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 depende nt on the presence of oxygen for the production of nitrite and nitrate , respectively. In oxygen-limited environments, they have to compete w ith each other as well as with other organotrophic bacteria for the av ailable oxygen. The outcome of the competition with be determined by t heir specific affinities for oxygen as well as by their population siz es. The effect of mixotrophic growth by the nitrite-oxidizing Nitrobac ter hamburgensis on the competition for limiting amounts of oxygen was studied in mixed continuous culture experiments with the ammonium-oxi dizing Nitrosomonas europaea at different levels of oxygen concentrati ons. The specific affinity for oxygen of N. europaea was in general hi gher than of N. hamburgensis. In transient state experiments, when oxi c conditions were switched to anoxic, N. hamburgensis was washed out a nd nitrite accumulated. However, grown at low oxygen concentration, th e specific affinity for oxygen of N. hamburgensis increased and became as great as that of N. europaea. Due to its larger population size, t he nitrite-oxidizing bacterium became the better competitor for oxygen and ammonium accumulated in the fermentor. It is suggested that conti nuously oxygen-limited environments present a suitable ecological nich e for the nitrite-oxidizing N. hamburgensis.