Gaseous NO2 as a regulator for ammonia oxidation of Nitrosomonas eutropha

Cells of Nitrosomonas eutropha strain N904 that were denitrifying under ano xic conditions with hydrogen as electron donor and nitrite as electron acce ptor were unable to utilize ammonium (ammonia) as an energy source. The rec overy of ammonia oxidation activity was dependent on the presence of NO2. A naerobic ammonia oxidation activity was observed in a helium atmosphere sup plemented with 25 ppm NO2 after 20 h. Ammonia oxidation activity was detect ed after 2-3 days using an oxic atmosphere with 25 ppm NO2. In contrast, am monia consumption started after 8-9 days under oxic conditions without the addition of NO2; in this case, small amounts of NO and NO2 were detected an d their concentrations increased with increasing ammonia oxidation activiti es. Hardly any ammonia oxidation was detected when nitrogen oxides were rem oved by intensive aeration. It would seem, therefore, that NO2 is the maste r regulatory signal for ammonia oxidation in Nitrosomonas eutropha. Anaerob ic ammonia oxidation activity was inhibited by the addition of NO. This inh ibition was partly compensated by either increasing the NO2 concentration o r by using 2,3-dimercapto-1-propane-sulfonic acid as a NO binding substrate . DMPS was inhibitory to nitrification under oxic conditions, while increas ed amounts of NO or NO2 led to increased oxidation activities.