Light-induced H<sup>+</sup> Efflux from Intact Cells of <italic>Cyanidium caldarium</italic>

Abstract

Light-induced pH changes in suspensions of an acidophilic unicellular alga, Cyanidium caldarium Geitler, were studied as a function of the pH of the medium. In the neutral pH region, alkalization of the medium due to photosynthetic CO2 uptake was observed. In the acidic pH region, illumination caused a significant decrease in the pH of the medium, indicating the efflux of H+ from the cells. Both the rate and extent of the pH decrease increased as the pH of the medium was lowered to 3.0. The H+ efflux was not affected by 3-(3',4'-dichlorophenyl)-l,l-dimethylurea, but was inhibited by phenylmercuric acetate. The fastest H+ efflux occurred at 45°C, whereas its extent was almost constant from 25 to 50°C. The activity decreased at temperatures above 50°C and was inactivated completely at 60°C. Its action spectrum corresponded the spectrum for chlorophyll a absorption. Results indicate that the light-induced H+ efflux is driven by photosystem I and is important in the maintenance of the intracellular pH at the functional neutral region against a steep pH gradient across the cell membrane.