KINETICS OF NITRITE OXIDATION IN 2 NITROBACTER SPECIES GROWN IN NITRITE-LIMITED CHEMOSTATS

The influence of growth rate, the presence of acetate and variation in the dissolved oxygen concentration on the kinetics of nitrite oxidation was studied in suspensions of intact cells of Nitrobacter winogradskyi and Nitrobacter hamburgensis. The cells were grown in nitrite-limited chemostats at different dilution rates under chemolithotrophic and mixotrophic conditions. Growth of N. hamburgensis in continuous culture was dependent on the presence of acetate. Acetate hardly affected the maximal nitrite oxidation rate per cell (V(max)), but displayed a distinctly negative effect on the saturation constants for nitrite oxidation (K(m)) of both Nitrobacter species. This effect was reversible; when acetate was removed from the suspensions the K(m)-values for nitrite oxidation returned to their original values. A reduction of the dissolved oxygen concentration from 100% to 18% air saturation slightly decreased the V(max) of chemolithotrophically grown N. winogradskyi cells, whereas a 2.3 fold increase was observed with mixotrophically grown cells of N. hamburgensis. It is suggested that the large variation in K(m) encountered in field samples could be due to this observed phenotypic variability. The V(max) per cell is not a constant, but apparently is dependent on growth rate and environmental conditions. This implies that potential nitrite oxidation activity and numbers of cells are not necessarily related. Considering their kinetic characteristics, it is unlikely that N. hamburgensis is able to compete succesfully with N. winogradskyi for limiting amounts of nitrite under mixotrophic conditions. However, at reduced partial oxygen tensions, N. hamburgensis may become the better competitor.