Nitrogen fixation is a chemical process by which molecular nitrogen (N
2), which has a strong triple covalent bond, is converted into ammonia (NH
3) or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. The nitrogen in air is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or diazotrophy is an important microbe-mediated process that converts dinitrogen (N2) gas to ammonia (NH3) using the nitrogenase protein complex (Nif).
Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia.
Klebsiella is a genus of Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule.
Azotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is inaccessible to plants, and releasing it in the form of ammonium ions into the soil. In addition to being a model organism for studying diazotrophs, it is used by humans for the production of biofertilizers, food additives, and some biopolymers. The first representative of the genus, Azotobacter chroococcum, was discovered and described in 1901 by Dutch microbiologist and botanist Martinus Beijerinck. Azotobacter species are Gram-negative bacteria found in neutral and alkaline soils, in water, and in association with some plants.
Microbial inoculants also known as soil inoculants or bioinoculants are agricultural amendments that use beneficial rhizosphericic or endophytic microbes to promote plant health. Many of the microbes involved form symbiotic relationships with the target crops where both parties benefit (mutualism). While microbial inoculants are applied to improve plant nutrition, they can also be used to promote plant growth by stimulating plant hormone production. Although bacterial and fungal inoculants are common, inoculation with archaea to promote plant growth is being increasingly studied.
Herbaspirillum seropedicae is a betaproteobacteria which is an endophytic diazotroph and forms nitrogen-fixing associations with maize, rice, sorghum, sugar cane, bananas (Musa) and pineapple. H. seropedicae is a potential nitrogen biofertilizer. Studies have shown that rice with H. seropedicae inoculated increases the yield to an equivalent of 40 kg N/ha.
Herbaspirillum autotrophicum is a bacterium which cannot fix nitrogen under laboratory conditions, like Herbaspirillum seropedicae, because it does not have the Nif gene.
Herbaspirillum frisingense is a nitrogen-fixing bacterium which was found in C4-fibre plants like prairie cordgrass, Chinese silver grass,, Amur silver-grass, and Napier grass. The specific name frisingense comes from Freising, a town in Germany where H. frisingense was first isolated from prairie cordgrass and Miscanthus plants.
Herbaspirillum lusitanum is a nitrogen-fixing bacterium found in root nodules of common beans. Phylogenetic analyses have shown this bacterium belongs to the genus Herbaspirillum. H. lusitanum lacks the nif gene. A nodD-like gene is present, but no other nod genes have been identified. The lack of nif and nod genes suggests H. lusitanum is an opportunistic bacterium capable of colonizing root nodules, but is unable to fix nitrogen.
Herbaspirillum rhizosphaerae is a Gram-negative mesophilic bacterium of the genus Herbaspirillum isolated from rhizosphere soil of Allium victorialis var. platyphyllum on Ulleung Island in Korea.
Herbaspirillum aquaticum is a bacterium of the genus Herbaspirillum.
Gluconacetobacter sacchari is a species of acetic acid bacteria first isolated from the leaf sheath of sugar cane and from the pink sugar-cane mealy bug on sugar cane growing in Queensland and northern New South Wales. The type strain of this species is strain SRI 1794T. It is notable for its production of bacterial cellulose and for being an endophyte in sugar cane.
Azospirillum brasilense is a well studied, nitrogen-fixing (diazotroph), genetically tractable, Gram-negative, alpha-proteobacterium bacterium, first described in Brazil by the group of Johanna Döbereiner and then receiving the name "brasilense". A. brasilense is able to fix nitrogen in the presence of low oxygen levels, making it a microaerobic diazotroph. An isolate from the genus Azospirillum was isolated from nitrogen poor soils in the Netherlands in 1925, however the species A. brasilense was first described in 1978 in Brazil, since this genus is widely found in the rhizospheres of grasses around the world where it confers plant growth promotion. Whether growth promotion occurs through direct nitrogen flux from the bacteria to the plant or through hormone regulation is debated. The two most commonly studied strains are Sp7 and Sp245, both are Brazilian isolates isolated from Tropical grasses from Seropedica, Brazil.
Azospirillum doebereinerae is a species of nitrogen-fixing bacteria associated with the roots of Miscanthus species. Its type strain is GSF71T.
Methylobacterium nodulans is an aerobic, facultatively methylotrophic, legume root nodule-forming and nitrogen-fixing bacteria.
Beijerinckia derxii is a nitrogen fixing strain of bacteria from the genus of Beijerinckia.
Bradyrhizobium oligotrophicum is a nitrogen-fixing bacteria from the genus of Bradyrhizobium which was isolated from rice paddy soil in Miyagi Prefecture in Japan.
Azospirillum is a Gram-negative, microaerophilic, non-fermentative and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae. Azospirillum bacteria can promote plant growth.
Bradyrhizobium stylosanthis is a nitrogen-fixing bacterium from the genus Bradyrhizobium which has been isolated from the nodules of the plant Stylosanthes guianensis.
Gluconacetobacter diazotrophicus is a bacterium with a rod-like shape, has rounded ends and belongs to Gram-negative bacteria. The bacterium is known for stimulating plant growth and being tolerant to acetic acid. With one to three lateral flagella, and known to be found on sugarcane, Gluconacetobacter diazotrophicus was discovered in Brazil by Vladimir A. Cavalcante and Johanna Dobereiner.
