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 membrane potential (Em) when cultured in saline medium (artificial Waldsea water) than when cultured in freshwater. The cell hyperpolarizes and membrane conductance (Gm) decreases when the external medium is changed from Waldsea control solution (WCS), a high-salt medium, to low-salt medium containing sufficient sorbitol to generate the same osmotic potential as WCS. Banding pattern and proton flux experiments show that C. buckellii has higher passive proton influx in the alkaline band in high-salt medium than in low-salt medium. Decrease of the passive proton influx by darkness or low external pH dramatically hyperpolarizes the membrane and decreases the conductance. The pH dependence curves of Em and Gm also indicate the existence of high passive proton conductance (GH) in C. buckellii. Ion substitution experiments show that Em and Gm of saltwater cells are not dependent on K+, Na+, Cl-, or SO42+. Mg2+ also affects Em and Gm, but its effect is probably on GH. We conclude that GH is the most important cause of the membrane depolarization and conductance increase in the saltwater alga C. buckellii.