Excitation Dynamics in the Core Antenna of PS I from Chlamydomonas reinhardtii CC 2696 at Room Temperature

Abstract

Photosystem I particles from a eukaryotic organism, the green alga Chlamydomonas reinhardtii CC 2696, were studied by transient hole-burning spectroscopy at room temperature. Global analysis of the spectra recorded after excitation of chlorophyll a molecules in Photosystem I at selected wavelengths between 670 and 710 nm reveals excitation dynamics with subpicosecond, 2−3 ps, and 20−23 ps components. The subpicosecond and 2−3 ps components are ascribed to energy equilibration within the core antenna, whereas the 20−23 ps component is ascribed to energy trapping by the reaction center. Energy equilibration components describe both uphill and downhill energy transfer depending of the excitation wavelength. The initial transient absorbance bands after direct excitation of the red tail of the Q y transition band of chlorophyll a (at 700, 705, and 710 nm) are 25 nm wide and structured, revealing strongly coupled excited states among a group of molecules, most likely reaction center chlorophyll molecules. Excitation at shorter wavelengths (670, 680, and 695 nm) results in only 5−7 nm wide initial absorbance bands originating from photobleaching and stimulated emission of antenna chlorophyll molecules. The results are compared to the excitation dynamics of Photosystem I from the cyanobacterium Synechocystis sp. PCC 6803. The most significant difference is that the 2−3 ps phase describes internal excitation dynamics within higher-energy antenna chlorophyll molecules in the algal PS I system rather than between bulk and red chlorophylls, as observed in cyanobacterial PS I. No indications of core antenna red pigments absorbing above 700 nm were found in the preparation from Chlamydomonas. Independent of excitation wavelength, after at most a few picoseconds, all excitons are distributed over the same pool of chlorophyll molecules centered at ∼682 nm.

Structural and functional changes of PSI-LHCI supercomplexes of Chlamydomonas reinhardtii cells grown under high salt conditions

“…Red-shifted pigments in photosynthetic systems typically arise from strongly coupled dimers of Chl molecules and may exert a signiWcant inXuence on the exciton trapping dynamics. Previous spectroscopic studies of green algal PSI (Gibasiewicz et al 2001(Gibasiewicz et al , 2002(Gibasiewicz et al , 2005 have indicated that the PSI core in C. reinhardtii does not contain any pigments with transition energies lower than that of P700-longer-wavelength pigments are associated only with the peripheral LHCI complexes. The loss of red-shifted emission in the salt samples can therefore be attributed to the change in pigment-pigment interactions of LHCI complexes under stress conditions.…”

“…While the decay lifetimes are relatively unchanged by cell growth under salt stress conditions, this component shows a decrease in emission intensity on the red side of the peak in the salt-stressed samples. Red emission in C. reinhardtii PSI has previously been associated with strongly coupled pigments bound by the LHCI proteins (Gibasiewicz et al 2001(Gibasiewicz et al , 2002(Gibasiewicz et al , 2005 and so this loss of red emission can be attributed to a disruption of the LHCI antenna system.…”

Spectral and Kinetic Analysis of the Energy Coupling in the PS I–LHC I Supercomplex from the Green Alga Chlamydomonas reinhardtii at 77 K

“…This suggests that structurally it is possible that similar clusters of the red pigments could be responsible for the low energy absorption bands in eukaryotic PS I with the exception of extremely red shifted species absorbing at 708, 714 or 719 nm in the cyanobacterial PS I core (Ra¨tsep et al 2000). The observed variation in the amount of the red Chl a species in the PS I core antenna from C. reinhardtii reported in a series of time-resolved studies (Hastings et al 1995;Melkozernov et al 1997;Mu¨ller et al 2003;Gibasiewicz et al 2001Gibasiewicz et al , 2002 can be explained by an accessibility of the red pigments in the PS I core to detergent treatment due to their peripheral location.…”

“…Figure taken from Kargul et al (2003). (Du et al 1993;Hastings et al 1995;Melkozernov et al 1997;Gibasiewicz et al 2001Gibasiewicz et al , 2002Mu¨ller et al 2003). Mechanisms of the delivery of the excitation energy from the LHCI peripheral antenna to the PS I core in the PS I-LHCI supercomplexes from C. reinhardtii are poorly understood.…”

Characterization of Low-Energy Chlorophylls in the PSI-LHCI Supercomplex from Chlamydomonas reinhardtii. A Site-Selective Fluorescence Study

“…This coupling was proposed to be observable due to the lack of red antenna Chls in the PSI core spectrally overlapping with electron-transfer Chls in this species. [30][31][32] In this contribution we applied site-selective (polarized) fluorescence spectroscopy 4 to characterize spectral properties of low-energy Chls in high-quality PSI-LHCI preparations from C. reinhardtii. Following a relatively simple modeling, this technique offers an access to parameters of the red Chl pool, such as absorption maximum, Stokes' shift, and ratio of homogeneous and inhomogeneous broadening, and allows conclusions on the electron-phonon coupling of the red Chls in this system.…”