Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy

In this work, the transfer of excitation energy was studied in native and cation-depletion induced, unstacked thylakoid membranes of spinach by steady-state and time-resolved fluorescence spectroscopy. Fluorescence emission spectra at 5 K show an increase in photosystem I (PSI) emission upon unstacking, which suggests an increase of its antenna size. Fluorescence excitation measurements at 77 K indicate that the increase of PSI emission upon unstacking is caused both by a direct spillover from the photosystem II (PSII) core antenna and by a functional association of light-harvesting complex II (LHCII) to PSI, which is most likely caused by the formation of LHCII-LHCI-PSI supercomplexes. Time-resolved fluorescence measurements, both at room temperature and at 77 K, reveal differences in the fluorescence decay kinetics of stacked and unstacked membranes. Energy transfer between LHCII and PSI is observed to take place within 25 ps at room temperature and within 38 ps at 77 K, consistent with the formation of LHCII-LHCI-PSI supercomplexes. At the 150-160 ps timescale, both energy transfer from LHCII to PSI as well as spillover from the core antenna of PSII to PSI is shown to occur at 77 K. At room temperature the spillover and energy transfer to PSI is less clear at the 150 ps timescale, because these processes compete with charge separation in the PSII reaction center, which also takes place at a timescale of about 150 ps. © 2007 Springer Science+Business Media B.V

Surplus Photosynthetic Antennae Complexes Underlie Diagnostics of Iron Limitation in a Cyanobacterium

Chlorophyll fluorescence from phytoplankton provides a tool to assess iron limitation in the oceans, but the physiological mechanism underlying the fluorescence response is not understood. We examined fluorescence properties of the model cyanobacterium Synechocystis PCC6803 and a ΔisiA knock-out mutant of the same species grown under three culture conditions which simulate nutrient conditions found in the open ocean: (1) nitrate and iron replete, (2) limiting-iron and high-nitrate, representative of natural high-nitrate, low-chlorophyll regions, and (3) iron and nitrogen co-limiting. We show that low variable fluorescence, a key diagnostic of iron limitation, results from synthesis of antennae complexes far in excess of what can be accommodated by the iron-restricted pool of photosynthetic reaction centers. Under iron and nitrogen co-limiting conditions, there are no excess antennae complexes and variable fluorescence is high. These results help to explain the well-established fluorescence characteristics of phytoplankton in high-nutrient, low-chlorophyll ocean regions, while also accounting for the lack of these properties in low-iron, low-nitrogen regions. Importantly, our results complete the link between unique molecular consequences of iron stress in phytoplankton and global detection of iron stress in natural populations from space

Spectral inhomogeneity of photosystem I and its influence on excitation equilibration and trapping in the cyanobacterium Synechocystis sp. PCC6803 at 77 K.

Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77 K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680 nm induces a subpicosecond energy transfer process that populates the Chl a spectral form at 685--687 nm within few transfer steps (300--400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4-6 ps, slightly slower than at room temperature. At low temperatures in the absence of uphill energy transfer the energy equilibration processes involve low-energy shifted chlorophyll spectral forms of the bulk antenna participating in a 30--50-ps process of photochemical trapping of the excitation by P(700). These spectral forms might originate from clustered pigments in the core antenna and coupled chlorophylls of the reaction center. Part of the excitation is trapped on a pool of the longest-wavelength absorbing pigments serving as deep traps at 77 K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720 nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708 nm and 714 nm (full-width at half-maximum (fwhm) approximately 200--300 cm(-1)). The origin of these two bands is attributed to the presence of two chlorophyll dimers, while the appearance of the early time bleaching bands at 683 nm and 678 nm under excitation into the red side of the absorption spectrum (>690 nm) can be explained by borrowing of the dipole strength by the ground-state absorption of the chlorophyll a monomers from the excited-state absorption of the dimeric red pigments