriales (Fig. 6.9).
The order Methanobacteriales has been proposed by
William E. Balch and Ralph S. Wolfe in 1981. Methanobacteriales are strict anaerobic mesophilic, thermophilic, or
hyperthermophilic microorganisms. They grow at temperature ranging from 15 to 97
C. Optimal growth of most
members of this order occurs at nearly neutral pH even if
alkaliphilic (e.g., Methanobacterium alcaliphilum) or moderately acidophilic (e.g., Methanobacterium espanolense)
strains have been characterized. They are found in anoxic
habitats (e.g., freshwater and marine sediments, hot springs,
groundwater, oil fields, peat bogs, rice paddies, terrestrial
subsurface environments, anaerobic sewages, sludge, and
gastrointestinal tracts of animals including humans, etc.).
Methanobacteriales are generally free-living, but some species are endosymbionts of anaerobic H 2 producer protists,
such as ciliates (e.g., Nyctotherus ovalis) thriving in the intestinal tracts of cockroaches, millipedes, and frogs. Cells are
either rod-shaped or coccoid, often forming chains or
aggregates. They can also form long filaments (up to
120 μm in length). Similarly to Methanopyrales but in contrast to other methanogens, the cell wall of Methanobacteriales contains pseudomurein. Most Methanobacteriales
use H 2 as electron donor to reduce CO 2 leading to CH 4
formation. Some representatives of this order can also use
formate, CO, or secondary alcohols as electron donors for
CO 2 reduction, whereas members of a single genus (e.g.,
Methanosphaera) can form CH 4 by using H 2 to reduce methanol. Methanobacteriales are divided into two families: the
Methanobacteriaceae and the Methanothermaceae. Methanobacteriaceae encompass rod-shaped mesophiles and
thermophiles. Most of them are nonflagellated. This family
is divided into four genera: Methanobacterium, Methanobrevibacter, Methanosphaera, and Methanothermobacter. They
are H 2 oxidizers, although some species can also oxidize
formate, CO, and/or secondary alcohols. Methanosphaera
reduce methanol, whereas other Methanobacteriaceae use
CO 2 as an electron acceptor. Methanobrevibacter smithii
and Methanosphaera stadtmanae are two main methanogens
found in human gut. The Methanothermaceae is represented
by a single genus, Methanothermus, which grows by reducing
CO 2 with H 2 . Members of this family are hyperthermophilic,
living at optimal temperature ranging from 80 to 85
C, at
pH
6.5.
In
contrast
to
Methanobacteriaceae,
Methanothermaceae are flagellates. From an evolutionary
point of view, Methanobacteriales belong to class I
methanogens and are closely related to Methanococcales
and Methanopyrales (Fig. 6.9). For additional information
on Methanobacteriales, see Dworkin and collaborators
(2006).
The order Methanopyrales has been proposed by Harald
Huber and Karl Setter in 2001. It is represented by a single
family Methanopyraceae and a single genus Methanopyrus,
a single species Methanopyrus kandleri that was isolated
from
hydrothermally
heated
deep-sea
sediment.
Methanopyrus are the only methanogens known to date
growing optimally at temperatures greater than 100
C.
Methanopyrus cells are rod-shaped and flagellated. Similarly
to Methanobacteriales but in contrast to other methanogens,
the cell wall of Methanopyrales contains pseudomurein.
Methanopyrus kandleri uses H 2 as electron donor to reduce
CO 2 leading to CH 4 formation. It is an obligate
chemolithoautotroph that uses CO 2 as sole carbon source.
From an evolutionary point of view, Methanopyrales belong
to class I methanogens and are closely related to
Methanococcales and Methanobacteriales (Fig. 6.9).
The order Methanococcales has been proposed by William
E. Balch and Ralph S. Wolfe in 1981. Representatives of this
order have been isolated from various anaerobic habitats (e.g.,
shores, estuary sediments, salt-marsh, coastal geothermally
heated marine sediments, reservoir water, deep-sea hydrothermal vents, high-temperature oil reservoirs, etc.).
Methanococcales have been divided into two families, each
being represented by two genera. Methanocaldococcaceae
gather Methanocaldococcus and Methanotorris, whereas
Methanococcaceae
encompass
Methanococcus
and
170
P. Caumette et al.