- 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
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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.
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
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.