Fractionation of the thylakoid membrane from Dunaliella salina – heterogeneity is found in Photosystem I over a broad range of growth irradiance

Thylakoids from the green alga, Dunaliella salina, were fragmented by sonication and the appressed grasna membranes separated from stroma lamellae by partitioning in aqueous two-phase systems. The concentration and antenna size of Photosystem I in the two membrane domains were determined for cultures grown at three different light intensities. Although the antenna size of both PS is decreased with increasing growth irradiance, the antenna size of Photosystem I in the grana was approximately 25-30% greater than the antenna size of Photosystem I in the stroma lamellae. Counter-current distribution analysis of sonicated thylakoids revealed that the amount of stroma lamellae increased whereas the amount of the stacked membranes decreased at higher growth irradiance. The overall decrease in the antenna size of PS I, in D. salina, at higher light intensities can therefore be explained by the combined effect of a decrease in the antenna size of Photosystem I, both in the grana and the stroma lamellae, and a relative increase in the amount of stroma lamellae which has smaller Photosystem I antennae than the Photosystem I centers found in the grana. Light-induced protein phosphorylation increased the relative amount of the stroma lamella fraction. This is interpreted as a result of partial unstacking of the grana. It is suggested that this may be a mechanism for increasing the cyclic electron transport around Photosystem I.