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

Abstract

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.

Genome Sequences of Pelagibaca bermudensis HTCC2601 T and Maritimibacter alkaliphilus HTCC2654 T , the Type Strains of Two Marine Roseobacter Genera

“…alkaliphilus was named because of its alkaline growth optimum at pH 10. Na ϩ /H ϩ antiporters have been shown to be involved in conferring alkaliphilic phenotypes for a variety of organisms by increasing internal cellular H ϩ concentrations in alkaline conditions where Na ϩ is present (2,7,8,14,16,17). As expected, the genome of M. alkaliphilus contains two putative Na ϩ /H ϩ antiporters, one homologous to nhaP, important for alkaliphily in several strains (2,16,17), and another located adjacent to predicted ABC transporter genes for capsular polysaccharide export.…”