Excitation energy transfer and trapping in Photosystem I of the green alga Bryopsis corticulans.

Photosystem I (PSI) from the siphonous green alga Bryopsis corticulans carries ten subunits of Light-Harvesting Complex I (LHCI) that enlarge its absorption cross-section yet must still deliver excitations rapidly to the P700 reaction center. Here we investigate the light-harvesting function of PSI-LHCI, the excitation energy transfer within the supercomplex and how it affects the overall photochemical trapping lifetime. We performed picosecond time-resolved fluorescence spectroscopy at room temperature and 77 K and femtosecond transient absorption spectroscopy on purified PSI-LHCI supercomplex. Ultrafast spectroscopy showed that spectral equilibration in the antenna occurs mainly in the first 1 ps after excitation and is virtually complete within 10 ps. Nonetheless, the fluorescence lifetime of the supercomplex was 49-55 (±1) ps, depending on excitation wavelength, compared to 22 ps in the core complex. This result suggests that energy transfer continues over tens of ps, long after thermal equilibration between high- and low-energy excited states. The fluorescence analysis suggests that two factors contribute to the increased excitation lifetime - a thermodynamic factor, directly stemming from the increased antenna size as well as a kinetic factor, i.e. a rate limitation of trapping imposed by the energy transfer from LHCI to PSI. The average time of excitation transfer from LHCI to the reaction center was estimated to be close to 30 ps. Therefore, energy transfer from the enlarged antenna does not impose a sizeable kinetic limitation to the photochemical quantum efficiency of the photosystem.