Sulfur Cycle

Importance of Sulfur

• Sulfur is an essential element in macromolecules for living organisms.
• It is a part of amino acids such as cysteine and methionine, which are involved in protein formation.

Sources of Sulfur

• Natural Sources: Include volcanic eruptions, mineral pyrite, and weathering of the Earth's crust.
• Biological Sources: Sulfur is released during the decomposition of dead organisms.

Steps in the Sulfur Cycle

1. Decomposition of Organic Compounds

• Process: Bacteria and fungi decompose dead organisms.
• Outcome: Sulfur groups are removed from amino acids, producing hydrogen sulfide (H₂S).

2. Oxidation of Hydrogen Sulfide (H₂S)

• Bacteria Involved: Photolithic riffs, aerobic purple sulfur bacteria, chromatium.
• Green Sulfur Bacteria: Chlorobium uses enzyme membrane-bound quinone oxido-reductase.
• Outcome: H₂S is oxidized to elemental sulfur.

3. Oxidation of Elemental Sulfur to Sulfates

• Process: Elemental sulfur cannot be directly used by plants and is converted to sulfates.
• Organisms Involved: Chemolithoautotrophs, aerobic Thiobacillus.
• Enzyme Involved: Membrane-bound sulfide oxido-reductase.

4. Reduction of Sulfates

• Process: Sulfates are reduced to hydrogen sulfide.
• Bacteria Involved:
  • Bisulfate-reducing bacteria
  • Chemolithoheterotrophs
  • Desulfovibrio, Desulfuromonas, Desulfotomaculum, Desulfobacter
• Enzyme Involved: Membrane-bound sulfide reductase.

Summary

• The sulfur cycle involves multiple biological and chemical steps where sulfur moves between organic and inorganic forms.
• The cycle is vital for the availability of sulfur to plants and maintaining ecological balance.