All known organisms require nitrogen for completion of the life cycle. The term global nitrogen economy describes the movement of this valuable resource through the various systems, living and non-living, in which it is contained. Although fixed nitrogen production by symbiotic cyanobacteria in association with plants contributes only slightly to the global pool of fixed nitrogen, in areas where nitrogen is severely limited they may form the dominant vegetation. Here they are invaluable in providing otherwise unavailable nitrogen by the eventual addition of their organic matter to the soil. Nitrogen fixation by cyanobacteria in symbiosis with cycads occurs at higher rates than observed in free-living forms. Rates of fixation on the order of 19kg N/ha/yr have been observed in natural populations in Australia
Measurements made by
Liu et al. (1993)
in China averaged 1.8 to 11.1 micromoles acetylene/g fresh weight during autumn. Cyanobacterial symbionts are excellent colonizers of nitrogen poor soils and, through their nitrogen input into the environment, they may help create habitats suitable for other species
(Rai et al., 2000).
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The enzyme Nitrogenase is composed of two iron-sulfur proteins, one of which requires molybdenum as a cofactor. In addition, ATP, Magnesium ions, ferredoxin and protons are required for catalytic activity. ATP provides energy by hydrolysis of high energy phosphate bonds. Magnesium ions are required as a cofactor for ATP hydrolysis. Ferredoxin acts as an electron donor, and protons are required to add hydrogen to nitrogen, forming ammonia. Electrons carried by ferredoxin are provided by NADPH. Recent discoveries have shown that there is not just one, but a family of nitrogenases, some of which could use Vanadium as a cofactor instead of Molybdenum
(Dilworth and Glenn, 1991).
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As in non-symbiotic cyanobacteria, nitrogenase is contained within the heterocysts of cycad symbionts, which occur at an increased frequency in symbiosis. In coralloid roots of cycads, heterocysts are most numerous nearest to the base of the root where they exist in multiple heterocyst complexes. Heterocysts nearest to the growing tip are located singly among vegetative cells or at the ends of filaments.
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After fixation to ammonium, nitrogen is
by various mechanisms, depending on the organism. Cyanobacteria and plants both utilize an enzyme system known as GS/GOGAT After being fixed from atmospheric nitrogen, ammonium is carried by a series of ammonium transport systems to the enzyme system. Here it is initially incorporated into the amide position of glutamine by glutamine synthetase (L-glutamate:ammonia ligase, ADP forming; also called GS). Next, the amide group of glutamine is transferred to the alpha position of alpha-ketoglutarate, yielding glutamate. This reaction is catalyzed by glutamate synthase (L-glutamine:ferredoxin oxidoreductase, transaminating), otherwise known as GOGAT. GS is found in both heterocysts and vegetative cells. GOGAT is most likely confined to vegetative cells
(Guerrero and Lara, 1987).