Diazotrophs are organisms capable of nitrogen fixation, i.e. converting the relatively inert diatomic nitrogen (N<sub>2</sub>) in Earth's atmosphere into bioavailable compound forms such as ammonia. Diazotrophs are typically microorganisms such as bacteria and archaea, with examples being rhizobia and Frankia and Azospirillum. All diazotrophs contain iron-molybdenum or iron-vanadium nitrogenase systems, and two of the most studied systems are those of Klebsiella pneumoniae and Azotobacter vinelandii due to their genetic tractability and their fast growth.

Etymology

The word diazotroph is derived from the words diazo ("di" = two + "azo" = nitrogen) meaning "dinitrogen (N<sub>2</sub>)" and troph meaning "pertaining to food or nourishment", in summary dinitrogen utilizing. The word azote means nitrogen in French and was named by French chemist and biologist Antoine Lavoisier, who saw it as the part of air which cannot sustain life.

Types

Diazotrophs are scattered across Bacteria taxonomic groups (as well as a couple of Archaea). Even within a species that can fix nitrogen there may be strains that do not. Nitrogen fixation is an energetically costly proccess and is thus reduced when other sources of nitrogen are available, and, for many species, when oxygen is at high partial pressure. Bacteria have different ways of dealing with the debilitating effects of oxygen on nitrogenases. Symbiotic diazotrophs have traditionally been considered to be more relevant to global nitrogen cycles, as the plant hosts may produce a favorable environment for diazotrophy, but free-living diazotrophs are also significant nitrogen fixers.

Symbiotic diazotrophs

  • Rhizobia—these are the species that associate with legumes, plants of the family Fabaceae. Oxygen is bound to leghemoglobin in the root nodules that house the bacterial symbionts, and supplied at a rate that will not harm the nitrogenase. Their role is less well established than for rhizobia. These footnotes suggest the ontogeny of these replicates rather than the phylogeny. In other words, an ancient gene (from before the angiosperms and gymnosperms diverged) that is unused in most species was reawakened and reused in these species.
  • Cyanobacteria—there are also symbiotic cyanobacteria. Some associate with fungi as lichens, with liverworts, with a fern, and with a cycad.
  • Facultative anaerobes—these species can grow either with or without oxygen, but they only fix nitrogen anaerobically. Often, they respire oxygen as rapidly as it is supplied, keeping the amount of free oxygen low. Examples include Klebsiella pneumoniae, Paenibacillus polymyxa, Bacillus macerans, and Escherichia intermedia. whilst other marine cyanobacteria, such as Trichodesmium and Cyanothece, are major contributors to oceanic nitrogen fixation.
  • Anoxygenic photosynthetic bacteria do not generate oxygen during photosynthesis, having only a single photosystem which cannot split water. Nitrogenase is expressed under nitrogen limitation. Normally, the expression is regulated via negative feedback from the produced ammonium ion but in the absence of N<sub>2</sub>, the product is not formed, and the by-product H<sub>2</sub> continues unabated [Biohydrogen]. Example species: Rhodobacter sphaeroides, Rhodopseudomonas palustris, Rhodobacter capsulatus.

Cultivation

Under the laboratory conditions, extra nitrogen sources are not needed to grow free-living diazotrophs. Carbon sources (such as sucrose or glucose) and a small amount of inorganic salt are added to the medium. Free-living diazotrophs can directly use atmospheric nitrogen (N<sub>2</sub>). However, while cultivating several symbiotic diazotrophs, such as rhizobia, it is necessary to add nitrogen because rhizobia and other symbiotic nitrogen-fixing bacteria can not use molecular nitrogen (N<sub>2</sub>) in free-living form and only fix nitrogen during symbiosis with a host plant.

Application

Biofertilizer

Diazotroph fertilizer is a kind of biofertilizer that can use nitrogen-fixing microorganisms to convert molecular nitrogen (N<sub>2</sub>) into ammonia (which is the formation of nitrogen available for the crops to use). These nitrogen nutrients then can be used in the process of protein synthesis for the plants. This whole process of nitrogen fixation by diazotroph is called biological nitrogen fixation. This biochemical reaction can be carried out under normal temperature and pressure conditions, so it does not require extreme conditions and specific catalysts in fertilizer production. Therefore, producing available nitrogen in this way can be cheap, clean and efficient. Nitrogen-fixing bacteria fertilizer is an ideal and promising biofertilizer.

Symbiosis between the roots of leguminous crops and rhizobia (a kind of diazotroph) has led agricultural societies to use legumes as a means of increasing soil fertility since ancient times. These rhizobia can be considered a natural biofertilizer which provides available nitrogen in the soil. By growing leguminous crops in association with non-leguminous crops, farmers could make use of the nitrogen-fixing properties of the rhizobia to increase soil fertility and crop yields.

thumb|Leguminous plants used to fertilize an abandoned land

Diazotroph biofertilizers used today include Rhizobium, Azotobacter, Azospirilium and Blue green algae (a genus of cyanobacteria). These fertilizer are widely used and commenced into industrial production. So far in the market, nitrogen fixation biofertilizer can be divided into liquid fertilizer and solid fertilizer. Most of the fertilizers are fermented in the way of liquid fermentation. After fermentation, the liquid bacteria can be packaged, which is the liquid fertilizer, and the fermented liquid can also be adsorbed with sterilized peat and other carrier adsorbents to form a solid microbial fertilizer. These nitrogen-fixation fertilizer has a certain effect on increasing the production of cotton, rice, wheat, peanuts, rape, corn, sorghum, potatoes, tobacco, sugarcane and various vegetables.

Importance

In organisms the symbiotic associations greatly exceed the free-living species, with the exception of cyanobacteria.

References

  • Marine Nitrogen Fixation - The Basics (USC Capone Lab)
  • Azotobacter
  • Rhizobia
  • Frankia & Actinorhizal Plants