The nitrogen, with symbol N and atomic number 7, is an essential chemical element for life on earth. Without nitrogen there is no amino acidsand without amino acids there is no proteins. Without nitrogen there are also no nitrogenous bases, and without nitrogenous bases there are no DNA neither RNA.
The abundance of nitrogen on Earth is not a problem. About 78% of the Earth's atmosphere is molecular nitrogen (Ntwo), but this form of inorganic nitrogen is only assimilable by nitrogen fixing organismsalso known as diazotrophs.
All known diazotrophic organisms are bacteriabut they do not form a taxonomic group with an evolutionary relationship between them, they only share the ability to use molecular nitrogen from the air and transform it into nitrogenous substances assimilable by other organismsfor example by plants, fungi and other bacteria.
Nitrogen-fixing bacteria, therefore, are those that introduce nitrogen into the food chain of an ecosystem. Its fixing activity is more efficient at low oxygen concentrations, a condition that usually occurs in the soil.
Nitrogen fixation
The nitrogen fixation is the set of processes that transform the atmospheric nitrogen (Ntwo) in nitrates (NO3–) assimilable by plants. Denitrification, also carried out by bacteria, is the opposite process in which nitrates are converted into gaseous nitrogen and returned to the atmosphere.
With nitrogen fixation and denitrification, the so-called cycle of nitrogenone of the most important biogeochemical cycles.
Nitrogen fixation occurs in two steps, ammonification and nitrification:
- ammonification: Nitrogen-fixing bacteria in the soil transform atmospheric nitrogen into ammonium (NH4+). Ammonium can also come from decomposing organisms, such as saprotrophs, which break down the remains of dead plants and animals.
- Nitrification: another type of bacteria, the nitrifying bacteriaoxidize ammonium and form nitrites (NOtwo–) and nitrates (NO3–).
Plants are capable of efficiently absorbing nitrates and ammonium, and use this nitrogen to synthesize amino acids, nitrogenous bases and other nitrogenous substances, thus leaving the nitrogen fixed in organic matter. Plants can also absorb small amounts of other nitrogenous substances, such as urea and certain amino acids.
Diazotrophic organisms are able to fix nitrogen thanks to the nitrogenasean enzyme exclusive to prokaryotic organisms that catalyzes the following reaction:
Ntwo + 16 ATP + 10 H+ + 8e– → 2NH4+ + Htwo + 16 Pi + 16 ADP
Once the nitrogen has become part of the plants, it will pass to animal life through herbivores, and from them to carnivores. When these living things die, decomposers transform organic nitrogen back into ammonium and nitrates.
When there is an excess of organic matter in the soil and anaerobic conditions exist, denitrifying bacteria transform nitrates into molecular nitrogen, which is returned back to the atmosphere.
Nitrogen-fixing bacteria also live in seas and oceans. The process is similar. Some of the gaseous nitrogen in the atmosphere dissolves in water, where fixing bacteria transform it into ammonium (ammonification).
At depth (below the euphotic zone), ammonium is transformed into nitrates (nitrification) and nitrates are assimilated by phytoplankton, thus entering the food chain of marine habitats and ecosystems.
Types of Nitrogen Fixing Bacteria
Nitrogenase is an enzyme complex that is very sensitive to oxygen. Nitrogen-fixing organisms typically require anaerobic conditionsbut there are many diazotrophic organisms that have developed mechanisms to counteract the effect of oxygen on nitrogenase and can also fix nitrogen under aerobic conditions.
Diazotrophic organisms or nitrogen fixers are usually studied in two large groups:
- symbiotic diazotrophs
- free-living diazotrophs
Symbiotic diazotrophs
The symbiotic nitrogen fixing bacteria They have a symbiotic way of life with plants. Plants offer food to bacteria and bacteria offer nitrogen to plants.
In general, the nitrogen fixation rate of symbiotic diazotrophs is 2-3 times higher than that of free-living diazotrophs.
- Rhizobia (order Rhizobiales): symbiotic bacteria in the roots of leguminous plants (clover, alfalfa, pea, etc.), where they form characteristic nodules. One of the most characteristic genera is the genus rhizobiumwhich is sometimes inoculated into soils to increase crop yields.
- frankies: the only known species of the family Frankiaceae it is frankia alni. This bacterium, like rhizobia, forms nodules on plants but can infect various plants, such as alder trees (genus Alnus), Australian pine (Casuarina equisetifolia) or gender Dryas (of the family Rosaceae).
- Cyanobacteria: there are some cyanobacteria capable of fixing nitrogen and that live in symbiosis with other organisms: with fungi forming lichens or with liver plants, ferns and cycads. Example: gender Anabaena live in symbiosis with Azolla (aquatic fern) and is used as fertilizer in rice crops.
- symbiosis with animals: Some nitrogen-fixing bacteria have been found in symbiosis with animals, such as termites and woodworms, although their contribution to the nitrogen cycle appears to be very low.
free-living diazotrophs
Free-living nitrogen-fixing organisms live independently in the environment. Among them there are anaerobes, facultative anaerobes and aerobes, as well as some photosynthetic cyanobacteria.
- Anaerobes: Nitrogen-fixing bacteria that do not tolerate the presence of oxygen. They live in habitats with little or no oxygen, such as soils and thick mats of dead plant matter. For example, Clostridium (I usually), Desulfovibrio (ocean bottoms) or methanogenic archaea.
- facultative anaerobes: they can live with or without oxygen, although they only fix nitrogen in anaerobic conditions. They often consume oxygen as soon as it is available, causing them to maintain low-oxygen environments. Examples: Klebsiella, Bacillus.
- aerobes: are nitrogen-fixing bacteria that need oxygen to live. Nitrogenase is very sensitive to oxygen, so these organisms have had to develop nitrogenase protection mechanisms so that it can function under aerobic conditions. One of the most studied aerobic nitrogen-fixing bacteria is Azotobacter vinelandii.
- photosynthetic: Cyanobacteria are bacteria that carry out oxygenic photosynthesis and generate molecular oxygen. Still, certain cyanobacteria are capable of fixing nitrogen. Some, like gender Pletonema, they only fix nitrogen in conditions of low light and low oxygen concentration. Others, like Anabaena cylindrica either Nostoc communeform colonies in which there are individuals specialized in nitrogen fixation (heterocysts); in these individuals the steps of photosynthesis that produce oxygen do not occur.