ATP‐driven proton pumping in two species of Chara differing in salt tolerance
Chara corallina is an obligate freshwater alga, while C. buckellii can be grown in salt and freshwater culture. When grown in fresh water, C. buckellii has electrophysiological properties similar to C. corallina, but when cultured in salt water, it has a less negative membrane potential and has a higher conductance. We show in internally perfused, tonoplast-free cells that the ATP-dependence of the two species cultured in fresh water is similar, although C. buckellii hyperpolarizes at lower ATP concentrations. We determined the pump parameters in perfused and intact cells. Using both techniques, C. corallina and C. buckellii cultured in fresh water show similar values of Ep, Gp and Ip. However, there is a significant difference between the two techniques: Ep is more negative (–400 to –700 mV) in perfused cells than in intact cells (–220 to –260mV); Gp is lower (0·1–0·2 versus 0·3–0·9 S m−2); and Ip is higher (40–60 versus 10–18 mA m−2). Salt-cultured C. buckellii was compared with freshwater C. buckellii using intact cells; Gp and Ep were similar, but Ip was much higher in salt-cultured cells (60 versus 15mA m−2). This higher pump rate is due to the depolarization of the membrane of salt-cultured algae, which is caused by a higher passive conductance. The significance of the less negative membrane potential and the higher rate of proton pumping is discussed with respect to the banding pattern and salt stress.