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Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses and aquatic ferns such as Azolla. [4] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. [5]
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant cells to form root nodules, where they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase.
Sulphate-reducing bacteria are important in ocean sediments (e.g. Desulfovibrio), and some Archean methanogens, like Methanococcus, fix nitrogen in muds, animal intestines [3] and anoxic soils. [4] Facultative anaerobes—these species can grow either with or without oxygen, but they only fix nitrogen anaerobically.
To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. [1] In general, they are gram negative, motile, non-sporulating rods. Rhizobia are a "group of soil bacteria that infect the roots of legumes to form root nodules". [2]
Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria convert nitrogen gas (N 2) to ammonia (NH 3) in a process called nitrogen fixation.
The host plant provides the bacteria with amino acids so they do not need to assimilate ammonia. [5] The amino acids are then shuttled back to the plant with newly fixed nitrogen. Nitrogenase is an enzyme involved in nitrogen fixation and requires anaerobic conditions. Membranes within root nodules are able to provide these conditions.
Bradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle . Plants cannot use atmospheric nitrogen (N 2 ); they must use nitrogen compounds such as nitrates .
Azotobacter species are free-living, nitrogen-fixing bacteria; in contrast to Rhizobium species, they normally fix molecular nitrogen from the atmosphere without symbiotic relations with plants, although some Azotobacter species are associated with plants. [38]