Sulfur cycle
The sulfur cycle refers to the system of chemical transformations of sulfur and sulfur-containing compounds in the lithosphere , hydrosphere , earth's atmosphere and biosphere as well as the exchange of these substances between these earth spheres .
In the following, the global, geochemical, largely biotic reactions of the chemical element sulfur and its main chemical compounds are briefly presented.
Occurrence
The chemical element sulfur occurs in the upper layers of the earth and in living things . The earth's crust (solid and loose rocks , the so-called lithosphere) contains on average about 0.5 g per kg, waters (hydrosphere) on average about 0.93 g per kg, living beings (biosphere) about 0.5 to 2 g per kg . Sulfur reaches the surface of the earth with volcanic gases from the depths.
Sulfur exists in the lithosphere, hydrosphere and biosphere mainly as sulfate (SO 4 2− ), hydrogen sulfide (H 2 S) and elemental sulfur (S). These inorganic substances are largely stable and are only subject to a very low degree of abiotic chemical reactions.
In biomass, sulfur occurs in organic compounds as a sulfhydryl group –SH and in heterocyclic compounds. Living beings convert sulfur and sulfur compounds in their energy and building metabolism to a large extent chemically and enzymatically. As a result, sulfur is subject to constant biotic change within the so-called sulfur cycle, which is of considerable geochemical global importance.
Biotic sulfur cycle
The following reactions take place in the biotic sulfur cycle :
1. Sulphate assimilation (PAPS): Some components of living beings contain sulfur, namely organic substances with sulphydryl groups (-SH), such as the amino acids L- methionine and L- cysteine , as well as sulfur-containing heterocycles such as biotin . For their structure, the sulfur is assimilated from sulfate (SO 4 2− ). For this purpose, the sulfate must be activated by binding to adenosine triphosphosulfate via adenosine phosphosulfate to 3'-phosphoadenosine-5'-phosphosulfate (PAPS) (difference to 6). A few bacteria can assimilate elemental sulfur from sulfur sources .
2. Putrefaction (desulfurylation): After the death of living beings, the sulfur contained in them is released from the organic substances as hydrogen sulfide (H 2 S) in the course of the breakdown of the biomass by the organism's own enzymes and by microorganisms ( e.g. Escherichia and Proteus ) . The hydrogen sulfide formed in this way accumulates under anoxic conditions, such as those found in poorly aerated soils or in oxygen-poor waters. It is poisonous to most living things.
3. Sulphide oxidation :
- A: Hydrogen sulfide is of particular chemoautotrophic , aerobic , sulfide-oxidizing bacteria (colorless sulfur bacteria of the genera Beggiatoa and Thiovulum for example) with oxygen (O 2 ) to elemental sulfur (S) is oxidized . During these reactions, energy is released that is used by the bacteria to assimilate carbon dioxide.
- b: Hydrogen sulfide is used by certain phototrophic ( anaerobic ) bacteria (for example of the genera Chromatium and Chlorobium ) under anoxic conditions in anoxygenic photosynthesis as a reducing agent for the assimilation of carbon dioxide (CO 2 ) and is oxidized to elemental sulfur or sulfate ( purple bacteria ).
- Example: green sulfur bacteria
4. Sulfur oxidation (sulfurization) :
- a: Certain sulfur-oxidizing aerobic bacteria (for example of the genera Thiobacillus and Acidithiobacillus ) and archaea (for example Acidianus ) oxidize elemental sulfur with oxygen (O 2 ) to sulfate. During this reaction, energy is released that is used by the microorganisms (chemotrophy).
- b: In the case of H 2 S deficiency, some of the bacteria mentioned under 3 can also oxidize the sulfur initially formed as an end product to sulfate (advantage: higher energy yield, disadvantage: acid formation ).
5. Sulphide oxidation to sulphate (sulphurication) : Hydrogen sulphide is oxidized to sulphate by certain aerobic, sulphide-oxidising bacteria (for example of the genera Thiobacillus and Acidithiobacillus ) and archaea (for example Sulfolobus ) with oxygen (O 2 ). During this reaction, energy is released that is used by the microorganisms (chemotrophy).
6. Desulfurication (sulfate reduction): Certain obligate anaerobic bacteria (so-called desulfuricants , for example the genera Desulfovibrio and Desulfobacter ) oxidize molecular hydrogen (H 2 ) or organic substances with sulfate to generate energy under anoxic conditions , whereby, for example, this to hydrogen sulfide (H 2 S ) is reduced (chemotrophy). To do this, the sulfate must be activated by binding to adenosine monophosphate to form adenosine phosphosulfate (APS) (in contrast to 1).
7. Sulfur reduction : Certain facultative or obligatory anaerobic bacteria (for example of the genus Desulfuromonas ) and archaea (for example the genus Pyrococcus ) oxidize molecular hydrogen (H 2 ) or organic substances with elemental sulfur under anoxic conditions to generate energy , which turns into hydrogen sulfide (H 2 S) is reduced (chemotrophy).
8. Heavy metal sulfide formation: In abiotic reactions with heavy metal ions (especially iron (II) ions), hydrogen sulfide forms heavy metal sulfides which are practically insoluble in water. This implementation protects living beings from the toxic effects of hydrogen sulfide.
9. Heavy metal sulphide dissolution : Heavy metal sulphides are attacked by iron and sulphide oxidising bacteria (for example Acidithiobacillus ferrooxidans ) and archaea (for example Acidianus ) and dissolved to sulphate by oxidation of the sulphide with oxygen (O 2 ), the heavy metals as Ions are dissolved. The microorganisms gain energy from this conversion (chemotrophy).
10. Volcanism : Hydrogen sulphide reaches the earth's surface with volcanic gases from the interior of the earth and thus also into the biotic sulfur cycle, into the biosphere. Heavy metal sulfides can also reach the earth's surface with hydrothermal solutions from the earth's interior and thus also enter the biotic material cycle.