Sulphur is an essential part of some of the amino acids in its organic form. It exists in various states like elemental sulphur, Sulphides, sulphites, sulphates, sulphur monooxide, Sulphur dioxide etc. Of these, elemental sulphur, Sulphides and Sulphates are very important in nature. The sulphur cycle exists in both sedimentary and gaseous phases.

The sedi­mentary phase is long termed and in this the sulphur is tied up in organic and inorganic deposits. When these deposits undergo weathering and decompo­sition sulphur is released and is carried into terrestrial and aquatic ecosys­tems in the form of salt solution. The gaseous phase of sulphur cycle is less pronounced and it allows global circulation.

Sulphur enters the atmosphere from various sources such as combustion of fossil fuels, volcanic eruption, surface of the oceans and gases released by decomposition. Initially sulphur enters the atmosphere in the form of Hy­drogen sulphide which gets quickly oxidised into sulphur dioxide. This sulphur dioxide is carried back to earth along with rain water as weak sulphuric acid (H2SO4).

In any case sulphur is absorbed by plant roots mostly in the form of sulphates. Within the plant sulphur gets incorporated into amino acids and then to proteins. From the plants sulphur gets into the con­sumers the excess being excreted.

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Death of the Organisms releases the sulphur in the living material back to the soil and also to aquatic ecosystems like ponds lakes etc., Here the organic material is decomposed by bacteria wherein the sulphydryl group is broken and is released from the amino acids. Aerobic bacteria oxidize Hydrogen sulphide is to sulphate.

H2S + 2O2 – SO4 +2H

The Sulphate produced by the above reaction can be re-absorbed by the autotrophic plants.

In anaerobic environments such as the bottom of lakes, ponds, etc., sulphide cannot be oxidized by the above method as the process requires oxygen. But if infra red radiation is present, photosynthetic bacteria can oxidize sulphide either to elemental sulphur or sulphate.

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The series of reactions which take place within the organic phase provide a ready mechanism for regulating the availability of sulphur to autotrophic plants.

From the organic phase sulphur is removed in the form of elemental sulphur which is insoluble and accumulates sediments. In these sediments if iron is also present it can combine with sulphide to form iron sulphides.