Significance of gaseous NO for ammonia oxidation by Nitrosomonas eutropha

Nitrification by the obligately lithoautotrophic ammonia oxidizer Nitrosomo nas eutropha was significantly inhibited when nitric oxide was removed from the culture medium by means of intensive aeration and turbulence. Nearly c omplete recovery of ammonia oxidation could be achieved by adding 100 ppm N O to the supplied air. Inhibition of ammonia oxidation occurred also upon a ddition of the NO binding agens 2,3-Dimercapto-1-propane-sulfonic acid (DMP S). Recovery of ammonia oxidation occurred within 3 h in the presence of 10 0 ppm NO and within 76 h in the absence of externally added NO. In co-cultu res of N. eutropha and the NO detoxifying bacterium Pseudomonas PS88, hardl y any nitrification was detectable and release of NO was extremely low when the heterotroph was provided with an organic substrate. When cells of Pseu domonas PS88 were added to a mixotrophically nitrifying culture of N. eutro pha the release of NO decreased drastically upon the addition and ammonia o xidation ceased. These results confirm for the first time the significance of NO in the course of ammonia oxidation by N. eutropha.