279the superficial sediments and in the dysoxic and anoxic
waters of Lake Pavin are bacterial C 16 fatty acids and
hopanoids (diploptene, diplopterol, C 32 homohopanols and
homohopanoic acid). Such^13 C-depleted lipids are generally
thought to be specific of (micro-) aerophilic methanotrophic
bacteria (e.g., Birgel and Peckmann 2008 ). However, the
more depleted δ^13 C values of some hopanoids in the anoxic
sediment of Lake Pavin compared to those of hopanoids
from the anoxic water column may indicate the presence of a
bacterial population capable of oxidizing CH 4 under strict
anaerobiosis. This intriguing hypothesis clearly needs to be
further investigated. Large volumes of water sampled at high
depth resolution in the oxic, dysoxic and anoxic parts of the
water column are currently being studied and should help
assessing the unconventional CH 4 consumers of Lake Pavin.
16.5 Conclusion and Perspectives
The Lake Pavin is an eccentric and original environment and
the study of the anaerobic biota of this biotope is a particu-
larly exciting prospect. In the point of view of the CH 4 cycle,
due to its specific characteristics, Lake Pavin is an excep-
tional site of high interest for scientists. In contrast to most of
freshwater lakes, Lake Pavin emits very low amounts of CH 4
in the atmosphere although high concentrations of this gas
are formed in the sediment and at the bottom of the water
column. The characteristics of the water column of Lake
Pavin provide, undoubtedly, particularly favorable condi-
tions to the oxidation of this metabolite. In particular, the
strong physicochemical gradients present at the oxic/anoxic
interface probably slow down the flow of CH 4 from the mon-
imolimnion, enabling a greater efficiency in its consumption.
Indeed, diffusion is the pathway of CH 4 which gives the most
accessibility to its consumption by methanotrophic microor-
ganisms (Bastviken 2009 ). Moreover, the geomorphological
characteristics of Lake Pavin, conditioning its meromixis,
may limit the contribution of other pathways of CH 4 trans-
port. As the overturning of the water column is incomplete in
Lake Pavin, only a small portion of the CH 4 accumulated in
the anoxic zone is transported during seasonal mixing events.
In addition, the low epilimnetic sediment surface limits the
bubbling process. Finally, considering the small area covered
by emergent macrophytes, transport by plants should be neg-
ligible. In addition to the relative importance of the different
pathways of CH 4 transport, we showed that the combination
of the two CH 4 oxidation processes (aerobic and anaerobic)
contributes to limit dramatically emissions of this gas from
the Lake Pavin.
Surprisingly, a small number of species are involved in
the biogeochemical CH 4 cycle in Lake Pavin, at least both in
the production and in the aerobic consumption of this metab-
olite; those involved in the anaerobic consumption being not
yet identified. This is probably the result of a long term evo-
lution of anaerobic microbial communities since the forma-
tion of the Lake. We can postulate that the steady state of the
monimolimnion illustrates that microbial communities reach
the climax in this anoxic ecosystem. Moreover, bottom-up
regulations are also probably crucial in determining the spe-
cies composition. Particularly, the temperature seems to play
an important role in the community composition of metha-
nogens and aerobic methanotrophs in Lake Pavin illustrat-
ing, if it was necessary, that physiological factors are crucial
to understand the structure, the diversity and the activities of
microbes in the environment.
A strong work remains to be done to identify the microor-
ganisms involved in the AOM in this Lake. Actors involved
in AOM in different other ecosystems (marine and freshwa-
ter systems) are not detected in the water column and in the
sediment of Lake Pavin. It seems therefore plausible that the
investigation of AOM in Lake Pavin will extend the knowl-
edge of the microorganisms and of the mechanisms acting to
consume CH 4 anaerobically in the biosphere. At time, our
hypothesis favor a coupling between AOM and denitrifica-
tion as results from geochemical modeling (Lopes et al.
2011 ) and enrichments cultures encourage such hypothesis.
Due to the greenhouse effect of CH 4 and in the context of
global warming, the understanding of microbial processes
involved in CH 4 biogeochemical cycle is of crucial impor-
tance both in terms of fundamental science and
applications.Acknowledgements This study includes results from Master, PhD and
postdoc researches supported by various instances: Région Auvergne,
CNRS, Université Blaise Pascal, Ministère de la Recherche et de la
Technologie. Part of the results presented was obtained with the col-
laboration of colleagues: Didier Jézéquel, Gilles Mailhot, Jonathan
Colombet, Keith Joblin, Paul Evans, Annie Guedon, Corinne Biderre-
Petit, Marion Rabiet, Karl Rockne, Christophe Guimbaud, Frédéric
Savoie, Stéphane Chevrier, Pierre Agrinier, Nelly Assayag, François
Prévot, Jean-Claude Romagoux, Guy Demeure, Aurélie Thénot,
Télesphore Sime-Ngando. The studies were granted from different
sources: French National Program EC2CO (Projects INTERLAC,
METHANOLAC) and ANR Program (Project METANOX). We thank
the reviewer, Bernard Ollivier, for his constructive comments, which
helped us to improve the manuscript.References
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