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Nitrogen Fixation by Free Living Blue Green Algae General Aspects Heterocysts Algal Nitrogenase Ammonia Assimilation

Algal Nitrogenase

	Algal Nitrogenase Considerable progress has been made in studies on the nitrogenase enzyme obtained from cell-free extracts of nitrogen-fixing algae. The first report of nitrogenase activity came from preparations of Anabaena cylindrica. Subsequently, nitrogenase activity was shown in isolated heterocysts and also in extracts from non-heterocystous Plectonema. The soluble nitrogenase from both heterocystous and non-heterocystous algae appear very similar. One distinctive feature of algal nitrogenase is its high oxygen sensitivity, which can be overcome by exclusion of oxygen from the medium containing cell-free extracts.
Blue-green algae evolve oxygen during photosynthesis and obviously, a built-in mechanism must exist at the cellular level to protect the highly sensitive nitrogenase from oxygen. Nitrogenase enzyme from filamentous forms such as Plectonema is more sensitive to oxygen than that from heterocystous algae indicating the protective mechanism afforded, by the heterocyst which prevents the oxygen inactivation of the nitrogen-fixing system under aerobic conditions. However, it is not yet clear whether the nitrogenase enzyme is exclusively located in the heterocyst, since many Workers have also detected nitrogenase activity in normal vegetative ceils of heterocyst bearing algae.
Nitrogenase of BGA is very similar to that of other N₂, fixing bacteria. It catalyses the reduction of protons, cyanide and C₂H₂ besides N₂ The enzyme requires ATP, reductant and Mg²⁺ and is oxygen sensitive. As in other bacteria, the Fe-protein component binds to MgATP and transfers electrons to the MoFe protein which binds to the substrate and reduces it. The MoFe and Fe proteins of Anabaena cylindrica and Ptectonema boryanum are known to cross react with the Fe proteins of Azotobacter and Clostridum. The O₂ sensitivity of nitrogenase is overcome in serval ways, the chief among which is the structural modification of some of the vegetative cells into thick walled heterocysts.
Further the physiological environment in heterocysts is conducive for N₂ fixation, due to the absence of photosystem II, the lack of photosynthetic O₂ evolution and the presence of uptake hydrogenase coupled with high activity of oxidative pentose pathway. Nitrogen Fixation by Free-Living Blue-Green Algae In non-heterocystous forms, compartmentalization or separation of photosynthesis and N₂ fixation has been suggested from studies in Plectonema and Trichodesmium as a means to overcome O₂ sensitivity of nitrogenase. Other explanations include internal membrane systems to serve as intracellular O₂ protective mechanisms. Such hypotheses are merely speculative and need confirmation.
	In heterocystous BGA, the source of reductant to nitrogenase can come from the photosystem I-dependent reaction in light, from oxidative pentose pathway in light or dark and likely from glycolysis or Kreb’s cycle or both. Since the heterocysts lack ribulose biphosphate (RuBP) carboxylase and Photosystem II, they receive carbon from photosynthetic vegetative cells. Indeed the transfer of photosynthates from vegetative cells to heterocysts has been shown but the nature of substances translocated is not clear. Substrates such as maltose, Sucrose and erythrose have been suggested as likely sugars that are received by the heterocysts.

Fixation of N₂ in dark for limited periods can take place in heterocystous BGA and the Source of reductant to generate reduced ferredoxin in dark has to come from non photosynthetic means. No inactivation of the Ferredoxin-NADP²⁺ reductase in heterocysts of Anabaena cylindrica by light has b en demonstrated, suggesting that a light-independent flow of electrons from glucose-6-phophate to NADP²⁺ and then to ferredoxin takes place in light as well as in dark

The possibility of hydrogen as another source of reductant for the supply of electrons to nitrogenase via an uptake hydrogenase system has also been suggested.

The major source of ATP in light for N₂ fixation is cyclic photophosphorylation as shown in Anabaena cylindrica and Anabaenopsis circularis in the presence of adequate supply of fixed carbon. Non-cyclic photophosphorylation. Cannot be the Source of ATP since heterocysts lack photosystem-II. In dark, oxidative phosphorylation can supply ATP since under aerobic surroundings. In summary, the supply of ATP for N₂ fixation by BGA may come from photophosphorylation, cyclic photophosphorylation, oxidative phosphorylation or from substrate level phosphorylation, as mentioned above.

Nitrogenase in BGA functions as an ATP dependent hydrogenase to generate hydrogen which can be recycled back in the main nitrogenase reaction in strains possessing an uptake hydrogenase system, As in Rhizobium, the presence uptake hydrogenase in BGA is beneficial as it helps to enhance N₂ fixation.

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Genetics Importance in Rice Cultivation The Through Method of Algal Production

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