Excitation energy transfer from phycobiliprotein to chlorophyll d in intact cells of Acaryochloris marina studied by time- and wavelength-resolved fluorescence spectroscopy.

The fluorescence decay spectra and the excitation energy transfer from the phycobiliproteins (PBP) to the chlorophyll-antennae of intact cells of the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina were investigated at 298 and 77 K by time- and wavelength-correlated single photon counting fluorescence spectroscopy. At 298 K it was found that (i) the fluorescence dynamics in A. marina is characterized by two emission peaks located at about 650 and 725 nm, (ii) the intensity of the 650 nm fluorescence depends strongly on the excitation wavelength, being high upon excitation of phycobiliprotein (PBP) at 632 nm but virtually absent upon excitation of chlorophyll at 430 nm, (iii) the 650 nm fluorescence band decayed predominantly with a lifetime of 70 +/- 20 ps, (iv) the 725 nm fluorescence, which was observed independent of the excitation wavelength, can be described by a three-exponential decay kinetics with lifetimes depending on the open or the closed state (F(0) or F(m)) of the reaction centre of Photosystem II (PS II). Based on the results of this study, it is inferred that the excitation energy transfer from phycobiliproteins to Chl d of PS II in A. marina occurs with a time constant of about 70 ps, which is about three times faster than the energy transfer from the phycobilisomes to PS II in the Chl a-containing cyanobacterium Synechococcus 6301. A similar fast PBP to Chl d excitation energy transfer was also observed at 77 K. At 77 K a small long-lived fluorescence decay component with a lifetime of 14 ns was observed in the 640-700 nm spectral range. However, it has a rather featureless spectrum, not typical for Chl a, and was only observed upon excitation at 400 nm but not upon excitation at 632 and 654 nm. Thus, this long-lived fluorescence component cannot be used as an indicator that the primary PS II donor of Acaryochloris marina contains Chl a.

Investigations on the reaction pattern of photosystem II in leaves from Arabidopsis thaliana wild type plants and mutants with genetically modified lipid content.

The role of digalactosyldiacylglycerol (DGDG) for the functional competence of photosystem II (PS II) has been analyzed in leaves of Arabidopsis thaliana plants where the lipid composition was selectively modified by genetic mutations. Measurements with a newly developed laser flash fluorometer and data evaluation within the framework of an extended "3-quencher" model lead to the following results: (i) the normalized fluorescence transients F(t)/F(0) induced by an actinic laser flash in dark adapted leaves are virtually the same in wild type (WT) and mutants with diminished (about 50%) monogalactosyldiacylglycerol (MGDG) content (mgd1 mutant); (ii) significant changes of the F(t)/F(0) curves are observed in mutants with a severely reduced DGDG content; (iii) in mutants dgd1 and dgd1 dgd2-1 with DGDG contents of 1/15 of the control and below the detection limit, respectively, the probability of the dissipative recombination reaction between P680(+)(*) and Q(A)(-) increases by factors of about two and four, respectively; (iv) the acceptor side reactions are only slightly affected; (v) excitation with actinic laser flash energies above the saturation level of photosynthesis gives rise to elevated carotenoid triplet formation in mutants dgd1 and dgd1 dgd2-1; and (vi) the relationship between DGDG content and functional effect(s) on PS II is strikingly nonlinear. A small fraction of DGDG molecules of the total pool is inferred to be specifically bound to PS II as an essential constituent for its functional competence.