The definitive version is available at http://www.febsletters.org/article/S0014-5793(98)00271-3/The light-induced Fourier transform infrared difference (FT-IR) spectrum originating from the donor side of O2-evolving photosystem (PS) II was obtained in non-depleted and CO2-depleted PSII membrane preparations. The observed spectrum free of contributions from the acceptor side signals was achieved by employing 2 mM/18 mM ferri-/ferrocyanide as a redox couple. This spectrum showed main positive bands at 1589 and 1365 cm−1 and negative bands at 1560, 1541, 1522 and 1507 cm−1. CO-depleted PSII preparations showed a quite different spectrum. The main positive and negative bands disappeared after depletion of bicarbonate. The addition of bicarbonate partially restored those bands again. Comparison between difference FT-IR spectra of untreated and bicarbonate-depleted PSII membranes indicated that the positive bands at 1589 and 1365 cm−1 can be assigned to COO− stretching modes from bicarbonate. The higher frequency corresponds to uas (COO−) and the lower frequency to us (COO−). 13C-Labeling FT-IR measurements confirmed these findings and also suggested that the negative band at 1560 cm−1 can be ascribed to uas (COO−). The data are discussed in the framework of the suggestion that bicarbonate can be a ligand to the Mn-containing water-oxidizing complex of PSII.This work was supported by the Dirección General de Investigación Científica y Técnica (Grant PB 95-0219), by Sabbatical Fellowships to V.V.K. and S.I.A. from the Spanish Ministry of Education and Culture and by the Russian Foundation of Basic Research (Grant 96-04-50394).Peer reviewed