In this article we will discuss about the sulphur cycle with the help of diagram.
Sulphur, like nitrogen and carbon, is an essential part of all living matter because sulphur containing amino acids are always present in almost all kinds of proteins. Plants can absorb directly the sulphur containing amino acids, e.g., cystine, cysteine, and methionine but these amino acids fulfill only a small proportion or requirements for sulphur.
To fulfill rest of the requirements of plants, sulphur passes through a cycle of transformation mediated by microorganisms. It accumulates in the soil mainly as a constituent of organic compounds and has to be converted to sulphates to become readily available to the plants.
The complete cycling of sulphur is schematically represented in Fig. 30.4 and some important steps are discussed as under:
(a) Degradation of Organic Compounds to Release H2S:
(i) Degradation of proteins (proteolysis) liberates amino acids which generally contain sulphur.
(ii) Enzymatic activity of many heterotrophic bacteria result in the release of H2S from further degradation of sulphur containing amino acids.
(iii) Sulphates may also be reduced to H2S by the action of Desulfotomaculum bacteria.
(b) Oxidation of Hydrogen Sulphide (H2S) to Elemental Sulphur:
Hydrogen sulphide undergoes decomposition to produce elemental sulphur by the action of certain photosynthetic sulphur bacteria, e.g., members belonging to the families Chlorobiaceae (Chlorobium) and Chromatiaceae (Chromatium).
Some non-sulphur purple bacteria, e.g., Rhodospirillum, Rhodopseudomonas, and Rhodomicrobium, which are facultative phototrophs and grow aerobically in the dark and anaerobically in the light, can also degrade H2S to elemental sulphur.
(c) Oxidation of Elemental Sulphur to Sulphates:
Elemental form of sulphur accumulated in soil by earlier described processes cannot be utilized as such by the plants. It is oxidized to sulphates by the action of chemolithotrophic bacteria of the family Thiobacteriaceae (Thiobcicillus thiooxidans).
Sulphates are the compounds that can readily be taken by the plants and are beneficial to agriculture in the following three ways:
(i) It is the most suitable source of sulphur and is readily available to plants.
(ii) Accumulation of sulphate solubilizes organic salts that contain plant nutrients such as phosphates and metals.
(iii) Sulphate is the anion of a strong mineral acid (H2SO4) and prevents excessive alkalinity due to ammonia formation by soil microorganisms.
Sulphate is assimilated by plants and is incorporated into sulphur containing amino acids and then into proteins. Animals fulfill their demand of sulphur by feeding on plants and plant products.
(d) Reduction of Sulphates:
Sulphate is first reduced to H2S by sulphate reducing microorganisms under anaerobic conditions. Many bacteria including species of Bacillus, Pseudomonas, Desulfovibrio do this work. The mechanism of sulphate reduction to hydrogen sulphide involves, firstly, the reduction of sulphate to sulphite utilizing ATP and, secondly, reduction of sulphite to hydrogen sulphide.
The whole mechanism of the reduction of sulphate to hydrogen sulphide by Desulfovibrio desulfuricans, the most important bacterium of this reduction, can be represented as follows: