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  Section: General Biotechnology / Plant Biotechnology
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Biological Nitrogen Fixation


Non-Symbiotic Nitrogen Fixation

Microorganisms which pass independent life and fix atmospheric nitrogen are known as free living diazotrophs. There are two groups of such micro-organisms : bacteria and cyanobacteria (blue-green algae). Based on the mode of nutrition (carbon, nitrogen and oxygen and requirement of reducing groups) bacteria are divided into (i) aerobic bacteria (Azonomas, Azotobacter, Beijerinckia, Mycobacterium, Methylomonas), (ii) facultative anaerobic bacteria (Bacillus,  Enterobacter, Klebsiella,  etc.), (iii) anaerobic bacteria (Clostridium, Desulfovibrio, etc.) and (iv) photosynthetic bacteria (Rhodomicrobium, Rhodopseudomonas, Rhodospirillum, Chromatium, Chlorobium, etc.).

Among cyanobacteria, both heterocystous and nonheterocystous forms fix atmospheric nitrogen, for example, Anabaena, Anabaenopsis, Aulosira, Calothrix, Cylindrospermum, Gloeocapsa, Lyngbya, Nostoc, Oscillatoria, Plectonema, Scytonema, Stegonema, Tolypothrix, Trichodesmium, etc. (Alexander, 1977).



Non-Symbiotic N2 fixation




Ecology of diazotrophs


Special features of diazotrophs



Sites of N2 fixation



Nitrogenase and reductants



Presence of hydrogenase



Self regulatory systems


Mechanism of N2 fixation

Symbiotic N2 fixation


Establishment of symbiosis



Host specificity and root hair curling



Infection of root hairs



Nodule development



Nodule development and maintenance


Factors affecting nodule development


Mechanism of N2 fixation in root nodules

Genetics of diazotrophs


Nod genes


Nif genes



Nif gene cloning


Hup genes


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