Sulfate-reducing bacteria
As Desulfurikation (Latin Sulphur: Sulphur ), and sulfate respiration or dissimilatory or bacterial sulfate reduction is defined as the reduction of sulfate to sulfide or hydrogen sulfide by certain bacteria and archaea ( Desulfurizierer ). As a reducing agent, various organic substances or elementary hydrogen are used (H2). Sum equations for examples of such redox reactions are:
These reactions are exergonic and serve the sulfate- reducing microorganisms as an energy source. The sulfate reduction proceeds as well as aerobic respiration via several intermediates.
For comparison, the aerobic oxidation of glucose provides a free enthalpy of 1140 kJ / mol.
Desulfurizierer and their properties
Desulfurizierer are obligate anaerobic bacteria, are found only in anoxic environments and use as a reducing agent mainly substances that are formed in the fermentative degradation of organic substances by fermentative bacteria as end products, especially alcohols, organic acids, elemental hydrogen. They can, by their ability to utilize sulfate as an oxidant, obtain energy from oxidation of substances that can not be used by the fermentative bacteria and are therefore eliminated from these as the final products. The ability to sulfate reduction have kinship -spaced bacterial genera. This may mean that the sulfate reduction was developed early in the evolution of organisms.
Within the Proteobacteria they are found in the Delta group and that in the orders Desulfobacterales, Desulfovibrionales and Syntrophobacterales. Examples of individual genera of sulfate-reducing bacteria of the Deltaproteobacteria are: Desulfovibrio, Desulfuromonas, Desulfobulbus, Desulfobacter, Desulfococcus, Desulfosarcina, Desulfonema and Desulfotomaculum.
Except for the delta proteobacteria sulfate respiration still occurs in the Phylum Thermodesulfobacteria and in the order Clostridiales the Department Firmicutes on ( genus Desulfotomaculum ).
Also in the Archaea domain, there are Desulfurizierer, for example the genus Archaeglobus.
Ecological Significance
The bacterial sulfate reduction is an important phase in the sulfur cycle of the upper soil layers. She is there beside volcanic degassing one of the main sources of hydrogen sulfide. As the gross equations, the pH is increased Desulfurikation (formation of OH - ions or consumption of protons). Desulfurikation takes place in almost all anoxic areas containing sulfate and recyclable organic materials or elemental hydrogen. Hydrogen sulphide acts on toxic creatures. The Desulfurizierer are sensitive to the hydrogen sulfide formed by themselves. In natural habitats, however, the hydrogen sulfide formed is mostly harmless because it is poorly water-soluble metal sulfides with many metal ions. In natural habitats is especially the precipitation of iron and sulphide ions as black iron sulphide FeS mono important:
This process is the cause of the blackening of anoxic water sediments, such as mud sediments of ponds and lakes and some deeper layers of silt watts. Well known are the black sediments of the Black Sea, which is said to have thus received its name. By further action of hydrogen sulphide on iron monosulfide can be formed via several intermediates iron disulfide FeS2, pyrite or marcasite so the minerals.
Technical significance
On the surface, base metals, elemental hydrogen is formed in contact with water to form metal ions. As an example, iron or an iron alloy, for example steel, selected
The hydrogen layer thus formed adheres mostly to the surface of the iron alloy, and prevent the progression of the process ( " passivation "). Under anoxic conditions in the presence of sulfate but it can lead to corrosion by sulfate-reducing bacteria that oxidize hydrogen with sulphate and so expose the metal surface so that it comes to the further formation of hydrogen and other metal dissolution (corrosion):
Sulfate-reducing bacteria by steel corrosion caused inter alia on the oil production is important. May also occur in oil reservoirs and oil production plants for the formation of hydrogen sulfide by bacterial sulfate reduction, whereby the hydrogen sulfide thus formed accumulates not only in the reservoir water, but also in crude oil and in the accompanying natural gas ( " acidification ", " sour gas "). This leads to disturbances due to the toxicity of hydrogen sulfide and due to its corrosive effect. In addition, a sulfur content in crude oil and natural gas in the combustion of which the emission of undesirable sulfur dioxide ( SO2) has the consequence (see acid rain), which is why the oil and natural gas must be desulfurized ( Erdölentschwefelung, gas desulphurisation ).