Ammonia-oxidizing bacteria are able to maintain a high oxidizing potential during starvation. nitrite has a short-lasting stimulatory effect on the oxidation of ammonia after starvation when supplied simultaneously with fresh ammonium. To examine whether nitrite-oxidizing bacteria as partners in naturally occuring nitrifying communities interfere with this stimulatory effect of nitrite, mixed culture experiments were performed in sand slurries. Following the consumption of ammonium, these mixed cultures were starved for 4 months in their own medium. After the starvation period, ammonium was supplied. In contrast to the former experiments in monocultures of Nitrosomonas europaea, no short-lasting peak in ammonium oxidation activity was observed in the presence of nitrite-oxidizing cells. When these nitrite-oxidizing cells had been pre-activated with 5 mM nitrite one week before ammonium was supplied to the starving cultures, ammonium-oxidizing activity was weakened for up to four days, depending on the culture examined. The possibility that ammonia-oxidizing cells are repressed in the presence of nitrite-oxidizing cells in natural environments such as nitrifying biofilms is discussed
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