8
Sulfur Cycling and Methane Oxidation
298
to SO 4
2in a proportion of 3:1 that maintains
electron balance (Eq. 8.19):
S 2 O 3
2+ H 2 O → H 2 S + SO 4
2(8.18)
4 S
0
+ 4 H 2 O → 3 H 2 S + SO 4
2+ 2 H
+
(8.19)
Disproportionation reactions do not cause a
net oxidation of the sulfur species, yet they have a
key function in sulfide oxidation. Disproportionation provides a shunt in the sulfur cycle
whereby the H 2 S formed by this reaction may be
oxidized again to the same sulfur intermediate by
metal oxides. Manganese oxide, for example,
rapidly oxidizes H 2 S to S
0
without participation of
bacteria, but does not oxidize the S
0
further to
sulfate (Burdige 1993). The elemental sulfur may,
however, be disproportionated (Eq. 8.19) whereby
a fourth of it is oxidized completely to sulfate
while the remaining three fourths return to the
sulfide pool. Through repeated partial oxidation of
sulfide to elemental sulfur with manganese oxide
and subsequent disproportionation of the elemental sulfur to sulfate and sulfide a complete oxidation of sulfide to sulfate by manganese oxide may
be achieved (Fig. 8.16; Thamdrup et al. 1993;
Böttcher and Thamdrup 2001):
4 H 2 S + 4 MnO 2 → 4 S
0
+ 4 Mn
2+
+ 8 OH
-
(8.20)
Sum of Eq. 8.20 and 8.19:
H 2 S + 4 MnO 2 + 2 H 2 O →
SO 4
2+ 4 Mn
2+
+ 6 OH
-
(8.21)
An interesting biological mechanism of sulfide
oxidation in coastal upwelling regions and other
high-productivity coastal ecosystems was discovered in the mid 1990’ies (Fossing et al. 1995). The
sediment underlying some of the most intensive
upwelling systems, e.g. off the Pacific coast of
South and Central America or the Atlantic coast of
southwest Africa, is densely populated with sulfur
bacteria (species of Thioploca, Thiomargarita,
and Beggiatoa) that have a peculiar mode of life
and an unusually large cell size (Schulz et al.
1999). Within a liquid vacuole inside each cell they
accumulate nitrate from the ambient sea water and
later use this nitrate down in the sediment as an
electron acceptor for sulfide oxidation from which
they gain energy. The sulfide is oxidized first to
elemental sulfur, which is stored transiently in the
cells, and then to sulfate. This adaptation is
unique in that it enables the motile types of the
bacteria to commute up and down between the
nitrate source in the sea water and the sulfide
source in the sediment without having access to
both at the same time (Jørgensen and Gallardo
1999; Schulz and Jørgensen 2001; see Chapter 6).
8.6
Determination of Process Rates
The rates of biogeochemical processes such as
sulfate reduction in marine sediments can be
determined by different approaches that all have
strengths and weaknesses and may demonstrate
fundamentally different aspects of the process. It
is important for each application to consider
carefully the properties of the sediment to be
analyzed and the limitations of the method
applied. The study of shallow and highly dynamic
surface sediments requires a different approach
than the study of deep sediments that have
undergone stable diagenesis over a long time
period. By the shallow sediment an experimental
measurement of the process rate may be optimal
whereas deep in the sediments generally a
modeling approach is preferred. Dependent on the
method, however, either the gross or the net rate
of sulfate reduction is measured. The extent to
which these differ has been determined only in a
few cases.
Gross sulfate reduction rates (gross SRR) can
be measured by experimental incubation of
recovered sediment samples on board ship or in
the laboratory. Today, this is mostly done by the
use of radioactively labeled sulfate,
35
SO 4
2, as a
tracer in order to obtain sufficiently high sensitivity of the method (see Chapter 5). Originally
developed by Sorokin (1962), Ivanov (1968),
Jørgensen (1978), this method has been modified
and refined over the years (e.g., Howarth 1979;
Canfield et al. 1986; Fossing and Jørgensen 1989;
Kallmeyer et al. 2004). By adaptation of the
radiotracer method it has been possible to directly
measure sulfate reduction rates that vary over
more than 7 orders of magnitude, for example on
the Peruvian shelf, from >1000 nmol SO 4
2cm
-3
day
-1
at the sediment surface to <0.001 nmol SO 4
2cm
-3
day
-1
at 100 m subsurface (Parkes et al. 2005; J.
Kallmeyer et al. in prep.).
The gross SRR as measured by
35
SO 4
2technique comes closest to the overall sulfate
reduction that takes place in the sediment. The
Sulfur Cycling and Methane Oxidation
298
to SO 4
2in a proportion of 3:1 that maintains
electron balance (Eq. 8.19):
S 2 O 3
2+ H 2 O → H 2 S + SO 4
2(8.18)
4 S
0
+ 4 H 2 O → 3 H 2 S + SO 4
2+ 2 H
+
(8.19)
Disproportionation reactions do not cause a
net oxidation of the sulfur species, yet they have a
key function in sulfide oxidation. Disproportionation provides a shunt in the sulfur cycle
whereby the H 2 S formed by this reaction may be
oxidized again to the same sulfur intermediate by
metal oxides. Manganese oxide, for example,
rapidly oxidizes H 2 S to S
0
without participation of
bacteria, but does not oxidize the S
0
further to
sulfate (Burdige 1993). The elemental sulfur may,
however, be disproportionated (Eq. 8.19) whereby
a fourth of it is oxidized completely to sulfate
while the remaining three fourths return to the
sulfide pool. Through repeated partial oxidation of
sulfide to elemental sulfur with manganese oxide
and subsequent disproportionation of the elemental sulfur to sulfate and sulfide a complete oxidation of sulfide to sulfate by manganese oxide may
be achieved (Fig. 8.16; Thamdrup et al. 1993;
Böttcher and Thamdrup 2001):
4 H 2 S + 4 MnO 2 → 4 S
0
+ 4 Mn
2+
+ 8 OH
-
(8.20)
Sum of Eq. 8.20 and 8.19:
H 2 S + 4 MnO 2 + 2 H 2 O →
SO 4
2+ 4 Mn
2+
+ 6 OH
-
(8.21)
An interesting biological mechanism of sulfide
oxidation in coastal upwelling regions and other
high-productivity coastal ecosystems was discovered in the mid 1990’ies (Fossing et al. 1995). The
sediment underlying some of the most intensive
upwelling systems, e.g. off the Pacific coast of
South and Central America or the Atlantic coast of
southwest Africa, is densely populated with sulfur
bacteria (species of Thioploca, Thiomargarita,
and Beggiatoa) that have a peculiar mode of life
and an unusually large cell size (Schulz et al.
1999). Within a liquid vacuole inside each cell they
accumulate nitrate from the ambient sea water and
later use this nitrate down in the sediment as an
electron acceptor for sulfide oxidation from which
they gain energy. The sulfide is oxidized first to
elemental sulfur, which is stored transiently in the
cells, and then to sulfate. This adaptation is
unique in that it enables the motile types of the
bacteria to commute up and down between the
nitrate source in the sea water and the sulfide
source in the sediment without having access to
both at the same time (Jørgensen and Gallardo
1999; Schulz and Jørgensen 2001; see Chapter 6).
8.6
Determination of Process Rates
The rates of biogeochemical processes such as
sulfate reduction in marine sediments can be
determined by different approaches that all have
strengths and weaknesses and may demonstrate
fundamentally different aspects of the process. It
is important for each application to consider
carefully the properties of the sediment to be
analyzed and the limitations of the method
applied. The study of shallow and highly dynamic
surface sediments requires a different approach
than the study of deep sediments that have
undergone stable diagenesis over a long time
period. By the shallow sediment an experimental
measurement of the process rate may be optimal
whereas deep in the sediments generally a
modeling approach is preferred. Dependent on the
method, however, either the gross or the net rate
of sulfate reduction is measured. The extent to
which these differ has been determined only in a
few cases.
Gross sulfate reduction rates (gross SRR) can
be measured by experimental incubation of
recovered sediment samples on board ship or in
the laboratory. Today, this is mostly done by the
use of radioactively labeled sulfate,
35
SO 4
2, as a
tracer in order to obtain sufficiently high sensitivity of the method (see Chapter 5). Originally
developed by Sorokin (1962), Ivanov (1968),
Jørgensen (1978), this method has been modified
and refined over the years (e.g., Howarth 1979;
Canfield et al. 1986; Fossing and Jørgensen 1989;
Kallmeyer et al. 2004). By adaptation of the
radiotracer method it has been possible to directly
measure sulfate reduction rates that vary over
more than 7 orders of magnitude, for example on
the Peruvian shelf, from >1000 nmol SO 4
2cm
-3
day
-1
at the sediment surface to <0.001 nmol SO 4
2cm
-3
day
-1
at 100 m subsurface (Parkes et al. 2005; J.
Kallmeyer et al. in prep.).
The gross SRR as measured by
35
SO 4
2technique comes closest to the overall sulfate
reduction that takes place in the sediment. The