The Fixation of nitrogen It is the set of biological and non-biological processes that produce chemical forms of nitrogen available to living beings. Nitrogen availability controls in an important way the functioning of ecosystems and global biogeochemistry, since nitrogen is a factor that limits the net primary productivity in terrestrial and aquatic ecosystems..
In the tissues of living organisms, nitrogen is part of amino acids, units of structural and functional proteins such as enzymes. It is also an important chemical element in the constitution of nucleic acids and chlorophyll..
Additionally, the biogeochemical reactions of carbon reduction (photosynthesis) and carbon oxidation (respiration), occur through the mediation of nitrogen-containing enzymes, since they are proteins.
In the chemical reactions of the biogeochemical cycle of nitrogen, this element changes its oxidation states from zero to Ntwo, a 3- in NH3, 3+ in NOtwo - and NH4+ , and 5+ in NO3 -.
Several microorganisms take advantage of the energy generated in these nitrogen oxide-reduction reactions and use it in their metabolic processes. It is these microbial reactions that collectively drive the global nitrogen cycle..
The most abundant chemical form of nitrogen on the planet is gaseous molecular diatomic nitrogen Ntwo, which constitutes 79% of the earth's atmosphere.
It is also the least reactive nitrogen chemical species, practically inert, very stable, due to the triple bond that joins both atoms. For this reason, the nitrogen so abundant in the atmosphere is not available to the vast majority of living beings..
Nitrogen in chemical forms available to living beings is obtained through "nitrogen fixation." Nitrogen fixation can occur through two main ways: abiotic forms of fixation and biotic forms of fixation..
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The lightning or "lightning" produced during electrical storms is not just noise and light; they are a powerful chemical reactor. Due to the action of lightning, nitrogen oxides NO and NO are produced during storms.two, generically called NOx.
These electric discharges, observed as lightning, generate conditions of high temperatures (30,000orC) and high pressures, which promote the chemical combination of oxygen Otwo and nitrogen Ntwo from the atmosphere, producing nitrogen oxides NOx.
This mechanism has a very low rate of contribution to the total rate of nitrogen fixation, but it is the most important among abiotic forms..
There is an anthropogenic contribution to the production of nitrogen oxides. We have already said that the strong triple bond of the nitrogen molecule Ntwo, can only break under extreme conditions.
The combustion of fossil fuels derived from petroleum (in industries and in commercial and private transport, maritime, air and land), produces enormous amounts of NO emissions.x to the atmosphere.
The ntwoOr emitted in the combustion of fossil fuels, it is a powerful greenhouse gas that contributes to global warming of the planet..
There is also a contribution of nitrogen oxides NOx by burning biomass in the area with the highest temperature of the flame, for example in forest fires, use of firewood for heating and cooking, incineration of organic waste and any use of biomass as a source of heat energy.
Nitrogen oxides NOx emitted into the atmosphere by anthropogenic routes, cause serious environmental pollution problems, such as photochemical smog in urban and industrial environments, and important contributions to acid rain.
Soil erosion and nitrogen-rich bedrock weathering expose minerals to the elements that can release nitrogen oxides. Bedrock weathering occurs due to exposure to environmental factors, caused by physical and chemical mechanisms acting together..
Tectonic movements can physically expose nitrogen-rich rocks to the elements. Subsequently, by chemical means, the precipitation of acid rain causes chemical reactions that release NOx, both from this type of rocks and from the ground.
There is recent research that assigns 26% of the planet's total bioavailable nitrogen to these mechanisms of soil erosion and rock weathering..
Some bacterial microorganisms have mechanisms capable of breaking the triple bond of Ntwo and produce NH ammonia3, which is easily transformed into ammonium ion, NH4+ metabolizable.
The forms of nitrogen fixation by microorganisms can occur through free-living organisms or through organisms that live in symbiotic associations with plants..
Although there are great morphological and physiological differences between nitrogen-fixing microorganisms, the fixation process and the nitrogenase enzyme system used by all of these are very similar..
Quantitatively, biotic nitrogen fixation through these two mechanisms (free life and symbiosis) is the most important globally..
Nitrogen-fixing microorganisms have strategic mechanisms to keep their nitrogenase enzymatic system active..
These mechanisms include respiratory protection, conformational chemical protection, reversible inhibition of enzyme activity, additional synthesis of an alternative nitrogenase with vanadium and iron as cofactors, creation of diffusion barriers for oxygen, and spatial separation of nitrogenase.
Some have microaerophilicity, such as the chemotrophic bacteria of the genus Azospirilium, Aquaspirillum, Azotobacter, Beijerinkia, Azomonas, Derxia, Crynebacterium, Rhizobium, Agrobacterium, Thiobacillus and phototrophs of the genres Gleocapsa, Anabaena, Spirulina, Nostoc, Oscillatoria, Calothrix, Lingbya.
Others present facultative anaerobiosis, such as the chemostrophic genera: Klebsiella, Citrobacter, Erwinia, Bacillus, Propionibacterium and phototrophs of the genres Rhodospirillum, Rhodopsuedomonas.
Nitrogen-fixing microorganisms that live in the soil in a free (asymbiotic) form are basically archaebacteria and bacteria..
There are several types of bacteria and cyanobacteria that can convert atmospheric nitrogen, Ntwo, in ammonia, NH3. According to the chemical reaction:
Ntwo+8H++8e-+16 ATP → 2 NH3+Htwo+16 ADP + 16Pi
This reaction requires the mediation of the nitrogenase enzyme system and a cofactor, vitamin B12. Additionally, this nitrogen fixation mechanism consumes a lot of energy, is endothermic and requires 226 Kcal / mol of Ntwo; that is, it carries a high metabolic cost, which is why it must be coupled to a system that produces energy.
The energy for this process is obtained from ATP, which comes from oxidative phosphorylation coupled to the electron transport chain (which uses oxygen as the final electron acceptor)..
The process of reducing molecular nitrogen to ammonia also reduces hydrogen in the proton form H+ to molecular hydrogen Htwo.
Many nitrogenase systems have coupled a hydrogen recycling system mediated by the hydrogenase enzyme. Nitrogen-fixing cyanobacteria couple photosynthesis to nitrogen fixation.
The nitrogenase enzyme complex has two components, component I, dinitrogenase with molybdenum and iron as cofactors (which we will call Mo-Fe-protein), and component II, dinitrogenase reductase with iron as cofactor (Fe-protein).
The electrons involved in the reaction are donated first to component II and later to component I, where the nitrogen reduction occurs..
For the transfer of electrons from II to I to occur, the Fe-protein is required to bind to a Mg-ATP at two active sites. This union generates a conformational change in the Fe-protein. An excess of oxygen can produce another unfavorable conformational change in the Fe-protein, since it cancels its electron accepting capacity.
This is why the nitrogenase enzyme complex is very susceptible to the presence of oxygen above tolerable concentrations and that some bacteria develop microaerophilic life forms or facultative anaerobiosis.
Among the free-living nitrogen-fixing bacteria, mention may be made of the chemotrophs belonging to the genera Clostridium, Desulfovibrio, Desulfotomaculum, Methanosarcina, and the phototrophs of the genres Chromatium, Thiopedia, Ectothiordospira, among other.
There are other nitrogen-fixing microorganisms that are capable of establishing symbiotic associations with plants, particularly with legumes and grasses, either in the form of ectosymbiosis (where the microorganism is located outside the plant), or endosymbiosis (where the microorganism lives within cells or intercellular spaces of the plant).
Most of the nitrogen fixed in terrestrial ecosystems comes from the symbiotic associations of bacteria of the genera Rhizobium, Bradyrhizobium, Sinorhizobium, Azorhizobium, Allorhizoium Y Mesorhizobium, with leguminous plants.
There are three interesting types of nitrogen-fixing symbioses: associative rhizocenoses, systems with cyanobacteria as symbionts, and mutualistic endorizobioses..
In associative rhizocenosis-like symbioses, specialized structures are not formed in the roots of plants.
Examples of this type of symbiosis are established between corn plants (Zea corn) and sugar cane (Saccharum officinarum) with Gluconacetobacter, Azoarcus, Azospirillum Y Herbaspirillum.
In rhizocenosis, the nitrogen-fixing bacteria uses the root exudate of the plant as a nutritive medium and colonizes the intercellular spaces of the root cortex..
In systems where cyanobacteria participate, these microorganisms have developed special mechanisms for the coexistence of anoxic nitrogen fixation and their oxygenic photosynthesis..
For example in Gleothece Y Synechococcus, they are temporarily separated: they carry out daytime photosynthesis and nighttime nitrogen fixation.
In other cases, there is spatial separation of both processes: nitrogen is fixed in groups of differentiated cells (heterocysts), where photosynthesis is not carried out.
Nitrogen-fixing symbiotic associations of cyanobacteria of the genus Nostoc with non-vascular plants (antóceras), as in cavities of Nothocerus endiviaefolius, with liverworts Gakstroemia magellanica and Chyloscyphus obvolutus in ectosymbiosis separately, with bryophytes (forming lichens in moss rhizoids), and with higher angiosperm plants, for example with the 65 perennial herbs of the genus Gunnnera.
For example, the nitrogen-fixing symbiotic association of cyanobacteria has been observed Anabaena with a bryophyte, non-vascular plant, on leaves of the small fern Azolla anabaenae.
As examples of endorhizobiosis, we can cite the association called actinorrhiza that is established between Frankia and some woody plants like casuarina (Casuarina cunninghamiana) Y alderAlnus glutinosa), and the association Rhizobium-legumes.
Most of the species in the family Leguminosae, form symbiotic associations with bacteria Rhizobium and this microorganismhas evolutionary specialization in giving nitrogen to the plant.
In the roots of plants associated with Rhizobium, the so-called radical nodules appear, the place where nitrogen fixation takes place.
In legumes Sesbania Y Aechynomene, additionally nodules are formed on the stems.
There is an exchange of chemical signals between the symbiote and the host. Plants have been found to exude certain types of flavonoids that induce the expression of nod genes in Rhizobium, which produce nodulation factors.
The nodulation factors generate modifications in the root hairs, formation of an infection channel and cell division in the root cortex, which promote the formation of the nodule.
Some examples of nitrogen-fixing symbiosis between higher plants and microorganisms are shown in the following table..
Mycorrhizobiosis
Additionally, in most ecosystems, there are nitrogen-fixing mycorrhizal fungi, belonging to the phyla Glomeromycota, Basidiomycota and Ascomycota.
Mycorrhizal fungi can live in ectosymbiosis, forming a hyphal sheath around the fine roots of some plants and spreading additional hyphae throughout the soil. Also in many tropical areas, plants host mycorrhizae in endosymbiosis, whose hyphae penetrate root cells.
It is possible that a fungus forms mycorrhizae with several plants simultaneously, in which case interrelationships are established between them; or that the mycorrhizal fungus is parasitized by a plant that does not photosynthesize, mycoheterotrophic, such as those of the genus Monotrope. Also several fungi can establish symbiosis with a single plant simultaneously.
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