193
respiration is the highest, –479 kJ mol
-1
, and that
of denitrification is nearly as high. Table 5.2
shows that respiration of one mol of organic
carbon with sulfate yields only a fraction of the
energy of respiration with oxygen. The remaining
potential chemical energy is not lost but is mostly
bound in the product, H 2 S. This energy may
become available to other microorganisms, such
as chemolithotrophic sulfur bacteria, when the
sulfide is transported back up towards the
sediment surface and comes into contact with
potential oxidants.
The quantitative importance of the different
oxidants for mineralization of organic carbon has
been studied intensely over the last few decades,
both by diagenetic modeling and by incubation
experiments. It is generally found that oxygen and
sulfate play the major role in shelf sediments,
where 25-50% of the organic carbon may be
mineralized anaerobically by sulfate reducing
bacteria (Jørgensen 1982). With increasing water
depth and decreasing organic influx down the
continental slope and into the deep sea, the depth
of oxygen penetration increases and sulfate
reduction gradually looses significance. Nitrate
seems to play a minor role as an oxidant of organic
matter. Manganese oxides occur in shelf
sediments mostly in lower concentrations than
iron oxides and, expectedly, the Mn(IV) reduction
rates should be lower than those of Fe(III)
reduction. Manganese, however, is recycled
nearer to the sediment surface and relatively faster
than iron. These solid-phase oxidants are both
dependent on bioturbation as the mechanism to
bring them from the sediment surface down to
their zone of reaction. The shorter this distance,
the faster can the metal oxide be recycled.
Table 5.6 shows an example of process rates in
a coastal marine sediment. In this comparison, it is
important to keep in mind that the different
oxidants are not equivalent in their oxidation
capacity. When for example the iron in Fe(III) is
reduced, the product is Fe
2+
and the iron atoms
have been reduced only one oxidation step from
+3 to +2. Sulfur atoms in sulfate, in contrast, are
reduced eight oxidation steps from +6 in SO 4
2to –
2 in H 2 S. One mol of sulfate, therefore, has 8-fold
higher oxidation capacity than one mol of iron
oxide. In order to compare the different oxidants
and their role for organic carbon oxidation, their
reduction rates were recalculated to carbon
equivalents on the basis of their change in
oxidation step. It is then clear that oxygen and
sulfate were the predominant oxidants in this
sediment, sulfate oxidizing about 44% of the
organic carbon in the sediment. Of the H 2 S formed,
15% was buried as pyrite, while the rest was
reoxidized and could potentially consume one
third of the total oxygen uptake. This redox
balance is typical for coastal sediments where up
to half of the oxygen taken up by the sediment is
used for the direct or indirect reoxidation of
sulfide and reduced manganese and iron
(Jørgensen and Nelson 2004).
Manganese and iron behave differently with
respect to their reactivity towards sulfide. Iron
binds strongly to sulfide as FeS or FeS 2 , whereas
manganese does not. Furthermore, the reduced
iron is more reactive than reduced manganese
when it reaches the oxic zone, and Fe
2+
generally
does not diffuse out of the sediment, although it
may escape by advective pore water flow (Huettel
et al. 1998). The Mn
2+
, in contrast, easily recycles
via the overlying water column and can thereby be
transported from the shelf out into deeper water.
This mechanism leads to the accumulation of
manganese oxides in some continental slope
sediments (see Chap. 11). The important role of
manganese and iron in slope sediments is evident
from Figure 5.12. Manganese constituted about
5% of the total dry weight of this sediment. Below
the manganese zone, iron oxides and then sulfate
took over the main role as oxidants. In the upper
few cm of the sediment, manganese oxide was the
dominant oxidant below the O 2 zone (Canfield
1993).
The consecutive reduction of oxidants with
depth in the marine sediment and the complex
reoxidation of their products constitute the ‘redox
cascade’ (Fig. 5.11). An important function of this
sequence of reactions is the transport of electrons
from organic carbon via inorganic species back to
oxygen. The potential energy transferred from the
organic carbon to the inorganic products is
thereby released and may support a variety of
lithotrophic microorganisms. These may make a
living from the oxidation of sulfide with Fe(III),
Mn(IV), NO 3
–
or O 2 . Others may be involved in the
oxidation of reduced iron with Mn(IV), NO 3
–
or O 2
etc. The organisms responsible for these reactions
are only partly known and new types continue to
be isolated.
The depth sequence of electron acceptors in
marine sediments, from oxygen to sulfate, is
accompanied by a decrease in the degradability of
the organic material remaining at that depth. This
5.5
Pathways of Organic Matter Degradation
respiration is the highest, –479 kJ mol
-1
, and that
of denitrification is nearly as high. Table 5.2
shows that respiration of one mol of organic
carbon with sulfate yields only a fraction of the
energy of respiration with oxygen. The remaining
potential chemical energy is not lost but is mostly
bound in the product, H 2 S. This energy may
become available to other microorganisms, such
as chemolithotrophic sulfur bacteria, when the
sulfide is transported back up towards the
sediment surface and comes into contact with
potential oxidants.
The quantitative importance of the different
oxidants for mineralization of organic carbon has
been studied intensely over the last few decades,
both by diagenetic modeling and by incubation
experiments. It is generally found that oxygen and
sulfate play the major role in shelf sediments,
where 25-50% of the organic carbon may be
mineralized anaerobically by sulfate reducing
bacteria (Jørgensen 1982). With increasing water
depth and decreasing organic influx down the
continental slope and into the deep sea, the depth
of oxygen penetration increases and sulfate
reduction gradually looses significance. Nitrate
seems to play a minor role as an oxidant of organic
matter. Manganese oxides occur in shelf
sediments mostly in lower concentrations than
iron oxides and, expectedly, the Mn(IV) reduction
rates should be lower than those of Fe(III)
reduction. Manganese, however, is recycled
nearer to the sediment surface and relatively faster
than iron. These solid-phase oxidants are both
dependent on bioturbation as the mechanism to
bring them from the sediment surface down to
their zone of reaction. The shorter this distance,
the faster can the metal oxide be recycled.
Table 5.6 shows an example of process rates in
a coastal marine sediment. In this comparison, it is
important to keep in mind that the different
oxidants are not equivalent in their oxidation
capacity. When for example the iron in Fe(III) is
reduced, the product is Fe
2+
and the iron atoms
have been reduced only one oxidation step from
+3 to +2. Sulfur atoms in sulfate, in contrast, are
reduced eight oxidation steps from +6 in SO 4
2to –
2 in H 2 S. One mol of sulfate, therefore, has 8-fold
higher oxidation capacity than one mol of iron
oxide. In order to compare the different oxidants
and their role for organic carbon oxidation, their
reduction rates were recalculated to carbon
equivalents on the basis of their change in
oxidation step. It is then clear that oxygen and
sulfate were the predominant oxidants in this
sediment, sulfate oxidizing about 44% of the
organic carbon in the sediment. Of the H 2 S formed,
15% was buried as pyrite, while the rest was
reoxidized and could potentially consume one
third of the total oxygen uptake. This redox
balance is typical for coastal sediments where up
to half of the oxygen taken up by the sediment is
used for the direct or indirect reoxidation of
sulfide and reduced manganese and iron
(Jørgensen and Nelson 2004).
Manganese and iron behave differently with
respect to their reactivity towards sulfide. Iron
binds strongly to sulfide as FeS or FeS 2 , whereas
manganese does not. Furthermore, the reduced
iron is more reactive than reduced manganese
when it reaches the oxic zone, and Fe
2+
generally
does not diffuse out of the sediment, although it
may escape by advective pore water flow (Huettel
et al. 1998). The Mn
2+
, in contrast, easily recycles
via the overlying water column and can thereby be
transported from the shelf out into deeper water.
This mechanism leads to the accumulation of
manganese oxides in some continental slope
sediments (see Chap. 11). The important role of
manganese and iron in slope sediments is evident
from Figure 5.12. Manganese constituted about
5% of the total dry weight of this sediment. Below
the manganese zone, iron oxides and then sulfate
took over the main role as oxidants. In the upper
few cm of the sediment, manganese oxide was the
dominant oxidant below the O 2 zone (Canfield
1993).
The consecutive reduction of oxidants with
depth in the marine sediment and the complex
reoxidation of their products constitute the ‘redox
cascade’ (Fig. 5.11). An important function of this
sequence of reactions is the transport of electrons
from organic carbon via inorganic species back to
oxygen. The potential energy transferred from the
organic carbon to the inorganic products is
thereby released and may support a variety of
lithotrophic microorganisms. These may make a
living from the oxidation of sulfide with Fe(III),
Mn(IV), NO 3
–
or O 2 . Others may be involved in the
oxidation of reduced iron with Mn(IV), NO 3
–
or O 2
etc. The organisms responsible for these reactions
are only partly known and new types continue to
be isolated.
The depth sequence of electron acceptors in
marine sediments, from oxygen to sulfate, is
accompanied by a decrease in the degradability of
the organic material remaining at that depth. This
5.5
Pathways of Organic Matter Degradation