Diazotrophy

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Diazotrophy is the name given to the growth of living things with elemental, molecular nitrogen (N 2 ) as a nitrogen source. So far, diazotrophy has only been known in prokaryotic organisms. The basis is the so-called nitrogen fixation . It consists of an enzymatic reduction of N 2 to ammonia (NH 3 ).

Ecological importance

Clostridia belong to the nitrogen-fixing bacteria .

There are both free- living (e.g. Azotobacter and Clostridium ) and symbiotically living nitrogen fixators ( Rhizobium , Frankia ). The symbiotic often live in and on the roots of higher plants. They provide the fixed nitrogen in the form of ammonium ions (NH 4 + ) for protein biosynthesis and for the synthesis of other N-containing body components and in return receive carbon compounds as an energy source for nitrogen fixation and their growth.

The nodule bacteria of the genus Rhizobium are of the highest quantitative importance due to their distribution and nitrogen-binding capacity. They usually live in symbiosis with plants of the Fabaceae family (butterflies, legumes). In agriculture , peas and broad beans are often grown either in mixed culture with other crops or incorporated into the crop rotation for soil improvement . Alfalfa is used as green manure or as fodder .

Less known, but very important, is the symbiosis of nitrogen-fixing cyanobacteria with algae fern for rice cultivation, and nitrogen fixation by actinomycetes of the genus Frankia in many shrubs and trees , such as alder and sea buckthorn .

Due to the ability to fix nitrogen, diazotrophic plants play a central role in the primary succession of terrestrial habitats , since the enrichment with plant-available nitrogen through atmospheric input is very slow. So even today on was moraine occurring Avens in late glacial Europe very widespread. Another typical diazotrophic pioneer plant is the green alder . A recent succession on a glacial moraine with dryas and alder can be observed in Glacier Bay National Park in the US state Alaska . The dunes of tropical islands are often populated by Casuarina early on .

Chemism

The N 2 fixation process requires the use of energy in the form of adenosine triphosphate (ATP) and a reducing agent in the form of NADPH, as well as specific enzymes. In the case of nodule bacteria, at least three enzyme systems must work together:

  1. Nitrogenase ; Mo-Fe-S containing enzyme complex of dinitrogenase and dinitrogenase reductase
  2. Leghemoglobin ; Hemoglobin-like dye with O 2 protective function for nitrogenase
  3. Ferredoxin ; Electron transporter

The NH 4 + formed by the nitrogenase is used by aminotransferases to synthesize amino acids with further consumption of ATP and NADPH .

α-ketoglutarate → glutamic acid → glutamine

Aspartic acid can also serve as an acceptor, the end product then being asparagine .

See also

literature

  • Walter Larcher: Ecophysiology of plants . 6th edition. Eugen Ulmer, Stuttgart 2001, ISBN 3-8252-8074-8 .
  • J. Postgate: Nitrogen Fixation. 3rd edition. Cambridge University Press, Cambridge UK 1998, ISBN 0-521-64047-4

Individual evidence

  1. ^ Glacier Bay National Park & ​​Preserve: Plant Succession in Pictures
  2. ↑ Species Information ( Memento from July 31, 2012 in the Internet Archive )