Nitrogen Fixing Bacteria

	Back to Home
Nitrogen Fixation in Free-Living and Associative Symbiotic Bacteria Nitrogen Fixing Bacteria Nitrogenase Genetics Mechanism of Nitrogen Fixation Response of Plants to Azotobacter Inoculation Nitrogen Fixation in the Root Zone of Rice

Nitrogen Fixing Bacteria

Nitrogen fixing Bacteria
The free-living bacteria having the ability to fix molecular nitrogen can be distinguished into obligate aerobic, facultative aerobic and anaerobic organisms. Obligate aerobic bacteria belong to the genera Azotobacter, Beijerinckia, Derxia, Achromobacter, Mycobacterium, Arthrobacter and Bacillus.

Among the facultative anaerobic bacteria are the genera Aerobacter, Klebsiella and Pseudomonas. Anaerobic nitrogen-fixing bacteria are represented by the genera Clostridium, Chlorobium, Chromatium, Rhodomicrobium, Rhodopseudomonas, Rhodospirillum, Desulfovibrio and Methanobacterium.

In some of these genera, nitrogen fixation takes place in a photoautotrophic manner by virtue of the presence in them of photosynthetic pigments as exemplified by the well-known genus Rhodopseudomonas. On the other hand, the genus Desulfovibrio fixes nitrogen in the process of reducing sulphates.

Bacteria of the family Azotobacteraceae constitute the majority of heterotrophic free-living nitrogen-fixing bacteria. They are grouped into three genera-Azotobacter, Beijerinckia and Derxia. Several species of Azotobacter are recognized as-A. chroococcum, mainly occurring in neutral or alkaline soils; A. agilis, an aquatic species; A. vinelandii and A. beijerinckii originally isolated from North American soils: A. insignis, isolated from Indonesian water samples; A. macrocytogenes isolated from Danish soils; and A.paspali from the rhizosphere of Paspalum spp. originally isolated from Brazilian soils

While the genus Beijerinckia has three species-B. indica (the earlier Azotobacter indicum), B. mobile and B. fluminensis, the genus Derxia has only one, D. gummosa.

Cell-size, flagellation, pigmentation and production of extra-cellular slime are considered as diagnostic features of these bacteria in distinguishing species, which could be summarized as follows:

A. Nitrogen Fixing Beijerinckia and Azotobacter	B. Beijerinckia colonies showing profuse gum formation	C. Foure different strains of A. Chroococcum showing melanin pigment	D. Azotobacter cells magnified	E. A. Chroococcum inoculation increases the growth of barley plants in pots	F. Beijerinckia cells magnified under a phase constrat microscope

Azotobacter chroococcum (peritrichous flagella, moderate slime and black-brown insoluble pigment): A. vinelandii, A. paspali and A. agilis (peritrichous flagella, little to moderate slime and green, fluorescent and soluble pigment) .

polar flagella, abundant slime and pink soluble pigment); Beijerinckia indica (peritrichous flagella, abundant slime and light rust-brown insoluble pigment) and Derxia gummosa (polar flagellum, abundant
slime and yellow-brown pigment).

Among the different nitrogen fixing bacteria, Clostridium pasteurianum and Azotobacter (a. chroococcum and A. vineland (ii) are the most intensively investigated genera. Earlier evidences mainly pointed out the ability of these bacteria to fix atmospheric nitrogen when cultured on a nitrogen-free medium. The amount of nitrogen fixed is usually estimated by the well-known Kjeldahl method.

The use of 15N tracer and acetylene reduction method have however enriched our knowledge regarding the biochemical pathway between N₂ and NH₃ but the exact nature of intermediate products have eluded even critical investigators. Nevertheless, the overall reaction in the enzymic reduction of atmospheric nitrogen to ammonia could be postulated as follows:

Nitrogen Fixing Bacteria

	Back to Home
Nitrogen Fixing Activity of Paddy Soil in Pots under Various Treatments Nitrogen Fixation in the Rhizosphere of Grasses and Weeds Associative Symbiosis Some New Nitrogen Fixing Bacterial Associations Several Nitrogen Fixing Bacteria Response of Plants Azospirillum Inoculation The Concept of C4 Dicarboxylic Acid Pathway Nitrogen Fixation by Rhizobium in a Free Living State

Home | Site map | Submit Article | Directory | Search