267- Temperature: the cultural approaches performed from
Pavin sediment samples led to the enrichment and isolation
of several methanogens including Methanobacteriales rep-
resentatives (Borrel et al. 2012b). However, this order was
not detected in clone libraries from the same samples (Fig.
16.1a) suggesting that the relative species are minoritary
among the methanogenic community. This hypothesis was
verified by quantification of 16SrDNA and 16S rRNA tran-
scripts. Nevertheless, a Methanobacteriales strain, repre-
sentative of the population inhabiting Pavin sediments, was
isolated and described as Methanobacterium lacus (Fig.
16.1b, Borrel et al. 2012b). The physiological preferenda of
M. lacus are compatible with the environmental conditions
prevailing in the sediment of Lake Pavin in terms of pH and
salinity. However, the in situ temperature (~5 °C) of Pavin
sediments is largely below the optimum temperature (30 °C)
of this strain might partially explain why Methanobacteriales
are poorly represented in this environment.
16.3.3.3 Cooperative Interactions with Other
Anaerobic Microbes (Fig. 16.3)
This section mainly focus on the “classical” fermentative pro-
cess and, because, of the central role of acetate in the anaero-
bic food chain, the acetogenesis process is also discussed.
- The particular case of syntrophy^6
Amongst heterotrophs it is as anaerobes that bacteria
specially excel.... It is in the use of hydrogen acceptors that
bacteria are specially developed as compared with animals
and plants.” (Stephenson 1947 ).
The methanogenic degradation of complex polymeric
materials involves a number of diverse, interacting microbial
species (see Sect. 16.3.1.1). The mutual dependence between
interacting species can be so extreme that species cannot
function without the activity of its partner, and the partners
perform together functions that species cannot do alone.
Syntrophy is a specialized case of tightly coupled mutualistic
interactions between two metabolic types, which depend on
each other to degrade certain substrates by transferring one
or more metabolic intermediates (such as H 2 ) between part-
ners (Fig. 16.2). For example, methanogens can interact with
obligate hydrogen producing bacteria. The latter phenomenon
is termed “interspecies hydrogen transfer” (Ianotti et al.
1973 ). Different genera of syntrophic bacteria have been
described as partnerships of methanogens, contributing to
the oxidation of fatty acids, benzoic acid, or fructose. In the
absence of a hydrogen scavenger, these reactions are ender-
gonic and cannot develop. When H 2 , is consumed, the reac-
tion becomes exergonic and the syntroph (i.e., methanogens)
can grow and oxidize the substrate. Syntrophic interactions
enable methanogenesis when methanogenic substrates are
limiting, sometimes leading to increased MPRs. The global
biogeochemical impact of syntrophic interactions involving
methanogenic Euryarchaeota is considerable as they enable
the complete degradation of complex organic molecules to
CO 2 and CH 4 in methanogenic habitats (Sieber et al. 2012 ).- Fermentation
Fermentation is a metabolism in which energy is derived
from the partial oxidation of an organic compound using
organic intermediates as electron donors and electron
acceptors. No external electron acceptor is involved; no
membrane or electron transport system is required; all ATP
is produced by substrate level phosphorylation.
for the simultaneous investigation of several samples of
a given living system (Wadsö 1985 ). It is with this type
of instrument (TAM III, Thermometric – TA Instruments)
that the samples coming from lake Pavin were examined
(in closed ampoules and under unstirred conditions).
Since the thermal power output corresponds to the sum
of all the heat fluxes arising from the different chemical
and physical processes that occur during incubation, the
shape of the power-time curve is very dependent on the
experimental conditions (Lamprecht 1980 ; Braissant
et al. 2013 ; Bricheux et al. 2013 ). During the growth
phase, the power-time curve shows the same exponential
shape as the biomass curve but beyond that phase it is
modulated by transition periods. Using complementary
analyses (cells density, for instance), the main metabolicperiods can be identified and the heat Q produced during
any of them or during the whole process can be obtained
by integration of the corresponding portion of the power-
time curve. In batch incubation, the thermal power out-
put curve finally returns to its baseline when the
metabolism of the exogeneous substrates ceases; in most
cases, a very small but non-zero final baseline is
observed indicating that the cells are still alive but that
they have switched over to time-limiting endogenous
metabolism (Lamprecht 1980 ). Recently, this type of
microcalorimetry showed to be not only a very effective
tool for the determination of growth rate constants
(Bricheux et al. 2013 ) but also a valuable probe for a
rapid detection of the metabolic perturbations induced
by xenobiotics (Lehours et al. 2010 ; Bricheux et al.
2013 ; Lescure et al. 2013 ).Box 16.2 (continued)(^6) For detailed information’s, see McInerney et al. 2008 , 2009 , 2011 ;
Sieber et al. 2012.
16 Methanogens and Methanotrophs in Lake Pavin
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