Effect of bicarbonate on the water-oxidizing complex of photosystem II in the super-reduced S-states

It is shown that the hydrazine-induced transition of the water-oxidizing complex (WOC) to super-reduced S-states depends on the presence of bicarbonate in the medium so that after a 20 min treatment of isolated spinach thylakoids with 3 mM NH2NH2 at 20 °C in the CO2/HCO3−-depleted buffer the S-state populations are: 42% of S−3, 42% of S−2, 16% of S−1 and even formal S−4 state is reached, while in the presence of 2 mM NaHCO3, the same treatment produces 30% of S−3, 38% of S−2, and 32% of S−1 and there is no indication of the S−4 state. Bicarbonate requirement for the oxygen-evolving activity, very low in untreated thylakoids, considerably increases upon the transition of the WOC to the super-reduced S-states, and the requirement becomes low again when the WOC returns back to the normal S-states using pre-illumination. The results are discussed as a possible indication of ligation of bicarbonate to manganese ions within the WOC.