Nitrogen fixation

[4] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots.

As part of the nitrogen cycle, it is essential for soil fertility and the growth of terrestrial and semiaquatic vegetations, upon which all consumers of those ecosystems rely for biomass.

Nitrogen fixation is thus crucial to the food security of human societies in sustaining agricultural yields (especially staple crops), livestock feeds (forage or fodder) and fishery (both wild and farmed) harvests.

It is also indirectly relevant to the manufacture of all nitrogenous industrial products, which include fertilizers, pharmaceuticals, textiles, dyes and explosives.

[11] "The protracted investigations of the relation of plants to the acquisition of nitrogen begun by de Saussure, Ville, Lawes, Gilbert and others, and culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887.

The mechanism proceeds via a series of protonation and reduction steps wherein the FeMoco active site hydrogenates the N2 substrate.

[41] Nitrogenase has three different forms (Nif, Anf, and Vnf) that correspond with the metal found in the active site of the protein (Molybdenum, Iron, and Vanadium respectively).

[47] Cyanobacteria, commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere.

In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as nitrate, nitrite, ammonium, urea, or some amino acids.

Several cyanobacteria strains are also capable of diazotrophic growth, an ability that may have been present in their last common ancestor in the Archean eon.

[48] Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine.

[52] Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen.

[53] Species of nitrogen fixing cyanobacteria in fresh waters include: Aphanizomenon and Dolichospermum (previously Anabaena).

[55][56] One type of organelle, originating from cyanobacterial endosymbionts called UCYN-A2,[57][58] can turn nitrogen gas into a biologically available form.

[63] Plants that contribute to nitrogen fixation include those of the legume family—Fabaceae— with taxa such as kudzu, clover, soybean, alfalfa, lupin, peanut and rooibos.

[66] The ability to fix nitrogen in nodules is present in actinorhizal plants such as alder and bayberry, with the help of Frankia bacteria.

They are found in 25 genera in the orders Cucurbitales, Fagales and Rosales, which together with the Fabales form a nitrogen-fixing clade of eurosids.

Fabales were the first lineage to branch off this nitrogen-fixing clade; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and physiological requirements were present in an incipient state in the most recent common ancestors of all these plants, but only evolved to full function in some of them.

[67] In addition, Trema (Parasponia), a tropical genus in the family Cannabaceae, is unusually able to interact with rhizobia and form nitrogen-fixing nodules.

[78] Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of lowering energy requirements.

[81] Lightning produces enough energy and heat to break this bond[81] allowing nitrogen atoms to react with oxygen, forming NOx.