The present investigation was carried out to understand the effect of ammonia concentrations on the rate of ammonia-oxidizing activity of ammonia oxidizing bacteria (AOB) isolates. Maximum ammonium removal by all the isolates was found at 3 ppm ammonium concentration. However, AOB-12 and Nitrosomonas europaea ATCC 19718 cultures were found to be efficient at low ammonium concentration (3 ppm and 8 ppm), whereas AOB-21and AOB consortia (AOB-12+AOB-21) performed well at high ammonium concentrations (25, 50, 100 and 200 ppm). These results were corroborated with ammonia monooxyginase (amoA) gene copy numbers estimated by quantitative real time PCR (q-PCR). The maximum amoA gene copy numbers were recorded in N. europaea ATCC 19718 at ammonia level of 50 ppm followed by AOB-12 at 25 ppm and the consortia having maximum copy numbers at 100 ppm. This study indicates the promise of using native AOB cultures to remove ammonia from the waste waters prior to release into the environment.

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