In this essay we will discuss about the sulphur cycle with the help of a diagram.
Sulphur is a component of three amino acids (Cysteine, cystine and methionine), so is a component of most proteins, some vitamins and enzymes.
Sulphur cycle is a sedimentary cycle as it is found in nature as element and also as sulphates in soil, water and rocks. So the reservoir pool of the elements is in the soil.
Producers (green plants) need sulphur in the form of sulphates (SO42-) from soil or from water (aquatic plants). Some plants get their sulphur in such organic forms e.g. amino acids. The animals get sulphur through food chain. Some animals get sulphur from water also. Sulphur is component of some proteins.
(a) After the death of plants and animals, they are decomposed by microbes like Aspergillus, Neurospora and Escherichia releasing hydrogen sulphide (H2S).
Aspergillus and Neurospora are aerobes while Escherichia operates in anaerobic conditions. Sulphate is also reduced under anaerobic conditions to elemental sulphur or to sulphides (including H2S) by certain heterotrophic bacteria i.e. Desulfavibrio, Escherichia and Acetobactor.
SO42- + 2H+ → H2S +2O2
The high concentration of H2S in deeper parts of aquatic ecosystems e.g. below 200 metres in the Black sea, does not allow the survival of higher animals. Similarly, fish is unable to survive.
(b) A part of H2S is oxidised to soluble sulphates by sulphur bacteria like Thiobacillus while Beggiatoa (colourless sulphur bacteria) oxidise a part of H2S to elemental sulphur.
The oxidation of H2S releases energy which is used in their chemosynthetic metabolism involving reduction of carbon dioxide:
6CO2 + 12H2S energy → C6H12O6 + 6H2O + 12 S
The remaining passes into reservoir pool in deep sediments. From the sea, sulphur goes back to land in three ways i.e. food chains, sea sprays and geological upheavals.
(c) Many industries release SO2 into atmosphere. As the lichens are very sensitive to SO2, they disappear in polluted air containing SO2.
(d) Fossil fuels on burning release SO2 into the atmosphere (Fig. 14.30).
(e) Volcanic emissions also add sulphates to soil and air.
Sulphur cycle is an imperfect cycle as sulphur has the potential for being bound under anaerobic conditions to cations like iron and calcium to form highly insoluble ferrous sulphide (FeS), ferric sulphide (Fe2, S3, pyrite) or calcium sulphate (CaSO4).
SO2 is a major source of air pollution. Atmospheric sulphur in the form of elemental sulphur or H2S or SO2 is oxidised to SO3 which combines with water to form sulphuric acid which comes on land as acid rain.