Comparative nitrogen fixation by mesophilic (HTS) vis-à-vis thermotolerant mutants (HTR) of Azotobacter chroococcum at high temperature and their effect on cotton biomass

Introduction and objective: The amount of nitrogen fixed by Azotobacter varied from 0.02-0.25 KgN/ha/d and has been reported to be variable with various physiological and environmental factors. The nitrogen fixation by mesophilic soil isolates and their analogue resistant mutants of Azotobacter chroococcum was compared with their thermotolerant mutants at three different temperatures (30oC, 37oC, 42oC). Material and methods: Mutants spontaneously resistant to methylammonium chloride were derived originally from a soil isolate of A. chroococcum. Ammonia excretion ability of different strains of A. chroococcum was determined by the method of Chaney and Marbach (1962). Nitrogenase activity of different strains of A. chroococcum was assayed by growing the cultures in the Burk medium. Effect of Azotobacter inoculation on cotton biomass was followed. Results: Growth and ammonia excretion were related in both mesophilic as well as thermotolerant mutants at 30oC but not at elevated temperature (42oC). A thermotolerant strain (HTR71) has excreted as much as 24.1µg ammonia/ml of culture broth in a sucrose supplemented synthetic Jensen’s medium at elevated temperature of 37oC under stationary conditions of growth while a thermotolerant mutant (HTR54) showed nitrogenase activity of 46.13 nmoles C2H4 /h/mg protein at 37oC while Mac68, a mesophilic strain from which HTR54 is derived by mutation has nitrogenase activity of 35.07 nmoles C2H4 /h/mg protein. Thermotolerant mutants showed better performance over mesophilic strains on the cotton biomass under pot house conditions. Conclusion: The present study suggest that these thermotolerant mutants would be the ideal diazotroph to be used as biofertilizers for crops in semiarid regions having high temperature (40oC or above).