AMMONIUM-INDUCED INHIBITION OF AMMONIUM-STARVED NITROSOMONAS-EUROPAEACELLS IN SOIL AND SAND SLURRIES

Ammonia-oxidising bacteria are poor competitors for limiting amounts o f ammonium. Hence, starvation for ammonium seems to be the regular con dition for these bacteria in natural environments. Long-term survival in the absence of ammonium will be dependent on the ability to maintai n large population sizes at the expense of endogenous energy sources a nd on the preservation of a relatively large capacity for ammonium oxi dation. The effect of freshly added ammonium on the performance of amm onia-oxidising bacteria was studied in ammonium-enriched slurries cons isting of samples taken from non-water-saturated soil and sand columns inoculated with Nitrosomonas europaea and Nitrobacter winogradskyi an d continuously percolated with mineral medium containing ammonium. Imm ediately after introduction of the nitrifying bacteria to the columns, ammonium oxidation started and nitrate leached from the columns. Afte r 6 weeks of incubation of the columns, 94% of the ammonium supplied w as recovered as nitrate in the effluent and net cell growth had ceased . In slurries with freshly added ammonium, ammonium oxidation decrease d after an initial period of relatively high oxidation rates, which la sted 6 at the most. This indicated that the cells had been starved for ammonium in the columns. After 3 days of slurry incubation the ammoni um-oxidising activity restarted, bur not in the presence of chloramphe nicol, indicating de novo synthesis of enzyme systems. Restart of acti vity after 3 days could not be attributed to the release of free-livin g cells from the sand particles or to the presence of organotrophic ba cteria in the slurries. (C) 1998 Federation of European Microbiologica l Societies. Published by Elsevier Science B.V. All rights reserved.