297
making the complete pyrite oxidation to sulfate
dependent on microbial catalysis (Schippers and
Jørgensen 2001):
FeS 2 + 7.5 MnO 2 + 11 H
+
→
Fe(OH) 3 + 2 SO 4
2+ 7.5 Mn
2+
+ 4 H 2 O (8.16)
Pyrite oxidation by nitrate could not be demonstrated through short-term sediment experiments
but FeS is readily oxidized, both by nitrate and
MnO 2 (Aller and Rude 1988; Schippers 2004). The
immediate product by FeS oxidation is not thiosulfate but polysulfide which subsequently degrades
into elemental sulfur:
FeS + 1.5 MnO 2 + 3 H
+
→
Fe(OH) 3 + S
0
+ 1.5 Mn
2+
(8.17)
The processes of sulfide oxidation described
here are reflected in the chemical zonations of
pore water chemistry, solid phase chemistry and
bacterial activity. As an example, Fig. 8.17
shows data from organic-rich coastal sediment
at the Baltic Sea - North Sea transition. Oxygen
penetrated less than 0.4 cm into the sediment and
showed no contact with the zone of detectable
H 2 S which started only from ca. 4 cm depth.
Nitrate penetrated only slightly deeper than
oxygen, with a peak at 0.2 cm depth due to aerobic
oxidation of ammonium (nitrification) diffusing up
from the sediment below. The upper 0.4-4 cm of
the sediment comprised the suboxic zone where
maxima in dissolved reduced manganese and iron
showed the zones where the reduction of these
metals was most intensive (peaks at 1- 2 cm depth
for manganese and 3-4 cm depth for iron). The
concentration of solid phase manganese oxide
dropped steeply with depth from the sediment
surface to 1 cm depth where its reduction was
most intensive. Oxidized iron decreased more
gradually as it was most intensively reduced in
the lower part of the suboxic zone. Measurements
of sulfate reduction showed that H 2 S was
produced throughout the sediment with maximum
rates at the top of the sulfidic zone. Sulfate
reduction also took place in the suboxic zone but
the produced H 2 S was here rapidly reoxidized and
did not reach detectable concentrations. Only by
the short-term experimental measurements using
35
SO 4
2could the SRR activity therefore be
demonstrated. Due to this sulfate reduction, a part
of the manganese and iron reduction was driven
by sulfide oxidation while another part was due to
the direct oxidation of organic matter by heterotrophic, metal-reducing bacteria (Thamdrup et al.
1994a).
The intermediate sulfur species in sulfide
oxidation, such as thiosulfate and elemental sulfur,
are not stable in the sediment but are further
transformed by microorganisms. Thiosulfate is
turned over within hours or days and generally
occurs only in sub-micromolar concentration in
the sediment pore water (Thamdrup et al. 1994b).
Elemental sulfur accumulates to much higher
concentration in the solid phase but may also turn
over on a seasonal or longer time scale in nearsurface sediments (Troelsen and Jørgensen 1982).
The preferred pathway of bacterial thiosulfate or
sulfur transformation depends strongly on the
chemical environment in the sediment. In the nearsurface sediment with suitable oxidants such as
oxygen, nitrate or metal oxides, these sulfur
species may be used as energy sources by
chemoautotrophic bacteria and be oxidized
completely to sulfate (Fig. 8.17, see also Chapter 5).
When formed below the suboxic zone, they may
be used as oxidants (electron acceptors) in bacterial respiration to oxidize organic material.
Through such a bacterial thiosulfate or sulfur
respiration these intermediates are reduced back
to sulfide. When the availability of oxidants and
organic material are both limited, the two sulfur
species may be disproportionated.
The ability of certain anaerobic bacteria to
disproportionate intermediate sulfur species such
as thiosulfate was discovered only in the late
1980’ies (Bak and Pfennig 1987; Krämer and
Cypionka 1989; Finster et al. 1998) but has since
been shown to play an important role in the sulfur
cycle of aquatic sediments (Jørgensen 1990;
Jørgensen and Bak 1991; Thamdrup et al. 1993). It
is characteristic for the disproportionation that
the sulfur species are concurrently reduced to
sulfide and oxidized to sulfate. This is an energy
yielding reaction under appropriate sediment
conditions and is independent of external
reductants or oxidants. The process can be
considered a type of inorganic fermentation and it
provides sufficient energy for bacteria to live on.
By thiosulfate disproportionation, the inner
(sulfonate) sulfur atom changes oxidation step
from +5 in S 2 O 3
2to +6 in SO 4
2, while the outer
(sulfane) atom changes from -1 in S 2 O 3
2to -2 in
H 2 S (Vairavamurthy at al. 1993; Eq. 8.18). By
elemental sulfur with an oxidation state of 0, some
of the atoms are reduced to H 2 S and some oxidized
8.5
Pathways of Sulfide Oxidation
making the complete pyrite oxidation to sulfate
dependent on microbial catalysis (Schippers and
Jørgensen 2001):
FeS 2 + 7.5 MnO 2 + 11 H
+
→
Fe(OH) 3 + 2 SO 4
2+ 7.5 Mn
2+
+ 4 H 2 O (8.16)
Pyrite oxidation by nitrate could not be demonstrated through short-term sediment experiments
but FeS is readily oxidized, both by nitrate and
MnO 2 (Aller and Rude 1988; Schippers 2004). The
immediate product by FeS oxidation is not thiosulfate but polysulfide which subsequently degrades
into elemental sulfur:
FeS + 1.5 MnO 2 + 3 H
+
→
Fe(OH) 3 + S
0
+ 1.5 Mn
2+
(8.17)
The processes of sulfide oxidation described
here are reflected in the chemical zonations of
pore water chemistry, solid phase chemistry and
bacterial activity. As an example, Fig. 8.17
shows data from organic-rich coastal sediment
at the Baltic Sea - North Sea transition. Oxygen
penetrated less than 0.4 cm into the sediment and
showed no contact with the zone of detectable
H 2 S which started only from ca. 4 cm depth.
Nitrate penetrated only slightly deeper than
oxygen, with a peak at 0.2 cm depth due to aerobic
oxidation of ammonium (nitrification) diffusing up
from the sediment below. The upper 0.4-4 cm of
the sediment comprised the suboxic zone where
maxima in dissolved reduced manganese and iron
showed the zones where the reduction of these
metals was most intensive (peaks at 1- 2 cm depth
for manganese and 3-4 cm depth for iron). The
concentration of solid phase manganese oxide
dropped steeply with depth from the sediment
surface to 1 cm depth where its reduction was
most intensive. Oxidized iron decreased more
gradually as it was most intensively reduced in
the lower part of the suboxic zone. Measurements
of sulfate reduction showed that H 2 S was
produced throughout the sediment with maximum
rates at the top of the sulfidic zone. Sulfate
reduction also took place in the suboxic zone but
the produced H 2 S was here rapidly reoxidized and
did not reach detectable concentrations. Only by
the short-term experimental measurements using
35
SO 4
2could the SRR activity therefore be
demonstrated. Due to this sulfate reduction, a part
of the manganese and iron reduction was driven
by sulfide oxidation while another part was due to
the direct oxidation of organic matter by heterotrophic, metal-reducing bacteria (Thamdrup et al.
1994a).
The intermediate sulfur species in sulfide
oxidation, such as thiosulfate and elemental sulfur,
are not stable in the sediment but are further
transformed by microorganisms. Thiosulfate is
turned over within hours or days and generally
occurs only in sub-micromolar concentration in
the sediment pore water (Thamdrup et al. 1994b).
Elemental sulfur accumulates to much higher
concentration in the solid phase but may also turn
over on a seasonal or longer time scale in nearsurface sediments (Troelsen and Jørgensen 1982).
The preferred pathway of bacterial thiosulfate or
sulfur transformation depends strongly on the
chemical environment in the sediment. In the nearsurface sediment with suitable oxidants such as
oxygen, nitrate or metal oxides, these sulfur
species may be used as energy sources by
chemoautotrophic bacteria and be oxidized
completely to sulfate (Fig. 8.17, see also Chapter 5).
When formed below the suboxic zone, they may
be used as oxidants (electron acceptors) in bacterial respiration to oxidize organic material.
Through such a bacterial thiosulfate or sulfur
respiration these intermediates are reduced back
to sulfide. When the availability of oxidants and
organic material are both limited, the two sulfur
species may be disproportionated.
The ability of certain anaerobic bacteria to
disproportionate intermediate sulfur species such
as thiosulfate was discovered only in the late
1980’ies (Bak and Pfennig 1987; Krämer and
Cypionka 1989; Finster et al. 1998) but has since
been shown to play an important role in the sulfur
cycle of aquatic sediments (Jørgensen 1990;
Jørgensen and Bak 1991; Thamdrup et al. 1993). It
is characteristic for the disproportionation that
the sulfur species are concurrently reduced to
sulfide and oxidized to sulfate. This is an energy
yielding reaction under appropriate sediment
conditions and is independent of external
reductants or oxidants. The process can be
considered a type of inorganic fermentation and it
provides sufficient energy for bacteria to live on.
By thiosulfate disproportionation, the inner
(sulfonate) sulfur atom changes oxidation step
from +5 in S 2 O 3
2to +6 in SO 4
2, while the outer
(sulfane) atom changes from -1 in S 2 O 3
2to -2 in
H 2 S (Vairavamurthy at al. 1993; Eq. 8.18). By
elemental sulfur with an oxidation state of 0, some
of the atoms are reduced to H 2 S and some oxidized
8.5
Pathways of Sulfide Oxidation