Chloroplasts of the yellow-green alga B^w/7/^r/^/?j/-y y77/^//7?As show a light-induced pH change similar to that described for chloroplasts of higher plants with regard to kinetics, relationship to illuminance, and maximum proton uptake. Upon weak illuminance the pH of the outer medium rises and reaches a plateau; upon darkening the pH decays to the original value, apparently with first order kinetics. Under intense illuminance the pH passes through a maximum; the dark decay shows a transient

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... ndershoot. Repeated strong illuminations diminish the extent of the pH shift and accelerate its dark decay. The action spectrum for the initial rate of the pH rise resembles that for NADP reduction with water as electron donor; it is different from the action spectrum for NADP reduction with ascorbate-DC!P as electron donor in the presence of DCMU. A joint action of photosystcm F and H in proton translocation is indicated. Light-induced pH changes in chloroplasts of higher plants have become increasingly interesting over the past years especially in view of the chemiosmotic theory of photophosphorylation (see JAGENDORF and UumE 1967; WALKER and CROns 1970; ScHWAnix 197!). One goal of this investigation was to find whether isolated algal chloroplasts arc suitable for studying this aspect of photosynthesis. Algae can easily be grown under controlled conditions, so that seasonal variations should not affect the results. The organism used in this work is the ycltow-grccn alga BMw///fr/o/?-y;'.y. Its chloroplasts catalyze light-dependent phosphorylation and Hill reactions at rates comparable to those obtained with spinach (BoGfR !969a,b). The alga consists of short filaments with up to seven chloroplasts in each cell. Since the observed pH changes seemed basically similar to those described for chloropiasts of higher plants, a more general question could be raised: Which pigment system is dramg the proton pump? Action spectra have been reported indicating a predominant participation oi pigment system [ in proton translocation (DiLLtY 1967; HhAin 1972); furthermore quantum yteld determinations corroborated these findings (DtLLEY and Vnn\o\ 1967; HEATH 1972). On the other hand, evidence has been presented for the involvement of both light reactions (S( m n i'HAK! #/. 1968). A knowledge of the site of the proton pump and of the pigment sysfcm(s) sensut/ing it gains further relevance if a proton gradient should really be driving photophosphoryiatton as suggested originally by NEUMANN and JAGENDORF (1964).