Nitrogen fixation symbionts

Only certain prokaryotes can fix atmospheric nitrogen. These include the cyanobacteria of soils and fresh and salt waters, other kinds of free-living soil bacteria such as Azotobacter species, and the nitrogen-fixing bacteria that live as symbionts in the root nodules of leguminous plants. The first important product of nitrogen fixation is ammonia, which can be used by all organisms either directly or after its conversion to other soluble compounds such as nitrites, nitrates, or amino acids. 

Terms in bold are defined in the nitrogen cycle 834 nitrogen fixation 834 symbionts 834 nitrogenase complex 835 leghemoglobin 836 glutamine synthetase 838 glutamate synthase 838 glutamine amidotransferases 840 5-phosphoribosyl-l-... 

Nitrogen fixation has also been reported from marine shipworms. Shipworms are bivalves, which live attached to wooden ships, in which they bore holes, and thus have a diet of wood alone. Cellulose is the principal component of wood, and is indigestible to animals. Certain bacterial species, however, contain the necessary enzymes to break down cellulose, and shipworms are often reported with gut associated bacterial symbionts. 

Villareal, T. A. (1991). Nitrogen-fixation by the cyanobacteria symbiont of the diatom genus Hemi-aulus. Mar. Ecol. Prog. Ser. 76, 201—204. 

Schubert, K. R., and H. J. Evans (1976). Hydrogen evolution a major factor affecting the efficiency of nitrogen fixation in modulated symbionts. Proc. Nat. Acad. Sci. U.S. 73, 1207-1211. 

The temi symbioses was first defined loosely by De Bary (1879) as two or more difFerendy named organisms bving together. Although symbiotic interactions are ubiquitous in nature, few of the marine planktonic systems have been well characterized, and comparatively less is known of the functional role of the symbiont for the host and vice versa. Many of the planktonic symbioses are between eukaryotic hosts and cyanobacterial symbionts, or cyanobionts. Cyanobacteria are photosynthetic, and many are capable of nitrogen (N2) fixation, thus often it is presumed... 

In symbiotic systems, like these where the association appears quite intimate or the symbiont population occupies a majority of the host cell volume, the relationship is assumed necessary (Douglas, 1998) and/or beneficial. The benefit of the DDA relationship is not fuUy understood nor characterized, and because N2 fixation has been measured when the DDAs are present, it is presumed that some of the nitrogen fixed by the symbiont is transferred to the host diatom. To date, there are only a few studies that have attempted to understand the nature of the symbiosis between the Richelia symbiont and the host diatom. 

The fixation of nitrogen by the symbionts legume — Rhizobium is of tremendous agricultural importance. Average amounts of nitrogen added to the soil by a leguminous crop are frequently between 120 and 600 kg N ha ... 

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