The midpoint potential ( E m ) of , the one-electron acceptor quinone of Photosystem II (PSII), provides the thermodynamic reference for calibrating PSII bioenergetics. Uncertainty exists in the literature, with two values differing by ∼80 mV. Here, we have resolved this discrepancy by using spectroelectrochemistry on plant PSII-enriched membranes. Removal of bicarbonate (HCO 3 −) shifts the E m from ∼−145 mV to −70 mV. The higher values reported earlier are attributed to the loss of HCO 3 − during the titrations (pH 6.5, stirred under argon gassing). These findings mean that HCO 3 − binds less strongly when Q A −• is present. Light-induced Q A −• formation triggered HCO 3 − loss as manifest by the slowed electron transfer and the upshift in the E m of Q A. HCO 3 −-depleted PSII also showed diminished light-induced 1O 2 formation. This finding is consistent with a model in which the increase in the E m of promotes safe, direct charge recombination at the expense of the damaging back-reaction route that involves chlorophyll triplet-mediated 1O 2 formation [Johnson GN, et al. (1995) Biochim Biophys Acta 1229:202–207]. These findings provide a redox tuning mechanism, in which the interdependence of the redox state of Q A and the binding by HCO 3 − regulates and protects PSII. The potential for a sink (CO 2) to source (PSII) feedback mechanism is discussed.