Sequential effects of sodium depletion on photosystem II in Synechocystis

Incubation of Synechocystis PCC 6714 in liquid medium devoid of Na+ results in a light-dependent loss in photosynthetic O2 evolving capacity within 1 h. Photosynthetic activity is fully restored and normal growth resumes after Na+ is supplied to culture medium of depleted cells. If external Na+ is provided as soon as inhibition becomes complete, normal photosynthesis is restored within 3 min. However, if cells are further illuminated for several h under Na+ stress, then full recovery takes much longer, and requires new protein synthesis. Electron transport assays using isolated membranes demonstrate that the immediate inhibition resulting from Na+ depletion involves the O2 evolving site, while the secondary effect requiring new protein synthesis occurs near the reaction center of Photosystem II. Experiments conducted at different pH values and in the absence of inorganic carbon demonstrate that within the short time duration of these experiments Na+ does not inhibit photosynthesis by restricting bicarbonate movement into the cells. These experiments extend previous results with other cyanobacteria which demonstrated that Ca2+ and Na+ stress cause reversible damage at a site near the reaction center of Photosystem II. The damage can be characterized as a primary ion effect at the oxygen evolving site and a secondary photoinhibition near the reaction center of Photosystem II.