The bioenergetics of ammonia and hydroxylamine oxidation in Nitrosomonas europaea at acid and alkaline pH

Abstract

Autotrophic ammonia oxidizers depend on alkaline or neutral conditions for optimal activity. Below pH 7 growth and metabolic activity decrease dramatically. Actively oxidizing cells of Nitrosomonas europaea do not maintain a constant internal pH when the external pH is varied from 5 to 8. Studies of the kinetics and pH-dependency of ammonia and hydroxylamine oxidation by N. europaea revealed that hydroxylamine oxidation is moderately pH-sensitive, while ammonia oxidation decreases strongly with decreasing pH. Oxidation of these oxogenous substrates results in the generation of higher proton motive force which is mainly composed of a ΔΨ. Hydroxylamine, but not ammonia, is oxidized at pH 5, which leads to the generation of a high proton motive force which drives energy-dependent processes such as ATP-synthesis and secondary transport of amino acids. Endogenoussubstrates can be oxidized between pH 5 to 8 and this results in the generation of a considerable proton motive force which is mainly composed of a ΔΨ. Inhibition of ammonia-mono-oxygenase or cytochrome aa3 does not influence the magnitude of this gradient or the oxygen consumption rate, indicating that endogenous respiration and ammonia oxidation are two distinct systems for energytransduction. The results indicate that the first step in ammonia oxidation is acid sensitive while the subsequent steps can take place and generate a proton motive force at acid pH.