ELECTROCHEMICAL POTENTIAL GRADIENTS OF H- ACROSS THE TONOPLAST OF THEGREEN-ALGA EREMOSPHAERA-VIRIDIS(, K+, CA2+, AND CL)

Using ion-selective microelectrodes, we measured the activity of H+, K +, Ca2+, and Cl- and the electrical potential both in the vacuole and in the cytoplasm of the unicellular green alga Eremosphaera viridis to obtain comparable values of the named parameters from the same object under identical conditions. The cytosol had a pH of 7.3, and activiti es of the other ions were 130 mM K+, 160 nM Ca2+, and 2.2 mM Cl-. We o bserved only small and transient light-dependent changes of the cytoso lic Ca2+ activity. The vacuolar K+ activity did not differ significant ly from the cytosolic one. The Ca2+ activity inside the vacuole was ap proximately 200 mu M, the pH was 5.0, and the Cl- activity was 6.2 mM. The concentrations of K+, Ca2+, and Cl- in cell extracts were measure d by induction-coupled plasma spectroscopy and anion chromatography. T his confirmed the vacuolar activities for K+ and Cl- obtained with ion -selective microelectrodes and indicated that approximately 60% of the vacuolar Ca2+ was buffered. The tonoplast potential was vanishingly l ow (less than or equal to 12 mV). There was no detectable electrochemi cal potential gradient for K+ across the tonoplast, but there was, how ever, an obvious electrochemical potential gradient for Cl- (-26 mV), indicating an active accumulation of Cl- inside the vacuole.