Sulphur cycle is defined as the biogeochemical cycle by which sulphur is exchanged between the biosphere, lithosphere, atmosphere and hydrosphere components of the earth. Sulphur is vital for the functioning of proteins and enzymes in plants, and in animals that depend upon plants for sulphur. Plants absorb sulphur when it is dissolved in water, and animals consume these plants, so that they take up enough sulphur from them to maintain their own health. Sulphur occurs in all living matter as a component of certain amino acids such as cysteine and methionine; and this element is abundant in the soil in protein. Sulphur is also a constituent of proteins, vitamins, hormones and cofactors that aid in enzyme reactions; and sulphur has the ability to act as an oxidizing agent and also as a reducing agent. Sulfur and its compounds including sulphuric acid and sulphur dioxide (sulphur (IV) oxide) are important elements of industrial processes. Sulfur dioxide (SO2) is a bleaching agent and is used to bleach wood pulp for paper and fiber for various textiles such as wool, silk and linen.
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SO2 is a colorless gas that creates a choking sensation when breathed; and it kills moulds and bacteria in the environment. It is also used to preserve dry fruits like apples, and SO2 can also be used to clean out vats used for preparing fermented foods such as cheese and wine. Sulfuric acid (H2SO4) is a very widely used chemical that absorbs water and is used in various industrial processes as a dehydrating agent. The major steps involved in sulphur cycle are mineralization of organic sulphur to inorganic sulphur (H2S); oxidation of sulphide and elemental sulphur and other related compounds to sulphate (SO42–); reduction of sulphate to sulphide; and microbial immobilization of the sulphur compounds and subsequent incorporation of sulphur compounds into the organic form of sulphur. These steps of sulphur cycle are often termed as (1) assimilative sulphate reduction (in which SO42- is reduced to organic sulphydryl groups (R-SH) by prokaryotes, plants and fungi; (2) desulphuration (in which organic molecules containing sulfur can be desulphurated H2S gas); (3) oxidation of H2S (in which elemental sulphur (S) is produced); (4) dissimilative sulphur reduction (in which elemental sulphur can be reduced to H2S); and (5) dissimilative sulphate reduction (in which sulphate reducers generate H2S from sulphate) (Figure 1).
Sulphur can be sourced naturally from sea water, rocks, mineral deposits and from ore deposits that are deep-seated in the earth and in the ocean. Several deposits of minerals in the earth crust contain sulphur in substantial amounts. Most of the earth’s sulphur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments. Through a series of microbial transformations occurring in the soil, sulphur ends up as sulphates that are usable by plants. Sulfur-containing proteins are degraded into their constituent amino acids by the action of a variety of soil organisms. During this reaction, the sulfur of the amino acids is converted to hydrogen sulphide (H2S) by another series of soil microorganisms such as sulphur reducing bacteria (SRB). And in the presence of oxygen, the H2S is converted to sulfur and then to sulphate again by SRB. The sulfate eventually becomes H2S, and the cycle continues (Figure 1).
Human activities such as burning of fossil fuels, natural gas and coal are some of the activities that have major impact on sulphur deposits and thus increase the amounts of sulphur in the atmosphere. Sulphur accumulates in the atmosphere through natural causes (such as volcanic eruptions and through the decay of dead organic matter) and human activities especially in industrial activities that emits large amounts of sulphur dioxide (SO2) and H2S into the atmosphere. These sulphur compounds or gases react with oxygen or other chemicals in the atmosphere to form sulphur trioxide (SO3) or salts of sulphur respectively; and in some extreme cases, the SO2 will react with water to form sulphuric acid (H2SO4) which fall back to the earth as acid rain. Acid rain affects soil biology by slowing the growth of plants; and it also affects soil microbes and some aquatic and terrestrial or arboreal life on earth.
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