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- Hydrogenotrophic methanogenesis: reduction of CO 2
with hydrogen (hydrogenotrophic methanogens) or for-
mate (formatotrophic methanogens) as electron donors.
The CO 2 reduction into CH 4 involves 4 H 2 and proceeds
via carrier-bound one carbon intermediates along the
C1-reductive pathway (Hedderich and Whitman 2006 ).
Hydrogenotrophic methanogens belong to the orders
Methanopyrales, Methanococcales, and
Methanobacteriales (Lang et al. 2015 ). The hydrogeno-
trophic pathway is found also in most of the derived lin-
eages of methanogens (Methanomicrobiales and
Methanocellales) and was most probably present already
in the common ancestor of the Euryarchaeota (Bapteste
et al. 2005 ). Hydrogenotrophic methanogenesis constitute
the main sink of H 2 produced by fermentative bacteria
(see Sect. 16.3.3.3). Methanogens and fermentative bac-
teria often grow in syntrophic consortia allowing metha-
nogens to maintain low H 2 concentrations, a necessary
condition for the fermentation process (Stams and Plugge
2009 ). Hydrogenotrophic methanogenesis is a commonly
used pathway and contributes to a huge part of CH 4 pro-
duction in many environments including freshwater lakes.
Theoretical values predict that hydrogenotrophic metha-
nogenesis accounts for 30 % of overall methanogenesis in
freshwater lakes, and the measured rates of this process
range from 0 to 100 % (Conrad 1999 ). - Acetotrophic methanogenesis: catabolization of acetate
by cleavage (i.e., disproportionation), with the carboxyl
group oxidized into CO 2 and the methyl group reduced
into CH 4. This pathway is only performed by members of
two genera, Methanosarcina and Methanosaeta (both
belonging to the order Methanosarcinales, Thauer et al.
2008 a). This pathway of CH 4 production often represents
the most important CH 4 source in cold and temperate
freshwater lakes (Conrad 1999 ). - Methylotrophic methanogenesis: disproportionation of
methyl compounds. The methyl group of C1-compounds
such as methanol, methylamines, dimethylsulfide, or
methanethiol is converted to CH 4 and CO 2. Methylotrophic
methanogens are a phylogenetically and biochemically
heterogeneous group comprising members of the
Methanosarcinaceae, Methermicoccaceae (both belong-
ing to the order Methanosarcinales, Sprenger et al. 2000 ;
2005 ), the genus Methanosphaera and some species of
the genus Methanobacterium (order Methanobacteriales,
Miller and Wolin 1985 ; Fricke et al. 2006 ), and members
of the recently discovered seventh order of methanogens,
the Methanomassiliicoccales (Paul et al. 2012 ; Borrel
et al. 2013 , 2014 ). Unlike hydrogenotrophic and aceto-
clastic methanogens, methylotrophic methanogens do not
compete with sulfate reducing bacteria for substrate
uptake (so-called “noncompetitive substrate”, Oremland
and Polcin 1982 ) and may thus grow in zones where
alternative electron acceptors are not depleted. This pro-
cess is expected to be low in freshwater lakes since pre-
cursors of methyl compounds (e.g., pectin, cholin,
osmoregulators such as glycine betaine) are not abundant
(Lomans et al. 1997 , 2001 ; Lovley and Klug 1983 ; Zinder
and Brock 1978 ). However, methylotrophic methanogen-
esis was observed in numerous freshwater lake sediments
(Lomans et al. 2001 ).16.3.2.4 Biochemical Aspects
of Methanogenesis^5
The complexity and uniqueness of methanogenesis as a form
of anaerobic respiration reside in the requirement of six
unusual coenzymes [ferredoxin (Fd), methanofuran (MFR),
tetrahydromethanopterin (H4MPT), coenzyme F420 (F420),
coenzyme M (CoM) and coenzyme B (CoB)]; a multistep
pathway and several unique membrane-bound enzyme com-
plexes coupled to the generation of a proton gradient driving
ATP synthesis (Ferry 2010 ; Nazaries et al. 2013 ). Although
the intermediates and enzymatic reactions of the hydrogeno-
trophic, acetotrophic and methylotrophic pathways are dif-
ferent, they share common features in the final steps of CH 4
production. The final enzymatic step is catalyzed by the
methyl-coenzyme M reductase (MCR) which is unique to
methanogens and, as developed later in this paper, to archaeal
anaerobic methanotrophs. Hence, MCR constitutes a func-
tional marker of microorganisms involved in these metabo-
lisms and is used to investigate the diversity, structure,
distribution and ecology of methanogenic communities in
freshwater lakes (Box 16.1).16.3.2.5 Environmental Distribution
of Methanogens
Methanogens are the only currently cultivated Archaea that
are truly cosmopolitan. The distribution of methanogens in
natural environments is highly dependent on their adaptation
to various temperature, pH and salinity ranges (Garcia 1990 ).
Methanogens are found in mesophilic as well as extreme
environments (Ollivier et al. 1994 ). Interestingly, methano-
genic archaea have also been found in oxic environments
such as various aerated soils (Angel et al. 2012 ) and the oxy-
genated water column of an oligotrophic lake (Grossart et al.
2011 ). These observations do not question the anaerobic
character of methanogens. In aerated soils, methanogens
become active under wet anoxic conditions whereas in oxy-
genated lake waters methanogens can be attached to photo-
autotrophs (e.g., Cyanobacteria, Chlorella), which may
provide methanogenic substrates and anaerobic niches.(^5) For detailed informations, see Ferry 2010.
A.-C. Lehours et al.