Na + /H + exchange activity in the alkaliphile halotolerant cyanobacterium Aphanothece halophytica

The activity of Na + /H + exchanger to remove toxic Na + is important for growth of organisms under high salinity. In this study, the halotolerant cyanobacterium Aphanothece halophytica was shown to possess Na + /H + exchange activity since exogenously added Na + could dissipate a pre-formed pH gradient, and decrease extracellular pH. Kinetic analysis yielded apparent K m (Na + ) and V max of 20.7 ± 3.1 mM and 3,333 ± 370 nmol H + min −1 mg −1 , respectively. For cells grown under salt-stress condition, the apparent K m (Na + ) and V max was 18.3 ± 3.5 mM and 3,703 ± 350 nmol H + min −1 mg −1 , respectively. Three cations with decreasing efficiency namely Li + , Ca 2+ , and K + were also able to dissipate pH gradient. Only marginal exchange activity was observed for Mg 2+ . The exchange activity was strongly inhibited by Na + -gradient dissipators, monensin, and sodium ionophore as well as by CCCP, a protonophore. A. halophytica showed high Na + /H + exchange activity at neutral and alkaline pH up to pH 10. Cells grown at pH 7.6 under high salinity exhibited higher Na + /H + exchange activity than those grown under low salinity during 15 days of growth suggesting a role of Na + /H + exchanger for salt tolerance in A. halophytica . Cells grown at alkaline pH of 9.0 also exhibited a progressive increase of Na + /H + exchange activity during 15 days of growth.