Environmental Engineering Reference
In-Depth Information
Fig. 4
Mechanism by
which sulfate reducer can
convert acetate to CO
2
and
reduced sulphur compounds
to electricity production in
sediment microbial fuel cells
in sulphide-rich sediments.
Data source
Lovley (
2006
)
A pure culture of
Desulfovibrio desulfuricans
can readily reduce Fe(III), but
Desulfobacter postgatei and Desulfobactercurvatus
cannot (Coleman et al.
1993
).
The experimental study showed the occurrence of the metabolism of Fe(III) and
sulphate by
D. desulfuricans
; at low concentrations of H
2
in aquatic sediments,
Fe(III) might be the predominant electron acceptor (Coleman et al.
1993
). It
has been evidenced that fermentation or methanogenesis do not metabolize the
organics rapidly (Lovley
2006
), but can produce a number of minor components
such as acetate, formate, methanol, CO
2
and H
2
at the end of the metabolic pro-
cess (Yang and Guo
1990
; Roden and Wetzel
1996
; Zeikus et al.
1975
; Lovley and
Klug
1986
; Lovley and Phillips
1987
). These products are subsequently used for
methane formation.
Methanogenic bacteria are a diverse subgroup of
archaebacteria (Archaea
) that
convert CO
2
into methane to provide energy (31 kcal/mol) for the cell (Eq.
2.29
)
(Thauer et al.
2008
; Thauer
1998
):
CO
2
+
4H
2
→
CH
4
+
2H
2
O
(2.29)
The conversion of glucose to alcohols and fatty acids during the fermenta-
tion allows the utilization of the standard Gibbs free energy content (Conrad
1999
; Thauer et al.
1977
). The degradation of alcohols and fatty acids to acetate
and H
2
caused by syntrophic bacteria is endoergonic under standard conditions
(Conrad
1999
; Thauer et al.
1977
), but it can take place when it is combined with
H
2
-consuming methanogenesis (Conrad
1999
). Hydrogenotrophic and acetotrophic
methanogenesis may convert fermentation products or glucose to CH
4
and/or CO
2
.
The mechanism for methane formation in methyl-coenzyme
M
reductase
(MCR) has been evidenced using the B3LYP hybrid density functional method
and chemical models consisting of 107 atoms (Pelmenschikov et al.
2002
). In this
mechanism, the reaction starts with CoB and methyl-CoM coenzymes and with
the active Ni(I) state of the tetrapyrrole F
430
prosthetic group, which then forms a
