Bicarbonate-Mediated CO(2) Formation on Both Sides of Photosystem II

[Image: see text] The effect of bicarbonate (HCO(3)(–)) 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 HCO(3)(–) enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO(2) and H(2)O. The other is for the electron-acceptor side, in which (i) reduction of the Q(A) quinone leads to the release of HCO(3)(–) from its binding site on the non-heme iron and (ii) the E(m) potential of the Q(A)/Q(A)(•–) couple increases when HCO(3)(–) dissociates. This suggested a protective/regulatory role of HCO(3)(–) 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 (13)C isotope labeling of HCO(3)(–) allows donor- and acceptor-side formation of CO(2) by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under in vivo conditions.