Photoacclimation in spathiphyllum

We studied photoacclimation in Spathiphyllum grown at an irradiance of 40 or 420 micromol/m2 s (LL or HL, respectively). All parameters studied responded to acclimation. Leaves at LL, in contrast to HL, were thinner and oriented perpendicular to the incident light, had more chlorophyll per g f w, fewer stomata on the upper leaf surface and a reduced layer of mesophyll cells. Their chloroplasts at HL had wider grana with less thylakoids per granum, and better organized photosystems than at LL. PSI and PSII activities per mg chlorophyll ( Vmax ), and PSI and PSII content (total activity per g f w), were lower at LL than at HL and so was the light requirement for saturation of the PSI or PSII partial photoreactions, suggesting that fewer photosystems with larger antenna size prevail at LL, but many more with smaller antenna size at HL. Analysis of chlorophyll distribution among the thylakoid pigment-protein complexes showed less antenna chlorophyll serving PSII (CPa+LHCP1+LHCP3) than that serving PSI (CPIa+CPI+LHCP2) at LL as compared to HL, and thus a lower PSII/PSI ratio at LL, in agreement with the general finding that LL plants, with larger PSII antenna size, have lower PSII/PSI ratio. The increase in PSI antenna size at LL was correlated with the increase in the distribution of chlorophyll in pigment-protein complexes serving PSI, and a very large chlorophyll/protein molar ratio in the isolated CPI complex. On the other hand, the PSII antenna chlorophyll (CPa+LHCP1+LHCP3) on a g f w basis, and the chlorophyll a/b ratio remained more or less constant at LL or HL. This may reflect our finding that Spathiphyllum contains mainly the 27 kDa inner LHCII antenna protein, the size of which remains unaffected by photoacclimation. The increase in the distribution of chlorophyll in pigment-protein complexes serving PSII at HL, therefore, reflects the higher population of PSII at HL. Very high PSI activity was found at HL, which we attribute to the highly organized small in size PSI.     

INCREASED ENERGY TRANSFER FROM PHYCOBILISOMES TO PHOTOSYSTEM II IN HIGH LIGHT ADAPTED Anabaena cylindrica

Excitation energy transfer from phycobilisomes to photosystem II in high-light adapted cells of Anabaena cylindrica was studied by fluorescence spectroscopy and compared to that of low-light adapted cells. Measurements were made on membrane fragments containing phycobilisomes, photosystem I and II, isolated in 0.75 M K-phosphate. Relative efficiency of 430 to 590 nm light in the excitation of F₆₈₀ chlorophyll fluorescence was compared in low and high light adapted cells, respectively. The values indicate that light energy absorbed by phycobilisomes is transferred to photosystem II antenna chlorophylls with higher efficiency in high-light adapted cells than in low-light adapted cells. Partial dissociation and uncoupling of energy transfer caused by low ion concentration were different in the membrane fragments isolated from the two kinds of cells and indicated a higher aggregation state of pigment-protein complexes of phycobilisomes in high-light adapted A. cylindrica cells.     

Comparative Time-Resolved Photosystem II Chlorophyll a Fluorescence Analyses Reveal Distinctive Differences between Photoinhibitory Reaction Center Damage and Xanthophyll Cycle-Dependent Energy Dissipation*

Abstract— The photosystem II (PSII) reaction center in higher plants is susceptible to photoinhibitory molecular damage of its component pigments and proteins upon prolonged exposure to excess light in air. Higher plants have a limited capacity to avoid such damage through dissipation, as heat, of excess absorbed light energy in the PSII light-harvesting antenna. The most important pho-toprotective heat dissipation mechanism, induced under excess light conditions, includes a concerted effect of the trans-thylakoid pH gradient (ΔpH) and the carotenoid pigment interconversions of the xanthophyll cycle. Co-incidentally, both the photoprotective mechanism and photoinhibitory PSII damage decrease the PSII chlorophyll a (Chi a) fluorescence yield. In this paper we present a comparative fluorescence lifetime analysis of the xanthophyll cycle- and photoinhibition-dependent changes in PSII Chi a fluorescence. We analyze multifrequency phase and modulation data using both multicomponent exponential and bimodal Lorentzian fluorescence lifetime distribution models; further, the lifetime data were obtained in parallel with the steady-state fluorescence intensity. The photoinhi-bition was characterized by a progressive decrease in the center of the main fluorescence lifetime distribution from ～2 ns to ～0.5 ns after 90 min of high light exposure. The damaging effects were consistent with an increased nonra-diative decay path for the charge-separated state of the PSII reaction center. In contrast, the ΔpH and xanthophyll cycle had concerted minor and major effects, respectively, on the PSII fluorescence lifetimes and intensity (Gilmore et ah, 1996, Photosynth. Res., in press). The minor change decreased both the width and lifetime center of the longest lifetime distribution; we suggest that this change is associated with the ΔpH-induced activation step, needed for binding of the deepoxidized xanthophyll cycle pigments. The major change increased the fractional intensity of a short lifetime distribution at the expense of a longer lifetime distribution; we suggest that this change is related to the concentration-dependent binding of the deepoxidized xanthophylls in the PSII inner antenna. Further, both the photoinhibition and xanthophyll cycle mechanisms had different effects on the relationship between the fluorescence lifetimes and intensity. The observed differences between the xanthophyll cycle and photoinhibition mechanisms confirm and extend our current basic model of PSII exciton dynamics, structure and function.     

Spectral characterization of chlorophyll fluorescence in barley leaves during linear heating. Analysis of high-temperature fluorescence rise around 60 degrees C

The spectral characteristics of chlorophyll fluorescence and absorption during linear heating of barley leaves within the range 25-75 degreesC (fluorescence temperature curve, FTC) were studied. Leaves with various content of light harvesting complexes (green, Chl b-less chlorina f2 and intermittent light grown) revealing different types of FTC were used. Differential absorption, emission and excitation spectra documented four characteristic phases of the FTC. The initial two FTC phases (a rise in the 46-49 degreesC region and a subsequent decrease to about 55 degreesC) mostly reflected changes in the fluorescence quantum yield peaking at about 685 nm. A steep second fluorescence rise at 55-61 degreesC was found to originate from a short-wavelength Chl a spectral form (emission maximum at 675 nm) causing a gradual blue shift of the emission spectra. In this temperature range, a clear correspondence of the blue shift in the emission and absorption spectra was found. We suggest that the second fluorescence rise in FTC reflects a weakening of the Chl a-protein interaction in the thylakoid membrane.     

Changes in the energy distribution between chlorophyll-protein complexes of thylakoid membranes from pea mutants with modified pigment content. I. Changes due to the modified pigment content

The low-temperature (77 K) emission and excitation chlorophyll fluorescence spectra in thylakoid membranes isolated from pea mutants were investigated. The mutants have modified pigment content, structural organization, different surface electric properties and functions [Dobrikova et al., Photosynth. Res. 65 (2000) 165]. The emission spectra of thylakoid membranes were decomposed into bands belonging to the main pigment protein complexes. By an integration of the areas under them, the changes in the energy distribution between the two photosystems as well as within each one of them were estimated. It was shown that the excitation energy flow to the light harvesting, core antenna and RC complexes of photosystem II increases with the total amount of pigments in the mutants, relative to the that to photosystem I complexes. A reduction of the fluorescence ratio between aggregated trimers of LHC II and its trimeric and monomeric forms with the increase of the pigment content (chlorophyll a, chlorophyll b, and lutein) was observed. This implies that the closer packing in the complexes with a higher extent of aggregation regulates the energy distribution to the PS II core antenna and reaction centers complexes. Based on the reduced energy flow to PS II, i.e., the relative increased energy flow to PS I, we hypothesize that aggregation of LHC II switches the energy flow toward LHC I. These results suggest an additive regulatory mechanism, which redistributes the excitation energy between the two photosystems and operates at non-excess light intensities but at reduced pigment content.     

Changes in the energy distribution in mutant thylakoid membranes of pea with modified pigment content. II. Changes due to magnesium ions concentration

Low-temperature (77K) steady-state chlorophyll fluorescence emission spectra, room temperature fluorescence and light scattering of thylakoid membranes isolated from pea mutants were studied as a function of Mg2+ concentration. The mutants have modified pigment content and altered structural organization of the pigment-protein complexes, distinct surface electric properties and functions. The analysis of the 77K emission spectra revealed that Mg2+-depletion of the medium caused not only an increased energy flow toward photosystem I in all investigated membranes but also changes in the quenching of the fluorescence, most probably by internal conversion. The results indicated that the macroorganization of the photosynthetic apparatus of mutants at supramolecular level (distribution and segregation of two photosystems in thylakoid membranes) and at supermolecular level (stacking of photosystem II supercomplexes) required different Mg ion concentrations. The data confirmed that the segregation of photosystems and the stacking of thylakoid membranes are two distinct phenomena and elucidated some features of their mechanisms. The segregation is initiated by changes in the lateral microorganization of light harvesting complexes II, their migration (repulsion from photosystem I) and subsequent separation of the two photosystems. Most likely 3D aggregation and formation of macrodomains, containing only photosystem II antenna complexes, play a certain precursory role for the increasing degree of the membrane stacking and the energy coupling between the light harvesting complexes II and the core complexes of photosystem II in the frame of photosystem II supercomplexes.     

Effects of ultraviolet-B light on photosystem II phosphoproteins in barley wild type and its chlorophyll b-less mutant chlorina f2.

The effects of ultraviolet-B light on the level and steady-state phosphorylation of photosystem II proteins have been studied in barley wild type and its chlorophyll b-less mutant chlorina f2. In the wild type, ultraviolet-B radiation is found to promote dephosphorylation of all thylakoid phosphoproteins. In addition, for reaction-centre proteins D1 and D2, dephosphorylation is paralleled by degradation. Photosystem II core proteins in the mutant are not found to be significantly phosphorylated in any experimental conditions, and loss of D1 and D2 reaction-centre proteins is slightly faster than in the wild type. These results are consistent with the possibility that phosphorylation of reaction-centre proteins affects their stability, possibly by slowing down the rate of degradation, as in the case of visible light.     

CHLOROPHYLL a FLUORESCENCE OF GONYAULAX POLYEDRA GROWN ON A LIGHT-DARK CYCLE AND AFTER TRANSFER TO CONSTANT LIGHT

Abstract— The chlorophyll a fluorescence properties of Gonyaulax polyedra cells before and after transfer from a lightdark cycle (LD) to constant dim light (LL) were investigated. The latter display a faster fluorescence transient from the level ‘I’ (intermediary peak) to ‘D’ (dip) to ‘P’ (peak) than the former (3 s as compared to 10 s), and a different pattern of decline in fluorescence from ‘I’ to ‘D’ and from ‘P’ to the steady state level with no clearly separable second wave of slow fluorescence change, referred to as ‘s' (quasi steady state)→‘M’ (maximum) →‘T’ (terminal steady state). The above differences are constant features of cells in LD and LL, and are not dependent on the time of day. They are interpreted as evidence for a greater ratio of photosystem II/photosystem I activity in cells in LL. After an initial photoadaptive response following transfer from LD to LL, the cell absorbance at room temperature and fluorescence emission spectra at 77 K for cells in LL and LD are comparable. The major emission peak is at 685–688 nm (from an antenna Chl a 680, perhaps Chl a-c complex), but, unlike higher plants and other algae, the emission bands at 696–698 nm (from Chl a_II complex, Chl a 685, close to reaction center II) and 710–720 nm (from Chl a₁, complexes, Chl a 695, close to reaction center I) are very minor and could be observed only in the fluorescence emission difference spectra of LL minus LD cells and in the ratio spectra of DCMU-treated to non-treated cells. Comparison of emission spectra of cells in LL and LD suggested that, in LL, there is a slightly greater net excitation energy transfer from the light-harvesting peridinin-Chl a (Chl a 670) complex, fluorescing at 675 nm, to the other antenna chlorophyll a complex fluorescing at 685–688 nm, and from the Chl a., complex to the reaction center II. Comparison of excitation spectra of fluorescence of LL and LD cells, in the presence of DCMU, confirmed that cells in LL transfer energy more extensively from the peridinin-Chl a complex to other Chl a complexes than do cells in LD.     

Excited-State Energy Level Does Not Determine the Differential Effect of Violaxanthin and Zeaxanthin on Chlorophyll Fluorescence Quenching in the Isolated Light-Harvesting Complex of Photosystem II

Abstract— The mechanism of action of xanthophyll cycle carotenoids in controlling the quenching of chlorophyll fluorescence in the major light-harvesting complex of photosystem II (LHCIIb) has been investigated. Auroxanthin, a diepoxy carotenoid with 7 conjugated carbon double bonds, violaxanthin (9 conjugated double bonds) and zeaxanthin (11 conjugated double bonds) have been compared with regard to their effects in vitro on fluorescence quenching and LHCIIb oligomerization. It was found that auroxanthin stimulated fluorescence quenching, similar to the effect of zeaxanthin and in contrast to the inhibition caused by violaxanthin. Auroxanthin caused an increase in the oligomerization of LHCIIb and an increase in relative emission of long-wavelength fluorescence at 77 K. It is concluded that auroxanthin can mimic the effect of zeaxanthin on LHCII, strongly suggesting that the xanthophyll cycle carotenoids control quenching in vitro by an indirect structural effect and not by direct quenching of chlorophyll excited states.     

Time-resolution of the antheraxanthin- and delta pH-dependent chlorophyll a fluorescence components associated with photosystem II energy dissipation in Mantoniella squamata

The electronic excited-state behavior of photosystem II (PSII) in Mantoniella squamata, as influenced by the xanthophyll cycle and the transthylakoid pH gradient (delta pH), was examined in vivo. Mantoniella is distinguished from other photosynthetic organisms by two main features namely (1) a unique light-harvesting complex that serves both photosystems I (PSI) and II (PSII); and (2) a violaxanthin (V) cycle that undergoes only one de-epoxidation step in excess light to accumulate the monoepoxide antheraxanthin (A) as opposed to the epoxide-free zeaxanthin (Z). The cells were treated first with high light to induce the delta pH and A accumulation, followed by herbicide-induced closure of PSII traps and a chilling treatment, to sustain and stabilize the delta pH and nigericin-sensitive fluorescence level in the dark. De-epoxidation was controlled with subsaturating concentrations of dithiothreitol (DTT) and was 5-10 times more sensitive to DTT than higher plant thylakoids. The PSII energy dissipation involved two steps: (1) the pH activation of the xanthophyll binding site that was associated with a narrowing and slight attenuation of the main 2 ns (ns = 10(-9) s) fluorescence lifetime distribution; and (2) the concentration-dependent binding of A to the activated binding site yielding a second distribution centered around 0.9 ns. Consistent with the model of Gilmore et al. (1998) (Biochemistry 37, 13,582-13,593), the fractional intensity of the 0.9 ns component depended almost entirely on the A concentration and correlated linearly with the decrease of the steady-state chlorophyll alpha fluorescence intensity.     

Ultraviolet Radiation Reduces the Photoprotective Capacity of the Marine Diatom Phaeodactylum tricornutum (Bacillariophyceae, Heterokontophyta)

This study demonstrates that UV radiation (UVR) reduces the photoprotective capacity of the diatom Phaeodactylum tricornutum by affecting xanthophyll cycle (XC) activity. The short‐term reduction of photosystem II (PSII) maximum efficiency of charge separation (F_v/F_m) in cells exposed to UVR could be explained mainly by a reduced photoprotective capacity under this condition. Phaeodactylum tricornutum cells acclimated to two different photosynthetically active radiation (PAR) intensities, high light (HL, 200 μmol quanta m^?2 s^?1) and low light (LL, 50 μmol quanta m^?2 s^?1), were exposed to saturating irradiance (1100 μmol quanta m^?2 s^?1) in the presence (PAR + UVR) and absence of UVR (PAR). HL cells exhibited a greater reduction in F_v/F_m in PAR + UVR when compared with the PAR treatment that was related to a reduction in the de‐epoxidation of XC pigments. In contrast, in LL cells, UVR did not considerably affect XC de‐epoxidation even though the reduction in F_v/F_m was greater than in HL cells. The negative effect of UVR on photoprotection was more pronounced in HL cells because they synthesized more XC pigments than LL cells. This was confirmed when XC activity was blocked with dithiothreitol and when PSII repair was inhibited with chloramphenicol (CAP). The differential reduction of F_v/F_m between PAR + UVR and PAR treatments disappeared when XC was blocked in HL cells. A higher reduction and an incomplete recovery of F_v/F_m were observed in cells incubated with CAP in the presence of UVR. Such responses confirm that UVR had a negative effect on photoprotective mechanisms causing an enhancement of damage by PAR, especially in HL‐acclimated cells in which heat dissipation is important for PSII regulation.     

ENERGY DISSIPATION AND PHOTOPROTECTION MECHANISMS DURING CHLOROPHYLL PHOTOBLEACHING IN THYLAKOID MEMBRANES

The kinetics of chlorophyll photobleaching were followed in whole thylakoid membranes as well as in photosystem I and photosystem II submembrane fractions. The onset of photobleaching was characterized by a slow rate which indicated the presence of energy traps implicated in the photoprotection of the bulk pigments. The pigments in photosystem I submembrane fractions bleached at a faster rate than those in photosystem II counterparts, the latter being more sensitive towards photoinhibition. An analysis of the pigment-protein complexes isolated from whole thylakoid membranes during the course of a photobleaching experiment has shown that the core-antenna complexes, including CP29, are more sensitive to illumination than the peripheral complexes. The absorption spectra of the CPI and CP29 complexes presented a blue shift of the red absorption maximum after partial photobleaching, indicative of a non-homogeneous bleaching of the holochromes in these complexes. An analysis of these data points towards the involvement of CP29 in a photoprotection mechanism at the level of photosystem II. The weaker resistance of photosystem I to photobleaching relative to photosystem II and its stronger resistance to photoinhibition is discussed in terms of an energy dissipation pathway in thylakoid membranes.     

PHOTOSYNTHETIC ACTION SPECTRA AND LIGHT-HARVESTING IN GRIFFITHSIA MONILIS (RHODOPHYTA)

Abstract— In cells of the red alga Griffithsia monilis the action spectrum of photosynthetic oxygen production at low light intensity shows that the phycobilins (including allophycocyanin) are the major light-harvesting pigments. As the light intensity is increased carotenoids and chlorophyll a contribute proportionately more to the spectrum, since the phycobilin activity becomes light-saturated. When action spectra are performed against a background light of various monochromatic wavelengths it can be shown that chlorophyll a increases in its light-harvesting activity. Nevertheless light absorbed at a single wavelength (487 nm) by phycoerythrin (and possibly a carotenoid) still shows the highest photosynthetic activity. Fluorescence measurements at 77K indicate that a chlorophyll a fluorescence is small and that the amount of chlorophyll a _ll (f 693) is very low. A model is proposed in which the phycobilins, in phycobilisomes, pass on absorbed light energy to either photosystem, whereas light absorbed by chlorophyll is passed on mainly to photosystem I.     

IS ZEAXANTHIN CAPABLE OF ENERGY TRANSFER TO CHLOROPHYLL a IN PARTIALLY GREENED LEAVES? A STUDY OF FLUORESCENCE EXCITATION SPECTRA DURING VIOLAXANTHIN DEEPOXIDATION

Absorbance spectra and excitation spectra of chlorophyll a fluoresence were recorded during the light-induced deepoxidation of violaxauthin to zeaxanthin in bean leaves (Phaseolus coccineus) greened under intermittent light. Light minus dark fluorescence excitation difference spectra showed distinct minima at 440, 465, and 500 nm corresponding to maxima in the absorbance difference spectra. Both difference spectra were prevented by the deepoxidase inhibitor dithiothreitol and were inverted when zeaxanthin was epoxidized. The fluorescence excitation difference spectra were successfully modeled by considering the absorbance differences between violaxanthin and zeaxanthin, assuming no energy transfer from the two pigments to chlorophyll a, and accounting for light-induced scattering changes. The pigment stoichiometry and the scattering changes of the simulation were in accordance with experimental data. The results indicate that, in the early stage of leaf development, light absorbed by the cycle pigments violaxanthin and zeaxanthin is not transferred to chlorophyll.     

Characterization of Two Light-Harvesting Subunits Isolated from the Brown Alga Pelvetia canaliculata: Heterogeneity of Xanthophyll Distribution

The main light-harvesting fraction from Pelvetia canaliculata was isolated on a sucrose density gradient from digitonin-solubilized chloroplasts. After further solubilization by dodecyl maltoside, the bulk fraction was separated into two subunits by preparative isoelectric focusing. The more acidic brown fraction was mainly composed of 22 kDa polypeptides having an apparent pI of 4.55. Its pigment composition was very simple, containing chlorophyll (Chi) a, Chi c and fucoxanthin. The in vivo spectral properties of fucoxanthin, namely a shift in light absorption to the green and efficient energy transmission to Chi a, were conserved in this subunit. No xanthophyll associated with photoprotection was found in this band, even when obtained from photoinhibited thalli. The less acidic green band contained predominantly 22 kDa polypeptides that were resolved into numerous components by denaturing isoelectric focusing. Its pigment composition was more complex, containing, in addition, pigments of the so-called xanthophyll cycle. In photoinhibited thalli, about half of the violaxanthin was converted into antheraxanthin and zeaxanthin. All the pigments of the xanthophyll cycle were specifically associated with this subunit, and it may thus have a central role in the thermal dissipation of the absorbed light energy as postulated for light-harvesting complex II isolated from green plants.     

Contribution of LHC II complex to the electric properties of thylakoid membranes: an electric light scattering study of Chl b-less barley mutant

Electric light scattering measurements demonstrate a strong decline in the permanent electric dipole moment and electric polarizability of both thylakoid membranes and photosystem II-enriched particles of the Chlorina f2 mutant which has severely reduced levels of light-harvesting chlorophyll a/b-binding proteins compared to the wild type barley chloroplasts. The shift in the electric polarizability relaxation to higher frequencies in thylakoids and photosystem II particles from Chlorina f2 reflects higher mobility of the interfacial charges of the mutant than that of the wild type membranes. The experimental data strongly suggest that the major light-harvesting complex of photosystem II directly contribute to the electric properties of thylakoid membranes.     

Effect of ultraviolet-B radiation on intact cells of the cyanobacterium Spirulina platensis: characterization of the alterations in the thylakoid membranes

Intact trichomes of Spirulina platensis are exposed to ultraviolet- B (UV-B) radiation (270-320 nm; 1.9 mW m(-2)) for 9 h. This UV-B exposure results in alterations in the pigment-protein complexes and in the fluorescence emission profile of the chlorophyll-protein complexes of the thylakoids as compared with thylakoids isolated from control dark-adapted Spirulina cells. The UV-B exposure causes a significant decrease in photosystem II activity, but no loss in photosystem I activity. Although there is no change in the photosystem I activity in thylakoids from UV-B-exposed cells, the chlorophyll a emission at room temperature and at 77 K indicates alterations associated with photosystem I. Additionally, the results clearly demonstrate that the photosystem II core antennae of chlorophyll proteins CP47 and CP43 are affected by UV-B exposure, as revealed by Western blot analysis. Furthermore, a prominent 94 kDa protein band appears in the sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) profile of UV-B-exposed cell thylakoids, which is absent from the control thylakoids. This 94 kDa protein appears not to be newly induced by UV-B exposure, but could possibly have originated from the UV-B-induced cross-linking of the thylakoid proteins. The exposure of isolated Spirulina thylakoids to the same intensity of UV-B radiation for 1-3 h induces losses in the CP47 and CP43 levels, but does not induce the appearance of the 94 kDa protein band in SDS-PAGE. These results clearly demonstrate that prolonged exposure of Spirulina cells to moderate levels of UV-B affects the chlorophyll a-protein complexes and alters the fluorescence emission spectral profile of the pigment-protein complexes of the thylakoid membranes. Thus, it is clear that chlorophyll a antennae of Spirulina platensis are significantly altered by UV-B radiation.     

Dynamic regulation of photoprotection determines thermal tolerance of two phylotypes of Symbiodinium clade A at two photon fluence rates

Coral bleaching is the manifestation of the dysfunction of the symbiosis between scleractinian corals and dinoflagellates of the diverse genus Symbiodinium and is induced by elevated temperatures and high irradiance. We investigated the photophysiological response of two genetically distinct Symbiodinium subtypes within clade A upon exposure to elevated temperatures at two light intensities for 3 weeks. While both subtypes displayed a characteristic photoacclimation to high light (HL) (decrease in light-harvesting pigments, lower photochemical efficiency of photosystem II, increased xanthophyll pool sizes), the tolerance toward thermal stress clearly differed between the two subtypes. Symbiodinium Ax was highly susceptible to chronic photoinhibition at temperatures ≥30°C, which was exacerbated under HL conditions. A1 showed a capacity for photoacclimation and high thermal tolerance, which might be related to higher cellular concentrations of photoprotective xanthophylls and the low-molecular antioxidant glutathione (GSx) along with the dynamic regulation of these photoprotective pathways. Whereas HL conditions induced both accumulation of diatoxanthin and GSx, thermal stress further stimulated xanthophyll cycling, which might compensate for diminished amounts of GSx at elevated temperatures. Our results show that the two clade A subtypes clearly differ in their strategies to cope with thermal stress in combination with high irradiance.     

ENERGY TRANSFER IN A LIGHT-HARVESTING CAROTENOID-CHLOROPHYLL c-CHLOROPHYLL a-PROTEIN OF PHAEODACTYLUM TRICORNUTUM

Abstract— The marine diatom Phaeodactylum tricornutum was readily disrupted in 0.1 N Tris-HCl buffer, pH 7.8, in a Braun Model MSK cell homogenizer at 0-5°C. Treatment of the suspension with sodium lauryol sarcosinate (3 molecules per 10000 daltons of protein) at 5°C in the dark and subsequent centrifugations produced a pigmented, protein fraction whose excitation spectrum exhibited energy transfer from carotenoids to chlorophyll a (Chi a ). Disruption of the pigment-protein complex by heating in 1% sodium dodecylsulfate resulted in loss of energy transfer. For each Chi a molecule this fraction had 1 Chi c , 4 fucoxanthin, and 6.7 accessory pigment molecules. Presence of the accessory complex of Photosystem II in this preparation is suggested by the high xanthophyll content. Further, based on Chl a concentrations, this fraction had about 18 times more apparent fluorescence emission at 680 nm when excited at 470 nm than the intact cells.     

Modeling of photosynthetic light-harvesting: From structure to function

In order to bridge the gap between the crystal structure of photosynthetic pigment-protein complexes and the data gathered in optical experiments, two essential problems need to be solved. On one hand, theories of optical spectra and excitation energy transfer have to be developed that take into account the pigment-pigment (excitonic) and the pigment-protein (exciton-vibrational) coupling on an equal footing. On the other hand, the parameters entering these theories need to be calculated from the structural data. Good agreement between simulations and experimental data then allows to draw conclusions on structure-function relationships of these complexes and to make predictions. In the development of theory, a delicate question is how to describe the interplay between the quantum dynamics of excitons and the dephasing of coherences by the coupling of excitons to protein vibrations. Quantum mechanic coherences are utilized for efficient light harvesting. In the reaction centers of purple bacteria an energy sink is created by a coherent coupling of exciton states to intermolecular charge transfer states. The dephasing of coherences can be monitored, e.g., by the temperature dependent shift of optical lines. In the Fenna-Matthews-Olson protein, which acts as an excitation energy wire between the outer chlorosome antenna and the reaction center complex, an energy funnel for efficient light-harvesting is formed by the pigment-protein coupling. The protein shifts the local transition energies of the pigments, the so-called site energies in a specific way, such that pigments facing the reaction center are redshifted with respect to those on the chlorosome side. In the light-harvesting complex of higher plants an excitation energy funnel is created by the use of two different types of chlorophyll (Chl) pigments, Chla and Chlb and by the pigment-protein coupling that creates an energy sink at Chla 610 located in the stromal layer at the periphery of the complex. The close contact between Chla and Chlb gives rise to ultrafast subpicosecond exciton transfer, whereas dynamic localization effects are inferred to lead to long ps relaxation times between the majority of Chla pigments.