MEMBRANE-POTENTIAL, PROTON AND SODIUM MOTIVE FORCES IN AZOSPIRILLUM-BRASILENSE SP7-S

Some biophysical features of the diazotroph Azospirillum brasilense Sp 7-S were studied. It maintained a stable but relatively low membrane p otential (Delta Psi) 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 p H 5.0 medium, decreasing to zero at pH 8.8. This proton motive force a ppears to be insufficient to drive metabolic processes such as cell gr owth and flagellar rotation in neutral and alkaline med:a. These resul ts prompted a study of the sodium motive force. Intracellular sodium i s kept very low relative to the medium, generating 8 large sodium moti ve force of about -180 mV at pH 7. Furthermore, A. brasilense motility was sensitive to amiloride and benzamil, inhibitors of Na+-driven fla gellar motors. We propose that a sodium motive force can be generated and utilised for metabolic processes in A. brasilense. These chemiosmo tic parameters may explain both the poor distribution of Azospirillum in acid soils and their requirement for sodium in alkaline environment s. (C) 1998 Federation of European Microbiological Societies, Publishe d by Elsevier Science B.V. All rights reserved.