Denitrification

Among denitrification refers to the conversion of the nitrate (NO ₃^- ) bound nitrogen to molecular nitrogen (N ₂ ) and nitrogen oxides , by certain heterotrophic and some autotrophic bacteria , which accordingly as denitrifying bacteria are referred to. The process is used by the bacteria to generate energy. In the absence of molecular oxygen (O ₂ ) ( anoxic conditions), various oxidizable substances ( electron donors ), such as organic substances, hydrogen sulfide (H ₂ S) and molecular hydrogen (H ₂ ), are oxidized with nitrate as the oxidant (oxidizing agent). The process is therefore a possibility of energy metabolism , namely an oxidative energy metabolism.

Reactions

The process is bound to the membranes of the bacteria; in its course, energy is conserved in the form of a difference in proton concentration between the spaces separated by the membrane. It is therefore a form of anaerobic breathing , which is also known as nitrate breathing .

The individual steps of the multi-stage reaction are catalyzed by the metalloenzymes nitrate reductase , nitrite reductase , nitrogen monoxide reductase and nitrous oxide reductase:

(1) nitrate reductase:

{\ displaystyle \ mathrm {\ NO_ {3} ^ {-} + \ 2 \ H ^ {+} + 2 \ e ^ {-} \ longrightarrow \ NO_ {2} ^ {-} + \ H_ {2} O }}

(2) nitrite reductase:

{\ displaystyle \ mathrm {\ NO_ {2} ^ {-} + 2 \ H ^ {+} + \ e ^ {-} \ longrightarrow \ NO + \ H_ {2} O}}

(3) nitric oxide reductase:

{\ displaystyle \ mathrm {\ 2 \ NO + \ 2 \ H ^ {+} + \ 2 \ e ^ {-} \ longrightarrow \ N_ {2} O + H_ {2} O}}

(4) nitrous oxide reductase:

{\ displaystyle \ mathrm {\ N_ {2} O + 2 \ H ^ {+} + 2 \ e ^ {-} \ longrightarrow \ N_ {2} + H_ {2} O}}

Since the redox potentials of all individual steps of denitrification are positive, these bacteria can use nitrate as an electron acceptor ( oxidizing agent ) for their oxidative energy metabolism ( oxidative phosphorylation ) when no or only limited molecular oxygen (O ₂ ) is available ( anoxic or hypoxic conditions). The oxidation of one mole of glucose with nitrate releases a maximum of 2670 kJ of energy ( ΔG ₀ ' = −2670 kJ).

The electrons (e ^- ) or the hydrogen (H) resulting from the oxidation of organic or inorganic substances are transferred by electron and hydrogen carriers, which differ depending on the different enzymes and bacteria. As a rule, cytochromes and quinones serve as electron and hydrogen carriers. The electron transport leads by means of the chemiosmotic coupling to the synthesis of ATP and thus to energy conservation. In addition to N ₂ , however, a smaller amount of the intermediate N ₂ O (gaseous) is always released. This process takes place in nature wherever nitrate and organic substances oxidizable by denitrifiers are available under anoxic or hypoxic conditions (e.g. swamps , soils , sediments in rivers and lakes ). Certain bacteria can also produce molecular hydrogen (H ₂ ), hydrogen sulfide (H ₂ S), ammonium (NH ₄⁺ ), iron (II) ions (Fe ²⁺ ) and methane with nitrate (NO ₃^- ) with the formation of molecular Nitrogen (N ₂ ) are oxidized.

Denitrifiers

Examples of denitrifying bacteria are:

Paracoccus denitrificans ( autotrophic , oxidation of H₂ or thiosulfate (S₂ O₃²⁻ ))
Thiobacillus denitrificans ( autotrophic , oxidation of sulfide (S²⁻ ) or thiosulfate (S₂ O₃²⁻ ))
Pseudomonas stutzeri ( heterotrophic , oxidation of organic substances)
Vogesella indigofera
Some types of Flavobacterium

In general, the ability to denitrify is widespread within prokaryotes ; There are clusters in the alpha, beta and gamma classes of proteobacteria .

Ecological and technical importance

The nitrogen bound in the nitrate is converted into molecular nitrogen (N ₂ ) through denitrification, i.e. converted into a form that is largely inert (Latin: inert, inactive) and cannot be used as a nutrient (nitrogen source) by most living things . It is therefore no longer available in the form of a fertilizer in waters and soils and is no longer environmentally relevant. Most of the resulting molecular nitrogen (N ₂ ) escapes into the atmosphere , in which it is the main component anyway. Denitrification and the anammox process, which has only recently been discovered, are the only metabolic pathways in which bound nitrogen is converted back into its molecular form and are therefore an essential part of the nitrogen cycle .

Technically, denitrification is used in wastewater treatment in sewage treatment plants to eliminate nitrate. It can also be used to remove nitrate when producing drinking water (see water treatment ). The reductant ( electron donor ) used is often alcohol, more rarely molecular hydrogen.

Individual evidence

^ Soil microbiology: biodiversity, ecophysiology and metagenomics; by Johannes CG Ottow; 2011; Springer Verlag; P. 314
^ Kristina L. Straub, Marcus Benz, Bernhard Schink, Friedrich Widdel: (1996): Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. In: Applied and Environmental Microbiology. Vol. 62, No. 4, 1996, pp. 1458-1460. PMID 16535298
↑ AA Raghoebarsing et al. (2006): A microbial consortium couples anaerobic methane oxidation to denitrification. In: Nature. Vol. 440, No. 7085, 2006, pp. 918-921. PMID 16612380
↑ Noel R. Krieg et al. (Ed.): Bergey's Manual of Systematic Bacteriology . 2nd edition, Volume 4: The Bacteroidetes, Spirochaetes, Tenericutes (Mollicutes), Acidobacteria, Fibrobacteres, Fusobacteria, Dictyoglomi, Gemmatimonadetes, Lentisphaerae, Verrucomicrobia, Chlamydiae, and Planctomycetes . Springer, New York 2010, ISBN 978-0-387-68572-4 , pp. 152-153 .

literature

Walter G. Zumft: Cell biology and molecular basis of denitrification. In: Microbiology and Molecular Biology Reviews. Vol. 61, No. 4, 1997, pp. 533-616. PMID 9409151 PMC 232623 (free full text)