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:

A)Mineralization

• 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.

References: 

i) https://www.britannica.com/science/sulfur-cycle

ii) https://www.lenntech.com/sulphur-cycle.htm

iii) https://www.frontiersin.org/articles/10.3389/fmicb.2019.00849/full

Sulfur Cycle