Agriculture Reference
In-Depth Information
The free energy change for the reaction is
+
2
.
303
RT
log
(
Ox
2
)(
Red
1
)
(
Ox
1
)(
Red
2
)
G
=
G
o
(
4
.
24
)
Combining Equations (4.23) and (4.24) gives
G
=−
2
.
303
RT
n(
pe
1
−
pe
2
)
−
log
(
Ox
2
)(
Red
1
)
(
Ox
1
)(
Red
2
)
(
4
.
25
)
As discussed earlier, the dependence of pe on the concentrations of reductants
and oxidants is often small in comparison with its dependence on pH. The term
in the square brackets in Equation (4.25) can therefore be replaced by pe
o
∗
terms
giving for the approximate standard free energy change:
G
o
∗
≈−
2
.
303
RT n(
pe
o
∗
−
pe
o
2
)
(
4
.
26
)
1
Figure 4.3 shows oxidation and reduction reactions used by microbes as energy
sources on a scale of pe
o
∗
(data from Table 4.1). The free energy changes for
the different complete reactions can be read from the
G
o
∗
scale, in accordance
with Equation (4.26). The energy expended by microbes in elaborating carbon,
for example through fixation of CO
2
, and other elements, for example nitrogen
through fixation of atmospheric N
2
, can be calculated in a similar way.
Such calculations indicate the maximum energy available from a reaction or the
minimum required to carry it out. The true gain to a microbe is smaller, or the cost
larger, because of the energy required for cell maintenance and reproduction and
other processes. The energetic efficiencies of biochemical processes are typically
of the order of 30 - 40 %. Nonetheless the ecological succession of microbes in
response to the stepwise oxidation of reduced compounds and exhaustion of
oxidants can be predicted from such calculations. Thus the succession of aerobic
organisms, denitrifiers, manganese reducers, iron reducers, sulfate reducers and
methanogenic bacteria following submergence of a soil directly matches the order
of decreasing pe
o
∗
for the corresponding redox couples in Figure 4.3(b): O
2
-H
2
O,
NO
3
−
-N
2
,MnO
2
(
s
)
-Mn
2
+
, Fe(OH)
3
(
s
)
-Fe
2
+
,SO
4
2
−
-HS
−
and CH
2
O-CH
4
.
Microorganisms and organisms in general can be classified according to the
principal sources of their energy, carbon and electrons. A hierarchical classifica-
tion is not possible because all combinations of these three occur. Thus all three
can be separate, as for green plants which obtain their energy from sunlight,
carbon from CO
2
and electrons by oxidizing water to O
2
;andallthreecanbe
the same, as for the majority of bacteria which use organic compounds as their
sources of energy, carbon and electrons.
Organisms that obtain their carbon from inorganic compounds, mainly CO
2
,are
called autotrophs. They are subdivided into photoautotrophs which obtain energy
from sunlight — for example, green plants and photosynthetic bacteria — and che-
moautotrophs which obtain energy from chemical processes — for example, in
Figure 4.3(a), nitrifying bacteria, which oxidize NH
4
+
to NO
3
−
, sulfur oxidizing
