Fucoxanthin-chlorophyll Protein Research Articles

Molecular events accompanying aggregation-induced energy quenching in fucoxanthin-chlorophyll proteins

In high light, the antenna system in oxygenic photosynthetic organisms switches to a photoprotective mode, dissipating excess energy in a process called non-photochemical quenching (NPQ). Diatoms exhibit very efficient NPQ, accompanied by a xanthophyll cycle in which diadinoxanthin is de-epoxidized into diatoxanthin. Diatoms accumulate pigments from this cycle in high light, and exhibit faster and more pronounced NPQ. The mechanisms underlying NPQ in diatoms remain unclear, but it can be mimicked by aggregation of their isolated light-harvesting complexes, FCP (fucoxanthin chlorophyll-a/c protein). We assess this model system by resonance Raman measurements of two peripheral FCPs, trimeric FCPa and nonameric FCPb, isolated from high- and low-light-adapted cells (LL,HL). Quenching is associated with a reorganisation of these proteins, affecting the conformation of their bound carotenoids, and in a manner which is highly dependent on the protein considered. FCPa from LL diatoms exhibits significant changes in diadinoxanthin structure, together with a smaller conformational change of at least one fucoxanthin. For these LL-FCPa, quenching is associated with consecutive events, displaying distinct spectral signatures, and its amplitude correlates with the planarity of the diadinoxanthin structure. HL-FCPa aggregation is associated with a change in planarity of a 515-nm-absorbing fucoxanthin, and, to a lesser extent, of diadinoxanthin. Finally, in FCPb, a blue-absorbing fucoxanthin is primarily affected. FCPs thus possess a plastic structure, undergoing several conformational changes upon aggregation, dependent upon their precise composition and structure. NPQ in diatoms may therefore arise from a combination of structural changes, dependent on the environment the cells are adapted to.

A key gene, violaxanthin de-epoxidase-like 1, enhances fucoxanthin accumulation in Phaeodactylum tricornutum

BackgroundFucoxanthin has been widely investigated owing to its beneficial biological properties, and the model diatom Phaeodactylum tricornutum, possessing fucoxanthin (Fux) chlorophyll proteins as light-harvesting systems, is considered to have the potential to become a commercial cell factory for the pigment production.ResultsHere, we compared the pigment contents in 10 different P. tricornutum strains from the globe, and found that strain CCMP631 (Pt6) exhibited the highest Fux content but with a low biomass. Comparison of mRNA levels revealed that higher Fux content in Pt6 was related with the higher expression of gene violaxanthin de-epoxidase-like (VDL) protein 1 (VDL1), which encodes the enzyme catalyzing the tautomerization of violaxanthin to neoxanthin in Fux biosynthesis pathway. Single nucleotide variants of VDL1 gene and allele-specific expression in strains Pt1 (the whole genome sequenced strain CCMP632) and Pt6 were analyzed, and overexpressing of each of the 4 VDL1 alleles, two from Pt1 and two from Pt6, in strain Pt1 leads to an increase in downstream product diadinoxanthin and channels the pigments towards Fux biosynthesis. All the 8 VDL1 overexpression (OE) lines showed significant increases by 8.2 to 41.7% in Fux content without compromising growth, and VDL1 Allele 2 OE lines even exhibited the higher cell density on day 8, with an increase by 24.2–28.7% in two Pt1VDL1-allele 2 OE lines and 7.1–11.1% in two Pt6VDL1-allele 2 OE lines, respectively.ConclusionsThe results reveal VDL1, localized in the plastid stroma, plays a key role in Fux over-accumulation in P. tricornutum. Overexpressing VDL1, especially allele 2, improved both the Fux content and growth rate, which provides a new strategy for the manipulation of Fux production in the future.

Two-Dimensional Electronic Spectroscopy Characterization of Fucoxanthin-Chlorophyll Protein Reveals Excitonic Carotenoid-Chlorophyll Interactions.

Fucoxanthin Chlorophyll Protein (FCP) is a Light Harvesting Complex found in diatoms and brown algae. It is particularly interesting for its efficiency in capturing the blue-green part of the light spectrum due to the presence of specific chromophores (fucoxanthin, chlorophyll a, and chlorophyll c). Recently, the crystallographic structure of FCP was solved, revealing the 3D arrangement of the pigments in the protein scaffold. While this information is helpful for interpreting the spectroscopic features of FCP, it has also raised new questions about the potential interactions between fucoxanthin and chlorophyll c. These interactions were suggested by their spatial closeness but have never been experimentally observed. To investigate this possible interaction mechanism, in this work, two-dimensional electronic spectroscopy (2DES) has been applied to study the ultrafast relaxation dynamics of FCP. The experiments captured an instantaneous delocalization of the excitation among fucoxanthin and chlorophyll c, suggesting the presence of a non-negligible coupling between the chromophores.

Fluorescence quenching in aggregates of fucoxanthin–chlorophyll protein complexes: Interplay of fluorescing and dark states

Diatoms, a major group of algae, account for about a quarter of the global primary production on Earth. These photosynthetic organisms face significant challenges due to light intensity variations in their underwater habitat. To avoid photodamage, they have developed very efficient non-photochemical quenching (NPQ) mechanisms. These mechanisms originate in their light-harvesting antenna – the fucoxanthin–chlorophyll protein (FCP) complexes. Spectroscopic studies of NPQ in vivo are often hindered by strongly overlapping signals from the photosystems and their antennae. Fortunately, in vitro FCP aggregates constitute a useful model system to study fluorescence (FL) quenching in diatoms. In this work, we present streak-camera FL measurements on FCPa and FCPb complexes, isolated from a centric diatom Cyclotella meneghiniana, and their aggregates. We find that spectra of non-aggregated FCP are dominated by a single fluorescing species, but the FL spectra of FCP aggregates additionally contain contributions from a redshifted emissive state. We relate this red state to a charge transfer state between chlorophyll c and chlorophyll a molecules. The FL quenching, on the other hand, is due to an additional dark state that involves incoherent energy transfer to the fucoxanthin carotenoids. Overall, the global picture of energy transfer and quenching in FCP aggregates is very similar to that of major light-harvesting complexes in higher plants (LHCII), but microscopic details between FCPs and LHCIIs differ significantly.

Origin of Energy Dissipation in the Oligomeric Fucoxanthin-Chlorophyll a/c Binding Proteins.

Fucoxanthin-chlorophyll proteins (FCPs) are a family of photosynthetic light-harvesting complex (LHC) proteins found in diatoms. They efficiently capture photons and regulate their functions, ensuring diatom survival in highly fluctuating light. FCPs are present in different oligomeric states in vivo, but functional differences among these FCP oligomers are not yet fully understood. Here we characterized two types of antenna complexes (FCP-B/C dimers and FCP-A tetramers) that coexist in the marine centric diatom Chaetoceros gracilis using both time-resolved fluorescence and transient absorption spectroscopy. We found that the FCP-B/C complex did not show fluorescence quenching, whereas FCP-A was severely quenched, via an ultrafast excitation energy transfer (EET) pathway from Chl a Qy to the fucoxanthin S1/ICT state. These results highlight the functional differences between FCP dimers and tetramers and indicate that the EET pathway from Chl a to carotenoids is an energy dissipation mechanism conserved in a variety of photosynthetic organisms.

Exogenous Arachidonic Acid Affects Fucoxanthin Biosynthesis and Photoprotection in Phaeodactylum tricornutum.

Fucoxanthin is an oxygenated carotenoid component that has been reported to play important roles in anti-oxidation, anti-obesity and anti-cancer in the human body. Fucoxanthin-chlorophyll protein (FCP) complexes participate in light harvesting and photoprotection in diatom. In order to better understand the change of fucoxanthin content and its role in photoprotection, the growth, fucoxanthin biosynthesis and photosynthetic phenotypes were studied in the diatom Phaeodactylum tricornutum under the treatment of exogenous arachidonic acid (AA). Our results demonstrated that even low concentration of AA at 0.1 mg/L strongly induced fucoxanthin accumulation in algal cells to a maximum of 1.1 mg/g dry weight (DW), which was 36.6% higher than that in the untreated ones. By principal component analysis (PCA), we also identified a close correlation between fucoxanthin accumulation and the expression of genes involved in fucoxanthin biosynthesis, especially phytoene synthase (PSY), suggesting that AA change the metabolism of fucoxanthin by inducing carotenoid metabolic enzymes at the transcriptional level. Furthermore, we found that the exogenous application of AA affected non-photochemical quenching (NPQ) and photoinhibition, which resulted from the changed diadinoxanthin (DD) and diatoxanthin (DT) cycle, and thus played an important role in photoprotection.

Green diatom mutants reveal an intricate biosynthetic pathway of fucoxanthin

Fucoxanthin is a major light-harvesting pigment in ecologically important algae such as diatoms, haptophytes, and brown algae (Phaeophyceae). Therefore, it is a major driver of global primary productivity. Species of these algal groups are brown colored because the high amounts of fucoxanthin bound to the proteins of their photosynthetic machineries enable efficient absorption of green light. While the structure of these fucoxanthin-chlorophyll proteins has recently been resolved, the biosynthetic pathway of fucoxanthin is still unknown. Here, we identified two enzymes central to this pathway by generating corresponding knockout mutants of the diatom Phaeodactylum tricornutum that are green due to the lack of fucoxanthin. Complementation of the mutants with the native genes or orthologs from haptophytes restored fucoxanthin biosynthesis. We propose a complete biosynthetic path to fucoxanthin in diatoms and haptophytes based on the carotenoid intermediates identified in the mutants and invitro biochemical assays. It is substantially more complex than anticipated and reveals diadinoxanthin metabolism as the central regulatory hub connecting the photoprotective xanthophyll cycle and the formation of fucoxanthin. Moreover, our data show that the pathway evolved by repeated duplication and neofunctionalization of genes for the xanthophyll cycle enzymes violaxanthin de-epoxidase and zeaxanthin epoxidase. Brown algae lack diadinoxanthin and the genes described here and instead use an alternative pathway predicted to involve fewer enzymes. Our work represents a major step forward in elucidating the biosynthesis of fucoxanthin and understanding the evolution, biogenesis, and regulation of the photosynthetic machinery in algae.

Structure-based model of fucoxanthin-chlorophyll protein complex: Calculations of chlorophyll electronic couplings.

Diatoms are a group of marine algae that are responsible for a significant part of global oxygen production. Adapted to life in an aqueous environment dominated by the blue-green light, their major light-harvesting antennae-fucoxanthin-chlorophyll protein complexes (FCPs)-exhibit different pigment compositions than of plants. Despite extensive experimental studies, until recently the theoretical description of excitation energy dynamics in these complexes was limited by the lack of high-resolution structural data. In this work, we use the recently resolved crystallographic information of the FCP complex from Phaeodactylum tricornutum diatom [Wang et al., Science 363, 6427 (2019)] and quantum chemistry-based calculations to evaluate the chlorophyll transition dipole moments, atomic transition charges from electrostatic potential, and the inter-chlorophyll couplings in this complex. The obtained structure-based excitonic couplings form the foundation for any modeling of stationary or time-resolved spectroscopic data. We also calculate the inter-pigment Förster energy transfer rates and identify two quickly equilibrating chlorophyll clusters.

Isolation of fucoxanthin chlorophyll protein complexes of the centric diatom Thalassiosira pseudonana associated with the xanthophyll cycle enzyme diadinoxanthin de-epoxidase.

In the present study, low concentrations of the very mild detergent n-dodecyl-α-d-maltoside in conjunction with sucrose gradient ultracentrifugation were used to prepare fucoxanthin chlorophyll protein (FCP) complexes of the centric diatom Thalassiosira pseudonana. Two main FCP fractions were observed in the sucrose gradients, one in the upper part and one at high sucrose concentrations in the lower part of the gradient. The first fraction was dominated by the 18 kDa FCP protein band in SDS-gels. Since this fraction also contained other protein bands, it was designated as fraction enriched in FCP-A complex. The second fraction contained mainly the 21 kDa FCP band, which is typical for the FCP-B complex. Determination of the lipid composition showed that both FCP fractions contained monogalactosyl diacylglycerol as the main lipid followed by the second galactolipid of the thylakoid membrane, namely digalactosyl diacylglycerol. The negatively charged lipids sulfoquinovosyl diacylglycerol and phosphatidyl glycerol were also present in both fractions in pronounced concentrations. With respect to the pigment composition, the fraction enriched in FCP-A contained a higher amount of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (Dt), whereas the FCP-B fraction was characterized by a lower ratio of xanthophyll cycle pigments to the light-harvesting pigment fucoxanthin. Protein analysis by mass spectrometry revealed that in both FCP fractions the xanthophyll cycle enzyme diadinoxanthin de-epoxidase (DDE) was present. In addition, the analysis showed an enrichment of DDE in the fraction enriched in FCP-A but only a very low amount of DDE in the FCP-B fraction. In-vitro de-epoxidation assays, employing the isolated FCP complexes, were characterized by an inefficient conversion of DD to Dt. However, in line with the heterogeneous DDE distribution, the fraction enriched in FCP-A showed a more pronounced DD de-epoxidation compared with the FCP-B.

Production of Carotenoids from Cultivated Seaweed.

Cladosiphon (C.) okamuranus, a brown alga endemic to the Nansei Islands, Japan, has been conventionally ingested as food. Nowadays, it is a major aquatic product of the Okinawa Prefecture with an annual production of around 20,000 tons. The life cycle of C. okamuranus comprises the macroscopic sporophyte (algal body) generation and the microscopic gametophyte generation. The germlings in the latter generation can proliferate when floating in seawater. This floating form has been exploited in techniques involved in the commercial production of C. okamuranus seedlings.Brown algae contain fucoxanthin, a carbonyl carotenoid known to have anticancer, anti-obesity, and antidiabetic effect in addition to the anti-oxidation effect. We found that the fucoxanthin content of cultivated floating form of C. okamuranus discoid germlings becomes up to 50 times that of the mature alga. Since the discoid germlings repeatedly grow like microorganisms, although they are large algae, they are utilized to produce fucoxanthin. We optimize the culture conditions by changing the temperature, light intensity, photoperiod, light wavelength, and nutrient salt conditions for optimal fucoxanthin productivity. The cultivation has been successful to industrial plant scale, culminating in the use of 1 ton of cultivating medium.In brown algal cells, fucoxanthin is primarily found bound to the photosynthetic pigment-protein complexes known as fucoxanthin-chlorophyll protein (FCP). Consequently cultivated floating form of C. okamuranus also shows high content of FCP. Isolation and characterization of pigments bound to the FCP were determined precisely, and ultrafast spectroscopies were applied to elucidate the photosynthetic function of fucoxanthin bound to the pigment-protein complexes. This cultivation method has also been applied to the other edible brown algae. We found that the optimal cultivation conditions as well as the yields of fucoxanthin and FCP highly depend on the species.The floating form cultivation was also applied to a large-sized edible green alga, Codium intricatum, which is uniquely producing a carbonyl carotenoid, siphonaxanthin. This has several anti-disease effects and is also a primal photosynthetic pigment which is found bound to photosynthetic antenna complex usually called siphonaxanthin-chlorophyll protein (SCP). We are working on the improvement of productivity, scale-up of production, and development of cultivation technology of new macro algae.

Pre-purification of diatom pigment protein complexes provides insight into the heterogeneity of FCP complexes

BackgroundAlthough our knowledge about diatom photosynthesis has made huge progress over the last years, many aspects about their photosynthetic apparatus are still enigmatic. According to published data, the spatial organization as well as the biochemical composition of diatom thylakoid membranes is significantly different from that of higher plants.ResultsIn this study the pigment protein complexes of the diatom Thalassiosira pseudonana were isolated by anion exchange chromatography. A step gradient was used for the elution process, yielding five well-separated pigment protein fractions which were characterized in detail. The isolation of photosystem (PS) core complex fractions, which contained fucoxanthin chlorophyll proteins (FCPs), enabled the differentiation between different FCP complexes: FCP complexes which were more closely associated with the PSI and PSII core complexes and FCP complexes which built-up the peripheral antenna. Analysis by mass spectrometry showed that the FCP complexes associated with the PSI and PSII core complexes contained various Lhcf proteins, including Lhcf1, Lhcf2, Lhcf4, Lhcf5, Lhcf6, Lhcf8 and Lhcf9 proteins, while the peripheral FCP complexes were exclusively composed of Lhcf8 and Lhcf9. Lhcr proteins, namely Lhcr1, Lhcr3 and Lhcr14, were identified in fractions containing subunits of the PSI core complex. Lhcx1, Lhcx2 and Lhcx5 proteins co-eluted with PSII protein subunits. The first fraction contained an additional Lhcx protein, Lhcx6_1, and was furthermore characterized by high concentrations of photoprotective xanthophyll cycle pigments.ConclusionThe results of the present study corroborate existing data, like the observation of a PSI-specific antenna complex in diatoms composed of Lhcr proteins. They complement other data, like e.g. on the protein composition of the 21 kDa FCP band or the Lhcf composition of FCPa and FCPb complexes. They also provide interesting new information, like the presence of the enzyme diadinoxanthin de-epoxidase in the Lhcx-containing PSII fraction, which might be relevant for the process of non-photochemical quenching. Finally, the high negative charge of the main FCP fraction may play a role in the organization and structure of the native diatom thylakoid membrane. Thus, the results present an important contribution to our understanding of the complex nature of the diatom antenna system.

Specific Lhc Proteins Are Bound to PSI or PSII Supercomplexes in the Diatom Thalassiosira pseudonana.

Despite the ecological relevance of diatoms, many aspects of their photosynthetic machinery remain poorly understood. Diatoms differ from the green lineage of oxygenic organisms by their photosynthetic pigments and light-harvesting complex (Lhc) proteins, the latter of which are also called fucoxanthin-chlorophyll proteins (FCP). These are composed of three groups of proteins: Lhcf as the main group, Lhcr that are PSI associated, and Lhcx that are involved in photoprotection. The FCP complexes are assembled in trimers and higher oligomers. Several studies have investigated the biochemical properties of purified FCP complexes, but limited knowledge is available about their interaction with the photosystem cores. In this study, isolation of stable supercomplexes from the centric diatom Thalassiosira pseudonana was achieved. To preserve in vivo structure, the separation of thylakoid complexes was performed by native PAGE and sucrose density centrifugation. Different subpopulations of PSI and PSII supercomplexes were isolated and their subunits identified. Analysis of Lhc antenna composition identified Lhc(s) specific for either PSI (Lhcr 1, 3, 4, 7, 10-14, and Lhcf10) or PSII (Lhcf 1-7, 11, and Lhcr2). Lhcx6_1 was reproducibly found in PSII supercomplexes, whereas its association with PSI was unclear. No evidence was found for the interaction between photosystems and higher oligomeric FCPs, comprising Lhcf8 as the main component. Although the subunit composition of the PSII supercomplexes in comparison with that of the trimeric FCP complexes indicated a close mutual association, the higher oligomeric pool is only weakly associated with the photosystems, albeit its abundance in the thylakoid membrane.

Fucoxanthin-Chlorophyll Protein Complexes of the Centric Diatom Cyclotella Meneghiniana Differ in Lhcx1 and Lhcx6_1 Content.

The ecological success of diatoms, key contributors to photosynthesis, is partly based on their ability to perfectly balance efficient light harvesting and photoprotection. Diatoms contain higher numbers of antenna proteins than vascular plants for light harvesting and for photoprotection. These proteins are arranged in fucoxanthin-chlorophyll protein (FCP) complexes. The number of FCP complexes, their subunit composition, and their interactions in the thylakoid membranes remain elusive in different diatoms. We used the recently available genome sequence of the centric diatom Cyclotella cryptica to analyze gene sequences for putative light-harvesting proteins in C. meneghiniana, and to elucidate the FCP complex composition. We analyzed two pools of FCP complexes that were trimeric (FCPa) and nonameric (FCPb). FCPa was composed of four different trimeric subtypes. Two different nonameric FCPb complexes were present. All were distinguished by their polypeptide composition and partly by pigmentation. With use of a milder purification method, two fractions composed of different FCP complexes were isolated. One was enriched in FCPs incorporating the photoprotective subunit Lhcx1, such as the newly identified nonameric FCPb2 and the major trimeric FCPa4 complex, which are predetermined to be involved in energy-dependent nonphotochemical quenching. The other fraction contained mainly FCPs that were devoid of Lhcx1, FCPa3, and FCPb1. Both fractions also included small amounts of trimeric FCPa complexes with the centric diatom-specific Lhcx protein, Lhcx6_1, as subunit. Thus, the antenna organization of centric diatoms, as well as the distribution of different photoprotective Lhcx proteins, differs from that of other diatoms, as well as from plants.

Photoelectrochemical Complexes of Fucoxanthin-Chlorophyll Protein for Bio-Photovoltaic Conversion with a High Open-Circuit Photovoltage.

Open-circuit photovoltage (Voc ) is among the critical parameters for achieving an efficient light-to-charge conversion in existing solar photovoltaic devices. Natural photosynthesis exploits light-harvesting chlorophyll (Chl) protein complexes to transfer sunlight energy efficiently. We describe the exploitation of photosynthetic fucoxanthin-chlorophyll protein (FCP) complexes for realizing photoelectrochemical cells with a high Voc . An antenna-dependent photocurrent response and a Voc up to 0.72 V are observed and demonstrated in the bio-photovoltaic devices fabricated with photosynthetic FCP complexes and TiO2 nanostructures. Such high Voc is determined by fucoxanthin in FCP complexes, and is rarely found in photoelectrochemical cells with other natural light-harvesting antenna. We think that the FCP-based bio-photovoltaic conversion will provide an opportunity to fabricate environmental benign photoelectrochemical cells with high Voc , and also help improve the understanding of the essential physics behind the light-to-charge conversion in photosynthetic complexes.

Quenching of chlorophyll triplet states by carotenoids in algal light-harvesting complexes related to fucoxanthin-chlorophyll protein.

We have used time-resolved absorption and fluorescence spectroscopy with nanosecond resolution to study triplet energy transfer from chlorophylls to carotenoids in a protective process that prevents the formation of reactive singlet oxygen. The light-harvesting complexes studied were isolated from Chromera velia, belonging to a group Alveolata, and Xanthonema debile and Nannochloropsis oceanica, both from Stramenopiles. All three light-harvesting complexes are related to fucoxanthin-chlorophyll protein, but contain only chlorophyll a and no chlorophyll c. In addition, they differ in the carotenoid content. This composition of the complexes allowed us to study the quenching of chlorophyll a triplet states by different carotenoids in a comparable environment. The triplet states of chlorophylls bound to the light-harvesting complexes were quenched by carotenoids with an efficiency close to 100%. Carotenoid triplet states were observed to rise with a ~5ns lifetime and were spectrally and kinetically homogeneous. The triplet states were formed predominantly on the red-most chlorophylls and were quenched by carotenoids which were further identified or at least spectrally characterized.

Influence of thylakoid membrane lipids on the structure of aggregated light-harvesting complexes of the diatom Thalassiosira pseudonana and the green alga Mantoniella squamata.

The study investigated the effect of the thylakoid membrane lipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulphoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) on the structure of two algal light-harvesting complexes (LHCs). In contrast to higher plants whose thylakoid membranes are characterized by an enrichment of the neutral galactolipids MGDG and DGDG, both the green alga Mantoniella squamata and the centric diatom Thalassiosira pseudonana contain membranes with a high content of the negatively charged lipids SQDG and PG. The algal thylakoids do not show the typical grana-stroma differentiation of higher plants but a regular arrangement. To analyze the effect of the membrane lipids, the fucoxanthin chlorophyll protein (FCP) complex of T. pseudonana and the LHC of M. squamata (MLHC) were prepared by successive cation precipitation using Triton X-100 as detergent. With this method, it is possible to isolate LHCs with a reduced amount of associated lipids in an aggregated state. The results from 77 K fluorescence and photon correlation spectroscopy show that neither the neutral galactolipids nor the negatively charged lipids are able to significantly alter the aggregation state of the FCP or the MLHC. This is in contrast to higher plants where SQDG and PG lead to a strong disaggregation of the LHCII whereas MGDG and DGDG induce the formation of large macroaggregates. The results indicate that LHCs which are integrated into thylakoid membranes with a high amount of negatively charged lipids and a regular arrangement are less sensitive to lipid-induced structural alterations than their counterparts in membranes enriched in neutral lipids with a grana-stroma differentiation.

The structure of FCPb, a light-harvesting complex in the diatom Cyclotella meneghiniana.

Diatoms possess fucoxanthin chlorophyll proteins (FCP) as light-harvesting systems. These membrane intrinsic proteins bind fucoxanthin as major carotenoid and Chl c as accessory chlorophyll. The relatively high sequence homology to higher plant light-harvesting complex II gave rise to the assumption of a similar overall structure. From centric diatoms like Cyclotella meneghiniana, however, two major FCP complexes can be isolated. FCPa, composed of Fcp2 and Fcp6 subunits, was demonstrated to be trimeric, whereas FCPb, known to contain Fcp5 polypeptides, is of higher oligomeric state. No molecular structure of either complex is available so far. Here we used electron microscopy and single particle analysis to elucidate the overall architecture of FCPb. The complexes are built from trimers as basic unit, assembling into nonameric moieties. The trimer itself is smaller, i.e. more compact than LHCII, but the main structural features are conserved.

Identification and characterization of multiple emissive species in aggregated minor antenna complexes

Aggregation induced conformational change of light harvesting antenna complexes is believed to constitute one of the pathways through which photosynthetic organisms can safely dissipate the surplus of energy while exposed to saturating light. In this study, Stark fluorescence (SF) spectroscopy is applied to minor antenna complexes (CP24, CP26 and CP29) both in their light-harvesting and energy-dissipating states to trace and characterize different species generated upon energy dissipation through aggregation (in-vitro) induced conformational change. SF spectroscopy could identify three spectral species in the dissipative state of CP24, two in CP26 and only one in CP29. The comprehensive analysis of the SF spectra yielded different sets of molecular parameters for the multiple spectral species identified in CP24 or CP26, indicating the involvement of different pigments in their formation. Interestingly, a species giving emission around the 730nm spectral region is found to form in both CP24 and CP26 following transition to the energy dissipative state, but not in CP29. The SF analyses revealed that the far red species has exceptionally large charge transfer (CT) character in the excited state. Moreover, the far red species was found to be formed invariably in both Zeaxanthin (Z)- and Violaxathin (V)-enriched CP24 and CP26 antennas with identical CT character but with larger emission yield in Z-enriched ones. This suggests that the carotenoid Z is not directly involved but only confers an allosteric effect on the formation of the far red species. Similar far red species with remarkably large CT character were also observed in the dissipative state of the major light harvesting antenna (LHCII) of plants [Wahadoszamen et al. PCCP, 2012], the fucoxanthin-chlorophyll protein (FCP) of brown algae [Wahadoszamen et al. BBA, 2014] and cyanobacterial IsiA [Wahadoszamen et al. BBA, 2015], thus pointing to identical sites and pigments active in the formation of the far red quenching species in different organisms.

The diadinoxanthin diatoxanthin cycle induces structural rearrangements of the isolated FCP antenna complexes of the pennate diatom Phaeodactylum tricornutum

The study investigated the influence of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (Dt) on the spectroscopic characteristics, structure and protein composition of isolated fucoxanthin chlorophyll protein (FCP) complexes of the pennate diatom Phaeodactylum tricornutum. 77 K fluorescence emission spectra revealed that Dt-containing FCP complexes showed a characteristic long wavelength fluorescence emission at 700 nm at a pH-value of 5 whereas DD-enriched FCPs retained the typical 680 nm fluorescence emission maximum of isolated FCPs. The 700 nm emission in Dt-containing FCPs indicates an aggregation of antenna complexes and is a typical feature of the quenching site Q1 in recent models for non-photochemical fluorescence quenching (NPQ). A comparable long-wavelength fluorescence emission was found in FCP complexes prepared with either triton X-100 or n-dodecyl β-D-maltoside as detergent. A treatment of the FCP complexes at low pH-values in the presence of a high concentration of Mg2+ ions showed that the extent of FCP aggregation which leads to the 700 nm fluorescence emission is different from the macro-aggregation of antenna complexes in higher plants. Protein analyses by mass spectrometry showed that the protein composition of the DD- and Dt-enriched FCP complexes was comparable. However, the Lhcf6 and Lhcr1 polypeptides were only found in Dt-enriched FCPs isolated with dodecyl maltoside whereas the Lhcf17 protein was only detected in DD-enriched FCPs prepared with triton. With respect to low pH-induced antenna aggregation it is important that the Lhcx1 protein was found in both DD- and Dt-enriched FCPs, albeit with only two peptides with confident scores.

Coherence and population dynamics of chlorophyll excitations in FCP complex: Two-dimensional spectroscopy study.

Energy transfer processes and coherent phenomena in the fucoxanthin-chlorophyll protein complex, which is responsible for the light harvesting function in marine algae diatoms, were investigated at 77 K by using two-dimensional electronic spectroscopy. Experiments performed on femtosecond and picosecond timescales led to separation of spectral dynamics, witnessing evolutions of coherence and population states of the system in the spectral region of Qy transitions of chlorophylls a and c. Analysis of the coherence dynamics allowed us to identify chlorophyll (Chl) a and fucoxanthin intramolecular vibrations dominating over the first few picoseconds. Closer inspection of the spectral region of the Qy transition of Chl c revealed previously not identified, mutually non-interacting chlorophyll c states participating in femtosecond or picosecond energy transfer to the Chl a molecules. Consideration of separated coherent and incoherent dynamics allowed us to hypothesize the vibrations-assisted coherent energy transfer between Chl c and Chl a and the overall spatial arrangement of chlorophyll molecules.