phs-201801-0024 ArticleInAPeriodical 2018 Photosynthetica 56 1 https://ps.ueb.cas.cz/artkey/phs-201801-0024.php https://doi.org/10.1007/s11099-018-0782-z 254-264 KotakisC. AkhtarP. ZsirosO. GarabG. LambrevP. H. Increased thermal stability of photosystem II and the macro-organization of thylakoid membranes, induced by co-solutes, associated with changes in the lipid-phase behaviour of thylakoid membranes The principal function of the thylakoid membrane depends on the integrity of the lipid bilayer, yet almost half of the thylakoid lipids are of non-bilayer-forming type, whose exact functions are not fully understood. Non-bilayer lipids can be extruded from the membrane in the presence of high concentrations of co-solutes. We applied 2 M sucrose to induce lipid phase separation in isolated thylakoid membranes, following consequent structural and physiological effects. Circular dichroism spectroscopy indicated significant changes in the chiral macro-arrangement of the pigment-protein complexes, which were reversed after washing out the co-solute. Similarly, merocyanine-540 fluorescence suggested reversible changes in the lipid phases. The PSII function, as tested by chlorophyll fluorescence induction transients and time-resolved fluorescence, was almost unaffected. However, the presence of sucrose dramatically increased the PSII thermostability, which can partly be explained by a direct osmolyte effect and partly by the lipid phase separation stabilizing the stacked membrane.