Sulfur cycle
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'''[[Sulfur]]''' is one of the constituents of many [[protein]]s, [[vitamin]]s and [[hormone]]s. It recycles as in other [[biogeochemical cycle]]s.
The essential steps of the '''sulfur cycle''' are:
* Mineralization of [[organic compound|organic]] [[sulfur]] to the inorganic form, [[hydrogen sulfide]]: (H<sub>2</sub>S).
* [[Oxidation]] of [[sulfide]] and elemental sulfur (S) and related compounds to [[sulfate]] (SO<sub>4</sub><sup>2–</sup>).
* Reduction of sulfate to sulfide.
* Microbial immobilization of the sulfur compounds and subsequent incorporation into the organic form of sulfur.
These are often termed as follows:
:''Assimilative sulfate reduction'' (see also [[sulfur assimilation]]) in which sulfate (SO<sub>4</sub><sup>2–</sup>) is reduced to organic [[sulfhydryl]] (otherwise known as thiol) groups (R–SH) by [[plant]]s, [[fungi]] and various [[prokaryote]]s. The oxidation states of sulfur are +6 in sulfate and –2 in R–SH.
:''Desulfuration'' in which organic molecules containing sulfur can be desulfurated, producing hydrogen sulfide gas (H<sub>2</sub>S), oxidation state = –2. Note the similarity to deamination.
:''Oxidation of hydrogen sulfide'' produces elemental sulfur (S<sup>o</sup>), oxidation state = 0. This reaction is done by the [[photosynthesis|photosynthetic]] green and purple sulfur [[bacteria]] and some [[chemolithotroph]]s.
:Further ''oxidation of elemental sulfur'' by sulfur oxidizers produces sulfate.
:''Dissimilative sulfur reduction'' in which elemental sulfur can be reduced to hydrogen sulfide.
:''Dissimilative sulfate reduction'' in which [[sulfate-reducing bacteria|sulfate reducers]] generate hydrogen sulfide from sulfate.
Human impact on the sulfur cycle is primarily in the production of [[sulfur dioxide]] (SO<sub>2</sub>) from industry (e.g. burning [[coal]]) and the [[internal combustion engine]]. Sulfur dioxide can precipitate onto surfaces where it can be oxidized to sulfate in the [[soil]] (it is also toxic to some plants), reduced to sulfide in the [[Earth's atmosphere|atmosphere]], or oxidized to sulfate in the atmosphere as [[sulfuric acid]], a principal component of [[acid rain]].
==External links==
*[http://www.lenntech.com/sulfur-cycle.htm Lenntech]
*[http://www.biosci.ohio-state.edu/~mgonzalez/Micro521/23.html Another Lecture]
{{Biogeochemical cycle}}
[[Category:Ecology]]
[[Category:Metabolism]]
[[Category:Soil biology]]
[[Category:Soil chemistry]]
[[Category:Sulfur]]
[[Category:Biogeography]]
[[de:Schwefelkreislauf]]
[[es:Ciclo del azufre]]
[[fr:Cycle du soufre]]
[[it:Ciclo dello zolfo]]
[[th:วัฏจักรกำมะถัน]]