Abstract
15N, 18O Tracer studies were applied to the aerobic oxidation of nitrite to nitrate by the chemolithotrophic bacterium, Nitrobacter agilis. It was established that, in conversion of nitrite to nitrate, one oxygen atom of nitrate arose from water and none from O2 or inorganic phosphate. This result confirms that of Kumar et al. [(1983) FEBS Lett. 152, 71–74]. Oxygen exchange between water and inorganic phosphate was small and that between water and nitrite or nitrate or any reaction intermediates between these two was not detected. Oxidation of nitrite was, therefore, effectively irreversible under the conditions employed. The uptake of extracellular phosphate was sufficient to allow significant transfer of 18O from phosphate to nitrate if oxidative phosphorylation had occurred by way of a PON anhydride between phosphate (or ADP) and nitrate. The results are, therefore, inconsistent with the occurrence of a reaction of this type during nitrite oxidation.