Essay on the Sulphur Cycle

After reading this essay you will learn about the sulphur cycle.

Sulphur is widely distributed in nature and is one of most abundant elements. It is essential to all living organisms because sulphur containing amino acids are always present in almost all types of proteins. It is found both in free as well as in combined states.

In the combined form it occurs in both inorganic and organic combinations. Cycling of sulphur is similar to that of nitrogen. Transformations between organic and elemental states and between oxidized and reduced states of sulphur are accomplished by various types of micro-organisms, especially bacteria.

Micro-organisms in the soil degrade proteins and liberate amino acids, some of which contain sulphur. Sulphur-containing amino acids are further metabolized with the release of hydrogen sulphide and amonia.

Bacteria which oxidize various forms of sulphur, especially hydrogen sulphide, belong to two different nutritional types— photosynthetic and chemolithotrophic. Photosynthetic bacteria which oxidize hydrogen sulphide to elemental sulphur belong to the families Chlorobiaceae and Chromatiaceae.

Hydrogen sulphide acts as an electron donor for CO₂ reduction in photosynthesis:

Sulphur is oxidized to sulphate by chemolithotrophic bacteria of the family Thiobacteriaceae. The most important organism is Thiobacillus thioxidans which oxidizes sulphur and thiosulphates to sulphuric acid under aerobic conditions.

2S + 3O₂ + 2H₂O → 2H₂SO₄

5Na₂S₂O₃ + H₂O + 4O₂→ 5Na₂SO₄ + H₂SO₄ + S

The formation of sulphate is beneficial to agriculture in three ways.

1. Sulphate is the most suitable source of sulphur for the plants.

2. Sulphate is the anion of a strong mineral acid (H₂SO₄) and prevents excessive alkalinity due to ammonia-formation by micro-organisms.

3. Accumulation of H₂SO₄ solubilizes inorganic salts that contain plant nutrients such as phosphates and metals.

On the other hand Thiobacillus denitrificans oxidizes sulphur with the reduction of nitrate. This is an important factor in the loss of fertility in certain anaerobic soils.

5S + 6HNO₃ + 2H₂O → 5H₂SO₄ + 3N₂

Sulphate in the soil is assimilated by plants and microorganisms and is incorporated into proteins. This is known as assimilatory sulphate reduction. There is also a dissimilatory sulphate reduction similar to dissimilatory nitrate reduction.

Organisms utilize sulphate as a terminal electron acceptor in anaerobic respiration with the production of hydrogen sulphide. Organic compounds are dehydrogenated and, hydrogen is used in the reduction of sulphate to hydrogen sulphide through sulphite.

1.

2. 4 H₂ + H₂SO₄→ H₂S + 4H₂O

The most important sulphate reducing bacteria are those belonging to the genus Desulfovibrio, and the best known species is Desulfovibrio desulfuricans. Certain obligate anaerobic organisms belonging to the genus Desulfotomaculum are also sulphate reducers.