Ammonium-induced inhibition of ammonium-starved Nitrosomonas europaea cells in soil and sand slurries

Ammonia-oxidising bacteria are poor competitors for limiting amounts of ammonium. Hence, starvation for ammonium seems to be the regular condition for these bacteria in natural environments. Long-term survival in the absence of ammonium will be dependent on the ability to maintain large population sizes at the expense of endogenous energy sources and on the preservation of a relatively large capacity for ammonium oxidation. The effect of freshly added ammonium on the performance of ammonia-oxidising bacteria was studied in ammonium-enriched slurries consisting of samples taken from non-watersaturated soil and sand columns inoculated with Nitrosomonas europaea and Nitrobacter winogradskyi and continuously percolated with mineral medium containing ammonium. Immediately after introduction of the nitrifying bacteria to the columns, ammonium oxidation started and nitrate leached from the columns. After 6 weeks of incubation of the columns, 94% of the ammonium supplied was recovered as nitrate in the effluent and net cell growth had ceased. In slurries with freshly added ammonium, ammonium oxidation decreased after an initial period of relatively high oxidation rates, which lasted 6 at the most. This indicated that the cells had been starved for ammonium in the columns. After 3 days of slurry incubation the ammoniumoxidising activity restarted, but not in the presence of chloramphenicol, indicating de novo synthesis of enzyme systems. Restart of activity after 3 days could not be attributed to the release of free-living cells from the sand particles or to the presence of organotrophic bacteria in the slurries