PHOTOINDUCED PH-CHANGES IN SUSPENSIONS OF THE ACID-RESISTANT GREEN-ALGA DUNALIELLA-ACIDOPHILA

The complex H+ translocations across the plasma membrane of the extrem ely acid resistant unicellular green alga Dunaliella acidophila (cytos olic pH 7.0; optimal growth and photosynthesis at pH 1.0) which develo p during dark/light/dark transitions were investigated with the aid of pH sensitive antimony micro electrodes. Despite of 100 mmol/l H+ in t he medium (pH 1.0), cells are able to produce in light a further acidi fication of the medium (maximal delta pH 0.005) which reaches a maximu m after 4 min. After the offset of illumination this reaction is rever sed: An alkalization takes place, the rate of which is in the order of 3-10% of the light-induced acidification. The light-induced acidifica tion of the external medium is sensitive to inhibitors of photosynthet ic electron transport and to inhibitors of the plasma membrane H+ ATPa se, and is stimulated by fusicoccin. It is concluded that the acidific ation is catalysed by the plasma membrane H+ ATPase of Dunaliella acid ophila the activity of which is stimulated by light. The photosyntheti c pigments are the photoreceptors of the reaction, but the signal tran sduction from the non-cyclic electron transport to the H+ ATPase in th e plasma membrane is unknown.