Probing the picosecond kinetics of the photosystem II core complex in vivo

Photosystems I (PSI) and II (PSII) are two major pigment-protein complexes of photosynthetic organisms that function in series to convert sunlight energy into chemical energy. We have studied the picosecond fluorescence behaviour of the core of both photosystems in vivo by using two Synechocystis PCC 6803 mutants: BE cells contain PSI but are lacking both PSII and the light-harvesting complexes called phycobilisomes (PBs) whereas PAL cells contain both PSI and PSII but lack the PBs. Measurements were performed at room temperature and at 77 K. The fluorescence kinetics of PSI and PSII can nicely be separated, en passant providing the PSI/PSII ratio. At room temperature, the PSI kinetics are identical to those of isolated PSI whereas the PSII kinetics can equally well be described by the in vitro trap-limited model of Y. Miloslavina, M. Szczepaniak, M. G. Muller, J. Sander, M. Nowaczyk, M. Rogner and A. R. Holzwarth, Biophys J., 2009, 96(2), 621-631, and by the transfer-to-the-trap-limited model of C. D. van der Weij-de Wit, J. P. Dekker, R. van Grondelle and I. H. M. van Stokkum, J. Phys. Chem. A, 2011, 115(16), 3947-3956, albeit that the in vivo kinetics turn out to be somewhat slower. At 77 K several low-energy pigments are observed in both photosystems which complicate the overall dynamics but the PSII kinetics can still be described by both a trap-limited and a transfer-to-the-trap-limited model.