Environmental Engineering Reference
In-Depth Information
Fig. 6
The mechanism for methane formation in methyl-coenzyme
M
reductase (MCR) in
methanogenesis in presence of natural substances. Data source Ebner et al. (
2010
)
(Ebner et al.
2010
). In this mechanism, the enzyme converts the thioether
methyl-coenzyme M, and the thiol coenzyme B, into methane and the heterodi-
sulfide of coenzyme M and coenzyme B (Ebner et al.
2010
). In the presence of
the competitive inhibitor coenzyme M instead of methyl-coenzyme M, addi-
tion of coenzyme B to the active Ni(I) state of MCR
red1
induces two new spe-
cies called MCR
red2a
and MCR
red2r
(Fig.
6
). The two MCR
red2
signals can also
be induced by the
S
-methyl- and the
S
-triluoromethyl analogs of coenzyme B. It
is thus suggested that the protein may undergo a conformational change upon for-
mation of MCR
red2
species in the transition from MCR
red1
, which opens up the
possibility that nickel coordination geometries other than square planar, tetragonal
pyramidal, or elongated octahedral might occur in intermediates of the catalytic
cycle (Ebner et al.
2010
).
The degradation of specific aliphatic carbon or functional groups by microbial
processes in natural waters may preferentially occur in macromolecules such as
fulvic and humic acids of terrestrial plant origin, as well as autochthonous fulvic
acid of algal origin. The microbial changes in the functional groups of organic
substances are typical phenomena in sediment pore waters, where a decrease of
the acidic functional groups as well as an increase of basic and neutral functional
groups occurs with depth (Rosenfeld
1979
; Burdige and Martens
1988
; Wu and
Tanoue
2001
; Maita et al.
1982
; Steinberg et al.
1987
). Such changes in functional
groups of autochthonous fulvic acid (C-like) can be understood from the vertical
increase in fluorescence intensity with depth, identified with excitation and emis-
sion matrix (EEM) of pore water samples and their parallel factor (PARAFAC)
modeling in the pore waters of lakes (Li W et al., unpublished data). The low val-
ues of fluorescence index for autochthonous fulvic acid (C-like) at deeper depth,
compared with upper sediment pore water, confirm the changes with depth in the
functional groups of that component (Li W et al., unpublished data). Such changes
might be a useful indicator for complex microbial processing of the functional
groups of autochthonous fulvic acids in the pore waters of lakes. Therefore, it is
suggested that microbial degradation may diagenetically alter either the minor
components (e.g. acetate) or the functional groups bound to macromolecules, such
as fulvic and humic acid from terrestrial plants and autochthonous fulvic acid from
algal biomass, generating CO
2
, CH
4
and other products.
