Photosynthetic action spectra of marine algae

A polarographic oxygen determination, with tissue in direct contact with a stationary platinum electrode, has been used to measure the photosynthetic response of marine algae. These were exposed to monochromatic light, of equal energy, at some 35 points through the visible spectrum (derived from a monochromator). Ulva and Monostroma (green algae) show action spectra which correspond very closely to their absorption spectra. Coilodesme (a brown alga) shows almost as good correspondence, including the spectral region absorbed by the carotenoid, fucoxanthin. In green and brown algae, light absorbed by both chlorophyll and carotenoids seems photosynthetically effective, although some inactive absorption by carotenoids is indicated. Action spectra for a wide variety of red algae, however, show marked deviations from their corresponding absorption spectra. The photosynthetic rates are high in the spectral regions absorbed by the water-soluble "phycobilin" pigments (phycoerythrin and phycocyanin), while the light absorbed by chlorophyll and carotenoids is poorly utilized for oxygen production. In red algae containing chiefly phycoerythrin, the action spectrum closely resembles that of the water-extracted pigment, with peaks corresponding to its absorption maxima (495, 540, and 565 mµ). Such algae include Delesseria, Schizymenia, and Porphyrella. In the genus Porphyra, there is a series P. nereocystis, P. naiadum, and P. perforata, with increasingly more phycocyanin and less phycoerythrin: the action spectra reflect this, with increasing activity in the orange-red region (600 to 640 mµ) where phycocyanin absorbs. In all these red algae, photosynthesis is almost minimal at 435 mµ and 675 mµ, where chlorophyll shows maximum absorption. Although the chlorophylls (and carotenoids) are present in quantities comparable to the green algae, their function is apparently not that of a primary light absorber; this role is taken over by the phycobilins. In this respect the red algae (Rhodophyta) appear unique among photosynthetic plants.     

THE PHOTOSYNTHETIC EFFICIENCY OF PHYCOCYANIN IN CHROOCOCCUS,AND THE PROBLEM OF CAROTENOID PARTICIPATION IN PHOTOSYNTHESIS

The absorption spectra of the principal pigment components extracted from Chroococcus cells have been measured, and their sum compared with the absorption of a suspension of living cells. The agreement was sufficiently close so that it was concluded the absorption spectra of the extracted and separated pigment components could be used to obtain estimates of the relative absorption of the various components in the living cells. The quantum yield of Chroococcus photosynthesis was measured at a succession of wave lengths throughout the visible spectrum, and the dependence of yield on wave length was compared with the proportions of light absorbed by the pigment components. This comparison showed beyond reasonable doubt that the light absorbed by phycocyanin is utilized in photosynthesis with an efficiency approximately equal to that of the light absorbed by chlorophyll. The light absorbed by the carotenoid pigments of Chroococcus seems for the most part to be unavailable for photosynthesis. The results leave open the possibility that light absorbed by the carotenoids is active in photosynthesis, but with an efficiency considerably lower than that of chlorophyll and phycocyanin. It is also possible that the light absorbed by one or a few of the several carotenoid components is utilized with a high efficiency, while the light absorbed by most of the components is lost for photosynthesis.     

Incompetence of dark-synthesized phycocyanin in excitation transfer to photosystem II chlorophyll

Summary When cells of Anabaena variabilis, all the phycobilin pigments of which had been newly synthesized in the dark, were excited by light absorbed in phycocyanin, the fluorescence emission spectrum showed a peak corresponding to the emission from allophycocyanin, but no emission from chlorophyll. These cells were active in photosynthesis and, when excited by light absorbed by chlorophyll, the emitted fluorescence was characteristic of photosystem II chlorophyll. This indicates that dark synthesized phycocyanin is capable of excitation transfer to allophycocyanin but not to photosystem II chlorophyll.Abbreviation CMU 3-(p-chlorophenyl)-1,1-dimethylurea     

DIFFERENCES BETWEEN IN VIVO ABSORPTION AND FLUORESCENCE EXCITATION SPECTRA IN NATURAL SAMPLES OF PHYTOPLANKTON

The fluorescence excitation spectrum of live phytoplankton cells represents the portion of light absorbed that has been effectively transferred to chlorophyll a of photosystem II, whereas light absorbed by photoprotective pigments will not lead to fluorescence. Therefore, the in vivo fluorescence excitation spectrum of phytoplankton has been used as a proxy for the action spectrum of phytoplankton in computations of primary production in the ocean. The distribution of chlorophyll a between photosystems, as well as variations in the pathway of energy inside the photosynthetic membrane, can also influence the fluorescence excitation spectrum. In this study, we investigated the contribution of photoprotective pigments to the differences found between in vivo absorption and fluorescence excitation spectra of phytoplankton measured during two cruises: one from Las Islas Canarias to Nova Scotia and another in the Labrador Sea. A comparison of normalized fluorescence excitation and absorption spectra showed high variability in the difference between absorption and fluorescence in the blue region of the spectrum for samples from the two cruises. This difference was not entirely correlated with the concentration of photoprotective carotenoids. In this paper, results are interpreted in terms of differences in pigment composition and known patterns of energy distribution in the photosystems of different algal groups.     

Picosecond energy transfer in Porphyridium cruentum and Anacystis nidulans.

Picosecond energy transfer is measured in Anacystis nidulans and Porphyridium cruentum. Fluorescence is sensitized by a 6-ps laser flash, at 530 nm. The time dependence of fluorescence is measured with reference to the laser pulse. Fluorescence is recorded from phycoerythrin (576 nm), R-phycocyanin (640 nm), allophycocyanin (666 nm), Photosystem II chlorophyll (690 nm) and long wave length chlorophyll (715 nm). Energy transfer measurements are made at 37 degrees C, 23 degrees C, and 0 degrees C, and 77 degrees K. It is shown that the rate of energy transfer can be varied with temperature. In both A. nidulans and P. cruentum there is a sequential transfer of excitation energy from phycoerythrin to phycocyanin to allophycocyan to Photosystem II chlorophyll fluorescence. The long wavelength chlorophyll fluorescence at 715 nm, however, does not always follow a sequential transfer of excitation energy. Depending on the temperature, fluorescence at 715 nm can precede fluorescence from phycocyanin.     

Transfer of the Excitation Energy in Anacystis nidulans Grown to Obtain Different Pigment Ratios

The blue-green alga, Anacystis nidulans, was grown in lights of different colors and intensities, and its absorption and fluorescence properties were studied. Strong orange light, absorbed mainly by phycocyanin, causes reduction in the ratio of phycocyanin to chlorophyll a; strong red light, absorbed mainly by chlorophyll, causes an increase in this ratio. This confirms the earlier findings of Brody and Emerson (12) on Porphyridum, and of Jones and Myers (8) on Anacystis. Anacystis cultures grown in light of low intensity show, upon excitation of phycocyanin, emission peaks at 600 mmu and 680 mmu, due to the fluorescence of phycocyanin and chlorophyll a, respectively. Changes in the efficiency of energy transfer from phycocyanin to chlorophyll a are revealed by changes in the ratios of these two bands. A decrease in efficiency of energy transfer from phycocyanin to chlorophyll a seems to occur whenever the ratio of chlorophyll a to phycocyanin deviates from the normal. Algae grown in light of high intensity show, upon excitation of phycocyanin, only a fluorescence band at 660 mmu and no band at 680 mmu. This suggests reduced efficiency of energy transfer from phycocyanin to the strongly fluorescent form of chlorophyll a (chlorophyll a(2)) and perhaps increased transfer to the weakly fluorescent form of chlorophyll a (chlorophyll a(1)).     

Energy transfers from Photosystem II to Photosystem I in Cryptomonas rufescens (Cryptophyceae)

In Cryptomonas rufescens (Cryptophyceae), phycoerythrin located in the thylakoid lumen is the major accessory pigment. Oxygen action spectra prove phycoerythrin to be efficient in trapping light energy.The fluorescence excitation spectra at ?196°C obtained by the method of Butler and Kitajima (Butler, W.L. and Kitajima, M. (1975) Biochim. Biophys. Acta 396, 72–85) indicate that like in Rhodophycease, chlorophyll a is the exclusive light-harvesting pigment for Photosystem I.For Photosystem II we can observe two types of antennae: (1) a light-harvesting chlorophyll complex connected to Photosystem II reaction centers, which transfers excitation energy to Photosystem I reaction centers when all the Photosystem II traps are closed. (2) A light-harvesting phycoerythrin complex, which transfers excitation energy exclusively to the Photosystem II reaction complexes responsible for fluorescence at 690 nm.We conclude that in Cryptophyceae, phycoerythrin is an efficient light-harvesting pigment, organized as an antenna connected to Photosystem II centers, antenna situated in the lumen of the thylakoid. However, we cannot afford to exclude that a few parts of phycobilin pigments could be connected to inactive chlorophylls fluorescing at 690 nm.     

Photochromic pigments in akinetes and pigment characteristics of akinetes in comparison with vegetative cells of Anabaena variabilis

Since akinete germination is triggered by light and the action spectrum for this process has features in common with the spectra of the two photochromic pigments, phycochromes b and d, a search was made for the presence of these phycochromes in akinetes of the blue-green alga. Anabaena variabilis Kützing. Allophycocyanin-B was also looked for, since the action spectrum for akinete germination points to a possible participation of this pigment too. Isoelectric focusing was used for purification of the pigments. The different fractions were investigated for phycochromes b and d by measuring the absorbance difference spectra: for phycochrome b. 500 nm irradiated minus 570 nm irradiated, and for phycochrome d, 650 nm irradiated minus 610 nm irradiated. For determination of allophycocyanin-B. fourth derivative analysis of absorption spectra was made for some of the fractions from the isoelectric focusing column. Phycochrome b was also assayed for by measuring in vivo absorption difference spectra. The assays were positive for all three pigments. The complete photosynthetic pigment systems were also studied by in vivo fluorescence measurements on both akinetes and vegetative cells of Anabaena variabilis. Fluorescence emission and excitation spectra at selected emission wavelengths were measured at room temperature and liquid nitrogen temperature. The energy transfer from phycoerythrocyanin to phycocyanin is very efficient under all conditions, as is the energy transfer from phycocyanin to allophycocyanin at room temperature. At low temperature, however, phycocyanin is partly decoupled from allophycocyanin, particularly in the akinetes; the energy transfer from allophycocyanin to chlorophyll a is less efficient at low temperature in both types of cells, but especially in akinetes. Delayed light emission was measured for both types of cells and found to be very weak in akinetes compared to vegetative cells. From this study it would seem that akinetes lack an active photosystem II, although the 691 nm peak in the 570 nm excited low temperature fluorescence emission spectrum proves the presence of photosystem II chlorophyll, and also its energetic connection to the phycobilisomes.     

Spectral properties of the CP43-deletion mutant of Synechocystis sp. PCC 6803

Spectral properties, particularly fluorescence spectra and their time-dependent behavior, were investigated for a mutant of the cyanobacterium Synechocystis sp. PCC 6803 lacking the 43 kDa chlorophyll-protein (CP43, PsbC). Lack of CP43 was confirmed by a size shift of the corresponding gene and by Western blotting. The CP43-deletion mutant grown under heterotrophic conditions accumulated a small amount of photosystem (PS) II, but virtually no PS II fluorescence was observed. A 686-nm fluorescence band was clearly observed by phycocyanin excitation, coming from the terminal pigments of phycobilisomes. In contrast, no PS I fluorescence was detected by phycocyanin excitation when accumulation of PS II components was not proved by a fluorescence excitation spectrum, indicating that energy transfer to PS I chlorophyll a was mediated by PS II chlorophyll a. Direct connection of phycobilisomes with PS I was not suggested. Based on these fluorescence properties, the energy flow in the CP43-deletion mutant cells is discussed.     

Regulation of the distribution of chlorophyll and phycobilin-absorbed excitation energy in cyanobacteria. A structure-based model for the light state transition

The light state transition regulates the distribution of absorbed excitation energy between the two photosystems (PSs) of photosynthesis under varying environmental conditions and/or metabolic demands. In cyanobacteria, there is evidence for the redistribution of energy absorbed by both chlorophyll (Chl) and by phycobilin pigments, and proposed mechanisms differ in the relative involvement of the two pigment types. We assayed changes in the distribution of excitation energy with 77K fluorescence emission spectroscopy determined for excitation of Chl and phycobilin pigments, in both wild-type and state transition-impaired mutant strains of Synechococcus sp. PCC 7002 and Synechocystis sp. PCC 6803. Action spectra for the redistribution of both Chl and phycobilin pigments were very similar in both wild-type cyanobacteria. Both state transition-impaired mutants showed no redistribution of phycobilin-absorbed excitation energy, but retained changes in Chl-absorbed excitation. Action spectra for the Chl-absorbed changes in excitation in the two mutants were similar to each other and to those observed in the two wild types. Our data show that the redistribution of excitation energy absorbed by Chl is independent of the redistribution of excitation energy absorbed by phycobilin pigments and that both changes are triggered by the same environmental light conditions. We present a model for the state transition in cyanobacteria based on the x-ray structures of PSII, PSI, and allophycocyanin consistent with these results.     

Analysis of photosynthetic pigments and chlorophyll fluorescence characteristics of different strains of Porphyra yezoensis

The levels of photosynthetic pigments and chlorophyll fluorescence of Porphyra yezoensis strains selected from high-light environments were investigated. Sutong and Sulian strains originated from the same high-light environment but were selected from different sites on the Yellow Sea coast of Jiangsu Province, China. In January (a low temperature period), the Sulian strain and the WT (a widely cultivated strain) had higher levels of chlorophyll a, phycoerythrin, phycocyanin, and allophycocyanin, and higher actual photochemical efficiency of PSII (ΔF/F _m′) than the Sutong strain. This indicated that Sulian and the WT may have better adaptation to low temperature. In March (an optimal temperature period), Sutong had higher levels of photosynthetic pigments and higher ΔF/F _m′ than the WT and Sulian strains. This suggested that Sutong had higher light use efficiency at optimal temperatures and that most energy absorbed by PSII was used for photosynthetic electron transport. The differing areas of origin of these strains may have resulted in these differences in temperature adaptation.     

SINGLE-CELL PIGMENTATION OF PORPHYRA LINEARIS ANALYZED BY FOURIER TRANSFORM MULTI-PIXEL SPECTROSCOPY AND IMAGE ANALYSIS1

The novel method of Fourier transform multi-pixel spectroscopy was used for the nondestructive analysis of and comparison of pigmentation in different regions of live thalli of the red alga Porphyra linearis. Because the thallus in this alga consists of a monolayer of nonoverlapping cells, we were able to analyze the pigmentation of single cells by combining light absorbance with natural fluorescence data. From the image of each cell in the vegetative male and female reproductive and holdfast regions, more than 4 ± 10⁴ fluorescence and absorbance spectra were obtained. Specific pigments in the different regions were localized by the use of a software program of similarity mapping followed by image construction. The reconstructed images revealed subcellular localization of each pigment according to specific spectroscopic fingerprints. The results showed that the vegetative and female reproductive cell types had a significantly higher content of phycoerythrin than of phycocyanin, and quite similar chlorophyll a levels. Most of the holdfast cells were poorly pigmented, but had more chlorophyll a than phycoerythrin or phycocyanin. The male reproductive cells contained only traces of pigments. Thus, by using Fourier transform multipixel spectroscopy, we were able to characterize the pigmentation of different regions of the thallus and follow the distribution patterns of the different pigments on the subcellular level along the differentiation gradient of the alga.     

Photosynthetic efficiency of marine plants

Multicellular marine plants were collected from their natural habitats and the quantum efficiency of their photosynthesis was determined in the laboratory in five narrow wave length bands in the visible spectrum. The results along with estimates of the relative absorption by the various plastid pigments show a fairly uniform efficiency of 0.08 molecules O₂ per absorbed quantum for (a) chlorophyll of one flowering plant, green algae, and brown algae, (b) fucoxanthol and other carotenoids of brown algae, and (c) the phycobilin pigments phycocyanin and phycoerythrin of red algae. The carotenoids of green algae are sometimes less efficient while those of red algae are largely or entirely inactive. Chlorophyll a of red algae is about one-half as efficient (_o2 = 0.04) as either the phycobilins, or the chlorophyll of most other plants. These results as well as those of high intensity and of fluorescence experiments are consistent with a mechanism in which about half the chlorophyll is inactive while the other half is fully active and is an intermediate in phycoerythrin- and phycocyanin-sensitized photosynthesis.     

A quantitative description of fluorescence excitation spectra in intact bean leaves greened under intermittent light

We present a simple approach for the calculation of in vivo fluorescence excitation spectra from measured absorbance spectra of the isolated pigments involved. Taking into account shading of the pigments by each other, energy transfer from carotene to chlorophyll a, and light scattering by the leaf tissue, we arrive at a model function with 6 free parameters. Fitting them to the measured fluorescence excitation spectrum yields good correspondence between theory and experiment, and parameter estimates which agree with independent measurements. The results are discussed with respect to the origin and the interpretation of in vivo excitation spectra in general.     

Effect of anthocyanins, carotenoids, and flavonols on chlorophyll fluorescence excitation spectra in apple fruit: signature analysis, assessment, modelling, and relevance to photoprotection

Whole apple fruit (Malus domestica Borkh.) widely differing in pigment content and composition has been examined by recording its chlorophyll fluorescence excitation and diffuse reflection spectra in the visible and near UV regions. Spectral bands sensitive to the pigment concentration have been identified, and linear models for non-destructive assessment of anthocyanins, carotenoids, and flavonols via chlorophyll fluorescence measurements are put forward. The adaptation of apple fruit to high light stress involves accumulation of these protective pigments, which absorb solar radiation in broad spectral ranges extending from UV to the green and, in anthocyanin-containing cultivars, to the red regions of the spectrum. In ripening apples the protective effect in the blue region could be attributed to extrathylakoid carotenoids. A simple model, which allows the simulation of chlorophyll fluorescence excitation spectra in the visible range and a quantitative evaluation of competitive absorption by anthocyanins, carotenoids, and flavonols, is described. Evidence is presented to support the view that anthocyanins, carotenoids, and flavonols play, in fruit with low-to-moderate pigment content, the role of internal traps (insofar as they compete with chlorophylls for the absorption of incident light in specific spectral bands), affecting thereby the shape of the chlorophyll fluorescence excitation spectrum.     

The photochemical and fluorescence properties of whole cells,spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum

The photochemical activities and fluorescence properties of cells, spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum were compared. The photochemical activities were measured in a whole range of wavelengths and expressed as quantum yield spectra (quantum yield vs. wavelength). The following reactions were measured: Photosynthesis (O₂ evolution) in whole cells; Hill reaction (O₂ evolution) with Fe(CN)₆^3? and NADP as electron acceptors (Photosystem II and Photosystem II+Photosystem I reactions); electron transfer from reduced 2,6-dichlorophenolindophenol to diquat (Photosystem I reaction). The fluorescence properties were emission spectra, quantum yield spectra and the induction pattern.On the basis of comparison between the quantum yield spectra and the pigments compositions the relative contribution of each pigment to each photosystem was estimated. In normal cells and spheroplasts it was found that Photosystem I (Photosystem II) contains about 90 % (10 %) of the chlorophyll a, 90 % (10 %) of the carotenoids and 15 % (85 %) of the phycocyanin. In spheroplast particles there is a reorganization of the pigments: they loose a certain fraction (about half) of the phycocyanin but the remaining phycocyanin attaches itself exclusively to Photosystem I (!). This is reflected by the loss of Photosystem II activity, a flat quantum yield vs. wavelength dependence and a loss of the fluorescence induction.The fluorescence quantum yield spectra conform qualitatively to the above conclusion. More quantitative estimation shows that only a fraction (20–40 %) of the chlorophyll of Photosystem II is fluorescent. Total emission spectrum and the ratio of variable to constant fluorescence are in agreement with this conclusion.The fluorescence emission spectrum shows characteristic differences between the constant and variable components. The variable fluorescence comes exclusively from chlorophyll a; the constant fluorescence is contributed, in addition to chlorophyll a, by phycocyanine and an unidentified long wavelength component.The variable fluorescence does not change in the transition from whole cells to spheroplasts. However, the constant fluorescence increases considerably. This indicates the release of a small fraction of pigments from the photosynthetic photochemical apparatus which then become fluorescent.     

Fluorometric Characterization of Two Picocyanobacteria Strains from Lakes of Different Underwater Light Quality

Unicellular autofluorescent picoplankton ranging from 0.6 to 0.9 ym in diameter were isolated from Lake Maggiore and from Lake Balaton. The cyanobacterial isolates contain two accessory pigments: phycoerythrin (PE) and phycocyanin (PC) respectively. The in vivo spectral properties of the two clones were compared to identify characteristics of the pigments. In vivo fluorescence excitation and emission spectra revealed that clones with PE are composed only of phycoerythrobilin chromophores and lack phycourobilin. Glycerol treatment enhances the fluorescence yield up to 3 times and improves the detection sensitivity of PE particularly at 436 and 520 nm and of PC at 600 nm. The vertical profiles of the underwater irradiance at different wavelengths were measured in both lakes to study the light quality of the natural environment of the two strains. Growth rates of both clones growing at different light intensities and wavelengths, selected by the same filters used for vertical profiles, were estimated. The results showed a difference in growth rate of phycoerythrin and phycocyanin containing cells exposed to an equal quantum flux of preferential illumination. In particular the maximum growth rate was reached by PE cells exposed to green light and by PC cells exposed to red light.     

In vitro reassociation of phycobiliproteins and membranes to form functional membrane-bound phycobilisomes

A membrane-bound phycobilisome complex has been isolated from the cyanobacterium Fremyella diplosiphon grown in green light, thus containing phycoerythrin in addition to phycocyanin and allophycocyanin. The complex was dissociated by lowering the salt concentration. In the mixture obtained, no energy transfer from phycoerythrin to chlorophyll (Chl) a was observed. Reassociation of the phycobiliproteins and membrane mixture was carried out by a gradual increase of the salt concentration. The complex obtained after reassociation was characterized by polypeptide composition, absorbance and fluorescence emission spectra and electron microscopy. These analyses revealed similar composition and structure for the original and reconstituted membrane-bound phycobilisomes. Fluorescence emission spectra and measurements of Photosystem II activity demonstrated energy transfer from phycoerythrin to Chl a (Photosystem II) in the reconstituted complex. Reassociation of mixtures with varying phycoerythrin / Chl ratio showed that the phycobiliprotein concentration was critical in the reassociation process. Measurements of the amount of phycobilisomes reassociated with the photosynthetic membrane did not show saturation of binding when increasing the phycobiliprotein concentration. The ratio phycoerythrin / Chl a in the native complex was 7:1 (mg / mg). When the phycobiliprotein concentration was increased during the reassociation process, a ratio of 13–15 mg phycoerythrin / mg Chl a could be obtained. Under these conditions, only part of the phycobilisomes attached to the thylakoids was able to transfer energy to Photosystem II.     

ACCLIMATION OF THE PHOTOSYNTHETIC APPARATUS OF PALMARIA PALMATA (RHODOPHYTA) TO LIGHT QUALITIES THAT PREFERENTIALLY EXCITE PHOTOSYSTEM I OR PHOTOSYSTEM II1

Acclimation of the photosynthetic apparatus to light absorbed primarily by phycobilisomes (which transfer energy predominantly to photosystem II) or absorbed by chlorophyll a (mainly present in the antenna of photosystem I) was studied in the macroalga Palmaria palmata L. In addition, the influence of blue and yellow light, exciting chlorophyll a and phycobilisomes, respectively, ivas investigated. All results were compared to a white light control. Complementary chromatic adaptation in terms of an enhanced ratio of phycoerythrin to phycocyanin under green light conditions was observed. Red light (mainly absorbed by chlorophyll a) and green light (mainly absorbed by phycobilisomes) caused an increase of the antenna system, which was not preferentially excited. Yellow and blue light led to intermediate states comparable to each other and white light. Growth was reduced under all light qualities in comparison to white light, especially under conditions preferably exciting phycobilisomes (green light-adapted algae had a 58% lower growth rate compared to white light-adapted algae). Red and blue light-adapted algae showed maximal photosynthetic capacity with white light excitation and significantly lower values with green light excitation. In contrast, green and yellow light-adapted algae exhibited comparable photosynthetic capacities at all excitation wavelengths. Low-temperature fluorescence emission analysis showed an increase of photosystem II emission in red light-adapted algae and a decrease in green light-adapted algae. A small increase of photosystem I emission teas also found in green light-adapted algae, but this was much less than the photosystem II emission increase observed in red light-adapted algae (both compared to phycobilisome emission). Efficiency of energy transfer from phycobilisomes to photosystem II was higher in red than in green light-adapted algae. The opposite was found for the energy transfer efficiency from phycobilisomes to photosystem I. Zeaxanthin content increased in green and blue light-adapted algae compared to red, white, and yellow light-adapted algae. Results are discussed in comparison to published data on unicellular red algae and cyanobacteria.     

Seasonal changes of excitation energy transfer and thylakoid stacking in the evergreen tree Taxus cuspidata: How does it divert excess energy from photosynthetic reaction center?

Photosystems must efficiently dissipate absorbed light energy under freezing conditions. To clarify the energy dissipation mechanisms, we examined energy transfer and dissipation dynamics in needles of the evergreen plant Taxus cuspidata by time-resolved fluorescence spectroscopy. In summer and autumn, the energy transfer processes were similar to those reported in other higher plants. However, in winter needles, fluorescence lifetimes became shorter not only in PSII but also in PSI, indicating energy dissipation in winter needles. In addition, almost the same fluorescence spectra were obtained with different excitation wavelengths. In contrast, the fluorescence spectrum showed a large difference due to excitation wavelength in spring needles. The fluorescence spectrum of spring needles in 550-nm excitation showed similar spectra to that of winter needles, however, red-chlorophyll fluorescence was not observed in chlorophyll excitation. These observations suggest that some complexes with some kind of red-shifted carotenoid and red-chlorophyll unlink from the core complex in spring. Seasonal changes of excitation energy dynamics are also discussed in relation to changes in thylakoid stacking.