COMPETITION FOR LIMITING AMOUNTS OF OXYGEN BETWEEN NITROSOMONAS-EUROPAEA AND NITROBACTER-WINOGRADSKYI GROWN IN MIXED CONTINUOUS CULTURES

Chemolithotrophic nitrifying bacteria are dependent on the presence of oxygen for the oxidation of ammonium via nitrite to nitrate. The success of nitrification in oxygen-limited environments such as waterlogged soils, will largely depend on the oxygen sequestering abilities of both ammonium- and nitrite-oxidizing bacteria. In this paper the oxygen consumption kinetics of Nitrosomonas europaea and Nitrobacter winogradskyi serotype agilis were determined with cells grown in mixed culture in chemostats at different growth rates and oxygen tensions. Reduction of oxygen tension in the culture repressed the oxidation of nitrite before the oxidation of ammonium was affected and hence nitrite accumulated. K(m) values found were within the range of 1-15 and 22-166 muM O2 for the ammonium- and nitrite-oxidizing cells, respectively, always with the lowest values for the N. europaea cells. Reduction of the oxygen tension in the culture lowered the half saturation constant K(m) for oxygen of both species. On the other hand, the maximal oxygen consumption rates were reduced at lower oxygen levels especially at 0 kPa. The specific affinity for oxygen indicated by the V(max)/K(m) ratio, was higher for cells of N. europaea than for N. winogradskyi under all conditions studied. Possible consequences of the observed differences in specific affinities for oxygen of ammonium- and nitrite-oxidizing bacteria are discussed with respect to the behaviour of these organisms in oxygen-limited environments.