SULFUR CYCLE

SULFUR CYCLE

• Sulfur is one of the most abundant elements on the earth.
• Sulfur is a yellow, brittle, tasteless, odourless non-metal.
• Sulfur is present in all kinds of Proteins.
• Plants directly absorb Sulfur-containing Amino acids such as Methionine, Cystine and Cysteine.
• Sulfur is released into the atmosphere by the (i) Burning of Fossil fuels; (ii) Volcanic activities and (iii) Decomposition of Organic molecules.
• On land, Sulphur is stored in underground rocks and minerals. It is released by Precipitation, Weathering of rocks and Geothermal vents.
• Photosynthetic and Chemosynthetic microorganisms contribute the environmental Sulfur cycle.

STEPS INVOLVED IN SULFUR CYCLE

• The sulfur cycle describes the movement of Sulfur through the atmosphere, mineral forms, and through living things.
• Microbial transformations of Sulfur are even more complex than those of Nitrogen because (i) Large number of Oxidation states of Sulfur and (ii) Several transformations of Sulfur also occur spontaneously (abiotically).
• Sulfur cycle has four steps. They are

1. Sulfur Dissimilation (Sulfates [SO₄^2–] to Hydrogen Sulphide [H₂S])
2. Oxidation of Hydrogen Sulphide (H₂S) to Elemental Sulfur
3. Oxidation of Elemental Sulfur to Sulfate (SO₄^2–)
4. Reduction of Sulfate (Sulfates [SO₄^2–] to Hydrogen Sulphide [H₂S])

1) Sulfur Dissimilation

• Protein degradation in Dead materials releases Amino acids that contain Sulfur (Sulfur containing Amino acids).
• Sulfur released from Amino acids forms Sulfate (SO₄^2–) which is converted to its most reduced form, Hydrogen sulfide (H₂S) by a process called Dissimilation (refers to degradation of Inorganic substance).
• Microorganisms involved in Sulfur Dissimilation process are Desulfovibrio sp., Desulfobacter sp. and Archaeoglobus sp. (Anaerobic condition)

2) Oxidation of Hydrogen Sulphide (H₂S) to Elemental Sulfur (S⁰)

• Hydrogen Sulphide (H₂S) is oxidized to Elemental sulfur (S⁰) under various conditions.
• Oxidation of Hydrogen Sulphide (H₂S) to Elemental sulfur (S⁰) was carried out under both Aerobic condition (Thiobacillus sp., Beggiatoa sp., Thiothrix sp. and Aerobic anoxygenic phototrophs) and Anaerobic condition (Photolithotrophic sulfur oxidizers such as Chromatium sp. and Chlorobium sp.).

3)  Oxidation of Elemental Sulfur (S⁰) to Sulfate (SO₄^2–)

• Oxidation of Elemental sulfur (S⁰) to Sulfate (SO₄^2–) was carried out under Anaerobic condition by Alteromonas sp., Clostridium sp., Desulfovibrio sp. and Desulfotomaculum sp.
• This process is a Two steps Anaerobic process

ü  Step - I: Elemental sulfur (S⁰) is oxidized to Sulfite (SO₃^2–).

ü  Step - II: Sulfite (SO₃^2–) is oxidized to Sulfate (SO₄^2–).

4) Reduction of Sulfate

• Sulfates are reduced to Hydrogen Sulphide (H₂S) anaerobically by Desulfovibrio desulfuricans, Desulfomonas sp. and Desulfotomaculum sp. 
• This reduction process occurs in two steps:

ü  Step - I: Sulfates (SO₄^2–) are reduced to Organic Sulfur (Sulphites) utilizing ATP by a process called Assimilation (refers to formation of Inorganic substance).

ü  Step - II: Reduction of Organic Sulfur (Sulphites) to Hydrogen Sulphide (H₂S) by a process called Mineralization (decomposition of Organic substances).