Ecology of Diazotrophs
Free living bacteria and cyanobacteria prefer a variety of habitats with varying degree of nutrients, pH, oxygen, etc. Photosynthetic nitrogen fixing bacteria are divided into : (a) non sulfur purple, (b) purple sulfur and (c) green bacteria. Since water is a major component of cytoplasm, adequate amount of water must be required for their vegetative growth. The amount of water governs the concentration of oxygen ; therefore, oversupply of water limits gas exchange, lowers the available supply, and finally, creates anaerobic condition. Due to water-logging conditions, number of anaerobic bacteria increases and that of aerobic ones decreases in soil (Alexander, 1977).
Environmental factors which influence number, community size, vegetative growth and activity of microorganisms are temperature, organic matter, pH, inorganic fertilizers, light, oxygen, season, soil and depth. In water-logging fields (anaerobic condition) such as flooded soils, lakes, ponds, rice fields, etc. non-sulfur purple bacteria grow luxuriantly. Some times their numbers increase from 104 to 105 cells/g soil (Kabayashi et al, 1967).
The azotobacters are the most intensively investigated heterotrophic group. They are the aerobic bacteria possessing highest respiratory rates. Members of these genera are mesophilic, which require optimum temperature of about 30°C for their growth. Density of Azotobacters ranges from 103 to 106 per gram soil. Other dominant N2 fixing aerobic bacteria present in soil are Beijerinckia and Derxia. Beijerinckia grows luxuriantly in acid soil in tropical region.
However, Derxia can tolerate a pH range of 5.0 to 9.0. The studies on facultative anaerobes have not been given due consideration. The presence of 20 to 18 x 103 Klebsiella cells and less than 103 Enterobacter and Bacillus cells per gram soil would be significant to utilize an adequate amount of N2 (Line and Loutit, 1973).
In water-logging conditions the number of clostridia (e.g. Clostridium acetobutylicum, C. butyricum, C. pasteurianum) increases in soil in a range from 102 to 106 cells per gram (Mishustin and Yemtsev, 1973). Under suitable conditions, N2 fixation has been estimated to be about 180 ppm in culture medium (Alexander, 1977).
Cyanobacteria are found commonly in well drained paddy; and other crop fields. Some of them possess heterocysts (e.g. Anabaena, Aulosira, Cylindrospermum, Nostoc, Tolypothrix, etc.) and some do not (e.g. Lyngbya, Oscillatoria, Plectonema, etc.).