270
radation of organic compounds, resulting in the produc-
tion of sulfide (H 2 S).- Ferric iron reduction^10 : Ferric iron (Fe3+) in soluble or
insoluble (e.g., FeOOH, Fe 2 O 3 -H 2 0) forms may be micro-
biologically reduced to ferrous iron (Fe2+). Dissimilatory
iron reduction may be a form of respiration in which fer-
ric iron serves as a dominant or exclusive terminal elec-
tron acceptor; or it may also accompany fermentation in
which ferric iron is used as a supplementary electron
acceptor (see above the example of strain BS2, Sect.
16.3.3.4) as opposed to dominant or exclusive terminal
electron acceptor.
16.3.3.6 Do Sulfate-Reducing and Ferric-Iron
Reducing Bacteria Compete
with Methanogens in the Water
Column of Lake Pavin?
- Sulfate-reducing bacteria (SRB) in Lake Pavin : using
fluorescent in situ hybridization (FISH) method, SRB
were demonstrated to account for 44 % of the Bacteria in
the chemocline of Lake Pavin (Lehours et al. 2005 ). These
high densities support a biogenic origin for the H 2 S found
in the anoxic zone of Lake Pavin. The genera
Desulfobulbus, Desulfobacter, and Desulfovibrio, tar-
geted with specific probes, represented only a small frac-
tion of the SRB quantified in the chemocline meaning that
other groups of SRB are involved in the sulfate-reduction
activity in this zone. The genera Desulfobulbus,
Desulfobacter, and Desulfovibrio presented density peaks
at the aerobic/anaerobic interface (>10^5 cells mlâ1). The
great abundance of the genus Desulfobulbus suggests a
major role of this genus in sulfate-reduction activity. - Ferric-Iron-reducing bacteria (FRB) in Lake Pavin :
Despite high concentrations of Fe (II) in the anoxic zone
of lake Pavin, both cloning-sequencing analyses and
culture- dependent approaches did not allow the detection
of members of known obligatory FRB, such as
Geobacteriaceae (Lehours et al. 2007 , 2009 ). This does
not mean that there is no known or unknown obligatory
FRB in the water column of lake Pavin, but suggests that
facultative FRB may substantially contribute to Fe(III)
reduction in this environment. In Fe(III)-enrichment cul-
tures, bacteria affiliated to the Planctomycetes, the
Firmicutes, the Actinobacteria, the Spirochaetes, and to
the five classes within the Proteobacteria were retrieved
and probably used ferric-iron reduction as an alternative
dissimilatory pathway. For example, several Pseudomonas,
Aeromonas and Serratia species, previously shown to
reduce iron under anaerobic conditions (Lovley 2006 ),
were enriched. Moreover, Desulfovibrio members, largely
detected in the chemocline of Lake Pavin (Lehours et al.
2005 ), were also enriched suggesting that some SRB may
be involved in ferric-iron reduction in Lake Pavin anoxic
zone.- Hypotheses explaining why acetoclastic methanogens
are the main CH 4 producers in Lake Pavin : Data
acquired from both the water column and the sediment of
Lake Pavin revealed that acetotrophic methanogenesis is
an important mineralization pathway and the main source
of CH 4 in this ecosystem. It is rather surprising when con-
sidering that acetotrophic methanogenesis is the less ther-
modynamically favorable pathway for methanogenesis
(Garcia et al. 2000 ). We formulate two hypotheses, in
relation to competitive interactions, to explain this
observation:
(1) FRB and methanogens are not in competition for acetate
uptakeIn most freshwater environments, Fe (III) reduction and
methanogenesis are the dominant processes but Fe (III)
reducers have a higher affinity for acetate providing them
some competitive advantages over methanogenesis (van
Bodegom et al. 2004 ). However, in the steady state environ-
ment of Lake Pavin anoxic zone, acetate may be preferen-
tially used for methanogenesis while ferric-iron reducers use
preferentially H 2 and alternative intermediate products such
as lactate or fumarate (Lehours et al. 2009 ). This hypothesis
is consistent with the dominance of Methanosaeta concilii, an
acetoclastic methanogenic species, within the methanogenic
communities (Lehours et al. 2005 , 2007 ). This hypothesis
also agrees with the results of van Bodegom et al. ( 2004 ) who
demonstrated that hydrogenotrophic methanogens are more
directly inhibited by Fe (III) than Methanosaeta concilii.(2) SRB are outcompeted by methanogens for acetate uptakeAcetate-utilizing sulfate reducers may also compete with
acetoclastic methanogens. However, this competition is not
straightfoward as for hydrogen. In experiments in which sul-
fate was added to a fully methanogenic anaerobic bioreactor,
the acetotrophic Methanosaeta species were out-competed
by sulfate reducers only several years after the beginning of
the incubation (Omil et al. 1998 ).
Is Lake Pavin an exception? Probably not. Indeed,
Methanosaeta species are probably the predominant CH 4 -
producers on Earth and up to two-thirds of biologically pro-
duced CH 4 released in the atmosphere each year is derived
from the methyl group of acetate (Smith and Ingram-Smith
2007 ). To understand why acetate is the main precursor of
methanogenesis, we also needed to come back to the physiol-
ogy of microorganisms which is important for modeling the
variation in MPRs according to environmental constraints.
For example, pH and temperature conditions prevailing in
Pavin sediments favor acetogenesis over hydrogenotrophic(^10) For detailed informations, see Ehrlich and Newman 2008. methanogenesis (Nozhevnikova et al. 2007 ).
A.-C. Lehours et al.