Nitrogen fixation

Schematic representation of the nitrogen cycle in nature

Under nitrogen fixation is generally understood as any conversion of the chemically inert elementary, molecular nitrogen (N ₂ ). Nitrogen fixation is the first and fundamental step in the nitrogen cycle .

One differentiates:

• biotic nitrogen fixation (by microorganisms )
• abiotic nitrogen fixation (formation of nitrogen oxides through burns or lightning strikes) and
• technical nitrogen fixation (for example in the Haber-Bosch process ).

It is estimated that around 200–300 million tons of N _{2 are} biologically fixed each year , around a third of which in the oceans . In comparison, the technical fixation ( Haber-Bosch process ) of N ₂ in 1998 was only about 30 million tons. The symbiotic nodule bacteria fix about 50 - 150 kg nitrogen per hectare and year and the free-living bacteria only 1 - 3 kg nitrogen per hectare and year.

Nitrogen fixation is to be distinguished from “ ammonium fixation ” , the binding of positively charged ammonium ions to negatively charged clay minerals in the soil (see also nutrient (plant) and cation exchange capacity ).

history

The Russian microbiologist Winogradski was the first to provide evidence of nitrogen fixation in a culture of Clostridium pasteurianum ( Bacillus amylobacter ) (outdated names for Clostridium butyricum, see butyric acid fermentation ).

Biotic nitrogen fixation

Some prokaryotic microorganisms reduce elemental, molecular nitrogen (N ₂ ) to compounds that are more reactive and, in particular, bioavailable .

${\displaystyle \mathrm {N_{2}+8\ H^{+}+8\ e^{-}\longrightarrow 2\ NH_{3}+H_{2}\!} }$

The conversion is catalyzed by the enzyme nitrogenase and is very energy-intensive due to the very stable triple bond of molecular, elemental nitrogen with 946 kilojoules per mole (kJ / mol) (under standard conditions for biology). As far as is known so far, at the same time as one molecule of N _{2 is} reduced to ammonia (NH ₃ ), 2 hydrogen ions (H ⁺ ) are inevitably also reduced to molecular hydrogen (H ₂ ), presumably for the initiation of N ₂ reduction. The N ₂ reduction takes place in three steps, in which 2 H atoms are added: N ₂ → NH = NH → NH ₂ -NH ₂ → 2 NH ₃ . The entire implementation is very energy-consuming, the energy is - as with most endergonic metabolic reactions - provided by the hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and ortho-phosphate (P _i ). The equation for the reduction of N ₂ and H ⁺ to NH ₃ and H ₂ when the required energy is made available through the hydrolysis of ATP to ADP + P _i is:

${\displaystyle \mathrm {N_{2}+8\ H^{+}+8\ e^{-}+16\ ATP\longrightarrow 2\ NH_{3}+H_{2}\ +16\ ADP+16\ P_{i}} }$

For the formation of the required reduction equivalents, shown here as electrons (e ^- ), the energy source required is the hydrolysis of 3 ATP to 3 ADP + 3 P _i , for 8 e ^- i.e. the hydrolysis of 8 × 3 = 24 ATP. For the formation of 2 moles of NH ₃ and 1 mole of H ₂ according to the above reaction equation, 16 + 24 = 40 moles of ATP must be hydrolyzed to ADP + P _i . With some N ₂ reducers (for example Klebsiella pneumoniae ) this remains so, the H _{2 formed} escapes. However, some (for example Azotobacter ) can then oxidize the formed H ₂ back to 2 H ⁺ , whereby the energy released is used to phosphorylate ADP to ATP, so these N ₂ reducers need a little less than 40 ATP for the N ₂ - Reduction.

Field research into nitrogen fixation by cyanobacteria in Svalbard (Spitzbergen) by comparison with closed organisms in glass cylinders, here called "incubation chamber"

So far, only prokaryotes are known as microorganisms that can fix nitrogen (nitrogen fixators); they are either free-living or live in symbiosis with plants. Well-known free-living representatives are the genera Azotobacter , Azomonas and Cyanobacteria (formerly called blue-green algae ). Cyanobacteria often fix nitrogen in specialized cells called heterocysts .

Other examples are: Aerobacter , Achromobacter , Bacillus polymixa (see Bacillus ), Pseudomonas , Clostridium pasteurianum (outdated name for butyricum Clostridium , see butyric acid fermentation ), methanobacterium (see methanogens ), Desulfovibrio , Rhodospirillum , Chromatium (see sulfur-reducing bacteria ), Chlorobium ( see Green Sulfur Bacteria ), Rhodomicrobium (see Iron Oxidizing Microorganisms ), Anabaena , Calothrix , Nostoc and Tolypothrix .

The best-known symbiotic nitrogen fixators are nodule bacteria (for example in legumes ) and Frankia (in woody plants such as alder ).

Since nitrogen fixation is very energy-intensive for living beings, it is strictly regulated and is only used when the living being has no other option for nitrogen supply.

The trace elements molybdenum and vanadium (and tungsten as a substitute) were identified as necessary agents for nitrogen fixation by Azotobacter.

See also : diazotrophy

Some authors have also shown nitrogen fixation for the body's own protein biosynthesis in insects (in aphids and uniforms ).

Abiotic nitrogen fixation

By lightning strikes during thunderstorms , combustion and volcanoes : nitrogen and oxygen in the air produce nitrogen oxides , which react with water droplets in the atmosphere to form nitrous acid or nitric acid and get into the ground as acid rain .

${\displaystyle \mathrm {N_{2}+O_{2}\quad \rightarrow \quad 2\,NO} }$

${\displaystyle \mathrm {4\,NO+3\,O_{2}+2\,H_{2}O\quad \rightarrow \quad 4\,HNO_{3}} }$

Technical nitrogen fixation

N ₂ can be reduced using the Haber-Bosch process . The process requires a temperature of 500 ° C, a pressure of 450 bar and catalysts . The reduction is similar to (2). Usually this ammonia is converted into fertilizers containing nitrates .

In azotation, nitrogen is fixed to represent calcium cyanamide according to the following reaction equation:

${\displaystyle \mathrm {CaC_{2}+N_{2}\longrightarrow \ Ca(CN)_{2}} }$ ${\displaystyle \mathrm {\longrightarrow \ Ca^{2+}\ +\ ^{-}N{=}C{=}N^{-}\ +C} }$

Further meaning

In addition, nitrogen fixation is the definition of the soil nitrogen in the organic substance when there is an unfavorable carbon-nitrogen ratio ( C / N ratio ). The reason for this lies in the nitrogen requirement of the degrading microorganisms . For example, when applying low -nitrogen mulch materials such as sawdust, wood chips or chopped bark, a nitrogen deficiency in the crop can be observed. It can therefore be beneficial to compost such materials beforehand or to add nitrogen fertilizer . The bound nitrogen is released again in the long term when the organic substances are broken down.

literature

• Entry to nitrogen fixation. In: Römpp Online . Georg Thieme Verlag, accessed on June 13, 2014.

Individual evidence

1. ^ Nitrogen fixation - Lexicon of Biology. In: Spektrum.de. Spektrum Verlag, accessed on February 14, 2016 . 
2. ^ ^{A b} Ruth Beutler: The metabolism. Springer-Verlag, 2013, ISBN 978-3-662-37018-6 , p. 988 ( limited preview in Google book search).
3. ↑ JL Slonczewski, John W. Foster: Microbiology - A science with a future (translation from English) . 2nd Edition. Springer Spectrum, Berlin, Heidelberg 2012, ISBN 978-3-8274-2909-4 , pp. 660-662 .
4. ↑ H. Bortels: molybdenum as a catalyst in the biological nitrogen fixation . In: Archives for Microbiology . tape 1 , no. 1 , January 1, 1930, p. 333-342 , doi : 10.1007 / BF00510471 .
5. ↑ H. Bortels: About the effect of molybdenum and vanadium fertilizers on the Azotobacter number and nitrogen fixation in soil . In: Archives for Microbiology . tape 8 , no. 1-4 , January 1, 1937, pp. 1-12 , doi : 10.1007 / BF00407188 .
6. ↑ E. Blanck: Handbuch der Bodenlehre . Springer, Berlin 1939, ISBN 978-3-642-99617-7 , pp. 525 ( limited preview in Google Book search).
7. ↑ L. Tóth, A. Wolsky, M. Bátori: Nitrogen binding from the air in aphids and homopterans (Rhynchota insecta) . In: Journal for Comparative Physiology . tape 30 , no. 1 , December 1, 1943, p. 67-73 , doi : 10.1007 / BF00338578 .