AMMONIUM LIMITATION RESULTS IN THE LOSS OF AMMONIA-OXIDIZING ACTIVITYIN NITROSOMONAS-EUROPAEA

The effects of limiting concentrations of ammonium on the metabolic ac tivity of Nitrosomonas europaea, an obligate ammonia-oxidizing soil ba cterium, were investigated. Cells were harvested during late logarithm ic growth and were incubated for 24 h in growth medium containing 0, 1 5, or 50 mM ammonium. The changes in nitrite production and the rates of ammonia- and hydroxylamine-dependent oxygen consumption were monito red, In incubations without ammonium, there was little change in the a mmonia oxidation activity after 24 h. With 15 mM ammonium, an amount t hat was completely consumed, there was an 85% loss of the ammonia oxid ation activity after 24 h. In contrast, there was only a 35% loss of t he ammonia oxidation activity after 24 h in the presence elf 50 mM amm onium, an amount that was not consumed to completion, There was little effect on the hydroxylamine oxidation activity in any of the incubati ons. The loss of ammonia oxidation activity was not due to differences in steady-state levels of ammonia monooxygenase (AMO) mRNA (amoA) or to degradation of the active site-containing subunit of AMO protein. T he incubations were also conducted at a range of pH values to determin e whether the loss of ammonia oxidation activity was correlated to the residual ammonium concentration. The loss of ammonia oxidation activi ty after 24 h was less at lower pH values (where the unoxidized ammoni um concentration was higher). When added in conjunction with limiting ammonium, short-chain alkanes, which are alternative substrates for AM O, prevented the loss of ammonia oxidation activity at levels correspo nding to their binding affinity for AMO. These results suggest that su bstrates of AMO can preserve the ammonia-oxidizing activity of N. euro paea in batch incubations by protecting either AMO itself or other mol ecules associated with ammonia oxidation.