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Dr. Mohammad Asadul Haque Associate Professor Department of Soil Science, PSTU Nitrogen fixation in nonlegume plants Nitrogen Fixation by Free-living Blue Green Algae Characteristics of BGA Blue green algae are unicellular, colonial or filamentous Filamentous BGA consist of many cells arranged one after another like a chain. Some of these filaments form special types of cells known as heterocyst and the organisms are called heterocystous BGA. Heterocysts are associated with N2 fixation. Heterocysts are large, thick walled, apparently empty cells and usually yellow in color. A mature heterocyst is surrounded by a multilayered envelope and connected by cytoplasmic bridges to neighboring photosynthetic vegetative cells. These bridges regulate the flow of molecules between two types of cells and therefore a series of physiological and biochemical changes lead to nitrogen fixation and assimilation. Heterocysts may terminal, intercalary and lateral Fig. Heterocystous BGA Heterocysts derive some reductant such as glucose 6 phosphate, pyruvate from photosynthetic vegetative cells to reduce atmospheric nitrogen to fixed nitrogen. The vegetative cells depend on heterocysts for nitrogen nutrition in the form of glutamine, glutamate and other amino acids. Dr. Mohammad Asadul Haque Associate Professor Department of Soil Science, PSTU Functions of heterocysts 1. Site of nitrogen fixation. BGA can fix N2 with the help of nitrogenase enzyme which is located in heterocysts of heterocystous BGA. 2. Heterocysts protect the nitrogenase enzyme from damage by oxygen. Nitrogenase enzyme is very sensitive to oxygen. It is quickly inactivated by oxygen, but heterocysts possess the reducing environment and there no photosynthesis takes place and does not evolve oxygen. 3. Heterocysts maintain the protoplasmic connection with vegetative cells with the help of cytoplasmic bridge situated at one or both ends. 4. They regulate spore formation. 5. Heterocysts along function as reproductive cell. List of cyanobacteria A. Heterocystous N2 fixing BGA Anabaena, Anabaenopsis, Aulosira, Chlorogloea, Cylindrospermum, Nostoc, Calothrix, Scytonema, Tolypothrix, Fischerella, Hapalosiphon, Mastigocladus, Stigonema and Westiellopsis B. Non-heterocystous N2 fixing BGA Plectonema, Trichodesmium, Lyngbya and Oscillatoria