278
for the growth of microorganisms performing AOM: anoxia,
availability of CH 4 and of alternative electron acceptors
other than O 2.
Microcalorimetric measurements (Box 16.2) performed
with water samples collected in the chemocline of Lake
Pavin revealed a greater metabolic activity in samples incu-
bated under 1 and 3 atm of CH 4 compared to those incubated
under N 2 (Fig. 16.5). These results, indicating a CH 4 -induced
metabolic activity under anoxic conditions, were completed
by enrichment cultures. Different culture conditions were
performed using anoxic water and sediment samples as inoc-
ula and labeled CH 4.^13 C-CH 4 was shown to be consumed
and partly transformed into^13 C-CO2 in both the sediment
and the water column. We also observed that CH 4 consump-
tion was greater when NO 3 − was provided as an electron
acceptor (Attard E. Personal communication), fitting well
with the model of CH 4 consumption provided by Lopes and
coworkers (2011) (Fig. 16.5).
16.4.2.5 Hypothetical Identity of Microbial
Actors Involved in AOM Process
in Lake Pavin
The microorganisms involved in AOM processes in Lake
Pavin are not presently identified. The model developed by
Lopes et al. ( 2011 ) shows that, probably, all currently known
AOM mechanisms occur (AOM coupled with sulfate-,
nitrate- and ferric-iron reduction), suggesting that different
microorganisms are able to perform AOM in this ecosystem
(Fig. 16.5).
- Observations and results favoring the involvement of
Archaea members
None sequence of 16S rDNA and of mcrA affiliated to the
ANME lineages was retrieved in the studies performed from
samples collected in the anoxic water column and sediment
of Lake Pavin (Lehours et al. 2007 ; Biderre-Petit et al. 2011 ).
However the AOM process in lacustrine environments
remains poorly known and may involve other archaeal popu-
lations than ANME.
- Are Marine Benthic group D (MBG-D) members involved
in AOM in the sediment?
Sequences belonging to the euryarchaeotal MBG-D, a
Thermoplasmatales-related group, were retrieved from the
sediments of Lake Pavin (Fig. 16.1a, Borrel et al. 2012a).
Members of the MBG-D represented the unique archaeal
members in clone library from the bottom of the AOM zone
of the Lake Cadagno (Schubert et al. 2011 ) and were detected
in several other marine environments and cultures where
AOM occurs (Inagaki et al. 2006 ; Harrison et al. 2009 ;
Webster et al. 2011 ; Zhang et al. 2011 ). These observations
coupled with that of Beal et al. ( 2009 ) who noted that
MBG-D clones represented up to 40 % of the archaeal com-
munity in clone libraries of their AOM enrichments from
methane-seep sediment might imply the involvement of
MBG-D members in AOM. This hypothesis is consistent
with the high activities of this group detected in the sediment
layer where CH 4 concentrations decreased (Fig. 16.1a).
However, the recent genomic analysis of three incomplete
genomes of MBG-D representatives suggests that represen-
tatives of this lineage are more likely involved in the degra-
dation of detrital protein (Lloyd et al. 2013 ). Future complete
genomes and activities determined through cultural
approaches will be necessary to have confirmation of their
metabolism.- Are Methanomassiliicoccales members involved in AOM
in the water column?
Sequence of mcrA and 16S rRNA affiliated to the
Methanomassiliicoccales were detected in the anoxic water
column of the Lake Pavin (Biderre-Petit et al. 2011 ) and
belong to a separate cluster than the sequences of cultured
representatives of this order (Borrel et al. 2013 ). These rep-
resentatives are all obligate methylotrophic methanogens
requiring an external H 2 source to reduce methyl-compound
(e.g., methanol, methylated amines) to CH 4 (Dridi et al.
2012 ; Poulsen et al. 2013 ; Brugère et al. 2013 ). Due to the
low number of data on this order and to the affiliation of the
sequences from Lake Pavin to a large sub-cluster of uncul-
tured representatives, it is not excluded that the member of
the Methanomassiliicoccales occurring in the Lake Pavin
might be involved in anaerobic methanotrophy.- Observations and results which do not favor the involve-
ment of archaeal members
We recently started to investigate the distribution and sta-
ble carbon isotopic composition (δ^13 C values) of diagnostic
prokaryotic lipids in the anoxic sediment and the different
redox zones of the water column (oxic, dysoxic, fully anoxic)
of Lake Pavin, in order to decipher which prokaryotic com-
munities are involved in the CH 4 cycle in this singular
meromictic Lake (see Box 16.4 for a rationale of this
approach). Preliminary results show a low and homogenous
diversity of specific archaeal lipid biomarkers in both the
superficial sediments and the anoxic waters of the lake, and
the absence of most biomarkers diagnostic of ANME (e.g.,
crocetane, pentamethylicosane or hydroxyarchaeol). The
stable carbon isotopic composition of isoprenoid glycerol
ether lipids present in the sediment (i.e., δ^13 C archaeol =
−18‰ and δ^13 C caldarchaeol = −31‰) argue against CH 4
consumption by archaeal members. The only^13 C-depleted
(−70‰ < δ^13 C < −55‰) lipid biomarkers detected so far inA.-C. Lehours et al.