Interrelated Roles of Chloride and Bicarbonate in Regulating Electron Transport Across Photosystem II in Limnospira...

Efficient charge separation and electron transfer in Photosystem II (PSII) depend on small inorganic cofactors that maintain redox balance and catalytic stability. Chloride facilitates water-oxidizing-complex turnover and minimizes charge recombination. Bicarbonate, coordinated to the non-heme iron, facilitates electron transfer between the plastoquinones Q_A and Q_B. This work investigates cooperativity between these cofactors across PSII in the hypercarbonate-requiring cyanobacterium Limnospira maxima. Bromide-for-chloride substitution induces a distinct kinetic limitation at the water oxidizing complex. While bicarbonate depletion inhibits electron transfer at the acceptor side, bromide-substituted cells maintain a measurable level of electron flow through the intersystem chain. The presence of bromide induces structural changes that allow partial electron transfer to continue even in the absence of the bicarbonate cofactor, which is not observed in the chloride system. However, this dual anion stress results in irreversible functional impairment in some centers, whereas full recovery of activity is observed with native chloride. When the donor side is restricted by bromide, the loss of bicarbonate, which is thought to function as a proton buffer for the donor side, compromises the overall stability of the reaction center. This leads to a permanent decrease in activity of the electron transfer chain, suggesting an interdependence between the roles of chloride and bicarbonate that is essential for protecting PSII during ionic stress.