Competition between heterotrophic and autotrophic nitrifiers for ammonia in chemostat cultures

Mixed cultures of a heterotrophic nitrifier/aerobic denitrifier, Thiosphaera pantotropha, and an autotrophic nitrifier, Nitrosomonas europaea, were grown in chemostats under dual ammonia-and acetate limitation. Because of simultaneous nitrification and denitrification by T. pantotropha, the activity of the cultures was evaluated from nitrogen balances as complete as possible. Under most conditions studied, no interaction took place between the two bacteria. Only above a critical C/N ratio of 10.4, T. pantotropha was able to outcompete N. europaea for ammonia (dilution rate = 0.04 h−1). At dissolved oxygen concentrations below 10 μM, the autotroph became oxygen-limited and the heterotroph dominated in the culture. Moreover, when the dilution rate was increased to 0.065 h−1, N. europaea could not maintain itself successfully in the chemostat, even when the C/N ratio was as low as 2.2. Nitrification by T. pantotropha was equivalent to that of N. europaea when the cell ratio of heterotrophs/autotrophs was 250. The relevance of these observations to the nitrogen cycle in natural environments is discussed.