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Sulfur Cycle and its Environmental Impacts

Sulfur is one of primary nutrients for both animal and plant life. It is component element that makes up some proteins, enzymes and vitamins. Sulfur accounts for 5 of dry weight of organisms. Plants take sulfur in the soluble form while animals depend on the sulfur containing plant and other animal materials.

After uptake of inorganic sulfur, it becomes organic sulfur in the plants and microorganisms. After decomposition of organic matter, sulfur compounds are converted again into inorganic sulfur or sulfur in cells of microorganisms.

Sulfur is entered into the atmosphere as a result of both natural and anthropogenic process.  Volcanic eruptions, organic matter decomposition, forest fire and microbial activities are the natural process that helps to cycle sulfur. Fossil fuel burning and industrial activities release sulfur dioxide and hydrogen sulfide in large scale into the environment.

The major reservoir pool of sulfur is ocean. Sulfur can be found in large concentration in as dissolved from or sediments forms. Gypsum and pyrite are the examples for sulfur bearing minerals. Leaching and rock sediments are the major sources that release sulfur into ocean. Oxidation number of sulfur varies from -2 to +6. But sulfate with oxidation number of +2 is most stable form of sulfur.

Main steps of sulfur cycle:

1)    Conversion of organic sulfur into inorganic sulfur (H2S)

2)    Oxidation of element sulfur, thiosulfate and hydrogen sulfide into sulfate

3)    Reduction of sulfate into hydrogen sulfide: Chemoorganotrophic anaerobic respiratory bacteria use non oxygen terminal electron acceptor at the end of their electron transport chain. They use sulfate as their terminal electron acceptor and reduce it into hydrogen sulfide.

4)    Sulfur assimilation: It is the reduction of sulfate into organic sulfhydryl groups. The oxidation number of sulfur changes from +6 to -2.

5)    Sulfur dissimilation: it is the reduction of organic sulfur containing substance into inorganic hydrogen sulfide.

6)    Oxidation of hydrogen sulfide into element sulfur: Green sulfur bacteria and purple sulfur bacteria perform photosynthesis to ATP synthesis. They use hydrogen sulfide as their electron donor. They reduce it into element sulfur. 

Sulfur compounds acts as the electron donors and electron acceptors for microorganisms. Anaerobic respiratory bacteria including Thiobacuillus thioxidans use element sulfur, thiosulfate or hydrogen sulfide as their substrate. They oxidize these sulfur compounds to run electron transport chains. They oxidize them into sulfate.  Theses sulfate dissolve and spread though the aquatic body. When sulfate reach into bottom where the anaerobic environmental condition exists anaerobic respiratory microorganisms use it as their terminal electron acceptor at their electron transport chain. They reduce sulfate into hydrogen sulfide. These hydrogen sulfide moves upward. Both green sulfur and purple sulfur bacteria which inhabit in the above layers of aquatic water body us use hydrogen sulfide as their electron donor in their photosynthesis. They oxidize hydrogen sulfide into element sulfur.

Effects of acidification of aquatic bodies: These sulfates make the environments extremely acidic. They dissolve minerals in the bottom of the aquatic bodies. Toxic metal ions including Al3+ release into the water. These toxic metal ions cause detrimental effects to some aquatic plants and animals. Acid dissolves the shells of aquatic animals including coelenterates, gastropods. Shell of eggs including fishes is dissolved by acids. pH below 5 does not allow fish eggs to hatch.

Oxidation of hydrogen sulfide is more beneficial than oxidation of element sulfur into sulfate because oxidation of hydrogen sulfide from -2 to +6 release eight electrons while oxidation of element sulfur from 0 to +6 releases six electrons. So hydrogen sulfide can take more electrons for their electron transport chin and produce more ATP.