Geoscience Reference
In-Depth Information
This corresponds to the transformations presented in Fig. 12.6. The
process is respiration if viewed from the microorganism's side, and
mineralization of organic matter if the product of the transformation
is considered.
If oxygen is absent (anaerobiosis), the electron acceptors are different
compounds (Fig. 12.7):
Fermentable sugars
CO
2
e
-
-
-
NO
,
NO
N O, N
2
3
2
2
MnO
2
Mn
2+
Fe
2+
H
2
Fe
3+
SO
4
2-
CO
2
Methane CH
4
Ethane C
26
Propane C
38
Ethylene C
24
Electron acceptors
Fig. 12.7
Decomposition of organic matter in absence of oxygen.
The value of
p
e, in the table by Bartlett and James (1993) shows that,
after O
2
/H
2
O, the following are involved, in order:
NO
3
/N
2
: this process corresponds to
denitrification
and is called
nitrate respiration;
it is a
dissimilative
(as opposed to
assimilative
)
pathway; actually, here nitrate is not a source of nitrogen for
the microorganisms but only an electron acceptor; in other
words, these nitrates are destroyed (in large amounts), mostly
to recover the corresponding energy; bacteria use the enzyme
nitrate reductase
for this purpose. The process is complex and
one must guard against excessive simplification. There are two
kinds of nitrate reductase. They can be present in the same
organism: one enables dissimilation by anaerobiosis and the
other, assimilation of nitrates when oxygen is present.
MnO
2
/Mn
2
+
: according to Bartlett and James (1993), the role
played by manganese in soils has been widely underestimated;
the darkening of phenolic compounds so commonly attributed
to iron, is produced in the laboratory with MnO
2
and not with
iron oxides!
