GENERAL MICROBIOLOGY - T.P. Pyroh - 2004

17. MOLECULAR NITROGEN FIXATION

17.2. NITROGEN FIXATION BY FREE-LIVING BACTERIA

The ability to fix nitrogen has been found in numerous Bacteria (anoxygenic phototrophs, cyanobacteria, facultative anaerobes, chemolithoautotrophs, methylotrophs, sulfate reducers, and methanogens).

Species of the genus Azotobacter are particularly active nitrogen fixers (fixing about 20 mg of nitrogen per 1 g of consumed sugar). Several Azotobacter species are distinguished, inhabiting various environments (table).

Class="center">Representatives of the Azotobacter group and their habitats

Species

Cell size

and Morphology

Colony characteristics

Habitat

Azotobacter chroococcum

3,1 х 2 мкм; mostly in pairs

Mucilaginous, dark-pigmented; may contain cysts

Soil

Azotobacter vinelandii

3,4 х 1,5 мкм; mostly in pairs

Large, mucilaginous; produce a yellow pigment with green fluorescence; may contain cysts

Soil and Water

Azotobacter paepall

2 мкм; highly pleomorphic

Pigment, яку Azotobac tervinelandil; may contain cysts

Soil, ROOT surface of Раsраlum notatum

Azotomonae agilis

3,3 х 2,8 мкм; single or in pairs

Produce a yellow pigment with white fluorescence

Soil and water

Beljerincbia іndica

2 мкм

Large, mucilaginous, colourless

Acidic tropical soils (pH 4,5)

Bacteria of the genus Azotobacter are Gram-negative, relatively large, strictly aerobic, and under certain conditions motile (possessing flagella); they are capable of oxidizing various Organic compounds. Mucus production and the presence of dark pigments give the colonies a characteristic appearance. Under nutrient depletion, they form resting stages known as cysts.

Nitrogen Fixation by free-living cyanobacteria is of great ecological importance in rice paddies. They can fix up to 30–50 kg of nitrogen per 1 ha annually. Overall, the ability to fix nitrogen has been identified in 40 species of cyanobacteria.



Last update: 12/08/2026

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