Index >> Nitrogen Fixation by Free Living Blue-Green Alage >> Hetercysts

	Heterocysts Heterocysts are large, thick-walled, apparently empty cells appearing amidst normal pigmented cells. However, when viewed through an electron microscope, they seem to have .a complex lamellar network. A large body of evidence has accumulated showing that heterocyst are the sites of nitrogen fixation and recent reports provide further direct evidence of nitrogenase activity in preparations made from isolated heterocysts.
A mature heterocyst is surrounded by a multilayered envelope and shows an elaborate cytoplasmic membrane system devoid of granular inclusions. Heterocysts provide a congenial environment for the effective functioning of nitrogenase, to generate energy and reductant required for nitrogen fixation, to bring the nitrogen fixed into organic combination and to maintain a dual transport system for getting carbon and sending out nitrogen into the vegetative cells. A - Chroococcus B-Gloeocapsa sp C - Gloeothece sp   D - Microcystis spp E - Nostoc sp F - Anabaena spp G - Cylindrospermum Sp H - Scytonema sp
Nitrogen-fixing genera of blue-green algae Unicellular Synechococcus, Gloeocapsa (Gleothece), Apheanothece, Dermocarpa, Xenoccoccus, Myxosarcina, Chroococcidiposis, Pleurocapsa group Filamentous, non-heterocystous Plectonema boryanum, Lyngbya, Trichodesmium, Oscillatoria, Pseudoanabaena, Microcoleus, Schizothrix, LPP group Filamentous, heterocystous Anabaena, Nostoc, Nodularia, Cylindrospermum, Scytonema, Calothrix, Anabacenopsis, Mastigocladus, Fischerella, Tolypothrix, Aulosira, Stigonema, Hapalosiphon, Chlorogloeopsis, Camptylonema, Gloeotrichia, Nostochopsis, Rivularia, Scytonematopsis, Westiella, Westiellopsis, Wollea, Chlorogloea
The transition or metamorphosis of a vegetative cell of a blue-green alga into a heterocyst is a gradual process from a CO2 fixing and O2 evolving cell into an anaerobic cell conducive for active nitrogen fixation. The heterocyst has a multilayered cell wall connected by cytoplasmic bridges to neighbouring photosynthetic vegetative cells. These bridges regulate the flow of molecules between the two types of cell and therefore a series of regulated physiological and biochemical changes led to nitrogen fixation and assimilation.
	Experiments carried out with Anabaena variabilis have shown that under light, nitrogenase activity in isolated heterocysts is dependent on a supply of H2 whereas in dark the activity is dependent on a supply of O2 and H2. The principal product of fixation of nitrogen which is translocated from heterocysts to vegetative cells is glutamine. The formation of glutamine in heterocysts is dependent on the transfer of glutamate from vegetative cells. These conclusions have been made by conducting experi­ments using N13 labelled nitrogen. A disaccharide probably maltose, a product of photosynthesis, moves from vegetative cells to the heterocysts where it is metabolized to glucose 6 phosphate and oxidized by the oxidative pentose pathway. It is believed that pyridine nucleotide (NADPH) reduced by this pathway can combine with O2and thus provide conducive environment for the reduction of electron carrier ferredoxin.

Heterocysts lack photosystem II activity but photosystem I which is present can also reduce ferredoxin. The reduced ferredoxin can donate electrons to the nitrogenase which reduces N₂ to NH₄⁺ as well as release H₂. Uptake hydrogenase, capable of recycling H₂ is present only in heterocysts. Heterocysts have high levels of glutamine synthetase (GS) and low levels of glutamine oxoglutarate amido transferase (GOGAT). Glutamate formed in vegetative cells and which gets transferred to the heterocysts reacts with NH₄⁺ to form glutamine. The latter moves into the vegetative cells where it reacts with alphaketoglutarate (alpha KG) to provide 2 molecules of glutamine. It is believed that glutamine or a metabolite of glutamine other than NH₄⁺ is a component of the system that represses heterocysts differentiation. These results have cone from the work of work and colleagues of the Michigan State University, East Lansing, Michigan and Haselkorn of the Chicago University, USA and schematically reproduced