Correlations between photophosphorylation and light-induced conformational changes of chloroplasts in whole cells of the halophilic green alga Dunaliella parva

Abstract

Summary In whole cells of the extremely halophilic unicellular green alga Dunaliella parva slow changes in apparent absorbance, peaking at 535 nm, can be separated from the well-known fast 515 nm shift. It is concluded that this 535 nm change is caused by light scattering and indicates conformational changes (shrinkage) of the chloroplasts. Properties of the 535 nm change are compared with those of the light-induced increase of the endogenous ATP-level in vivo. From similar responses of both reactions to electron transport inhibitors and uncouplers it is concluded that the 535 nm change is correlated to light-induced ATP-formation and can be taken as a useful indicator for photophosphorylation in whole cells. In far-red light, exciting predominantly photosystem I, neither distinct conformational changes of chloroplasts nor an increase of the ATP-level could be observed. In red light, exciting both photosystem II and I, both the 535 nm change and ATP-formation are strongly suppressed by DCMU. These results suggest that in the presence of O2 cyclic photophosphorylation plays a minor role in Dunaliella. DBMIB, which inhibits photosynthetic electron transport between plastoquinone and cytochrome f, inhibits both ATP-formation and the 535 nm change. However, both reactions could be reactivated in vivo 1) by the addition of the Hill-reagent benzoquinone, which restored O2-evolution and 2) by the addition of the electron donor DAD, which caused O2-uptake. The former system was sensitive, the latter one insensitive to DCMU. FCCP inhibited ATP-formation and the 535 nm change in both systems. Photosynthetic control (stimulation of electron transport by FCCP) was observed only in the DBMIB/benzoquinone-system. These results seem to indicate two different ATP-sites in Dunaliella. One is located between the site of water oxidation and photosystem II. The second one is located after the DBMIB-inhibition site in the region of photosystem I.