Bicarbonate-Mediated CO₂ Formation on Both Sides of Photosystem II

The effect of bicarbonate (HCO₃^-) on photosystem II (PSII) activity was discovered in the 1950s, but only recently have its molecular mechanisms begun to be clarified. Two chemical mechanisms have been proposed. One is for the electron-donor side, in which mobile HCO₃^- enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO₂ and H₂O. The other is for the electron-acceptor side, in which (i) reduction of the Q_A quinone leads to the release of HCO₃^- from its binding site on the non-heme iron and (ii) the E_m potential of the Q_A/Q_A^•- couple increases when HCO₃^- dissociates. This suggested a protective/regulatory role of HCO₃^- as it is known that increasing the E_m of Q_A decreases the extent of back-reaction-associated photodamage. Here we demonstrate, using plant thylakoids, that time-resolved membrane-inlet mass spectrometry together with ¹³C isotope labeling of HCO₃^- allows donor- and acceptor-side formation of CO₂ by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under in vivo conditions.