Abstract
Ion-sensitive microelectrodes were used to measure Cl− and H+ activities in the cytoplasm of the unicellular green alga Eremosphaera viridis de Bary. In the light, cytoplasmic Cl− activity was 2.2 millimolar at most and cytoplasmic H+ activity was about 5.4·10−8 molar (pH 7.3). Darkening resulted in a permanent increase of the Cl− activity to 3.2 millimolar and in a transient acidification, which was compensated within 3 to 5 minutes. Switching light on again decreased the Cl− activity to the light level (2.2 millimolar). Simultaneously, a transient alkalization of the cytoplasm was observed. The transient character of the light-dependent pH changes was probably caused by pH-stat mechanisms, whereas the light-dependent Cl− activity changes were compensated to a much smaller degree. Studies with different inhibitors (3-(3,4-dichlorophenyl)-1, 1-dimethylurea, piretanide, venturicidin) indicated a direct relation between the light-driven H+ flow across the thylakoid membrane and the observed light-dependent Cl− and H+ activity changes in the cytoplasm. It is suggested that light-driven H+ flux across the thylakoid membrane was in part electrically compensated by a parallel Cl− flux. The resulting Cl− and H+ activity changes in the stroma were compensated by Cl− and H+ fluxes across the chloroplast envelope giving rise to the observed Cl− and H+ activity changes in the cytoplasm.
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Selected References
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