Energy coupling in the PSI-LHCI supercomplex from the green alga Chlamydomonas reinhardtii

Energy transfer and trapping in the PSI-LHCI supercomplex from the green alga Chlamydomonas reinhardtii have been studied using femtosecond transient absorption and picosecond fluorescence spectroscopy at room temperature. Data suggest that excitations of the PSI-LHCI supercomplex at 700 nm have similar probabilities of excitation of either the primary donor in the PSI core (absorbing at 697 nm) or low-energy Chls in the LHCI (red pigments) that presumably absorb at this spectral region. Both transient absorption and picosecond fluorescence spectroscopy indicate a biphasic overall decay of the excitation in the PSI-LHCI. The process includes a photochemical trapping in the PSI core antenna occurring with a typical lifetime of 25 +/- 3 ps and a significantly slower excitation decay phase in the PSI-LHCI supercomplex occurring with a lifetime of 104 20 ps and maximum of absorption changes around 685 nm. The slow excitation decay process suggests presence of an energy transfer pathway from the LHCI to the PSI core, which introduces a diffusion-limited step, in contrast to optimized and energetically well coupled excitation dynamics in the PSI core and CP43'-PSI supercomplexes from iron-stress-induced cyanobacteria. Although LHCI in green algae seems to be similar to those isolated from higher plants the data demonstrate apparent differences in the excitation dynamics suggesting differences in molecular organizations and causes of the red spectral shift associated with PSI. Data of time-resolved spectroscopy are discussed based on the available structural models of PSI-LHCI supercomplexes.