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An important distinction between cycling of sulfur and cycling of
nitrogen and carbon is that sulfur is "already fixed". That is, plenty of
sulfate anions (SO42-) are available for living organisms to utilize. By
contrast, the major biological reservoirs of nitrogen atoms (N2) and
carbon atoms (CO2) are gases that must be pulled out of the
atmosphere.
Characteristics
 sulfur, S, 16Chemical seriesnonmetals
 Group 16,Period 3,Block p
 Appearance lemon yellow
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Atomic mass 32.065 g/mol
Electron configuration [Ne] 3s2 3p4
It has two isotopes 32S and 34S, the light isotope constitutes 96%
It is believed that oxygen accumulation in the atmosphere is the result
of the reduction of sulfate ions in seawater by anaerobic bacteria
This results in the formation of biogenic sulfur deposits characterized
by elevated 32S ratio and lower 34S.
Oxidation states of sulfur
 sulfur has nine oxidation states
compound
formula
Oxidation state
Sulfate
SO42-
+VI
Sulfite
SO32-
+ IV
Dithionite
S2O42-
+III
Thiosulfate
S2O32-
+II
Polythionate
SnO62-
=10/n (n=3-6)
S0 (=S8 )
0
Polysulfide
H2Sn
-2/n (n=3-6)
Disulfide
S22-
-I
Pyrite
FeS2
-I
Iron momosulfide
FeS
-II
Hydrogen sulfide
H 2S
-II
Elemental sulfur
Importance of Sulfur
1- As component of the living cells
 S is one of the constituents of many proteins, vitamins and hormones.
It recycles as in other biogeochemical cycles.
 Despite its importance it does represent between 1-2% of the living
organisms
 It is concentrated in the amino acids such as Cysteine and Methionine
 Organisms reduces sulfate ions to sulfhydril group (-SH)
 O2 is liberated and used in oxidative metabolism
2- Modification of environmental characteristics
 Sulfate average concentration in the world ocean is ~2700 mg/l (third
of the major constituents and second of the anions)
 S is the 4th most important element (900 mg/l, 28mM)
 Sulfur is not a limiting elements, it is widely found in the marine
environment
 Bacterial mineralization of organic sulfur and anaerobic reduction of
sulfate ions in water and sediments modifies environmental
characteristic and influences the cycles of nutrient elements and
consequently influences the biological productivity
Example: oxidation of pyrite
 FeS2 (s) + 3.5 O2 + H2O
 Fe2+ +0.5 O2 + 2H+
 Fe3+ +3 H2O
Fe2+ + 2SO42- + 2H+
Fe3+ + 0.5 H2O
Fe(OH)3 + 3H+
 FeS2 (s) + 14 Fe3+ + 8 H2O
15 Fe2+ + 2SO42- + 16H+
The result of these interactions is the dramatic decrease of pH(~ 3)
Elemental sulfur if present will also be oxidized to sulfuric acid according to the
equation:
2So + 3O2 + 2H2O
2H2SO4
Sources of sulfur
 Weathering of sediments and sedimentary rocks
 Rain water
 Atmospheric particles
 Agricultural fertilizers
 Volcanic emanations (H2S- SO2)
 Burning of fossil fuel (95% as SO2, which when dissolves in rain
water gives H2SO4, essential constituent of the acidity of acid
rains)
Overview: Important reactions of the sulfur cycle
 Sulfate Reduction
 Assimilative Reduction: sulfate ions are reduced during building
of living cells where sulfate is reduced to sulfhydril group (-SH)
in some amino acids such as Cysteine and Methionine
• Sulfate is firstly activated by a double phosphorylation in the
presence of ATP before it is reduced to SO32• Sulfide ion is the product of this process
• Formed sulfide ions are then introduced in the living cells
 Dissimilative sulfate reduction
from sulfate
sulfate reducers (SRB) generate hydrogen sulfide
 Bacterial anaerobic respiration process
 Sulfate plays the role of electron acceptor
H2SO4 + 4H2 → 4H2O + H2S
SO42- + H2+ + 2CH2O → 2CO2 + 2H2O + H2S
 The first step in the reduction resembles that of the assimilative
reduction and results in the formation of sulfite
 Energy is liberated at the final step of H2S production
 This step is achieved through two possible mechanisms
• One-step reduction to in the presence of bisulfite reductase
• Three-step reduction through the formation of thiosulfate and
trithionate
 Thermochemical reduction
o Abiotic process assisted by thermal energy
o Sulfate is used for the oxidation of organic matter as a source of
oxygen
o Sulfate serves as electron acceptor in the oxidation of ferrous
iron when seawater circulates in the subsurface crustal rocks at
more than 300 oC
o Thermal sulfate reduction by the organic matter needs high
temperature ranging between 80 and 200 oC
o Increase of temperature accelerates the reduction process and
reduces the half life of sulfate
o Biological reduction is more efficient than the thermochemical
reduction
Chemistry of Hydrogen sulfide
 H2S is a weak acid
 It dissociates in two steps according to the following reactions:
H2S
H+ + HS-
HS-
H+ + S2-
pk = 7
pk = 12.41-17.1
∑ H2S = H2S + HS- + S2 In seawater at 25 oC and pH= 8.1, HS- constitutes 96.9 of ∑ H2S and
the rest is H2S. Sulfide ions does not constitute any appreciable
quantities.
 Hydrogen Sulfide Oxidation
H2s is unstable and is subjected to different kinds of oxidation
I.
Biological oxidation

Chemosynthetic bacteria (chromatiaceae): BeggiatoaThiobacillus
Non-pigmented aerobic bacteria that uses oxygen in the
oxidation of H2S that firstly produces elemental So which
accumulates either inside the cells (Beggiatoa) according to
the reaction:
H2S + 1/2 O2 → H2O + So
when all H2S has been used, S is oxidized to sulfate
So + 1.5 O2 + H2O → H2SO4
H2S + 2 O2 → SO42- + 2H+
∆Go = -797 kJ mol-1
Produced energy is stored as ATP
 S may also accumulate outside the cells (Thiobacillus)
 May also oxidize other sulfur reduced compounds in the presence of
oxygen
2Na2S2O3 + O2 → 2S + 2Na2S2O4
These bacteria lives at the interface between the oxygenated and non
oxygenated layers and secretes a separating film to avoid competition
with auto-oxidation
Oxidation may also take place in anaerobic environment by nitrate
reducing bacteria (Thiobacillus denitrificans) that may oxidize thiosulfate
and S as follows:
5S2O32- + 8NO3- + 2HCO3- → 10SO42- + 2CO2 + H2O + 4N2
5So + 8NO3- + 2CO32- → 5SO42- + 2CO2 + 4N2
 Photosynthetic bacteria (autotrophes)
 Chlorobacteriaceae (green bacteria)
 Thiorhodaceae (purple bacteria)
Green bacteria needs light as energy source and uses H2S as
electron source in the anaerobic photosynthetic process in the
reduction of CO2
CO2 + 2H2S → CH2O + H2O + 2S
CO2 + 2H2S + H2O → CH2O + H2SO4
Green bacteria stokes S outside the cells and supports high
concentration of H2S. Purple bacteria stokes S inside the cells, it
supports pH higher than 9.5
Since light and H2S rarely exist in the same time in natural system,
photosynthetic autotrophic bacteria is of little importance in the
Sulfur cycle.
 Anaerobic bacteria
 Thiobacillus denitrificans
 Thiomicrospira denitrificans
 These bacteria isolated from sediments are able to use the NO3- as
electron
acceptor in the oxidation of H2S and other reduced S forms
 Oxides of iron and Mn can also serve as electron acceptor in this
oxidation Process
Anaerobic bacterial oxidation of H2S is particularly important in
the S cycle since more than 90% of the H2S produced by
dissimilative reduction is returned to the environment as SO42- by
this process.
II. Chemical Oxidation
 Auto-oxidation
o At normal temperature of seawater, H2S reacts with O2 to form
SO42- ions and other sulfur compounds at different levels of
oxidation such as elemental S, sulfite and thiosulfate
o Half life of H2S in oxygenated seawater at 25 oC and pH of 8 is 26±9
h
o Oxidation rate increases with temperature, salinity, pH and in
the presence of trace elements such as Mn and Fe
o In seawater containing these elements H2S half life ranges
between 8 and 20 min.
 Anaerobic oxidation
 Interaction between H2S with iron and manganese oxides results
in the formation of elemental So.
 Accumulation of elemental So below the oxic layer in the
sediments was reported in marine and estuarine sediments
 SO42- may result from the oxidation of H2S in the presence of
hematite α-Fe2O3
 Generally, chemical anaerobic oxidation of hydrogen sulfide
produces mainly elemental So
III- Disproportionation
 Bacterially (SRB) mediated reactions resulting in the
transformation of elemental S and other intermediate S
compounds such as sulfite and thiosulfate
4SO32- + H+ → 3SO42- + HS-
S2O32- + H2O → SO42- + HS- + H+
4So + 4H2O → SO42- + 3H2S + 2H+
∆ Go = -58.9 kJ mol-1
∆ Go = -219 kJ mol-1
∆ Go = 41 kJ mol-1
The last reaction is exothermic, however, it can take place in the
presence of Fe II and III and Mn IV because these elements
interact with the resulting H2S and maintains its concentration in
the medium very low rendering the reaction exothermic