Energy Transfer and Trapping in Synechococcus WH 7803

Alonso M. Acuñaa, Claire Lemaireb, Rienk van Grondellea, Bruno Roberta,b, Ivo H.M. van Stokkuma
aInstitute for Lasers, Life and Biophotonics, Faculty of Sciences, Vrije Universiteit Amsterdam; bCEA, Institut de Biologie et de Technologies de Saclay, and CNRS, 91191 Gif/Yvette Cedex France

Energy transfer and trapping in Synechococcus WH 7803 whole cells and isolated Photosystem I (PSI) complexes have been studied by time-resolved emission spectroscopy at room temperature (RT). With the help of global and target analysis the pathways of energy transfer and the charge separation dynamics have been modelled. When the cells are in excellent condition, the fraction of the energy absorbed by functionally uncoupled phycobilisomes (PBS) is about ≈1%, and ≈13% of the energy is absorbed by functionally uncoupled PE545 (panels A,B). In worse condition, the “free PBS” contribution increases to ≈10% (panels C,D). Exciting with 550 nm we have resolved the energy transfer from PBS to PS II. The energy is funneled from PE545 to PC645 to APC660 to APC680 to PS II. The slowest energy transfer step is from PE545 to PC645 (dominantly 12/ns (black), a small fraction (grey) with 8/ns). When coupled to PSII, the population of the APC680 intermediate is low, because it transfers its energy faster to PS II (≈54/ns) than it is populated (≈32/ns). Only in the functionally uncoupled PBS it becomes clearly visible (green in panels C,D).

Figure: (A,C) Populations and (B,D) Species Associated Spectra (SAS) of PE545 (black,grey), PC645 (red), APC660 (blue), APC680 (green) and PS II (magenta). The PE545 SAS amplitude differs because of a filter. Note that the time axis is linear until 150 ps (after the maximum of the Instrument Response Function which was 28 ps full width at half maximum), and logarithmic thereafter.