Na+ and K+ transport in Nitrosomonas europaea and Nitrobacter agilis

Potassium-depleted cells of Nitrosomonas europaea and Nitrobacter agilis were prepared by diethanolamine treatment and contained less than 5 mM intracellular K⁺. The addition of K⁺ to K⁺-depleted cells of N. europaea and N. agilis resulted in a depolarization of membrane potential (ΔΨ) by about 5 and 10 mV, respectively. This depolarization was, however, compensated by an equivalent increase in transmembrane pH gradient (ΔpH), so that the total proton-motive force (Δp) remained constant, indicating that K⁺ transport was electrogenic in both bacteria. Using ²²Na⁺-loaded cells, it is shown that both bacteria lack a respiration-dependent Na⁺ pump; however, antiporters for Na⁺/H⁺, K⁺/Na⁺ and K⁺/H⁺ were detected. Of these, at least the K⁺/Na⁺ antiporter required an electrochemical gradient for its operation. It is also shown that the unprotonated form of NH₄⁺ is transported into these bacteria by a simple diffusion mechanism.