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.     

ISOLATION AND CHARACTERIZATION OF NATIVE PIGMENT-PROTEIN COMPLEXES FROM TWO EUSTIGMATOPHYCEAE1

Pigment-protein complexes were isolated from two species of Eustigmatophyceae, Monodus subterraneus Peterson and Vischeria punctata Vischer, by digitonin treatment followed by density gradient centrifugation. Absorption and fluorescence spectra of the samples were monitored at various steps of preparation, and pigment composition was analyzed by reverse phase HPLC. Although the fluorescence emission spectra were very different in the two species, the absorption spectra were similar, and each exhibited an absorption band with a maximum at 487 nm attributable to violaxanthin and vaucheriaxanthin ester (the molar concentration of these pigments in Monodus was, respectively, 28 and 10 per 100 Chl a). The light-harvesting role of these xanthophylls was ascertained by fluorescence excitation spectra. The light-harvesting fractions (LH) collected in the upper part of the gradient were depleted in β-carotene, whereas their xanthophyll/chlorophyll ratio was almost the same as in whole cells. This is consistent with the presence in these algae of large LH antennae and relatively small core antennae in the photosystems. In Monodus, a polypeptide of 23 kDa, immunologically related to the major LH polypeptide of brown algae, constituted the majority of the LH protein moiety.     

Pigment analysis of chloroplast pigment-protein complexes in wheat

Pigment-protein complexes separated from wheat (Triticum aestivum L. selection ND96-25 by two gel electrophoresis techniques were analyzed by high-performance liquid chromatography for chlorophylls and carotenoids. The two techniques are compared, and pigment analyses are given for the major reaction centers and light-harvesting complexes. Reaction centers contain mostly chlorophyll a, carotene, and lutein, whereas light-harvesting complexes contain chlorophyll a, chlorophyll b, lutein, and neoxanthin. The amounts of violaxanthin are variable.     

Characterization of the photosynthetic apparatus of the Eustigmatophycean Nannochloropsis gaditana: Evidence of convergent evolution in the supramolecular organization of photosystem I

Nannochloropsis gaditana belongs to Eustigmatophyceae, a class of eukaryotic algae resulting from a secondary endosymbiotic event. Species of this class have been poorly characterized thus far but are now raising increasing interest in the scientific community because of their possible application in biofuel production. Nannochloropsis species have a peculiar photosynthetic apparatus characterized by the presence of only chlorophyll a, with violaxanthin and vaucheriaxanthin esters as the most abundant carotenoids. In this study, the photosynthetic apparatus of this species was analyzed by purifying the thylakoids and isolating the different pigment-binding complexes upon mild solubilization. The results from the biochemical and spectroscopic characterization showed that the photosystem II antenna is loosely bound to the reaction center, whereas the association is stronger in photosystem I, with the antenna-reaction center super-complexes surviving purification. Such a supramolecular organization was found to be conserved in photosystem I from several other photosynthetic eukaryotes, even though these taxa are evolutionarily distant. A hypothesis on the possible selective advantage of different associations of the antenna complexes of photosystems I and II is discussed.     

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.     

Changes in Pigment Composition and Photosynthesis Induced by Iron-Deficiency in the Blue-Green Alga Anacystis nidulans

Absorption spectra and photosynthetic action spectra have been determined for living Anacystis grown in complete and iron-deficient inorganic media. The absorption studies have shown a spectral shift from 679 nm to 673 nm in the chlorophyll a absorption peak when the algae had to grow without iron. The shift is believed to reflect a changed ratio between at least two chlorophyll a forms denoted Ca670 and Ca680 in this work. Action spectra determinations have revealed a similar shift from 677 nm to 672 nm in the photosynthetic activity peak of chlorophyll a when Anacystis was transferred to a medium without iron. It is proposed that both Ca670 and Ca680 participate in light absorption for photo-system I.     

Spectral and functional studies on siphonaxanthin-type light-harvesting complex of photosystem II from Bryopsis corticulans

Carotenoids with conjugated carbonyl groups possess special photophysical properties which have been studied in some water-soluble light-harvesting proteins (Polívka and Sundström, Chem Rev 104:2021–2071, 2004). However, siphonaxanthin-type light-harvesting complexes of photosystem II (LHCII) in siphonous green alga have received fewer studies. In the present study, we determined sequences of genes for several Bryopsis corticulans Lhcbm proteins, which showed that they belong to the group of major LHCII and diverged early from green algae and higher plants. Analysis of pigment composition indicated that this siphonaxanthin-type LHCII contained in total 3 siphonaxanthin and siphonein but no lutein and violaxanthin. In addition, 2 chlorophylls a in higher plant LHCII were replaced by chlorophyll b. These changes led to an increased absorption in green and blue-green light region compared with higher plant LHCII. The binding sites for chlorophylls, siphonaxanthin, and siphonein were suggested based on the structural comparison with that of higher plant LHCII. All of the ligands for the chlorophylls were completely conserved, suggesting that the two chlorophylls b were replaced by chlorophyll a without changing their binding sites in higher plant LHCII. Comparisons of the absorption spectra of isolated siphonaxanthin and siphonein in different organic solutions and the effect of heat treatment suggested that these pigments existed in a low hydrophobic protein environment, leading to an enhancement of light harvesting in the green light region. This low hydrophobic protein environment was maintained by the presence of more serine and threonine residues in B. corticulans LHCII. Finally, esterization of siphonein may also contribute to the enhanced harvesting of green light.     

Energy transfer dynamics in a red-shifted violaxanthin-chlorophyll a light-harvesting complex

Photosynthetic eukaryotes whose cells harbor plastids originating from secondary endosymbiosis of a red alga include species of major ecological and economic importance. Since utilization of solar energy relies on the efficient light-harvesting, one of the critical factors for the success of the red lineage in a range of environments is to be found in the adaptability of the light-harvesting machinery, formed by the proteins of the light-harvesting complex (LHC) family. A number of species are known to employ mainly a unique class of LHC containing red-shifted chlorophyll a (Chl a) forms absorbing above 690?nm. This appears to be an adaptation to shaded habitats. Here we present a detailed investigation of excitation energy flow in the red-shifted light-harvesting antenna of eustigmatophyte Trachydiscus minutus using time-resolved fluorescence and ultrafast transient absorption measurements. The main carotenoid in the complex is violaxanthin, hence this LHC is labeled the red-violaxanthin-Chl a protein, rVCP. Both the carotenoid-to-Chl a energy transfer and excitation dynamics within the Chl a manifold were studied and compared to the related antenna complex, VCP, that lacks the red-Chl a. Two spectrally defined carotenoid pools were identified in the red antenna, contributing to energy transfer to Chl a, mostly via S₂ and hot S₁ states. Also, Chl a triplet quenching by carotenoids is documented. Two separate pools of red-shifted Chl a were resolved, one is likely formed by excitonically coupled Chl a molecules. The structural implications of these observations are discussed.     

Excitation spectra of chlorophyll fluorescence in spinach and barley chloroplasts at 4 K

Excitation spectra of chlorophyll a fluorescence in chloroplasts from spinach and barley were measured at 4.2 K. The spectra showed about the same resolution as the corresponding absorption spectra. Excitation spectra for long-wave chlorophyll a emission (738 or 733 nm) indicate that the main absorption maximum of the photosystem (PS) I complex is at 680 nm, with minor bands at longer wavelengths. From the corresponding excitation spectra it was concluded that the emission bands at 686 and 695 nm both originate from the PS II complex. The main absorption bands of this complex were at 676 and 684 nm. The PS I and PS II excitation spectra both showed a contribution by the light-harvesting chlorophyll $\text{a}\text{b}$ protein(s), but direct energy transfer from PS II to PS I was not observed at 4 K. Omission of Mg²⁺ from the suspension favored energy transfer from the light-harvesting protein to PS I. Excitation spectra of a chlorophyll b-less mutant of barley showed an average efficiency of 50–60% for energy transfer from β-carotene to chlorophyll a in the PS I and in the PS II complexes.     

Optical spectroscopic studies of light-harvesting by pigment-reconstituted peridinin-chlorophyll-proteins at cryogenic temperatures

Low temperature, steady-state, optical spectroscopic methods were used to study the spectral features of peridinin-chlorophyll-protein (PCP) complexes in which recombinant apoprotein has been refolded in the presence of peridinin and either chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll d (Chl d), 3-acetyl-chlorophyll a (3-acetyl-Chl a) or bacteriochlorophyll a (BChl a). Absorption spectra taken at 10 K provide better resolution of the spectroscopic bands than seen at room temperature and reveal specific pigment–protein interactions responsible for the positions of the Q_y bands of the chlorophylls. The study reveals that the functional groups attached to Ring I of the two protein-bound chlorophylls modulate the Q_y and Soret transition energies. Fluorescence excitation spectra were used to compute energy transfer efficiencies of the various complexes at room temperature and these were correlated with previously reported ultrafast, time-resolved optical spectroscopic dynamics data. The results illustrate the robust nature and value of the PCP complex, which maintains a high efficiency of antenna function even in the presence of non-native chlorophyll species, as an effective tool for elucidating the molecular details of photosynthetic light-harvesting.     

Nomenclature for membrane-bound light-harvesting complexes of cyanobacteria

Accessory chlorophyll-binding proteins (CBP) in cyanobacteria have six transmembrane helices and about 11 conserved His residues that might participate in chlorophyll binding. In various species of cyanobacteria, the CBP proteins bind different types of chlorophylls, including chlorophylls a, b, d and divinyl-chlorophyll a, b. The CBP proteins do not belong to the light-harvesting complexes (LHC) superfamily of plant and algae. The proposed new name of CBP for this class of proteins, which is a unique accessory light-harvesting superfamily in cyanobacteria, clarifies the confusion of names of prochlorophytes chlorophyll binding protein (Pcb), PSII-like light-harvesting proteins and iron-stress-induced protein A (IsiA). The CBP complexes are a member of a larger family that includes the chlorophyll a-binding proteins CP43 and CP47 that function as core antennas of photosystem II.     

Minor complexes at work: light-harvesting by carotenoids in the photosystem II antenna complexes CP24 and CP26

Plant photosynthesis relies on the capacity of chlorophylls and carotenoids to absorb light. One of the roles of carotenoids is to harvest green-blue light and transfer the excitation energy to the chlorophylls. The corresponding dynamics were investigated here for the first time, to our knowledge, in the CP26 and CP24 minor antenna complexes. The results for the two complexes differ substantially. In CP26 fast transfer (80 fs) occurs from the carotenoid S₂ state to chlorophylls a absorbing at 675 and 678 nm, whereas transfer from the hot S₁ state to the lowest energy chlorophylls is observed in <1 ps. In CP24, energy transfer from the S₂ state leads in 80 fs to the population of chlorophylls b and high-energy chlorophylls a absorbing at 670 nm, whereas the low-energy chlorophylls a are populated only in several picoseconds. The results suggest that CP26 has a structural and functional organization similar to that of LHCII, whereas CP24 differs substantially from the other Lhc complexes, especially regarding the lutein L1 binding domain. No energy transfer from the carotenoid S₁ state to chlorophylls was observed in either complex, suggesting that this state is energetically below the chlorophyll Qy state and therefore may play a role in the quenching of chlorophyll excitations.     

Photophysics of the carotenoids associated with the xanthophyll cycle in photosynthesis

Green plants use the xanthophyll cycle to regulate the flow of energy to chlorophylla within photosynthetic proteins. Under conditions of low light intensity violaxanthin, a carotenoid possessing nine conjugated double bonds, functions as an antenna pigment by transferring energy from its lowest excited singlet state to that of chlorophylla within light-harvesting proteins. When the light intensity increases, violaxanthin is biochemically transformed into zeaxanthin, a carotenoid that possesses eleven conjugated double bonds. The results presented here show that extension of the conjugation of the polyene lowers the energy of the lowest excited singlet state of the carotenoid below that of chlorophylla. As a consequence zeaxanthin can act as a trap for the excess excitation energy on chlorophylla pigments within the protein, thus regulating the flow of energy within photosynthetic light-harvesting proteins.     

A new evaluation of chlorophyll absorption in photosynthetic membranes

The three major chlorophyll-proteins of spinach chloroplasts were solubilized with digitonin and isolated by electrophoresis with deoxycholate. The gel bands were identified from their absorption and fluorescence spectra measured at 77 K. The slowest moving band was a Photosystem I complex (CPI); the second, a Photosystem II complex (Cpa); and the third, a chlorophyll a-b, antenna complex (LHCP). When absorption spectra (630–730 nm) of the bands were added in the proportions found in the gel, the sum closely matched the absorption of the chloroplasts both before and after solubilization. Thus these spectra represent the native absorption of the major antenna chlorophyll-proteins of green plants. Each of these spectra was resolved with a computer assisted, curve-fitting program into 8 mixed Gaussian-Lorentzian shaped components. The major, Chl a components in the 3 fractions were different both in peak positions and bandwidths. This result suggests that each chlorophyll-protein has its own unique set of chlorophyll a spectral forms or components.Abbreviations Chl chlorophyll - CPI Photosystem I Chl-protein - CPa Photosystem II Chl-protein - LHCP light-harvesting Chl a-b protein - DOC sodium deoxycholate - SDS sodium dodecylsulfate CIW-DPB No. 819     

Chlorophyll a and carotenoid triplet states in light-harvesting complex II of higher plants.

Laser-flash-induced transient absorption measurements were performed on trimeric light-harvesting complex II to study carotenoid (Car) and chlorophyll (Chl) triplet states as a function of temperature. In these complexes efficient transfer of triplets from Chl to Car occurs as a protection mechanism against singlet oxygen formation. It appears that at room temperature all triplets are being transferred from Chl to Car; at lower temperatures (77 K and below) the transfer is less efficient and chlorophyll triplets can be observed. In the presence of oxygen at room temperature the Car triplets are partly quenched by oxygen and two different Car triplet spectral species can be distinguished because of a difference in quenching rate. One of these spectral species is replaced by another one upon cooling to 4 Ki demonstrating that at least three carotenoids are in close contact with chlorophylls. The triplet minus singlet absorption (T-S) spectra show maxima at 504-506 nm and 517-523 nm, respectively. In the Chl Qy region absorption changes can be observed that are caused by Car triplets. The T-S spectra in the Chl region show an interesting temperature dependence which indicates that various Car's are in contact with different Chl a molecules. The results are discussed in terms of the crystal structure of light-harvesting complex II.     

Temperature-dependent adjustment of the thermal stability of photosystem II in vivo: possible involvement of xanthophyll-cycle pigments

Moderately elevated temperatures induce a rapid increase in the heat and light resistance of photosystem II (PSII) in higher-plant leaves. This phenomenon was studied in intact potato leaves exposed to 35 °C for 2 h, using chlorophyll fluorometry, kinetic and difference spectrophotometry and photoacoustics. The 35 °C treatment was observed to cause energetic uncoupling between carotenoids and chlorophylls: (i) the steady-state chlorophyll fluorescence emission excited by a blue light beam (490 nm) was noticeably reduced as compared to fluorescence elicited by orange light (590 nm) and (ii) the quantum yield for photosynthetic oxygen evolution in blue light (400–500 nm) was preferentially reduced relative to the quantum yield measured in red light (590–710 nm). Analysis of the chlorophyll-fluorescence and light-absorption characteristics of the heated leaves showed numerous analogies with the fluorescence and absorption changes associated with the light-induced xanthophyll cycle activity, indicating that the carotenoid species involved in the heat-induced pigment uncoupling could be the xanthophyll violaxanthin. More precisely, the 35 °C treatment was observed to accelerate and amplify the non-photochemical quenching of chlorophyll fluorescence (in both moderate red light and strong white light) and to cause an increase in leaf absorbance in the blue-green spectral region near 520 nm, as do strong light treatments which induce the massive conversion of violaxanthin to zeaxanthin. Interestingly, short exposure of potato leaves to strong light also provoked a significant increase in the stability of PSII to heat stress. It was also observed that photosynthetic electron transport was considerably more inhibited by chilling temperatures in 35 °C-treated leaves than in untreated leaves. Further, pre-exposure of potato leaves to 35 °C markedly increased the amplitude and the rate of light-induced changes in leaf absorbance at 505 nm (indicative of xanthophyll cycle activity), suggesting the possibility that moderately elevated temperature increased the accessibility of violaxanthin to the membrane-located de-epoxidase. This was supported by the quantitative analysis of the xanthophyll-cycle pigments before and after the 35 °C treatment, showing light-independent accumulation of zeaxanthin during mild heat stress. Based on these results, we propose that the rapid adjustment of the heat resistance of PSII may involve a modification of the interaction between violaxanthin and the light-harvesting complexes of PSII. As a consequence, the thermoresistance of PSII could be enhanced either directly through a conformational change of PSII or indirectly via a carotenoid-dependent modulation of membrane lipid fluidity.Abbreviations and Symbols Fo and Fm initial and maximal level of chlorophyll fluorescence, respectively - Fv = Fm — Fo variable chlorophyll fluorescence - LHC(II) light-harvesting chlorophylla/b-protein complexes (of PSII) - photoacoustically measured quantum yield of photosynthetic oxygen evolution (in relative values) - _P fluorimetrically measured quantum yield of PSII photochemistry in the light - PFD photon flux density - q_E pH dependent quenching of chlorophyll fluorescence We thank Dr. J-L Montillet (CEA-Cadarache) for the use of his HPLC apparatus and Professor Y. Lemoine (University of Lille, France) for technical advice on HPLC.     

Highly efficient energy transfer from a carbonyl carotenoid to chlorophyll a in the main light harvesting complex of Chromera velia

We report on energy transfer pathways in the main light-harvesting complex of photosynthetic relative of apicomplexan parasites, Chromera velia. This complex, denoted CLH, belongs to the family of FCP proteins and contains chlorophyll (Chl) a, violaxanthin, and the so far unidentified carbonyl carotenoid related to isofucoxanthin. The overall carotenoid-to-Chl-a energy transfer exhibits efficiency over 90% which is the largest among the FCP-like proteins studied so far. Three spectroscopically different isofucoxanthin-like molecules were identified in CLH, each having slightly different energy transfer efficiency that increases from isofucoxanthin-like molecules absorbing in the blue part of the spectrum to those absorbing in the reddest part of spectrum. Part of the energy transfer from carotenoids proceeds via the ultrafast S₂ channel of both the violaxanthin and isofucoxanthin-like carotenoid, but major energy transfer pathway proceeds via the S₁/ICT state of the isofucoxanthin-like carotenoid. Two S₁/ICT-mediated channels characterized by time constants of ~ 0.5 and ~ 4 ps were found. For the isofucoxanthin-like carotenoid excited at 480 nm the slower channel dominates, while those excited at 540 nm employs predominantly the fast 0.5 ps channel. Comparing these data with the excited-state properties of the isofucoxanthin-like carotenoid in solution we conclude that, contrary to other members of the FCP family employing carbonyl carotenoids, CLH complex suppresses the charge transfer character of the S₁/ICT state of the isofucoxanthin-like carotenoid to achieve the high carotenoid-to-Chl-a energy transfer efficiency.     

Purification and Characterization of a Light-Harvesting Chlorophyll-Protein Complex from the Marine Eustigmatophyte Nannochloropsis sp.

Light-harvesting chlorophyll-protein was purified from thylakoidmembranes of the marine unicellular alga Nannochloropsis sp.(Eustigmatophyceae), which contains neither chlorophyll b norchlorophyll c. Solubilization of thylakoid membranes with octyl-ß-D-glucopyranosideor with digitonin followed by separation on sucrose densitygradient yielded a chlorophyll-protein complex composed of anapoprotein of 26 kDa and an average of 9 chlorophyll a and 4violaxanthin molecules per apoprotein. Excitation spectra ofchlorophyll a fluorescence for the algal thylakoid membranesindicated energy transfer from the xanthophylls; however, anyattempt to solubilize the membranes greatly decreased energytransfer which was further reduced as the purification proceeded.The 26 kDa polypeptide of the isolated light-harvesting complexdid not cross-react with polyclonal antibodies raised againstanalogous proteins from higher plants and chlorophyll a/c alga.The N-terminus amino acid sequence of the apoprotein shows significantstructural similarity to the N-termini of the mature light harvestingfucoxanthin, chlorophyll a/c proteins from the diatom Phaeodactylumtricornutum, but not with the N-termini of light-harvestingproteins from chlorophyll a/b containing organisms. (Received June 25, 1992; Accepted July 28, 1992)     

The Light-harvesting Chlorophyll a/b-Protein Complex of Chlamydomonas reinhardii

The molecular organization of chlorophyll in Chlamydomonas reinhardii has been shown to be essentially similar to that in higher plants. Some 50% of the chlorophyll in Chlamydomonas reinhardii chloroplast membranes has been shown to be located in a chlorophyll a/b-protein complex. The complex was isolated in a homogeneous form by hydroxylapatite chromatography of sodium dodecyl sulfate extracts of the chloroplast membranes. Its absorption spectrum exhibits two maxima in the red region at 670 and 652 nm due to the presence of equimolar quantities of chlorophylls a and b in the complex. Preparations of the chlorophyll-protein also contain some of each of the carotenoids observed in the intact chloroplast membrane, but not in the same proportions. The native complex (S value = 2.3S) exhibits a molecular weight of 28,000 ± 2,000 on calibrated sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, on the basis of its amino acid composition and other data a more probable molecular weight of about 35,000 was calculated. Each 35,000 dalton unit contains three chlorophyll a and three chlorophyll b molecules, and on the average one carotenoid molecule conjugated with probably a single polypeptide of 29,000 daltons. Comparison of spectral and biochemical characteristics demonstrates that this algal chlorophyll-protein is homologous to the previously described major light-harvesting chlorophyll a/b-protein of higher plants. It is anticipated that the Chlamydomonas complex functions solely in a light-harvesting capacity in analogy to the function determined for the higher plant component.     

Orientation of pigments in the thylakoid membrane and in the isolated chlorophyll-protein complexes of higher plants. II. Linear dichroism spectra of isolated pigment-protein complexes oriented in polyacrylamide gels at 300 and 100 K

The absorption and linear dichroism (LD) spectra (380–780 nm) of isolated light-harvesting complex (LHC), Photosystem I (PS I), Photosystem II (PS II), as well as intact thylakoids have been determined at 300 and 100 K. The samples were oriented in squeezed polyacrylamide gel. The low-temperature spectra of LHC and PS I present LD signals which are characteristic enough to be recognized in the LD spectrum of thylakoids. Tentative assignments of the various features of the LD spectra to the major photosynthetic pigments are discussed. A shoulder in the low-temperature absorption spectra is observed at about 673 nm in all the systems under investigation. The absence of an associated LD signal suggests that this ubiquitous chlorophyll (Chl) a form is non-dichroic. Furthermore, in the three isolated chlorophyll-protein complexes described in this study the sign of the LD signal indicates that both the Q_y transition of the Chl a and the carotenoid molecules are preferentially oriented parallel to the largest dimension(s) of the particles.