Environmental Engineering Reference
In-Depth Information
46 kJ)
than the oxidation of a carbohydrate with oxygen as an electron acceptor. Therefore, sulfate
reduction occurs only after the more energetically favorable electron acceptors have been
depleted.
Similarly, organisms can use electron donors other than C, and when a suitable electron
acceptor is present the summed reaction can yield energy, as in the following example of
nitrification:
Note that the sum of these two half-reactions yields much less energy (net
Δ
G
is
NH
4
1
NO
3
1
2H
1
2O
2
!
H
2
O
½
Δ
G
347 kJ
=
mole NH
4
oxidized
1
52
Microorganisms use a large number of electron donors and acceptors to capture energy
to support life-processes (
Table A.1
). These different reactions yield different amounts of
energy, and the reaction that yields the most energy possible under the ambient environ-
mental conditions tends to prevail. As a result, these microbially mediated redox reactions
tend to occur in a predictable sequence (
Figure A.2
). Thus, organic carbon is usually bro-
ken down chiefly by aerobic respiration (the reaction with the highest energy yield) until
oxygen is exhausted, then by denitrification, manganese reduction, iron reduction, sulfate
reduction, and finally methanogenesis, as successively less profitable (in terms of energy
yield) electron acceptors are used and then depleted. This sequence of reactions is often
TABLE A.1
Examples of some ecologically important biologically mediated redox reactions (many other
examples exist and are important in some ecosystems). Most of these reactions (other than aerobic
photosynthesis and oxygenic photosynthesis) are performed Primarily or exclusively by bacteria.
Process
Chemical Reaction
Location or Conditions
A. Oxidation of organic matter (organic carbon is the electron donor)
Aerobic
respiration
CH
2
O
1
O
2
!
CO
2
1
H
2
O
Where oxygen and OM
are present; widespread
4NO
3
2
1
4H
1
!
Denitrification
5CH
2
O
1
5CO
2
1
7H
2
O
1
2N
2
Usually where O
2
is low
or absent and NO
3
and
OM are present (aquatic
sediments, wet soils,
wetlands)
2Mn
1
2
Manganese
reduction
CH
2
O
2MnO
2
1
4H
1
!
CO
2
1
3H
2
O
Where O
2
is absent and
OM and Mn are abundant
(marine sediments,
ground waters)
1
1
8H
1
!
4Fe
1
2
Iron reduction
CH
2
O
4Fe(OH)
3
1
CO
2
1
11H
2
O
Where O
2
is absent and
iron is abundant
(wetlands, ground waters,
wet soils)
1
1
(
Continued
)
