Sulfur cycle

With sulfur cycle is called the system of chemical transformations of sulfur and sulfur-containing compounds in the lithosphere, hydrosphere, atmosphere and biosphere, and the exchange of these materials between these Erdsphären.

The following are the global, geochemical, largely biotic transformations of the chemical element sulfur and its mainly occurring chemical compounds are briefly presented.

Occurrence

The chemical element sulfur is present in the upper layers of the earth and in living things. The earth's crust (fixed and loose rocks, called the lithosphere ) contains an average of about 0.5 g per kg, water ( hydrosphere ) is on average about 0.93 g per kg, living organisms ( biosphere ) about 0.5 to 2 g per kg. Sulfur comes with volcanic gases from the depths to the surface. Sulfur is found in these areas ( litho -, hydro-and biosphere ) mainly present as sulfate ( SO42 - ), hydrogen sulfide (H2S) and elemental sulfur (S ) in biomass coming into organic compounds (as a sulfhydryl -SH and heterocyclic compounds ). The inorganics sulfate, hydrogen sulfide and elemental sulfur are largely stable and are subject only to a very limited degree abiotic chemical reactions. Creatures set but to a considerable extent sulfur and sulfur compounds in the course of their energy and building material change to chemical- enzymatic. This sulfur is subject to constant biotic change in a kind of cycle, the so-called sulfur cycle, which is global, geochemically of considerable importance.

Biotic sulfur cycle

The biotic sulfur cycle is composed of the following reactions:

1 sulfate assimilation ( PAPS ): Some components of living things contain sulfur, namely organic compounds with sulfhydryl groups (- SH), such as the amino acids L -methionine and L- cysteine, and sulfur - containing heterocycles, such as biotin. For its construction of the sulfur from sulfate ( SO42 - ) is assimilated. For this purpose, the sulfate must by binding to Adenosintriphosphosulfat about Adenosinphosphosulfat to 3'- phosphoadenosine -5'- phosphosulfate ( PAPS ) are activated ( contrast to 6). A few bacteria can assimilate elemental sulfur from sulfur springs.

2 rot ( Desulfurylation ): After the death of living creatures, the sulfur contained in them in the course of the degradation of the biomass is released by organisms own enzymes and microorganisms (eg, Escherichia and Proteus ) from the organic substances as hydrogen sulfide (H2S). Under anoxic conditions, such as prevail in poorly aerated soils or in low-oxygen waters, the hydrogen sulfide thus formed accumulates. It is toxic to most living things.

3 sulfide oxidation:

4 sulfur oxidation ( Sulfurikation ):

5 sulphide oxidation to sulphate ( Sulfurikation ): Hydrogen sulphide is oxidized (for example of the genera Thiobacillus and Acidithiobacillus ) and Archaea (e.g., Sulfolobus ) with oxygen (O2) to sulfate by certain aerobic sulphide - oxidising bacteria. This reaction releases energy that is used by the microorganisms ( Chemotrophie ).

6 Desulfurikation ( sulfate reduction ): Certain obligate anaerobic bacteria ( so-called sulfate-reducers, for example the genera Desulfovibrio and Desulfobacter ) oxidized for energy production under anoxic conditions molecular hydrogen (H2) or organic matter with sulfate to give this to hydrogen sulfide (H2S ) is reduced ( Chemotrophie ). To the sulphate to be activated ( difference 1) by binding to adenosine monophosphate to Adenosinphosphosulfat (APS).

7 sulfur reduction: Certain facultative or obligate anaerobic bacteria (for example, the genus Desulfuromonas ) and archaea ( for example the genus Pyrococcus ) oxidized under anoxic conditions for energy molecular hydrogen (H2 ) or organic substances with elemental sulfur, said to hydrogen sulphide ( is H2S) reduced ( Chemotrophie ).

8 heavy metal sulfide formation: hydrogen sulphide forms in abiotic reactions with heavy metal ions (particularly iron ( II ) ions) heavy metal sulphides, which are practically insoluble in water. This reaction protects human beings from the toxic effects of hydrogen sulfide.

9 heavy metal sulfide Resolution: heavy metal sulfides are iron and sulfide - oxidizing bacteria dissolved (eg Acidithiobacillus ferrooxidans ) and archaea (for example, Acidianus ) oxidatively attacked and oxidation of sulfide with oxygen ( O2) to sulfate, with the heavy metals as ion be solved. The microorganisms gain energy from this reaction ( Chemotrophie ).

10 volcanism: Hydrogen sulfide enters with volcanic gases from the Earth's interior to the surface and thus also in the biotic sulfur cycle in the biosphere. Even heavy metal sulfides can with hydrothermal solutions from the Earth's interior to the surface and thus also reach biotic material cycles.

Carbon cycle • hydrogen cycle nitrogen cycle • • • oxygen cycle phosphorus cycle • sulfur cycle • Water circulation • mercury cycle

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• Geochemistry