Oxygen consumption kinetics of Nitrosomonas europaea and Nitrobacter hamburgensis grown in mixed continuous cultures at different oxygen concentrations

Chemolithotrophic ammonium- and nitrite-oxidizing bacteria are dependent on the presence of oxygen for the production of nitrite and nitrate, respectively. In oxygen-limited environments, they have to compete with each other as well as with other organotrophic bacteria for the available oxygen. The outcome of the competition will be determined by their specific affinities for oxygen as well as by their population sizes. The effect of mixotrophic growth by the nitrite-oxidizing Nitrobacter hamburgensis on the competition for limiting amounts of oxygen was studied in mixed continuous culture experiments with the ammonium-oxidizing Nitrosomonas europaea at different levels of oxygen concentrations.The specific affinity for oxygen of N. europaea was in general higher than of N. hamburgensis. In transient state experiments, when oxic conditions were switched to anoxic, N. hamburgensis was washed out and nitrite accumulated. However, grown at low oxygen concentration, the specific affinity for oxygen of N. hamburgensis increased and became as great as that of N. europaea. Due to its larger population size, the nitrite-oxidizing bacterium became the better competitor for oxygen and ammonium accumulated in the fermentor. It is suggested that continuously oxygen-limited environments present a suitable ecological niche for the nitrite-oxidizing N. hamburgensis.