Nitrogen Fixation By Azotobacter Species In Mixed Culture


John S. Lacy

Date of Graduation

Spring 1976


Master of Science in Biology



Committee Chair

Roar Irgens


It has been demonstrated that ammonia, the natural endproduct of nitrogen fixation, acts as a corepressor of nitrogenase synthesis. It has also been shown that ammonia will accumulate in the medium of pure cultures of Azotobacter vinelandii until levels become high enough to repress nitrogenase synthesis. A. vinelandii and A. chroococcum were grown in mixed culture with yeasts and bacterial soil isolates in a nitrogen free medium. It was theorized that the yeast or the soil isolates would utilize the excess ammonia produced by Azotobacter, thus keeping ammonia levels low and nitrogenase synthesis active. The amounts of nitrogen fixed in the mixed cultures were compared to the amounts fixed in a pure culture of the Azotobacter sp. tested. The mixed cultures of A. vinelandii did not fix any more nitrogen than A. vinelandii in pure culture. However, the mixed cultures of A. chroococcum did fix a significantly greater amount of nitrogen (164 ug/ml) than the pure culture of A. chroococcum (134 ug/ml). The specific increase in fixed nitrogen appears to be a function of the ability of the added "contaminant" both to utilize ammonia at low concentrations, thereby preventing the Azotobacter nif genes from being corepressed, and to hydrolyze Azotobacter capsular slime for carbon and energy.

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© John S. Lacy