PASSIVE PROTON CONDUCTANCE IS THE MAJOR REASON FOR MEMBRANE DEPOLARIZATION AND CONDUCTANCE INCREASE IN CHARA-BUCKELLII IN HIGH-SALT CONDITIONS

Chara buckellii G.O.A., a salt-tolerant alga, has a less negative memb rane potential (E(m)) when cultured in saline medium (artificial Walds ea water) than when cultured in freshwater. The cell hyperpolarizes an d membrane conductance (G(m)) decreases when the external medium is ch anged from Waldsea control solution (WCS), a high-salt medium, to low- salt medium containing sufficient sorbitol to generate the same osmoti c potential as WCS. Banding pattern and proton flux experiments show t hat C buckellii has higher passive proton influx in the alkaline band in high-salt medium than in low-salt medium. Decrease of the passive p roton influx by darkness or low external pH dramatically hyperpolarize s the membrane and decreases the conductance. The pH dependence curves of E(m) and G(m) also indicate the existence of high passive proton c onductance (GH) in C buckellii. Ion substitution experiments show that E(m) and G(m) of saltwater cells are not dependent on K+, Na+, Cl-, o r SO42+. Mg2+ also affects E(m) and G(m), but its effect is probably o n G(H). We conclude that G(H) is the most important cause of the membr ane depolarization and conductance increase in the saltwater alga C bu ckellii.