SULPHUR CYCLE

Sulphur Cycle

The sulphur cycle is the collection of processes by which sulphur moves to and from minerals (including the waterways) and living systems. Such biogeochemical cycles are important in geology because they affect many minerals. Biogeochemical cycles are also important for life because sulphur is an essential element, being a constituent of many proteins and cofactors.

Importance of Sulphur:

• Plants utilize sulphur in the dissolved form as sulphate.
• Sulphur is important constitute of certain amino acids like cystine, methionine, thiamine and biotin.
• Indirectly influence nodulation in legumes, chlorophyll and other pigment formation.
• Animal life also requires sulphur.

Steps of Sulphur Cycle: Various transformations of the sulphur in soil results mainly due to microbial activity, although some chemical transformations are also possible (eg. oxidation of iron sulphide) the major types of transformations involved in the cycling of sulphur are:

1. Mineralization
2. Oxidation
3. Reduction
4. Assimilation

Figure: Sulphur cycle

1. Mineralization: Organic sulphur changes into inorganic forms, such as hydrogen sulphide (H₂S), elemental sulphur, as well as sulphide minerals.
2. Oxidation:

• Oxidation of elemental sulphur and inorganic sulphur compounds (such as H₂S, sulphite and thio-sulphate) to sulphate (SO4) is brought about by chemoautotrophic and photosynthetic bacteria (Oxidation of hydrogen sulphide produces elemental sulphur and further oxidation of elemental sulphur produces sulphate).
• Sulphides may be oxidized to elemental sulphur aerobically by species like Thiothrix and Beggiatoa and anaerobically by the Purple Sulfur Bacteria. Both of these groups are primarily aquatic microbes.
• In soil, the predominant microbes involved in the oxidation of sulphide to elemental sulfur belong to the genus Thiobacillus.
• For the following two "generic" reactions, the first is typical of oxidation of sulphide to sulphur, and the second of oxidation of sulphide to sulphate. As shown in the diagram to the left, the rate-limiting step is mediated by the enzyme sulfite oxidase (cofactors for the enzyme are Mo and Fe).

2S + 3O₂ +2H₂O =>         2H₂SO₄ =>     2H⁺ + SO₄^—           (Aerobic)

CO₂ + 2H₂S        =>         (CH₂O) + H₂O + 2S

Or, H₂ + S + 2CO₂ + H₂O =>              H₂SO₄ +2 (CH₂O)  (Anaerobic)

• The major Sulphur Oxidiser microorganisms are: Thiobacillus, Beggiatoa, Thiothrix, Thioploca, Aspergillus, Penicillium, Microsporum etc.

3. Reduction:

• Sulphate can be reduced to hydrogen sulphide (H₂S) by sulphate reducing bacteria (e.g. Desulfovibrio and Desulfatomaculum) and may diminish the availability of sulphur for plant nutrition.
• The reduction process is particularly favoured by the alkaline and anaerobic conditions of the soil, as the organisms use sulphate as a source of oxygen supply.
• Besides the strict autotrophs of the genus Thiobacillus, there are several other members which are facultative autotrophs capable of reducing H₂S and drive energy from it. For example, calcium sulphate is attacked under anaerobic condition by the members of the genus Desulfovibrio and Desulfatomaculum to release H₂S.

CaSO₄ + 4H₂       =>                  Ca(OH)₂ + H₂S + H₂O

• Hydrogen sulphide produced by the reduction of sulphate and sulphur containing amino acids decomposition is further oxidized by some species of green and purple phototrophic bacteria (e.g. Chlorobium, Chromatium) to release elemental sulphur.

CO₂ + 2H₂ + H₂S        =>        (CH₂O) + H₂O + 2S

4. Assimilation:

• Sulphates and sulphuric acid, when dissolved in water, are made available for plant growth.
• Plants utilize the sulphates to form various amino acids, hormones, growth factors, etc.
• They are either eaten away by the animals are also in some form or other returned to the soil.

When the various complex organic sulphur compounds reach the soil, they are attacked by the soil organisms and the cycle of events continues.