- Sulfur cycle
Sulfuris one of the constituents of many proteins, vitamins and hormones. It recycles as in other biogeochemical cycles.
The essential steps of the sulfur cycle are:
* Mineralization of organic
sulfurto the inorganic form, hydrogen sulfide: (H2S).
Oxidationof sulfideand elemental sulfur (S) and related compounds to sulfate(SO42–).
* 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 (SO42–) is reduced to organic sulfhydryl(otherwise known as thiol) groups (R–SH) by plants, fungiand various prokaryotes. 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 (H2S), oxidation state = –2. Note the similarity to deamination.:"Oxidation of hydrogen sulfide" produces elemental sulfur (So), oxidation state = 0. This reaction is done by the photosynthetic green and purple sulfur bacteriaand some chemolithotrophs.: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 reducers generate hydrogen sulfide from sulfate.
Human impact on the sulfur cycle is primarily in the production of
sulfur dioxide(SO2) 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 atmosphere, or oxidized to sulfate in the atmosphere as sulfuric acid, a principal component of acid rain.
* [http://www.lenntech.com/sulfur-cycle.htm Lenntech]
* [http://www.biosci.ohio-state.edu/~mgonzalez/Micro521/23.html Another Lecture]
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