Membrane potential, proton and sodium motive forces inAzospirillum brasilenseSp7-S

Abstract

Some biophysical features of the diazotroph Azospirillum brasilense Sp7-S were studied. It maintained a stable but relatively low membrane potential (ΔΨ) of about −85 mV in conditions typical of aerated soil solutions. Internal pH was also well regulated at about 7.4 from pH 6–8. The calculated proton motive force was maximal at −220 mV in pH 5.0 medium, decreasing to zero at pH 8.8. This proton motive force appears to be insufficient to drive metabolic processes such as cell growth and flagellar rotation in neutral and alkaline media. These results prompted a study of the sodium motive force. Intracellular sodium is kept very low relative to the medium, generating a large sodium motive force of about −180 mV at pH 7. Furthermore, A. brasilense motility was sensitive to amiloride and benzamil, inhibitors of Na+-driven flagellar motors. We propose that a sodium motive force can be generated and utilised for metabolic processes in A. brasilense. These chemiosmotic parameters may explain both the poor distribution of Azospirillum in acid soils and their requirement for sodium in alkaline environments.