268
Syntrophic associations between fermentative bacteria
and methanogenic archaea involve the exchange of hydro-
gen, formate or acetate between the partners. These
metabolites are produced by the fermentative syntrophic
metabolizer from its growth substrates (e.g.,, propionate,
butyrate, and benzoate) and are consumed by the methano-
genic partner keeping them at concentrations low enough
to permit the metabolism of the substrate by the syntrophic
metabolizer to be sufficiently exergonic to support adenos-
ine triphosphate (ATP) synthesis, anabolism, and growth.
For example, the syntrophic transformation of butyrate to
acetate and H 2 [at pH 7, 1 atm H 2 (101 kPa), and 1 M of
acetate and butyrate] is thermodynamically unfavorable with
a Gibbs free energy change of +48.6 kJ per mole of butyrate
metabolized. When hydrogen and formate concentrations are
kept low by methanogens or other hydrogen/formate users,
butyrate degradation becomes thermodynamically favorable
with a Gibbs free energy change of −39.2 kJ per mole of
butyrate.
- Acetogenesis^7
Acetogenesis has only been described in organisms
belonging to the Bacteria and, whereas most acetogens are
affiliated to the Firmicutes, they constitute paraphyletic
groups including Spirochaetes, δ-Proteobacteria like
Desulfotignum phosphitoxidans, and Acidobacteria like
Holophaga foetida. Encountered in most anaerobic environ-
ments, including extreme ones, acetogens reduce one-carbon
compounds using the reductive acetyl-CoA (or
Wood- Ljungdahl) pathway as their main mechanism for
energy conservation and for the synthesis of acetyl-CoA, a
metabolic precursor of acetate and biomass.
If acetogenesis is often perceived as a fermentative pro-
cess, the use of CO 2 as final electron acceptor implies a
strong dissimilarity with conventional fermentation process.
Acetogens are sometimes called “homoacetogens” (meaning
that they produce only acetate as fermentation product) or
“CO 2 -reducing acetogens”. Important for the biology of eco-
systems, particularly of freshwater lakes, acetogens contrib-
ute globally to 10 % of the output of acetate on Earth
(Ragdsdale and Pierce 2008 ).
16.3.3.4 Example of Interactions
Between Abiotic Factor (Iron)-
Fermentation and Methanogenesis
in Lake Pavin
A wide variety of fermentative microorganisms are able to
reduce Fe(III) during anaerobic growth, but, Fe(III) reduc-
tion appears to be a minor pathway for electron flows in fer-
mentative microorganisms that are not considered to be
conservers of the process energy (Lovley 1987 ). Cultures of
these microorganisms can accumulate significant amounts of
Fe(II) but less than 5 % of the reducing equivalents are trans-
ferred to Fe(III) (Lovley 1987 , 2006 ; Lovley and Phillips
1988 ). Although thermodynamic calculations demonstrated
that fermentation is more energetically favorable with than
without Fe(III) reduction (Lovley and Phillips 1988 ), it has
been postulated that the minor electron transfer from reduc-
ing equivalents to Fe(III) during fermentation does not cause
any increase the cell yield (Lovley 2006 ).
A comparative study has been performed on the growth,
fermentative profile and heat production of the facultative
iron-reducing bacterial strain BS2 (Fig. 16.4e), isolated from
the anoxic zone of the iron-rich Lake Pavin and cultured on
glucose in presence and absence of Fe(III), was performed
(Fig. 16.4a–d, Lehours et al. 2010 ). The possible ecological
benefit for this microorganism to use Fe(III) as an electron
acceptor was clearly demonstrated, and modifications of the
fermentative balance between glucose and glucose + Fe(III)
growth conditions were recorded (e.g., a higher gas produc-
tion (H 2 and CO 2 ) and differences in acetate and lactate
amounts, Fig. 16.4f). Given that fermentation products are
used as electron donors for terminal electron-accepting pro-
cesses like methanogenesis, sulfate-reduction or dissimila-
tory Fe(III) reduction, their availability may have a major
impact on terminal electron-accepting processes. Previous
studies on the anoxic zone of Lake Pavin have highlighted
that acetate might be the main precursor for methanogenesis
(Lehours et al. 2007 , 2009 ) and that lactate and H 2 favor the
Fe(III) reduction process (Lehours et al. 2009 ), suggesting
that Fe (III) reduction by fermentative bacteria may not only
affect their physiology but also the competitive relationships
among terminal electron-accepting microorganisms of the
anaerobic microbial food-web.16.3.3.5 Competitive Interactions with Other
Anaerobic Microbes (Fig. 16.3)
This section only presents the main terminal-electron-
accepting processes that may be competitive of
methanogenesis.- Denitrification^8 : Nitrogen is available in nature at differ-
ent degrees of oxidation. Denitrification in the strict sense
of the term corresponds to the reduction of nitrate to
nitrite which is often used as respiratory acceptor and is
reduced, in turn, into gaseous compounds such as NO,
N 2 O, N 2. - Sulfate-reduction^9 : Sulfate-reducing bacteria (SRB) use
sulfate (SO 4 2−) as a terminal electron acceptor for the deg-
(^7) For a detailed review, see Drake et al. 1994 , 2006 ; Müller et al. 2004 ;
Ragsdale and Pierce 2008.
(^8) For detailed informations see Zumft 1992 , 1997 ; Burgin and Hamilton
2007.
(^9) For detailed informations, see Widdel 1988 ; Muyzer and Stams 2008.
A.-C. Lehours et al.