Sulfur Cycle

  • Sulfur is an essential nutrient for plant as well as animal.
  • It is one of the constituent of many proteins and co-factors.
  • The major reserve of the element is the soil.
  • Sulfur compounds can be used in various ways as oxidants or reductants in microbial respiration.
  • The dominance of one or another transformation is governed to a large extend by the environmental circumstances.
  • This affects the composition and activity of the micro-flora.
  • Sulfur cycle consists of 4 distinct processes. They are as follows:


  • Sulfur in its elemental form cannot be used by plants and animals.
  • These should be converted into assimilable form which is known by the process mineralization.
  • Certain bacteria such as Thiobacillus thiooxidans, an autotroph are able to oxidize sulfur to sulfates.
  • The reaction involved is :

2S + H2O + 3O2  →   2H2SO4

sulfur cycle | Definition, Steps, Diagram, Importance, & Facts | Britannica

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B)Assimilation of sulfate by plant and animals

  • Sulfate is assimilated by plants and is incorporated into sulfur containing amino acids and then into proteins.
  • These are also incorporated into several co-factors such as thiamine, biotin, and lipoic acid and several types of carbohydrates.
  • Microbial assimilation is the immobilization of simple compounds of sulfur and their incorporation into bacterial, fungal or actinomycetes cells.
  • Different types of sulfur compounds can serve as a source of sulfur for the growth of micro-organisms like inorganic compounds– sulfate, sulfite, hyposulfite, thiosulfate, sulfide, tetrathionate, thiocyanate, organic compounds like amino acids, undecomposed proteins.


C)Enzymatic degradation of organic sulfates

  • Due to the death or excretion by organisms, degradation of proteins occurs which liberate amino acids some of which contain sulfur.
  • This sulfur is released from the amino acid by enzymatic activity of many heterotrophic bacteria.

reaction showing oxidation of inorganic sulfur to elemntal formको लागि तस्बिर परिणाम

D)Reduction of inorganic sulfate

  • Mineralized sulfate may also be reduced to hydrogen sulfide by soil microorganisms.
  • An example of bacteria involved in this process is the genus of Desulfotomaculum and the reaction suggested is:

4H2 + CaSO4  →   H2S + Ca(OH)2 +2H2O

sulfur cycleको लागि तस्बिर परिणाम


E)Oxidation of inorganic sulfur to elemental sulfur

  • Hydrogen sulfide resulting from sulfate reduction and amino acid decomposition is oxidized to elemental sulfur.
  • This reaction is characteristic of certain pigmented photosynthetic sulfur bacteria such as Chromatium and Chlorobium.





Sulfur Cycle