Light-Activated H+Transport into the Vacuole ofValonia ventricosa

Abstract

Using glass capillary microelectrodes for the measurement of potential differences (PD) and antimony microelectrodes for the measurement of pH, we investigated the light-induced changes of PD between the central vacuole and the external medium, of pH in the vacuole (pHv), as well as of pH in the external medium (pH0) of the green marine alga Valonia ventricosa. PD in the dark was about +30 to +40 mV (vacuole positive), pHv 6-3, and the resistance of the protoplast (cell wall-plasmalemma-tonoplast) 17-8 kOhm cm2. Illumination caused an increase of the positive PD (after a few oscillations) up to +80 to +100 mV, acidification of the vacuolar sap, alkalinization of the external medium, and a decrease in the resistance of the protoplast to 7-6 kOhm cm2. The kinetics of the changes of PD, pHv, and pH0 were similar to each other. It is concluded that a light-stimulated active H+ flow occurs from the external medium into the central vacuole of Valonia ventricosa as a result of the onset of photosynthetic activity.