259Fig. 16.1 Methane profiles and methanogens in Lake Pavin. (a)
Schematic representation of CH 4 profiles in the oxic water column
(0–60 m), in the anoxic water column (60–92 m) and in the sediment
(0–40 cm) (Note that the scales for CH 4 concentrations are different
between the three layers). The abundances of the Methanobacteriales,
Methanosarcinales and Methanomicrobiales are indicated according to
Lehours et al. ( 2005 ). The composition of archaeal clone libraries [cal-
culated as follows: (number of sequences of the targeted group/number
of total archaeal sequences) x 100] is also indicated for the anoxic water
column and the sediment according to Lehours et al. ( 2007 ) and Borrel
et al. (2012a), respectively. (b) Transmission electron micrographs of
the strain Methanobacterium lacus isolated from the sediments of Lake
Pavin (adapted from Borrel et al. 2011 ). M. lacus is representative of
the population of Methanobacteriales in these sediments as the mcrA
sequences of M. lacus exhibited > 98 % similarity with those of
Methanobacteriales inhabiting Pavin sediments. (c) Transmission elec-
tron micrographs of pleiomorphic ellipsoid infective viruses in a visibly
infected filamentous sheathed prokaryote, identified as a Methanosaeta-
like cell, observed in the sediment of Lake Pavin (adapted from Borrel
et al. 2012c). (d) Schematic representation of organic matter (OM) and
organic carbon (OC) content in the fortieth first centimeters of Lake
Pavin sediments (adapted from Borrel et al. 2012a). (e) Thermograms
representing the heat production in three sediment layers (1, 5 and 11
cm), measured using microcalorimetry (Box 16.2). The total heat pro-
duced is 460 mJ, 365 mJ and 255 mJ for the sediment samples collected
at 1, 5 and 11 cm depth, respectively (unpublished results). This
decrease of heat production according to depth is the result of a decrease
in the microbial activity. As the organic matter content does not signifi-
cantly change with depth, the decrease in microbial activity could be
attributed to a change in organic matter quality16 Methanogens and Methanotrophs in Lake Pavin