Nitrogen Fixation

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Nitrogen Fixation


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'Introduction N2 are found 78.084% on atmosphere of earff. N2 are found in N2 ( N N) form in soil. x Dinitrogen are don't more active in chemically form. x Dinitrogen is more stable, so we of nitrogen fixation. x Nitrogen fixaäon is a procßs by whiå nitrogen of the Eardl's atrnosphere is converted into ammonia (NH3), nitrogen or other available living organisms.

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Biological nitrogen tion Fixation of atmospheric Nitrogen into nitrogenous salts with the help of micro-organisms. Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. Biological Nitrogen fixation in all micm-organism are prokaryotes. They micro-organism are called is diazotrophs.

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Some nitrogen fixing organisms

Free living aerobic bacteria Azotobacter Beijerinckia Klebsiella Cyanobacteria (lichens) Free living anaerobic bacteria Clostridium Desulfovibrio Purple sulphur bacteria Purple non-sulphur bacteria Green sulphur bacteria

Free living associative bacteria Azospirillum Symbionts Rhizobium (legumes) Frankia (alden trees)

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Nitrogen Fixation

The growth of all organisms depend on the availability of Nitrogen (e.g. amino acids) Nitrogen in the form of Dinitrogen (N 2 ) makes up 78% of the air we breathe but is essentially inert due to the triple bond (N  N) In order for nitrogen to be used for growth it must be "fixed" (combined) in the form of ammonium (NH 4 ) or nitrate (NO 3 ) ions.

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Mechanism of symbiotic N 2 -fixation :

1 . N 2 -fixation is essentially anaerobic process. 2.The oxygen supply to bacteriod is excluded due to presence of leghaemoglobin around it. 3. The quantum of N 2 -fixed is closely related to the amount of legheamoglobin and the extent of bacteriod tissues in nodules. 4.Reduction of N 2 to NH 3 is mediated through enzyme ` nitrogenase ’. 5.The first stable intermediate in N 2 -fixation is ammonia. 6.This nitrogen is then converted into amino acids and proteins thereby plants are benefited.

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Formation of a Root Nodule to Bact«ia "Ito cells

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Rhizobium Root Nodules


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Infection Process

Attachment Root hair curling Localized cell wall degradation Infection thread Cortical cell differentiation Rhizobia released into cytoplasm Bacterioid differentiation (symbiosome formation) Induction of nodulins

CSS 455 Nitrogen04

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B.Nitrogen Fixation Cont.

All nitrogen fixing bacteria use highly conserved enzyme complex called Nitrogenase Nitrogenase is composed of two subunits: an iron-sulfur protein and a molybdenum-iron-sulfur protein Aerobic organisms face special challenges to nitrogen fixation because nitrogenase is inactivated when oxygen reacts with the iron component of the proteins

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Rhizobium species Cross inoculation group Genus R. trifoli Clover group Trifolium R. meliloti Alfalfa group Melilotus, Meticago R. phaseoli Bean group Phaseolus R. lupini Lupine group Lupinous R. leguminosarum Pea group Pisum R. japonicum Soybean group Glycine R. species Cowpea Vigna

Cross inoculation group of Rhizobium