Nitrification

As nitrification is referred to, the bacterial oxidation of ammonia (NH ₃ ) or ammonium - ions (NH ₄⁺ ) to nitrate (NO ₃^- ). It consists of two coupled sub-processes: First, ammonia is oxidized to nitrite, which is oxidized to nitrate in the second sub-process. Both sub-processes provide sufficient energy that is used by the organisms involved for growth and other life processes. In the nitrogen cycle of ecosystems nitrification plays a major role as they carried the decomposers of dead biomass converted ammonia released back into nitrate. This creates nitrogen-containing mineral nutrients for plants . Sergei Winogradsky was the first to realize that nitrification is a concerted process in which different groups of bacteria take part. In his (from today's perspective) classic publications on nitrification, he not only described the sub-processes, but almost all of the genera known to date were described for the first time. Winogradsky divided the bacteria involved, the nitrifiers , into two groups: he used the prefix nitroso as a common connecting element in the generic name for the organisms involved in the first sub-process; the genera of the second sub-process received the prefix nitro- in the generic name.

In 2015 the complete oxidation of ammonia to nitrate by a single bacterium was discovered in the genus Nitrospira . These bacteria catalyze both nitrification steps and are therefore referred to as complete ammonia oxidizers or comammox bacteria.

First sub-process

The first sub-process consists of the oxidation of ammonia with molecular oxygen to nitrite (equation 1). Under standard conditions (see energy metabolism ), 235 kJ of energy are released per mole of ammonia converted , corresponding to the change in free energy : ΔG ° '= −235 kJ / mol.

Equation 1 :

${\ displaystyle \ mathrm {\ 2 \ NH_ {3} +3 \ O_ {2} \ longrightarrow \ 2 \ H ^ {+} + \ 2 \ NO_ {2} ^ {-} + 2 \ H_ {2} O }}$

The bacteria responsible for the first sub-process are called ammonia oxidizing bacteria or nitrite bacteria . All representatives are aerobic and obligate chemolithoautotroph. The following genera, recognizable by the word part nitroso , belong to it:

The oxidation of ammonia using molecular oxygen takes place in two steps. In the first step (equation 2) the ammonia is oxidized to hydroxylamine by the enzyme ammonium monooxygenase (AMO). In this reaction, one oxygen atom from the oxygen molecule is incorporated into the hydroxylamine, the other is reduced to water. In the second step (equation 3), hydroxylamine is oxidized to nitrite, catalyzed by hydroxylamine oxidoreductase (HAO). Of the 4 mol of electrons obtained from the oxidation of hydroxylamine (equation 3), 2 mol of electrons are used for the AMO reaction (equation 2), about 1.7 mol of electrons are transferred to oxygen via cytochrome c (equation 4), the rest 0.3 mol of electrons flow into the retrograde electron transport to obtain NADPH or NADH .

Equation 2 :

${\ displaystyle \ mathrm {\ NH_ {3} +2 \ H ^ {+} + 2 \ e ^ {-} + \ O_ {2} \ longrightarrow \ NH_ {2} OH + H_ {2} O}}$

Equation 3:

${\ displaystyle \ mathrm {\ NH_ {2} OH + H_ {2} O \ longrightarrow \ H ^ {+} + \ NO_ {2} ^ {-} + \ 4 \ H ^ {+} + \ 4 \ e ^ {-}}}$

Equation 4:

${\ displaystyle \ mathrm {\ 4 \ H ^ {+} + 4 \ e ^ {-} + \ O_ {2} \ longrightarrow \ 2 \ H_ {2} O}}$

Second sub-process

The second sub-process consists in the oxidation of nitrite with molecular oxygen to nitrate, in which under standard conditions 76 kJ of energy are released per mole of oxidized nitrite (the change in free energy ΔG ° 'is −76 kJ / mol).

Equation 5:

${\ displaystyle \ mathrm {\ 2 \ NO_ {2} ^ {-} + \ O_ {2} \ longrightarrow \ 2 \ NO_ {3} ^ {-}}}$

This conversion is catalyzed by the enzyme nitrite oxidase. The bacteria responsible for the second sub-process are called nitrite oxidizing bacteria or nitrate bacteria . All representatives are aerobic and, except for Nitrobacter, are obligate chemolithoautotroph. The following genera, recognizable by the word part Nitro- , belong to it:

Overall implementation

Both sub-processes (equation 1 and equation 5) add up to:

Equation 6:

${\ displaystyle \ mathrm {NH_ {3} +2 \ O_ {2} \ longrightarrow \ NO_ {3} ^ {-} + \ H ^ {+} + \ H_ {2} O}}$

During this conversion, 311 kJ of energy are released per mole of oxidized ammonia under standard conditions (the change in free energy ΔG ° 'is −311 kJ / mol).

In nature, representatives of both bacterial groups, ammonia and nitrite oxidizers, occur together under normal conditions and work together in such a way that nitrite does not accumulate.

Physiology of nitrifiers

The nitrifying bacteria carry out a chemolithoautotrophic metabolism : The inorganic nitrogen compounds serve both as an electron donor ( lithotrophy ) and, together with oxygen O _2, as an energy source ( chemotrophy ). The energy released during the oxidation is required for the synthesis of ATP from ADP and phosphate . ATP is mainly used to build biomass from carbon dioxide . The nitrifying bacteria can meet their carbon needs from carbon dioxide alone. This means that they are autotrophic and carry out so-called chemosynthesis ( chemotrophy ). The carbon dioxide is assimilated via the Calvin cycle .

Ecological and technical importance

Nitrifying bacteria are found in many aerobic ecosystems. The availability of the substrates ammonia or ammonium or nitrite required for their energy generation depends to a large extent on the ammonification. Many ammonia oxidizers can also use urea as a primary substrate. In natural systems, ammonium oxidation could also be observed under conditions that were no longer tolerated by the pure cultures examined . Ammonium oxidation has been observed in acidic soils, in cold ecosystems, on the surface of acidic sandstones or in hot springs at 50 - 60 ° C. In eutrophic systems, nitrification can lead to a significant consumption of oxygen, which can promote denitrification .

Nitrification is associated with the production of acid (H ⁺ formation, see equation 1). The pH value is lowered if the acid formed is not neutralized, for example through reaction with calcium carbonate (CaCO ₃ ). The acid formed puts a strain on the buffer capacity of the water and can acidify the water or the soil. Since nitrifiers only metabolize in the neutral to slightly alkaline range, the acidification can prevent the complete conversion of the fish-toxic ammonium / ammonia in sewage treatment plants (autoinhibition). The nitric acid formed by nitrification can have a destructive effect on mineral materials (e.g. building materials), especially calcareous ones, by dissolving carbonates. Buildings are particularly affected in an environment in which ammonia occurs and nitrification is possible, for example sewage systems and animal stalls. Even if carbonate-containing building materials or sculptures come into contact with nitrogen oxides and water, their surface is often destroyed by nitrification: the nitrogen oxides form nitric acid (HNO ₂ ) with water , which is oxidized to nitric acid (HNO ₃ ) by nitrite oxidizers . As a result, the carbonates dissolve and the structure or image is damaged. In the sandstones of buildings, the outer areas are often interspersed with nitrifying agents. At Cologne Cathedral, ammonium oxidizers were detected in the sandstone up to 15 cm deep. Along with the infestation of nitrifiers, the pH value in the stone was also reduced, as a result of which the dolomite (CaMg (CO ₃ ) ₂ ) present in the sandstone is dissolved over time.

In the aquarium hobby , the main part of the filtration, the so-called biological filtration of the aquarium water, is based on nitrification.

During nitrification, 4.33 g of oxygen (O ₂ ) per gram of NO ₃ -N formed are consumed (oxygen serves as an electron acceptor ). For each gram of NO ₃ -N formed by nitrifying agents , nitrifying agent biomass increases corresponding to 0.24 g of chemical oxygen demand (COD) ( cell yield ). 1.42 grams of COD correspond to one gram of biotic dry matter.

literature

S. Winogradsky: Sur les organismes de la nitrification . In: Comptes rendus de séances l'Academie des Sciences . Vol. 110, 1890, pp. 1013-1016.
Georg Fuchs (Ed.): General microbiology , founded by Hans Günter Schlegel , 8th edition. Georg Thieme Verlag, Stuttgart / New York 2007, ISBN 978-3-13-444608-1 , p. 329 ff.

Individual evidence

^ S. Winogradsky: Recherches sur les organismes de la nitrification. In: Annales de l'Institut Pasteur Vol. 4, 1892, pp. 213-231, 257-275 and 760-771.
^ S. Winogradsky: Contributions à la morphologie des organismes de la nitrification. In: Archives des sciences biologiques St. Petersburg Vol. 1, 1892, pp. 88-137.
↑ Holger Daims et al .: Complete nitrification by Nitrospira bacteria . Nature 528, pp. 504-509, December 2015
↑ GA Kowalchuk, JR Stephen: AMMONIA-OXIDIZING BACTERIA: A Model for Molecular Microbial Ecology In: Annual Review of Microbiology . Vol. 55, 2001, pp. 485-529.
↑ N. Walker, KN Wickramasinghe: Nitrification and autotrophic nitrifying bacteria in acid tea soils. In: Soil Biology and Biochemistry . Vol. 11, 1978, pp. 231-236.
^ RD Jones, RY Morita: Low-temperature growth and whole-cell kinetics of a marine ammonium oxidizer. In: Marine Ecology-Progress Series . Vol. 21, 1985, pp. 239-243.
^ E. Bock: Biologically induced corrosion of natural stones - heavy infestation with nitrifying agents. In: Building protection / building renovation . Vol. 1, 1987, pp. 24-27.
↑ RS Golovacheva: Thermophilic nitrifying bacteria from hot springs. In: Microbiologiya . Vol. 45, 1976, pp. 329-331.
↑ Holger Brill (ed.): Microbial material destruction and material protection - damage mechanisms and protective measures . Gustav Fischer Verlag, Jena, Stuttgart 1995, ISBN 3-334-60940-5 , pp. 87 and 89-90.

Web links

Wiktionary: nitrification - explanations of meanings, word origins, synonyms, translations

[1] S. Winogradsky: Recherches sur les organismes de la nitrification. In: Annales de l'Institut Pasteur Vol. 4, 1892, pp. 213-231, 257-275 and 760-771.

[2] S. Winogradsky: Contributions à la morphologie des organismes de la nitrification. In: Archives des sciences biologiques St. Petersburg Vol. 1, 1892, pp. 88-137.

[3] Holger Daims et al .: Complete nitrification by Nitrospira bacteria . Nature 528, pp. 504-509, December 2015

[4] GA Kowalchuk, JR Stephen: AMMONIA-OXIDIZING BACTERIA: A Model for Molecular Microbial Ecology In: Annual Review of Microbiology . Vol. 55, 2001, pp. 485-529.

[5] N. Walker, KN Wickramasinghe: Nitrification and autotrophic nitrifying bacteria in acid tea soils. In: Soil Biology and Biochemistry . Vol. 11, 1978, pp. 231-236.

[6] RD Jones, RY Morita: Low-temperature growth and whole-cell kinetics of a marine ammonium oxidizer. In: Marine Ecology-Progress Series . Vol. 21, 1985, pp. 239-243.

[7] E. Bock: Biologically induced corrosion of natural stones - heavy infestation with nitrifying agents. In: Building protection / building renovation . Vol. 1, 1987, pp. 24-27.

[8] RS Golovacheva: Thermophilic nitrifying bacteria from hot springs. In: Microbiologiya . Vol. 45, 1976, pp. 329-331.

[9] Holger Brill (ed.): Microbial material destruction and material protection - damage mechanisms and protective measures . Gustav Fischer Verlag, Jena, Stuttgart 1995, ISBN 3-334-60940-5 , pp. 87 and 89-90.