Spectral analysis on origination of the bands at 437 nm and 475.5 nm of chlorophyll fluorescence excitation spectrum in Arabidopsis chloroplasts

Chlorophyll fluorescence has been often used as an intrinsic optical molecular probe to study photosynthesis. In this study, the origin of bands at 437 and 475.5 nm in the chlorophyll fluorescence excitation spectrum for emission at 685 nm in Arabidopsis chloroplasts was investigated using various optical analysis methods. The results revealed that this fluorescence excitation spectrum was related to the absorption characteristics of pigment molecules in PSII complexes. Moreover, the excitation band centred at 475.5 nm had a blue shift, but the excitation band at 437 nm changed relatively less due to induction of non‐photochemical quenching (NPQ). Furthermore, fluorescence emission spectra showed that this blue shift occurred when excitation energy transfer from both chlorophyll b (Chl b) and carotenoids (Cars) to chlorophyll a (Chl a) was blocked. These results demonstrate that the excitation band at 437 nm was mainly contributed by Chl a, while the excitation band at 475.5 nm was mainly contributed by Chl b and Cars. The chlorophyll fluorescence excitation spectrum, therefore, could serve as a useful tool to describe specific characteristics of light absorption and energy transfer between light‐harvesting pigments. Copyright © 2015 John Wiley & Sons, Ltd.     

A fluorometric method for the differentiation of algal populations in vivo and in situ

Fingerprints of excitation spectra of chlorophyll (Chl) fluorescence can be used to differentiate `spectral groups' of microalgae in vivo and in situ in, for example, vertical profiles within a few seconds. The investigated spectral groups of algae (green group, Chlorophyta; blue, Cyanobacteria; brown, Heterokontophyta, Haptophyta, Dinophyta; mixed, Cryptophyta) are each characterised by a specific composition of photosynthetic antenna pigments and, consequently, by a specific excitation spectrum of the Chl fluorescence. Particularly relevant are Chl a, Chl c, phycocyanobilin, phycoerythrobilin, fucoxanthin and peridinin. A laboratory-based instrument and a submersible instrument were constructed containing light-emitting diodes to excite Chl fluorescence in five distinct wavelength ranges. Norm spectra were determined for the four spectral algal groups (several species per group). Using these norm spectra and the actual five-point excitation spectrum of a water sample, a separate estimate of the respective Chl concentration is rapidly obtained for each algal group. The results of dilution experiments are presented. In vivo and in situ measurements are compared with results obtained by HPLC analysis. Depth profiles of the distribution of spectral algal groups taken over a time period of few seconds are shown. The method for algae differentiation described here opens up new research areas, monitoring and supervision tasks related to photosynthetic primary production in aquatic environments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Excitation relaxation dynamics and energy transfer in fucoxanthin–chlorophyll a/c-protein complexes,probed by time-resolved fluorescence

In algae, light-harvesting complexes contain specific chlorophylls (Chls) and keto-carotenoids; Chl a, Chl c, and fucoxanthin (Fx) in diatoms and brown algae; Chl a, Chl c, and peridinin in photosynthetic dinoflagellates; and Chl a, Chl b, and siphonaxanthin in green algae. The Fx–Chl a/c-protein (FCP) complex from the diatom Chaetoceros gracilis contains Chl c₁, Chl c₂, and the keto-carotenoid, Fx, as antenna pigments, in addition to Chl a. In the present study, we investigated energy transfer in the FCP complex associated with photosystem II (FCPII) of C. gracilis. For these investigations, we analyzed time-resolved fluorescence spectra, fluorescence rise and decay curves, and time-resolved fluorescence anisotropy data. Chl a exhibited different energy forms with fluorescence peaks ranging from 677 nm to 688 nm. Fx transferred excitation energy to lower-energy Chl a with a time constant of 300 fs. Chl c transferred excitation energy to Chl a with time constants of 500–600 fs (intra-complex transfer), 600–700 fs (intra-complex transfer), and 4–6 ps (inter-complex transfer). The latter process made a greater contribution to total Chl c-to-Chl a transfer in intact cells of C. gracilis than in the isolated FCPII complexes. The lower-energy Chl a received excitation energy from Fx and transferred the energy to higher-energy Chl a. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

A diatom light-harvesting pigment-protein complex : purification and characterization

A light-harvesting pigment-protein complex was isolated from the diatom Phaeodactylum tricornutum using the zwitterionic detergent CHAPS (3-[3-cholamidopropyl)dimethylammonio]-1-propanesulfonate). Detergent-solubilized membranes were fractionated by sucrose density gradient centrifugation into three components. The medium density fraction contained chlorophyll a, chlorophyll c, and fucoxanthin. This fraction was purified by DEAE-ion exchange chromatography, and contained chlorophyll a, chlorophyll c, and fucoxanthin in a molar ratio of 2.4:1.0:4.8. Fluorescence emission and excitation spectra of the isolated complex demonstrated that light energy absorbed by chlorophyll c and fucoxanthin was coupled to chlorophyll a fluorescence. Upon denaturation, the apoprotein yielded a polypeptide doublet at 17.5 to 18.0 kilodaltons which accounted for 30 to 40% of the toal membrane protein. These findings indicate that this pigment-protein complex is a major component of the diatom photosynthetic lammellae. The quantitative amino acid composition of the apoprotein was very similar to those reported for other membrane-bound pigment-protein complexes. Based on the protein to chlorophyll a ratio of 7700 grams protein per mole chlorophyll a for the complex, each apoprotein molecule contains, to the nearest integer, two chlorophyll a, one chlorophyll c, and five fucoxanthin molecules. Polyclonal antibodies raised against the 17.5 to 18.0 kilodaltons apoprotein showed a monospecific reaction with only the 17.5 to 18.0 protein zone from denatured P. tricornutum membranes as well as to the nondenatured pigment-protein complex. It appears that this complex is common to other diatom species.     

Comparison of chlorophyll a spectra in wild-type and mutant barley chloroplasts grown under day or intermittent light

The absorption (640–710 nm) and fluorescence emission (670–710 nm) spectra (77 K) of wild-type and Chl b-less, mutant, barley chloroplasts grown under either day or intermittent light were analysed by a RESOL curve-fitting program. The usual four major forms of Chl a at 662, 670, 678 and 684 nm were evident in all of the absorption spectra and three major components at 686, 693 and 704 nm in the emission spectra. A broad Chl a component band at 651 nm most likely exists in all chlorophyll spectra in vivo. The results show that the mutant lacks not only Chl b, but also the Chl a molecules which are bound to the light-harvesting, Chl a/b, protein complex of normal plants. It also appears that the absorption spectrum of this antenna complex is not modified appreciably by its isolation from thylakoid membranes.Abbreviations Chl chlorophyll - DL daylight - ImL intermittent light - WT wildtype - LHC light-harvesting Chl a/b protein complex - S.E. standard error of the mean DBP-CIW No. 763.     

Using absorbance and fluorescence spectra to discriminate microalgae

The utility of absorbance and fluorescence-emission spectra for discriminating among microalgal phylogenetic groups, selected species, and phycobilin- and non-phycobilin-containing algae was examined using laboratory cultures. A similarity index algorithm, in conjunction with fourth-derivative transformation of absorbance spectra, provided discrimination among the chlorophyll [Chl] a/phycobilin (cyanobacteria), Chl a/Chl c/phycobilin (cryptophytes), Chl a/Chl b (chlorophytes, euglenophytes, prasinophytes), Chl a/Chl c/fucoxanthin (diatoms, chrysophytes, raphidophytes) and Chl a/Chl c/peridinin (dinoflagellates) spectral classes, and often between}among closely related phylogenetic groups within a class. Spectra for phylogenetic groups within the Chl a/Chl c/fucoxanthin, Chl a/Chl c/peridinin, Chl a/phycobilins and Chl a/Chl c/phycobilin classes were most distinguishable from spectra for groups within the Chl a/Chl b spectral class. Chrysophytes/diatoms/raphidophytes and dinoflagellates (groups within the comparable spectral classes, Chl a/Chl c/fucoxanthin and Chl a/Chl c/peridinin, respectively) displayed the greatest similarity between/among groups. Spectra for phylogenetic groups within the Chl a/Chl c classes displayed limited similarity with spectra for groups within the Chl/phycobilin classes. Among the cyanobacteria and chlorophytes surveyed, absorbance spectra of species possessing dissimilar cell morphologies were discriminated, with the greatest range of differentiation occurring among cyanobacteria. Among the cyanobacteria, spectra for selected problematic species were easily discriminated from spectra from each other and from other cyanobacteria. Fluorescence-emission spectra were distinct among spectral classes and the similarity comparisons involving fourth-derivative transformation of spectra discriminated the increasing contribution of distinct cyanobacterial species and between phycobilin- and non-phycobilin-containing species within a hypothetical mixed assemblage. These results were used to elucidate the application for in situ moored instrumentation incorporating such approaches in water quality monitoring programmes, particularly those targeting problematic cyanobacterial blooms.     

PHOTOSYNTHETIC LIGHT-HARVESTING FUNCTION OF VIOLAXANTHIN IN NANNOCHLOROPSIS SPP. (EUSTIGMATOPHYCEAE)1

Whole cell absorption spectra of the Eustigmatophycean algae Nannochloropsis salina Bourrelly and Nannochloropsis sp. reveal the presence of a distinct absorption peak at 490 nm. The lack of chlorophylls b and c in these species indicates that this peak must be attributed to carotenoid absorption. In vivo fluorescence excitation spectra for chlorophyll a emission show a corresponding maximum at 490 nm. This peak is more clearly resolved than carotenoid maxima in other algal classes due to the absence of accessory chlorophylls. The carotenoid composition of the two Nannochloropsis species shows that violaxanthin and vaucheriaxanthin are the main contributors to 490 nm absorption. Violaxanthin accounts for approximately 60% of the total carotenoid in both clones. We conclude that light absorption by violaxanthin, and possibly by vaucheriaxanthin, is coupled in energy transfer to chlorophyll a and that violaxanthin is the major light-harvesting pigment in the Eustigmatophyceae. This is the first report of the photosynthetic light-harvesting function of this carotenoid.     

The development of antenna complexes of barley (Hordeum vulgare cv. Akcent) under different light conditions as judged from the analysis of 77 K chlorophyll a fluorescence spectra

Using 77 K chlorophyll a (Chl a) fluorescence spectra in vivo, the development was studied of Photosystems II (PS II) and I (PS I) during greening of barley under intermittent light followed by continuous light at low (LI, 50 μmol m⁻² s⁻¹) and high (HI, 1000 μmol m⁻² s⁻¹) irradiances. The greening at HI intermittent light was accompanied with significantly reduced fluorescence intensity from Chl b excitation for both PS II (F685) and PS I (F743), in comparison with LI plants, indicating that assembly of light-harvesting complexes (LHC) of both photosystems was affected to a similar degree. During greening at continuous HI, a slower increase of emission from Chl b excitation in PS II as compared with PS I was observed, indicating a preferred reduction in the accumulation of LHC II. The following characteristics of 77 K Chl a fluorescence spectra documented the photoprotective function of an elevated content of carotenoids in HI leaves: (1) a pronounced suppression of Soret region of excitation spectra (410–450 nm) in comparison with the red region (670–690 nm) during the early stage of greening indicated a strongly reduced excitation energy transfer from carotenoids to the Chl a fluorescing forms within PS I and PS II; (2) changes in the shape of the excitation band of Chl b and carotenoids (460–490 nm) during greening under continuous light confirmed that the energy transfer from carotenoids to Chl a within PS II remained lower as compared with the LI plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alteration of Components of Chlorophyll-Protein Complexes and Distribution of Excitation Energy Between the Two Photosystems in Two New Rice Chlorophyll b-less mutants

Two new yellow rice chlorophyll (Chl) b-less (lack) mutants VG28-1 and VG30-5 differ from the other known Chl b-less mutants with larger amounts of soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase small sub-unit and smaller amounts of Chl a. We investigated the altered features of Chl-protein complexes and excitation energy distribution in these two mutants, as compared with wild type (WT) rice cv. Zhonghua 11 by using native mild green gel electrophoresis and SDS-PAGE, and 77 K Chl fluorescence in the presence of Mg²⁺. WT rice revealed five pigment-protein bands and fourteen polypeptides in thylakoid membranes. Two Chl b-less mutants showed only CPI and CPa pigment bands, and contained no 25 and 26 kDa polypeptides, reduced amounts of the 21 kDa polypeptide, but increased quantities of 32, 33, 56, 66, and 19 kDa polypeptides. The enhanced absorption of CPI and CPa and the higher Chl fluorescence emission ratio of F685/F720 were also observed in these mutants. This suggested that the reduction or loss of the antenna LHC1 and LHC2 was compensated by an increment in core component and the capacity to harvest photon energy of photosystem (PS) 1 and PS2, as well as in the fraction of excitation energy distributed to PS2 in the two mutants. 77 K Chl fluorescence spectra of thylakoid membranes showed that the PS1 fluorescence emission was shifted from 730 nm in WT rice to 720 nm in the mutants. The regulation of Mg²⁺ to excitation energy distribution between the two photosystems was complicated. 10 mM Mg²⁺ did not affect noticeably the F685/F730 emission ratio of WT thylakoid membranes, but increased the ratio of F685/F720 in the two mutants due to a reduced emission at 685 nm as compared to that at 720 nm.     

Antheraxanthin, a light harvesting carotenoid found in a chromophyte alga

The pigments of the chromophyte freshwater alga, Chrysophaera magna Belcher were analyzed by thin layer chromatography (TLC) and high performance liquid chromatography (HPLC) to reveal the presence of chlorophylls a and c, β-carotene, fucoxanthin, and antheraxanthin. The presence of antheraxanthin was verified by comparison of TLC R_F values, HPLC retention times, and absorption features to those of authentic, synthetic antheraxanthin. Antheraxanthin accounted for about 15% of the total carotenoid content of C. magna. The molar ratio of the major carotenoids was antheraxanthin:fucoxanthin:β-carotene, 1:2.3:3.3. The whole-cell absorption spectrum revealed a broad band between 470 and 520 nanometers which was attributed to fucoxanthin and antheraxanthin in vivo. Upon extraction in hydrocarbon, this broad absorption region was lost. The in vivo fluorescence excitation spectrum for 680 nm emission revealed the energy transfer activities and light harvesting roles of chlorophylls a and c, and fucoxanthin. In addition, an excitation band was resolved at 487 nanometers which could be attributed only to antheraxanthin. Comparison of whole-cell fluorescence excitation spectra of C. magna with the diatom Phaeodactylum tricornutum, which possesses fucoxanthin but not antheraxanthin, supports the assignment of the 487 nm band to antheraxanthin. This is the first report of a photosynthetic light harvesting function of the xanthophyll, antheraxanthin. This carotenoid broadens the absorption cross-section for photosynthesis in C. magna and extends light harvesting into the green portion of the spectrum.     

Fluorescence properties of the allenic carotenoid fucoxanthin: Implication for energy transfer in photosynthetic pigment systems

The fluorescence spectrum of an allenic carotenoid, all-trans-fucoxanthin isolated from a brown alga, has been reported for the first time. This carotenoid is known to function efficiently as a primary photosynthetic antenna pigment in marine algae. The emission bands were located around 630, 685 and 750 nm in CS₂ at 20°C, absorption bands being located at 448, 476 and 505 nm. The energy difference between the 0-0 bands of absorption and emission spectra was about 3900 cm^-1 and location of the emission maximum was less sensitive to the polarizability of solvents than that of the absorption maximum. These clearly indicate that the emission originates from the optically forbidden singlet state (2A_g). This is in contrast to other carotenoids whose emission is assigned to 1B_u state, probably due to the symmetric structure of the conjugated double bond responsible for the absorption in the visible region. A rapid internal conversion from 1B_u to 2A_g state might be facilitated by distorted structure of the conjugated double bond of fucoxanthin. The energy level responsible for the emission is almost identical to the Q_y level of the acceptor molecule (Chl a), thus we propose an energy transfer pathway from the optically forbidden 2A_g state of the carotenoid to the Q_y transition of Chl a in algal pigment systems.     

Photosynthetic light-response curves and photoinhibition of the deep-water laminaria abyssalis and the intertidal laminaria digitata (phaeophyceae)

Photosynthetic light‐response curves of the deep‐water Laminaria abyssalis Oliveira and of the intertidal L. digitata Lamoroux were determined and related to photoinhibition phenomena as monitored by oxygen evolution and photosystem II efficiency (F_V/F_M). L. abyssalis has half the pigment content, number of cells and plastids, and photosynthetic capacity per unit area compared with L. digitata. L. abyssalis showed a higher in vivo Chl a absorption coefficient and higher photosynthetic efficiency on a Chl a basis, although the two algae showed somewhat similar light‐response curves on a Chl a basis. Both species showed similar Chl a/Chl c and Chl a/fucoxanthin ratios, and similar dark respiration rates and light compensation points. In addition, they also showed similar convexities in their light‐response curves and no differences in their light saturation of F_V/F_M. Room temperature chlorophyll fluorescence induction measurements of fronds incubated in 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) suggest that both species may have a similar PSII absorption cross section. Thus, L. abyssalis appears to optimize its light absorption at very low light intensities, not by increasing the pigment content, but by absorbing light more efficiently. However, L. abyssalis was more sensitive to photoinhibition than L. digitata and showed no recovery of F_V/F_M and O₂ evolution after a photoinhibitory treatment, even with a subsequent exposure to 24 h of dim light. L. digitata, on the other hand, recovered its photosynthetic capacity within 6 h under dim light. These results suggest that photosynthetic light‐induction curves based on Chl a are not a good indicator of either the photosynthetic capacity or the sensitivity to photoinhibition when macroalgae of different species are being compared. Based on their light‐response and photoinhibition characteristics, we suggest that L. abyssalis, a deep‐water oceanic macroalgae, is an atypical shade alga whereas L. digitata has the properties of a sun alga.     

Reconstitution of the Peridinin–chlorophyll a Protein (PCP): Evidence for Functional Flexibility in Chlorophyll Binding

The coding regions for the N-domain, and full length peridinin–chlorophyll a apoprotein (full length PCP), were expressed in Escherichia coli. The apoproteins formed inclusion bodies from which the peptides could be released by hot buffer. Both the above constructs were reconstituted by addition of a total pigment extract from native PCP. After purification by ion exchange chromatography, the absorbance, fluorescence excitation and CD spectra resembled those of the native PCP. Energy transfer from peridinin to Chl a was restored and a specific fluorescence activity calculated which was ~86% of that of native PCP. Size exclusion analysis and CD spectra showed that the N-domain PCP dimerized on reconstitution. Chl a could be replaced by Chl b, 3-acetyl Chl a, Chl d and Bchl using the N-domain apo protein. The specific fluorescence activity was the same for constructs with Chl a, 3-acetyl Chl a, and Chl d but significantly reduced for those made with Chl b. Reconstitutions with mixtures of chlorophylls were also made with eg Chl b and Chl d and energy transfer from the higher energy Qy band to the lower was demonstrated.     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

Variations in biochemical parameters of Heterocapsa sp. and Olisthodiscus luteus grown in 12:12 light:dark cycles II. Changes in pigment composition

Photosynthetic pigment composition was studied in batch cultures of Heterocapsa sp. and Olisthodiscus luteus growing exponentially in a 12:12 light:dark cycle. Both species divided in the dark. The synthesis of pigments was continuous for both species. However for chlorophyll c and peridinin, in Heterocapsa sp., and chlorophyll c and fucoxanthin, in O. luteus, (pigments belonging to light harvesting complexes) the synthesis was significantly higher during the light period. Concentrations per total cell volume (TCV) of chlorophyll a, chlorophyll c, peridinin and diadinoxanthin in Heterocapsa sp., and chlorophyll a, chlorophyll c, fucoxanthin and violaxanthin in O. luteus, showed a maximum at the onset of light and decreased during the light period. The values of the chlorophyll a:chlorophyll c, chlorophyll a:peridinin and chlorophyll a:fucoxanthin ratios are compared with data reported in the literature.     

Light-harvesting systems of brown algae and diatoms. Isolation and characterization of chlorophyll ac and chlorophyll afucoxanthin pigment-protein complexes

The present study examined the protein associations and energy transfer characteristics of chlorophyll c and fucoxanthin which are the major light-harvesting pigments in the brown and diatomaceous algae. It was demonstrated that sodium dodecyl sulfate (SDS)-solubilized photosynthetic membranes of these species when subjected to SDS polyacrylamide gel electrophoresis yielded three spectrally distinct pigment-protein complexes. The slowest migrating zone was identical to complex I, the SDS-altered form of the P-700 chlorophyll a-protein. The zone of intermediate mobility contained chlorophyll c and chlorophyll a in a molar ratio of 2 : 1, possessed no fucoxanthin, and showed efficient energy transfer from chlorophyll c to chlorophyll a. The fastest migrating pigment-protein zone contained fucoxanthin and chlorophyll a, possessed no chlorophyll c, and showed efficient energy transfer from fucoxanthin to chlorophyll a. It is demonstrated that the chlorophyll $\text{a}\text{c-}\text{protein}$ and the chlorophyll $\text{a}\text{fucoxanthin-protein}$ complexes are common to the brown algae and diatoms examined, and likely share similar roles in the photosynthetic units of these species.     

Spectral and kinetic parameters of phosphorescence of triplet chlorophyll a in the photosynthetic apparatus of plants

Spectral and kinetic parameters and quantum yield of IR phosphorescence accompanying radiative deactivation of the chlorophyll a (Chl a) triplet state were compared in pigment solutions, greening and mature plant leaves, isolated chloroplasts, and thalluses of macrophytic marine algae. On the early stages of greening just after the Shibata shift, phosphorescence is determined by the bulk Chl a molecules. According to phosphorescence measurement, the quantum yield of triplet state formation is not less than 25%. Further greening leads to a strong decrease in the phosphorescence yield. In mature leaves developing under normal irradiation conditions, the phosphorescence yield declined 1000-fold. This parameter is stable in leaves of different plant species. Three spectral forms of phosphorescence-emitting chlorophyll were revealed in the mature photosynthetic apparatus with the main emission maxima at 955, 975, and 995 nm and lifetimes ～1.9, ～1.5, and 1.1–1.3 ms. In the excitation spectra of chlorophyll phosphorescence measured in thalluses of macrophytic green and red algae, the absorption bands of Chl a and accessory pigments — carotenoids, Chl b, and phycobilins — were observed. These data suggest that phosphorescence is emitted by triplet chlorophyll molecules that are not quenched by carotenoids and correspond to short wavelength forms of Chl a coupled to the normal light harvesting pigment complex. The concentration of the phosphorescence-emitting chlorophyll molecules in chloroplasts and the contribution of these molecules to chlorophyll fluorescence were estimated. Spectral and kinetic parameters of the phosphorescence corresponding to the long wavelength fluorescence band at 737 nm were evaluated. The data indicate that phosphorescence provides unique information on the photophysics of pigment molecules, molecular organization of the photosynthetic apparatus, and mechanisms and efficiency of photodynamic stress in plants.     

Photosystem I complexes associated with fucoxanthin-chlorophyll-binding proteins from a marine centric diatom, Chaetoceros gracilis

Diatoms occupy a key position as a primary producer in the global aquatic ecosystem. We developed methods to isolate highly intact thylakoid membranes and the photosystem I (PS I) complex from a marine centric diatom, Chaetoceros gracilis. The PS I reaction center (RC) was purified as a super complex with light-harvesting fucoxanthin-chlorophyll (Chl)-binding proteins (FCP). The super complex contained 224 Chl a, 22 Chl c, and 55 fucoxanthin molecules per RC. The apparent molecular mass of the purified FCP-PS I super complex (∼ 1000 kDa) indicated that the super complex was composed of a monomer of the PS I RC complex and about 25 copies of FCP. The complex contained menaquinone-4 as the secondary electron acceptor A₁ instead of phylloquinone. Time-resolved fluorescence emission spectra at 77 K indicated that fast (16 ps) energy transfer from a Chl a band at 685 nm on FCP to Chls on the PS I RC complex occurs. The ratio of fucoxanthin to Chl a on the PS I-bound FCP was lower than that of weakly bound FCP, suggesting that PS I-bound FCP specifically functions as the mediator of energy transfer between weakly bound FCPs and the PS I RC.     

Modifications pigmentaires et ultrastructurales chez la diatomée Detonula sp. cultivée en lumière rouge

Résumé La diatomée Detonula sp. lorsqu'elle est cultivée en lumière rouge (650 nm), est enrichie en holochrome chlorophyllien Ca 705 (pour la nomenclature des holochromes voir French et al., 1971). La composition pigmentaire est alors modifiée par rapport à celle de l'algue cultivée en lumière blanche. Si les mêmes composants, Chlorophylle a, Chlorophylle c, Carotène, Diadinoxanthine, Diatoxanthine, Fucoxanthine, Violaxanthine, sont tous présents, on note un abaissement du taux de Chlorophylle c (le rapport Chl c/Chl a passe de 30% à 5%) et un abaissement du taux de Fucoxanthine (le rapport Fucoxanthine/Chlorophylle a passe de 85% à 60%). Cette modification pigmentaire s'accompagne d'une modification ultrastructurale des plastes. Au lieu d'une vingtaine de thylakoïdes disposés par groupe de 4 au milieu d'un stroma abondant, on observe une disparition du stroma accompagnée d'une augmentation très importante du nombre de thylakoïdes, tous juxtaposés en un seul ensemble.La diminution importante du taux de Chlorophylle c et de la Fucoxanthine, pigments du photosystème II, liée à l'enrichissement en Chlorophylle a 705, indique une modification de l'équilibre entre les deux photosystèmes au profit du photosystème I. Cet enrichissement en photosystème I s'accompagne d'une modification ultrastructurale qui est rapprochée des modifications observées dans d'autres organismes lorsque l'équilibre des deux photosystèmes est perturbé.

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Pigments and ultrastructure modifications in diatom Detonula sp. cultivated in red light

Summary Cultivation of the diatom Detonula sp. with red light, which causes an enrichment of the chlorophyll holochrome Ca 705 (for nomenclature of chlorophyll holochromes see French et al., 1971), also modifies the gross pigment composition of the cells. Relative to chlorophyll a, the cellular contents of chlorophyll c and fucoxanthin are considerably lower than in cells grown with white light; and this is accompanied by a change in plastid ultrastructure. The plastids of cells grown in white light contain about 20 thylakoids disposed in groups of 4, within an abundant stroma; those of cells grown in red light contain a much greater number of thylakoids (about 40), evenly distributed throughout the plastid.The diminution of chlorophyll c and fucoxanthin, pigments associated with photosystem II, coupled with the enrichment in chlorophyll a 705, points to a shift in the ratio between photosystems I and II in favor of the former. The accompanying ultrastructural modification of the plastid resembles that observed in other organisms as a result of changes in the relative amounts of the two photosystems.

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Cet article recouvre en partie le travail d'une thèse de Doctorat d'Etat ès Sciences Naturelles qui sera soutenue à la Faculté des Sciences de Paris. VI.