Pigment organization in fucoxanthin chlorophyll a/c2 proteins (FCP) based on resonance Raman spectroscopy and sequence analysis

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.

Probing the Fucoxanthin-Chlorophyll a/c-Binding Proteins (FCPs) of the Marine Diatom Fragilariopsis sp. by Resonance Raman Spectroscopy

We report resonance Raman spectra of the light-harvesting fucoxanthin-chlorophyll a/c-binding proteins (FCPs) of marine diatom Fragilariopsis sp. The Raman shifts in the 15N-isotope-enriched diatom provide the first spectroscopic evidence for the characterization of the Ca-N marker bands and, thus, of the penta- and hexacoordinated states of chlorophylls a/c in the FCPs. Under 405 and 442 nm Raman excitations, all of the marker bands of Chl a/c are observed and the isotope-based assignments provide new information concerning the structure of Chls a/c in the FCPs and their interactions with the protein environment. Therefore, the Raman spectrum at 405 nm originates from the π-π* transitions of Chl a/c and not from a different, non π-π* electronic transition, as previously reported (BBA Bioenergetics, 2010, 1797, 1647-1656). Based on the 15N isotope shifts of the Ca-N and in conjunction with other marker bands, two distinct conformations of five- and six-coordinated Chl a and Chl c are observed. In addition, two keto carbonyls were observed at 1679 (strong H-bonded) and 1691 cm-1 (weak H-bonded) in both the 405 and 442 nm Raman spectra, respectively. Collectively, the results provide solid evidence of the nature of the vibrational modes of the active Chl a/c photosynthetic pigments in the FCPs.

Conservation of triplet-triplet energy transfer photoprotective pathways in fucoxanthin chlorophyll-binding proteins across algal lineages

Detailed information on the photo-generated triplet states of diatom and haptophyte Fucoxanthin Chlorophyll-binding Proteins (FCPs and E-FCPs, respectively) have been obtained from a combined spectroscopic investigation involving Transient Absorption and Time-Resolved Electron Paramagnetic Resonance. Pennate diatom Phaeodactylum tricornutum FCP shows identical photoprotective Triplet-Triplet Energy Transfer (TTET) pathways to the previously investigated centric diatom Cyclotella meneghiniana FCP, with the same two chlorophyll a-fucoxanthin pairs that involve the fucoxanthins in sites Fx301 and Fx302 contributing to TTET in both diatom groups. In the case of the haptophyte Emilianina huxleyi E-FCP, only one of the two chlorophyll a-fucoxanthins pairs observed in diatoms, the one involving chlorophyll a409 and Fx301, has been shown to be active in TTET. Furthermore, despite the marked change in the pigment content of E-FCP with growth light intensity, the TTET pathway is not affected. Thus, our comparative investigation of FCPs revealed a photoprotective TTET pathway shared within these classes involving the fucoxanthin in site Fx301, a site exposed to the exterior of the antenna monomer that has no equivalent in Light-Harvesting Complexes from the green lineage.

The redox state of the plastoquinone (PQ) pool is connected to thylakoid lipid saturation in a marine diatom

The redox state of the plastoquinone (PQ) pool is a known sensor for retrograde signaling. In this paper, we asked, "does the redox state of the PQ pool modulate the saturation state of thylakoid lipids?" Data from fatty acid composition and mRNA transcript abundance analyses suggest a strong connection between these two aspects in a model marine diatom. Fatty acid profiles of Phaeodactylum tricornutum exhibited specific changes when the redox state of the PQ pool was modulated by light and two chemical inhibitors [3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB)]. Data from liquid chromatography with tandem mass spectrometry (LC-MS/MS) indicated a ca. 7-20% decrease in the saturation state of all four conserved thylakoid lipids in response to an oxidized PQ pool. The redox signals generated from an oxidized PQ pool in plastids also increased the mRNA transcript abundance of nuclear-encoded C16 fatty acid desaturases (FADs), with peak upregulation on a timescale of 6 to 12 h. The connection between the redox state of the PQ pool and thylakoid lipid saturation suggests a heretofore unrecognized retrograde signaling pathway that couples photosynthetic electron transport and the physical state of thylakoid membrane lipids.

Novel Approach to Freshwater Diatom Profiling and Identification Using Raman Spectroscopy and Chemometric Analysis

(1) An approach with great potential for fast and cost-effective profiling and identification of diatoms in lake ecosystems is presented herein. This approach takes advantage of Raman spectroscopy. (2) The study was based on the analysis of 790 Raman spectra from 29 species, belonging to 15 genera, 12 families, 9 orders and 4 subclasses, which were analysed using chemometric methods. The Raman data were first analysed by a partial least squares regression discriminant analysis (PLS-DA) to characterise the diatom species. Furthermore, a method was developed to streamline the integrated interpretation of PLS-DA when a high number of significant components is extracted. Subsequently, an artificial neural network (ANN) was used for taxa identification from Raman data. (3) The PLS interpretation produced a Raman profile for each species reflecting its biochemical composition. The ANN models were useful to identify various taxa with high accuracy. (4) Compared to studies in the literature, involving huge datasets one to four orders of magnitude larger than ours, high sensitivity was found for the identification of Achnanthidium exiguum (67%), Fragilaria pararumpens (67%), Amphora pediculus (71%), Achnanthidium minutissimum (80%) and Melosira varians (82%).

A kaleidoscope of photosynthetic antenna proteins and their emerging roles

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-products. The major challenge in this approach lies in the application of fundamental discoveries in light-harvesting systems for the improvement of plant or algal photosynthesis. Here, we underline some of the latest fundamental discoveries on the molecular mechanisms and regulation of light harvesting that can potentially be exploited for the optimization of photosynthesis.

Electronic and Vibrational Properties of Allene Carotenoids

Carotenoids are conjugated linear molecules built from the repetition of terpene units, which display a large structural diversity in nature. They may, in particular, contain several types of side or end groups, which tune their functional properties, such as absorption position and photochemistry. We report here a detailed experimental study of the absorption and vibrational properties of allene-containing carotenoids, together with an extensive modeling of these experimental data. Our calculations can satisfactorily explain the electronic properties of vaucheriaxanthin, where the allene group introduces the equivalent of one C=C double bond into the conjugated C=C chain. The position of the electronic absorption of fucoxanthin and butanoyloxyfucoxanthin requires long-range corrections to be found correctly on the red side of that of vaucheriaxanthin; however, these corrections tend to overestimate the effect of the conjugated and nonconjugated C=O groups in these molecules. We show that the resonance Raman spectra of these carotenoids are largely perturbed by the presence of the allene group, with the two major Raman contributions split into two components. These perturbations are satisfactorily explained by modeling, through a gain in the Raman intensity of the C=C antisymmetric stretching mode, induced by the presence of the allene group in the carotenoid C=C chain.

Environmental diagnosis with Raman Spectroscopy applied to diatoms

Freshwater quality has been changing due to the ever greater use of water resources and the contamination load resulting from human activities. Management of these systems, thus, requires constant diagnose of water quality with fast and efficient methodologies. The conventional methods adopted are, however, time-consuming, often very expensive, and require specialised expertise. Raman spectroscopy (RS) is a simple, fast and label-free technique that can be applied to environmental diagnosis using diatoms. Here, we developed a diagnostic method based on Raman spectroscopy applied to freshwater diatoms. For this, Raman spectra were recorded from diatoms of three lakes of a natural city park. The data acquired was analysed by chemometrics methods to describe the data (Partial Least Squares Regression), infer relationships in the dataset (Cluster Analysis) and produce classification models (Artificial Neural Network). The classification models developed diagnosed the lakes with excellent accuracy (89%) without requiring taxonomic information about the diatom species recorded. This study provides a proof-of-concept for the application of diatom Raman spectroscopy to diagnosing water quality, laying an important foundation for future environmental studies aiming at assessing freshwater systems, to be replicated at larger scales and to varied geographic settings.

Photosensitization of fucoxanthin-graphene complexes: A computational approach

Photosensitization of fucoxanthin-graphene (FX-GR) complexes were investigated in this work for detecting their roles of irradiating energy absorptions. To this aim, density functional theory (DFT computational approach as employed to obtain the optimized structures and their corresponding molecular orbital features. Both of original linear models of FX and its broken models, LFX and RFX, were investigated for attaching to a brigading GR molecular model. In this regard, the models were optimized to obtain the minimized energy configurations, in which for double-attachment of FG to the GR coroner atoms, Cis and Trans configurations were obtained for the FX-GR complex models. Based on the obtained achievements of molecular orbitals photosensitization features, the models were varied by the absorbed wavelengths making them suitable for various applications. In this regard, both of shorter and longer irradiated wavelengths were applicable for the purpose.

Evidence for reversible light-dependent transitions in the photosynthetic pigments of diatoms

Marine diatoms contribute to oxygenic photosynthesis and carbon fixation and handle large changes under variable light intensity on a regular basis. The unique light-harvesting apparatus of diatoms are the fucoxanthin–chlorophyll a/c-binding proteins (FCPs). Here, we show the enhancement of chlorophyll a/c (Chl a/c), fucoxanthin (Fx), and diadinoxanthin (Dd) marker bands in the Raman spectra of the centric diatom T. pseudonana, which allows distinction of the pigment content in the cells grown under low- (LL) and high-light (HL) intensity at room temperature. Reversible LL–HL dependent conformations of Chl c, characteristic of two conformations of the porphyrin macrocycle, and the presence of five- and six-coordinated Chl a/c with weak axial ligands are observed in the Raman data. Under HL the energy transfer from Chl c to Chl a is reduced and that from the red-shifted Fxs is minimal. Therefore, Chl c and the blue-shifted Fxs are the only contributors to the energy transfer pathways under HL and the blue- to red-shifted Fxs energy transfer pathway characteristic of the LL is inactive. The results indicate that T. pseudonana can redirect its function from light harvesting to energy-quenching state, and reversibly to light-harvesting upon subsequent illumination to LL by reproducing the red-shifted Fxs and decrease the number of Dds. The LL to HL reversible transitions are accompanied by structural modifications of Chl a/c and the lack of the red-shifted Fxs.

A reversible light-intensity behavior of Dds and Fxs composition in the cells of T. pseudonana. The observed LL to HL reversible transitions are accompanied by structural modifications of Chls a/c and the lack of the red-shifted Fxs.

Differential Membrane Lipid Profiles and Vibrational Spectra of Three Edaphic Algae and One Cyanobacterium

The membrane glycerolipids of four phototrophs that were isolated from an edaphic assemblage were determined by UPLC-MS after cultivation in a laboratory growth chamber. Identification was carried out by 18S and 16S rDNA sequencing. The algal species were Klebsormidium flaccidum (Charophyta), Oocystis sp. (Chlorophyta), and Haslea spicula (Bacillariophyta), and the cyanobacterium was Microcoleus vaginatus (Cyanobacteria). The glycerolipid profile of Oocystis sp. was dominated by monogalactosyldiacylglycerol (MGDG) species, with MGDG(18:3/16:4) accounting for 68.6%, whereas MGDG(18:3/16:3) was the most abundant glycerolipid in K. flaccidum (50.1%). A ratio of digalactosyldiacylglycerol (DGDG) species to MGDG species (DGDG/MGDG) was shown to be higher in K. flaccidum (0.26) than in Oocystis sp. (0.14). This ratio increased under high light (HL) as compared to low light (LL) in all the organisms, with its highest value being shown in cyanobacterium (0.38-0.58, LL-HL). High contents of eicosapentaenoic acid (EPA, C20:5) and hexadecenoic acid were observed in the glycerolipids of H. spicula. Similar Fourier transform infrared (FTIR) and Raman spectra were found for K. flaccidum and Oocystis sp. Specific bands at 1629.06 and 1582.78 cm-1 were shown by M. vaginatus in the Raman spectra. Conversely, specific bands in the FTIR spectrum were observed for H. spicula at 1143 and 1744 cm-1. The results of this study point out differences in the membrane lipid composition between species, which likely reflects their different morphology and evolutionary patterns.

Raman spectroscopy applied to diatoms (microalgae, Bacillariophyta): Prospective use in the environmental diagnosis of freshwater ecosystems

Diatom species are good pollution bioindicators due to their large distribution, fast response to changes in environmental parameters and different tolerance ranges. These organisms are used in ecological water assessment all over the world using autoecological indices. Such assessments commonly rely on the taxonomic identification of diatom species-specific shape and frustule ornaments, from which cell counts, species richness and diversity indices can be estimated. Taxonomic identification is, however, time-consuming and requires years of expertise. Additionally, though the diatom autoecological indices are region-specific, they are often applied indiscriminately across regions. Raman spectroscopy is a simpler, fast and label-free technique that can be applied to environmental diagnosis with diatoms. However, this approach has been poorly explored. This work reviews Raman spectroscopy studies involving the structure, location and conformation of diatom cell components and their variation under different conditions. A critical appreciation of the pros and cons of its application to environmental diagnosis is also given. This knowledge provides a strong foundation for the development of environmental protocols using Raman spectroscopy in diatoms. Our work aims at stimulating further research on the application of Raman spectroscopy as a tool to assess physiological changes and water quality under a changing climate.

Pigment structure in the light-harvesting protein of the siphonous green alga Codium fragile

The siphonaxanthin-siphonein-chlorophyll-a/b-binding protein (SCP), a trimeric light-harvesting complex isolated from photosystem II of the siphonous green alga Codium fragile, binds the carotenoid siphonaxanthin (Sx) and/or its ester siphonein in place of lutein, in addition to chlorophylls a/b and neoxanthin. SCP exhibits a higher content of chlorophyll b (Chl-b) than its counterpart in green plants, light-harvesting complex II (LHCII), increasing the relative absorption of blue-green light for photosynthesis. Using low temperature absorption and resonance Raman spectroscopies, we reveal the presence of two non-equivalent Sx molecules in SCP, and assign their absorption peaks at 501 and 535 nm. The red-absorbing Sx population exhibits a significant distortion that is reminiscent of lutein 2 in trimeric LHCII. Unexpected enhancement of the Raman modes of Chls-b in SCP allows an unequivocal description of seven to nine non-equivalent Chls-b, and six distinct Chl-a populations in this protein.

Confronting FCP structure with ultrafast spectroscopy data: evidence for structural variations

Diatoms are a major group of algae, responsible for a quarter of the global primary production on our planet. Their adaptation to marine environments is ensured by their light-harvesting antenna - the fucoxanthin-chlorophyll protein (FCP) complex, which absorbs strongly in the blue-green spectral region. Although these essential proteins have been the subject of many studies, for a long time their comprehensive description was not possible in the absence of structural data. Last year, the 3D structures of several FCP complexes were revealed. The structure of an FCP dimer was resolved by crystallography for the pennate diatom Phaeodactylum tricornutum [W. Wang et al., Science, 2019, 363, 6427] and the structure of the PSII supercomplex from the centric diatom Chaetoceros gracilis, containing several FCPs, was obtained by electron microscopy [X. Pi et al., Science, 2019, 365, 6452; R. Nagao et al., Nat. Plants, 2019, 5, 890]. In this Perspective article, we evaluate how precisely these structures may account for previously published ultrafast spectroscopy results, describing the excitation energy transfer in the FCP from another centric diatom Cyclotella meneghiniana. Surprisingly, we find that the published FCP structures cannot explain several observations obtained from ultrafast spectroscopy. Using the available structures, and results from electron microscopy, we construct a trimer-based FCP model for Cyclotella meneghiniana, consistent with ultrafast experimental data. As a whole, our observations suggest that the structures from the proteins belonging to the FCP family display larger variations than the equivalent LHC proteins in plants, which may reflect species-specific adaptations or original strategies for adapting to rapidly changing marine environments.

A distinctive pathway for triplet-triplet energy transfer photoprotection in fucoxanthin chlorophyll-binding proteins from Cyclotella meneghiniana

Fucoxanthin chlorophyll-binding proteins (FCPs) are the major light-harvesting complexes of diatoms. In this work, FCPs isolated from Cyclotella meneghiniana have been studied by means of optically detected magnetic resonance (ODMR) and time-resolved electron paramagnetic resonance (TR-EPR), with the aim to characterize the photoprotective mechanism based on triplet-triplet energy transfer (TTET). The spectroscopic properties of the chromophores carrying the triplet state have been interpreted on the basis of a delved analysis of the recently solved crystallographic structures of FCP. The results point toward a photoprotective role for two fucoxanthin molecules exposed to the exterior of the FCP monomers. This shows that FCP has adopted a structural strategy different from that of related light-harvesting complexes from plants and other microalgae, in which the photoprotective role is carried out by two highly conserved carotenoids in the interior of the complex.

Effects of nitrogen on interspecific competition between two cell-size cyanobacteria: Microcystis aeruginosa and Synechococcus sp

Micro-cyanobacteria and pico-cyanobacteria coexist in many lakes throughout the world. Their distinct cell sizes and nutrient utilization strategies may lead to dominance of one over the other at varying nutrient levels. In this study, Microcystis aeruginosa and Synechococcus sp. were chosen as representative organisms of micro- and pico-cyanobacteria, respectively. A series of nitrate and ammonia conditions (0.02, 0.1, 0.5, and 2.5 mg N L^-1) were designed in mono- or co-cultured systems, respectively. Growth rates of the two species were calculated and fitted by the Monod and Logistic equations. Furthermore, the interspecific competition was analyzed using the Lotka-Volterra model. In mono-cultures, the two cyanobacteria displayed faster growth rates in ammonia than in nitrate. Meanwhile, Synechococcus sp. showed faster growth rates compared to M. aeruginosa in lower N groups (≤ 0.5 mg N L^-1). However, in the highest nitrate treatment (2.5 mg N L^-1), M. aeruginosa achieved much higher biomass and faster growth rates than Synechococcus sp.. In co-cultures, Synechococcus sp. dominated in the lowest N treatment (0.02 mg N L^-1), but M. aeruginosa dominated under the highest nitrate condition (2.5 mg N L^-1). Based on the analysis of Raman spectra of living cells in mono-cultures, nitrate (2.5 mg N L^-1) upgraded the pigmentary contents of M. aeruginosa better than ammonia (2.5 mg N L^-1), but nitrogen in different forms showed little effects on the pigments of Synechococcus sp.. Findings from this study can provide valuable information to predict cyanobacterial community succession and aquatic ecosystem stability.

Tuning antenna function through hydrogen bonds to chlorophyll a

We describe a molecular mechanism tuning the functional properties of chlorophyll a (Chl-a) molecules in photosynthetic antenna proteins. Light-harvesting complexes from photosystem II in higher plants - specifically LHCII purified with α- or β-dodecyl-maltoside, along with CP29 - were probed by low-temperature absorption and resonance Raman spectroscopies. We show that hydrogen bonding to the conjugated keto carbonyl group of protein-bound Chl-a tunes the energy of its Soret and Q_y absorption transitions, inducing red-shifts that are proportional to the strength of the hydrogen bond involved. Chls-a with non-H-bonded keto C13¹ groups exhibit the blue-most absorption bands, while both transitions are progressively red-shifted with increasing hydrogen-bonding strength - by up 382 & 605 cm^-1 in the Q_y and Soret band, respectively. These hydrogen bonds thus tune the site energy of Chl-a in light-harvesting proteins, determining (at least in part) the cascade of energy transfer events in these complexes.

Insights into the mechanisms and dynamics of energy transfer in plant light-harvesting complexes from two-dimensional electronic spectroscopy

During the past two decades, two-dimensional electronic spectroscopy (2DES) and related techniques have emerged as a potent experimental toolset to study the ultrafast elementary steps of photosynthesis. Apart from the highly engaging albeit controversial analysis of the role of quantum coherences in the photosynthetic processes, 2DES has been applied to resolve the dynamics and pathways of energy and electron transport in various light-harvesting antenna systems and reaction centres, providing unsurpassed level of detail. In this paper we discuss the main technical approaches and their applicability for solving specific problems in photosynthesis. We then recount applications of 2DES to study the exciton dynamics in plant and photosynthetic light-harvesting complexes, especially light-harvesting complex II (LHCII) and the fucoxanthin-chlorophyll proteins of diatoms, with emphasis on the types of unique information about such systems that 2DES is capable to deliver. This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.

Light harvesting complexes in chlorophyll c-containing algae

Besides the so-called 'green lineage' of eukaryotic photosynthetic organisms that include vascular plants, a huge variety of different algal groups exist that also harvest light by means of membrane intrinsic light harvesting proteins (Lhc). The main taxa of these algae are the Cryptophytes, Haptophytes, Dinophytes, Chromeridae and the Heterokonts, the latter including diatoms, brown algae, Xanthophyceae and Eustigmatophyceae amongst others. Despite the similarity in Lhc proteins between vascular plants and these algae, pigmentation is significantly different since no Chl b is bound, but often replaced by Chl c, and a large diversity in carotenoids functioning in light harvesting and/or photoprotection is present. Due to the presence of Chl c in most of the taxa the name 'Chl c-containing organisms' has become common, however, Chl b-less is more precise since some harbour Lhc proteins that only bind one type of Chl, Chl a. In recent years huge progress has been made about the occurrence and function of Lhc in diatoms, so-called fucoxanthin chlorophyll proteins (FCP), where also the first molecular structure became available recently. In addition, especially energy transfer amongst the unusual pigments bound was intensively studied in many of these groups. This review summarises the present knowledge about the molecular structure, the arrangement of the different Lhc in complexes, the excitation energy transfer abilities and the involvement in photoprotection of the different Lhc systems in the so-called Chl c-containing organisms. This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.

Circular spectropolarimetric sensing of higher plant and algal chloroplast structural variations

Photosynthetic eukaryotes show a remarkable variability in photosynthesis, including large differences in light-harvesting proteins and pigment composition. In vivo circular spectropolarimetry enables us to probe the molecular architecture of photosynthesis in a non-invasive and non-destructive way and, as such, can offer a wealth of physiological and structural information. In the present study, we have measured the circular polarizance of several multicellular green, red, and brown algae and higher plants, which show large variations in circular spectropolarimetric signals with differences in both spectral shape and magnitude. Many of the algae display spectral characteristics not previously reported, indicating a larger variation in molecular organization than previously assumed. As the strengths of these signals vary by three orders of magnitude, these results also have important implications in terms of detectability for the use of circular polarization as a signature of life.

Structural basis for blue-green light harvesting and energy dissipation in diatoms

Diatoms are abundant photosynthetic organisms in aquatic environments and contribute 40% of its primary productivity. An important factor that contributes to the success of diatoms is their fucoxanthin chlorophyll a/c-binding proteins (FCPs), which have exceptional light-harvesting and photoprotection capabilities. Here, we report the crystal structure of an FCP from the marine diatom Phaeodactylum tricornutum, which reveals the binding of seven chlorophylls (Chls) a, two Chls c, seven fucoxanthins (Fxs), and probably one diadinoxanthin within the protein scaffold. Efficient energy transfer pathways can be found between Chl a and c, and each Fx is surrounded by Chls, enabling the energy transfer and quenching via Fx highly efficient. The structure provides a basis for elucidating the mechanisms of blue-green light harvesting, energy transfer, and dissipation in diatoms.

Biological aspects and biotechnological potential of marine diatoms in relation to different light regimens

As major primary producers in marine environments, diatoms are considered a valuable feedstock of biologically active compounds for application in several biotechnological fields. Due to their metabolic plasticity, especially for light perception and use and in order to make microalgal production more environmentally sustainable, marine diatoms are considered good candidates for the large-scale cultivation. Among physical parameters, light plays a primary role. Even if sunlight is cost-effective, the employment of artificial light becomes a winning strategy if a high-value microalgal biomass is produced. Several researches on marine diatoms are designed to study the influence of different light regimens to increase biomass production enriched in biotechnologically high-value compounds (lipids, carotenoids, proteins, polysaccharides), or with emphasised photonic properties of the frustule.

Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids

Fucoxanthin is a carotenoid that is mainly found in light-harvesting complexes from brown algae and diatoms. Due to the presence of a carbonyl group attached to polyene chains in polar environments, excitation produces an excited intra-molecular charge transfer. This intra-molecular charge transfer state plays a key role in the highly efficient (∼95%) energy-transfer from fucoxanthin to chlorophyll a in the light-harvesting complexes from brown algae. In purple bacterial light-harvesting systems the efficiency of excitation energy-transfer from carotenoids to bacteriochlorophylls depends on the extent of conjugation of the carotenoids. In this study we were successful, for the first time, in incorporating fucoxanthin into a light-harvesting complex 1 from the purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied to this reconstituted light-harvesting complex in order to determine the efficiency of excitation energy-transfer from fucoxanthin to bacteriochlorophyll a when they are bound to the light-harvesting 1 apo-proteins.

Carotenoid–chlorophyll energy transfer in the fucoxanthin–chlorophyll complex binding a fucoxanthin acyloxy derivative

A fucoxanthin derivative has negligible charge-transfer character of the S₁/ICT state resulting in slowing down of the carotenoid–chlorophyll energy transfer.

Rapid regulation of excitation energy in two pennate diatoms from contrasting light climates

Non-photochemical quenching (NPQ) is a fast acting photoprotective response to high light stress triggered by over excitation of photosystem II. The mechanism for NPQ in the globally important diatom algae has been principally attributed to a xanthophyll cycle, analogous to the well-described qE quenching of higher plants. This study compared the short-term NPQ responses in two pennate, benthic diatom species cultured under identical conditions but which originate from unique light climates. Variable chlorophyll fluorescence was used to monitor photochemical and non-photochemical excitation energy dissipation during high light transitions; whereas whole cell steady state 77 K absorption and emission were used to measure high light elicited changes in the excited state landscapes of the thylakoid. The marine shoreline species Nitzschia curvilineata was found to have an antenna system capable of entering a deeply quenched, yet reversible state in response to high light, with NPQ being highly sensitive to dithiothreitol (a known inhibitor of the xanthophyll cycle). Conversely, the salt flat species Navicula sp. 110-1 exhibited a less robust NPQ that remained largely locked-in after the light stress was removed; however, a lower amplitude, but now highly reversible NPQ persisted in cells treated with dithiothreitol. Furthermore, dithiothreitol inhibition of NPQ had no functional effect on the ability of Navicula cells to balance PSII excitation/de-excitation. These different approaches for non-photochemical excitation energy dissipation are discussed in the context of native light climate.

Isolating and Incorporating Light-Harvesting Antennas from Diatom Cyclotella Meneghiniana in Liposomes with Thylakoid Lipids

The photosynthetic performance of plants, algae and diatoms strongly depends on the fast and efficient regulation of the light harvesting and energy transfer processes in the thylakoid membrane of chloroplasts. The light harvesting antenna of diatoms, the so called fucoxanthin chlorophyll a/c binding proteins (FCP), are required for the light absorption and efficient transfer to the photosynthetic reaction centers as well as for photo-protection from excessive light. The switch between these two functions is a long-standing matter of research. Many of these studies have been carried out with FCP in detergent micelles. For interaction studies, the detergents have been removed, which led to an unspecific aggregation of FCP complexes. In this approach, it is hard to discriminate between artifacts and physiologically relevant data. Hence, more valuable information about FCP and other membrane bound light harvesting complexes can be obtained by studying protein-protein interactions, energy transfer and other spectroscopic features if they are embedded in their native lipid environment. The main advantage is that liposomes have a defined size and a defined lipid/protein ratio by which the extent of FCP clustering is controlled. Further, changes in the pH and ion composition that regulate light harvesting in vivo can easily be simulated. In comparison to the thylakoid membrane, the liposomes are more homogenous and less complex, which makes it easier to obtain and understand spectroscopic data. The protocol describes the procedure of FCP isolation and purification, liposome preparation, and incorporation of FCP into liposomes with natural lipid composition. Results from a typical application are given and discussed.

Nitrogen starvation induces distinct photosynthetic responses and recovery dynamics in diatoms and prasinophytes

Nitrogen stress is an important control on the growth of phytoplankton and varying responses to this common condition among taxa may affect their relative success within phytoplankton communities. We analyzed photosynthetic responses to nitrogen (N) stress in two classes of phytoplankton that often dominate their respective size ranges, diatoms and prasinophytes, selecting species of distinct niches within each class. Changes in photosynthetic structures appeared similar within each class during N stress, but photophysiological and growth responses were more species- or niche-specific. In the coastal diatom Thalassiosira pseudonana and the oceanic diatom T. weissflogii, N starvation induced large declines in photosynthetic pigments and Photosystem II (PSII) quantity and activity as well as increases in the effective absorption cross-section of PSII photochemistry (σ'PSII). These diatoms also increased photoprotection through energy-dependent non-photochemical quenching (NPQ) during N starvation. Resupply of N in diatoms caused rapid recovery of growth and relaxation of NPQ, while recovery of PSII photochemistry was slower. In contrast, the prasinophytes Micromonas sp., an Arctic Ocean species, and Ostreococcus tauri, a temperate coastal eutrophile, showed little change in photosynthetic pigments and structures and a decline or no change, respectively, in σ'PSII with N starvation. Growth and PSII function recovered quickly in Micromonas sp. after resupply of N while O. tauri failed to recover N-replete levels of electron transfer from PSII and growth, possibly due to their distinct photoprotective strategies. O. tauri induced energy-dependent NPQ for photoprotection that may suit its variable and nutrient-rich habitat. Micromonas sp. relies upon both energy-dependent NPQ and a sustained, energy-independent NPQ mechanism. A strategy in Micromonas sp. that permits photoprotection with little change in photosynthetic structures is consistent with its Arctic niche, where low temperatures and thus low biosynthetic rates create higher opportunity costs to rebuild photosynthetic structures.

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 ~5 ns 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.

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.

Excited state characterization of carbonyl containing carotenoids: a comparison between single and multireference descriptions

Carotenoids can play multiple roles in biological photoreceptors thanks to their rich photophysics. In the present work, we have investigated six of the most common carbonyl containing carotenoids: echinenone, canthaxanthin, astaxanthin, fucoxanthin, capsanthin and capsorubin. Their excitation properties are investigated by means of a hybrid density functional theory (DFT) and multireference configuration interaction (MRCI) approach to elucidate the role of the carbonyl group: the bright transition is of ππ* character, as expected, but the presence of a C[double bond, length as m-dash]O moiety reduces the energy of nπ* transitions which may become closer to the ππ* transition, in particular as the conjugation chain decreases. This can be related to the presence of a low-lying charge transfer state typical of short carbonyl-containing carotenoids. The DFT/MRCI results are finally used to benchmark single-reference time-dependent DFT-based methods: among the investigated functionals, the meta-GGA (and in particular M11L and MN12L) functionals show to perform the best for all six investigated systems.

How reduced excitonic coupling enhances light harvesting in the main photosynthetic antennae of diatoms

Strong excitonic interactions are a key design strategy in photosynthetic light harvesting, expanding the spectral cross-section for light absorption and creating considerably faster and more robust excitation energy transfer. These molecular excitons are a direct result of exceptionally densely packed pigments in photosynthetic proteins. The main light-harvesting complexes of diatoms, known as fucoxanthin-chlorophyll proteins (FCPs), are an exception, displaying surprisingly weak excitonic coupling between their chlorophyll (Chl) a's, despite a high pigment density. Here, we show, using single-molecule spectroscopy, that the FCP complexes of Cyclotella meneghiniana switch frequently into stable, strongly emissive states shifted 4-10 nm toward the red. A few percent of isolated FCPa complexes and ∼20% of isolated FCPb complexes, on average, were observed to populate these previously unobserved states, percentages that agree with the steady-state fluorescence spectra of FCP ensembles. Thus, the complexes use their enhanced sensitivity to static disorder to increase their light-harvesting capability in a number of ways. A disordered exciton model based on the structure of the main plant light-harvesting complex explains the red-shifted emission by strong localization of the excitation energy on a single Chl a pigment in the terminal emitter domain due to very specific pigment orientations. We suggest that the specific construction of FCP gives the complex a unique strategy to ensure that its light-harvesting function remains robust in the fluctuating protein environment despite limited excitonic interactions.

Contrasting effects of copper limitation on the photosynthetic apparatus in two strains of the open ocean diatom Thalassiosira oceanica

There is an intricate interaction between iron (Fe) and copper (Cu) physiology in diatoms. However, strategies to cope with low Cu are largely unknown. This study unveils the comprehensive restructuring of the photosynthetic apparatus in the diatom Thalassiosira oceanica (CCMP1003) in response to low Cu, at the physiological and proteomic level. The restructuring results in a shift from light harvesting for photochemistry—and ultimately for carbon fixation—to photoprotection, reducing carbon fixation and oxygen evolution. The observed decreases in the physiological parameters F_v/F_m, carbon fixation, and oxygen evolution, concomitant with increases in the antennae absorption cross section (σ_PSII), non-photochemical quenching (NPQ) and the conversion factor (φ_e:C/η_PSII) are in agreement with well documented cellular responses to low Fe. However, the underlying proteomic changes due to low Cu are very different from those elicited by low Fe. Low Cu induces a significant four-fold reduction in the Cu-containing photosynthetic electron carrier plastocyanin. The decrease in plastocyanin causes a bottleneck within the photosynthetic electron transport chain (ETC), ultimately leading to substantial stoichiometric changes. Namely, 2-fold reduction in both cytochrome b₆f complex (cytb₆f) and photosystem II (PSII), no change in the Fe-rich PSI and a 40- and 2-fold increase in proteins potentially involved in detoxification of reactive oxygen species (ferredoxin and ferredoxin:NADP⁺ reductase, respectively). Furthermore, we identify 48 light harvesting complex (LHC) proteins in the publicly available genome of T. oceanica and provide proteomic evidence for 33 of these. The change in the LHC composition within the antennae in response to low Cu underlines the shift from photochemistry to photoprotection in T. oceanica (CCMP1003). Interestingly, we also reveal very significant intra-specific strain differences. Another strain of T. oceanica (CCMP 1005) requires significantly higher Cu concentrations to sustain both its maximal and minimal growth rate compared to CCMP 1003. Under low Cu, CCMP 1005 decreases its growth rate, cell size, Chla and total protein per cell. We argue that the reduction in protein per cell is the main strategy to decrease its cellular Cu requirement, as none of the other parameters tested are affected. Differences between the two strains, as well as differences between the well documented responses to low Fe and those presented here in response to low Cu are discussed.

Identification of genes coding for functional zeaxanthin epoxidases in the diatom Phaeodactylum tricornutum

Phaeodactylum tricornutum like other diatoms synthesizes fucoxanthin and diadinoxanthin as major carotenoid end products. The genes involved have recently been assigned for early pathway steps. Beyond β-carotene, only gene candidates for β-carotene hydroxylase, zeaxanthin epoxidase and zeaxanthin de-epoxidase have been proposed from the available genome sequence. The two latter enzymes may be involved in the two different xanthophyll cycles which operate in P. tricornutum. The function of three putative zeaxanthin epoxidase genes (zep) was addressed by pathway complementation in the Arabidopsis thaliana Zep mutant npq2. Genes zep2 and zep3 were able to restore zeaxanthin epoxidation and a functional xanthophyll cycle but the corresponding enzymes exhibited different catalytic activities. Zep3 functioned as a zeaxanthin epoxidase whereas Zep2 exhibited a broader substrate specificity additionally converting lutein to lutein-5,6-epoxide. Although zep1 was transcribed and the protein could be identified after import into the chloroplast in A. thaliana, Zep1 was found not to be functional in zeaxanthin epoxidation. The non-photochemical quenching kinetics of wild type A. thaliana was only restored in transformant npq2-zep3.

Quantum Chemical Studies of Light Harvesting

The design of optimal light-harvesting (supra)molecular systems and materials is one of the most challenging frontiers of science. Theoretical methods and computational models play a fundamental role in this difficult task, as they allow the establishment of structural blueprints inspired by natural photosynthetic organisms that can be applied to the design of novel artificial light-harvesting devices. Among theoretical strategies, the application of quantum chemical tools represents an important reality that has already reached an evident degree of maturity, although it still has to show its real potentials. This Review presents an overview of the state of the art of this strategy, showing the actual fields of applicability but also indicating its current limitations, which need to be solved in future developments.

The nitrogen costs of photosynthesis in a diatom under current and future pCO2

With each cellular generation, oxygenic photoautotrophs must accumulate abundant protein complexes that mediate light capture, photosynthetic electron transport and carbon fixation. In addition to this net synthesis, oxygenic photoautotrophs must counter the light-dependent photoinactivation of Photosystem II (PSII), using metabolically expensive proteolysis, disassembly, resynthesis and re-assembly of protein subunits. We used growth rates, elemental analyses and protein quantitations to estimate the nitrogen (N) metabolism costs to both accumulate the photosynthetic system and to maintain PSII function in the diatom Thalassiosira pseudonana, growing at two pCO2 levels across a range of light levels. The photosynthetic system contains c. 15-25% of total cellular N. Under low growth light, N (re)cycling through PSII repair is only c. 1% of the cellular N assimilation rate. As growth light increases to inhibitory levels, N metabolite cycling through PSII repair increases to c. 14% of the cellular N assimilation rate. Cells growing under the assumed future 750 ppmv pCO2 show higher growth rates under optimal light, coinciding with a lowered N metabolic cost to maintain photosynthesis, but then suffer greater photoinhibition of growth under excess light, coincident with rising costs to maintain photosynthesis. We predict this quantitative trait response to light will vary across taxa.

Evolution and function of light harvesting proteins

Photosynthetic eukaryotes exhibit very different light-harvesting proteins, but all contain membrane-intrinsic light-harvesting complexes (Lhcs), either as additional or sole antennae. Lhcs non-covalently bind chlorophyll a and in most cases another Chl, as well as very different carotenoids, depending on the taxon. The proteins fall into two major groups: The well-defined Lhca/b group of proteins binds typically Chl b and lutein, and the group is present in the 'green lineage'. The other group consists of Lhcr/Lhcf, Lhcz and Lhcx/LhcSR proteins. The former are found in the so-called Chromalveolates, where they mostly bind Chl c and carotenoids very efficient in excitation energy transfer, and in their red algae ancestors. Lhcx/LhcSR are present in most Chromalveolates and in some members of the green lineage as well. Lhcs function in light harvesting, but also in photoprotection, and they influence the organisation of the thylakoid membrane. The different functions of the Lhc subfamilies are discussed in the light of their evolution.

Supramolecular organization of fucoxanthin-chlorophyll proteins in centric and pennate diatoms

Fucoxanthin-chlorophyll proteins (FCP) are the major light-harvesting proteins of diatom algae, a major contributor to marine carbon fixation. FCP complexes from representatives of centric (Cyclotella meneghiniana) and pennate (Phaeodactylum tricornutum) diatoms were prepared by sucrose gradient centrifugation and studied by means of electron microscopy followed by single particle analysis. The oligomeric FCP from a centric diatom were observed to take the form of unusual chain-like or circular shapes, a very unique supramolecular assembly for such antennas. The existence of the often disputed oligomeric form of FCP in pennate diatoms has been confirmed. Contrary to the centric diatom FCP, pennate diatom FCP oligomers are very similar to oligomeric antennas from related heterokont (Stramenopila) algae. Evolutionary aspects of the presence of novel light-harvesting protein arrangement in centric diatoms are discussed.