Environmental Engineering Reference
In-Depth Information
increasing depth in anoxic sediment or across the metalimnion of a eutrophic
stratified lake. Changes in redox also occur over time, giving rise to an or-
derly sequence of preferred nutrient transformations. The order that com-
pounds can serve as electron acceptors in the oxidation of organic carbon is
related to relative energy yield and redox and was presented in Fig. 12.4.
Redox gradients are sites of high rates and diverse types of metabolic
activities. These microbial “hot spots” occur because of the dependence of
many aquatic microbial geochemical processes on either reduced chemicals
in oxidized environments or oxidized chemicals in reduced environments.
Hypothetical distribution of some of these activities and populations of mi-
crobes (Fig. 13.13) illustrates part of the complexity of the situation. Ad-
ditional complexity arises from the variety of fermentative processes that
can occur and the fact that many different microbial strains or species can
be responsible for each of the processes.
Gradients of redox can occur in a variety of habitats (discussed pre-
viously). Consideration of redox adds more complexity to the view of bio-
geochemical cycling because of the series of redox potentials. In the ab-
sence of O
2
, redox gradients still provide a habitat gradient with different
metabolically driven fluxes depending on the amount of potential energy
that can be extracted. Because many of these organisms have a long evo-
lutionary history, natural selection dictates that organisms relying on inef-
ficient metabolic pathways will be outcompeted and are unlikely to domi-
nate in areas where other processes are more efficient.
O
2
(mg L
-1
)
0369 2
0
0
0
0
FePO
4
2
2
2
2
SO
4
2-
4
Sulfur oxidizers
4
4
4
H
2
S
6
6
6
6
Sul
fate
r
educ
ers
PO
4
3-
8
8
8
8
Fe
S
10
10
10
10
0.0 0.2 0.4 0.6 0.8 1.0
PO
4
3-
or FePO
4
(mg L
-1
)
0.0 0.2 0.4 0.6 0.8 1.0
SO
4
2-
, H
2
S, FeS
2
(mg L
-1
)
0
30000
60000
90000
-100 100
300 500
700
# microbial cells
Redox (mV)
0
0
0
0
2
2
2
2
NO
3
-
Fe
3+
4
4
Fe(OH)
3
Nitrifiers
4
4
6
6
6
6
Iron oxidizers
NH
4
+
Denitrifiers
8
8
F
e2
+
8
8
10
10
10
10
0.0 0.2 0.4 0.6 0.8 1.0
NO
3
-
or NH
4
+
(mg L
-1
)
0
30 000
60000
90000
0.0 0.2 0.4 0.6 0.8 1.0
0
30000
60000
90000
#
microbial cells
# microbial cells
Fe
2+
, Fe
3+
, Fe(OH)
3
(mg L
-1
)
0
0
2
2
Methanotrophs
4
4
6
6
Methanogens
8
8
10
10
0.0 0.2 0.4 0.6 0.8 1.0
0
30000
60000
90000
# mi
robial cells
CH
4
(mg L
-1
)
FIGURE 13.13
Hypothetical patterns of chemical and microbial distributions with depth in
sediments.
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