The lifetimes and energies of the first excited singlet states of diadinoxanthin and diatoxanthin: the role of these molecules in excess energy dissipation in algae

Rapid, high-throughput phenotypic profiling of endosymbiotic dinoflagellates (Symbiodiniaceae) using benchtop flow cytometry

Endosymbiotic dinoflagellates (Family Symbiodiniaceae) are the primary producer of energy for many cnidarians, including corals. The intricate coral-dinoflagellate symbiotic relationship is becoming increasingly important under climate change, as its breakdown leads to mass coral bleaching and often mortality. Despite methodological progress, assessing the phenotypic traits of Symbiodiniaceae in-hospite remains a complex task. Bio-optics, biochemistry, or "-omics" techniques are expensive, often inaccessible to investigators, or lack the resolution required to understand single-cell phenotypic states within endosymbiotic dinoflagellate assemblages. To help address this issue, we developed a protocol that collects information on cell autofluorescence, shape, and size to simultaneously generate phenotypic profiles for thousands of Symbiodiniaceae cells, thus revealing phenotypic variance of the Symbiodiniaceae assemblage to the resolution of single cells. As flow cytometry is adopted as a robust and efficient method for cell counting, integration of our protocol into existing workflows allows researchers to acquire a new level of resolution for studies examining the acclimation and adaptation strategies of Symbiodiniaceae assemblages.

Excitation relaxation dynamics of carotenoids constituting the diadinoxanthin cycle

Carotenoids (Cars) exhibit two functions in photosynthesis, light-harvesting and photoprotective functions, which are performed through the excited states of Cars. Therefore, increasing our knowledge on excitation relaxation dynamics of Cars is important for understanding of the functions of Cars. In light-harvesting complexes, there exist Cars functioning by converting the π-conjugation number in response to light conditions. It is well known that some microalgae have a mechanism controlling the conjugation number of Cars, called as the diadinoxanthin cycle; diadinoxanthin (10 conjugations) is accumulated under low light, whereas diatoxanthin (11 conjugations) appears under high light. However, the excitation relaxation dynamics of these two Cars have not been clarified. In the present study, we investigated excitation relaxation dynamics of diadinoxanthin and diatoxanthin in relation to their functions, by the ultrafast fluorescence spectroscopy. After an excitation to the S₂ state, the intramolecular vibrational redistribution occurs, followed by the internal conversion to the S₁ state. The S₂ lifetimes were analyzed to be 175 fs, 155 fs, and 140 fs in diethyl ether, ethanol, and acetone, respectively, for diadinoxanthin, and 155 fs, 135 fs, and 125 fs in diethyl ether, ethanol, and acetone, respectively for diatoxanthin. By converting diadinoxanthin to diatoxanthin, the absorption spectra shift to longer wavelengths by 5-7 nm, and lifetimes of S₂ and S₁ states decrease by 11-13% and 52%, respectively. Differences in levels and lifetimes of excited states between diadinoxanthin and diatoxanthin are small; therefore, it is suggested that changes in the energy level of chlorophyll a are necessary to efficiently control the functions of the diadinoxanthin cycle.

Enhanced pigment content estimation using the Gauss-peak spectra method with thin-layer chromatography for a novel source of natural colorants

Alternative pigment sources that are harmless to human health and can be produced in an eco-responsible way are of great research interest. The experiments undertaken in this study were conducted using autumn leaves of Aesculus hippocastanum as potential novel colorant sources. This study focused on improving the Gauss-peak spectra method (a less expensive alternative to high-pressure liquid chromatography) in combination with thin-layer chromatography, leading to the development of a new methodology. The collected leaves were stored at two different temperatures: 20°C and −20°C. The data obtained by spectrophotometric scanning of the samples were analyzed using the Gauss-peak spectra method in the R program with three wavelength ranges: 350–750 nm, 390–710 nm, and 400–700 nm. The results were then assessed for statistically significant differences in the estimated concentrations for the different wavelength ranges regarding (1) total pigment, carotenoid, and chlorophyll concentration (two-sample t-test) and (2) concentration of each indicated pigment (two-way analysis of variance). The results were also tested for differences between the estimated concentrations of samples stored under the different conditions. The Gauss-peak spectra results with and without thin-layer chromatography were statistically compared using a paired t-test. The results showed that thin-layer chromatography greatly enhanced the efficiency of the Gauss-peak spectra method for estimating the major and minor pigment composition without generating high additional costs. A wavelength range of 400–700 nm was optimal for all Gauss-peak spectra methods. In conclusion, the proposed method is a more successful, inexpensive alternative to high-pressure liquid chromatography.

Pigment configuration in the light-harvesting protein of the xanthophyte alga Xanthonema debile

The soil chromophyte alga Xanthonema (X.) debile contains only non-carbonyl carotenoids and Chl-a. X. debile has an antenna system denoted Xanthophyte light-harvesting complex (XLH) that contains the carotenoids diadinoxanthin, heteroxanthin, and vaucheriaxanthin. The XLH pigment stoichiometry was calculated by chromatographic techniques and the pigment-binding structure studied by resonance Raman spectroscopy. The pigment ratio obtained by HPLC was found to be close to 8:1:2:1 Chl-a:heteroxanthin:diadinoxanthin:vaucheriaxanthin. The resonance Raman spectra suggest the presence of 8-10 Chl-a, all of which are 5-coordinated to the central Mg, with 1-3 Chl-a possessing a macrocycle distorted from the relaxed conformation. The three populations of carotenoids are in the all-trans configuration. Vaucheriaxanthin absorbs around 500-530 nm, diadinoxanthin at 494 nm and heteroxanthin at 487 nm at 4.5 K. The effective conjugation length of heteroxanthin and diadinoxanthin has been determined as 9.4 in both cases; the environment polarizability of the heteroxanthin and diadinoxanthin binding pockets is 0.270 and 0.305, respectively.

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.

Biodiversity of NPQ

In their natural environment plants and algae are exposed to rapidly changing light conditions and light intensities. Illumination with high light intensities has the potential to overexcite the photosynthetic pigments and the electron transport chain and thus induce the production of toxic reactive oxygen species (ROS). To prevent damage by the action of ROS, plants and algae have developed a multitude of photoprotection mechanisms. One of the most important protection mechanisms is the dissipation of excessive excitation energy as heat in the light-harvesting complexes of the photosystems. This process requires a structural change of the photosynthetic antenna complexes that are normally optimized with regard to efficient light-harvesting. Enhanced heat dissipation in the antenna systems is accompanied by a strong quenching of the chlorophyll a fluorescence and has thus been termed non-photochemical quenching of chlorophyll a fluorescence, NPQ. The general importance of NPQ for the photoprotection of plants and algae is documented by its wide distribution in the plant kingdom. In the present review we will summarize the present day knowledge about NPQ in higher plants and different algal groups with a special focus on the molecular mechanisms that lead to the structural rearrangements of the antenna complexes and enhanced heat dissipation. We will present the newest models for NPQ in higher plants and diatoms and will compare the features of NPQ in different algae with those of NPQ in higher plants. In addition, we will briefly address evolutionary aspects of NPQ, i.e. how the requirements of NPQ have changed during the transition of plants from the aquatic habitat to the land environment. We will conclude with a presentation of open questions regarding the mechanistic basis of NPQ and suggestions for future experiments that may serve to obtain this missing information.

Reactive oxygen species generation and signaling in plants

The introduction of molecular oxygen into the atmosphere was accompanied by the generation of reactive oxygen species (ROS) as side products of many biochemical reactions. ROS are permanently generated in plastids, peroxisomes, mitochiondria, the cytosol and the apoplast. Imbalance between ROS generation and safe detoxification generates oxidative stress and the accumulating ROS are harmful for the plants. On the other hand, specific ROS function as signaling molecules and activate signal transduction processes in response to various stresses. Here, we summarize the generation of ROS in the different cellular compartments and the signaling processes which are induced by ROS.

Role of carotenoids in light-harvesting processes in an antenna protein from the chromophyte Xanthonema debile

Chromophytes are an important group of microorganisms that contribute significantly to the carbon cycle on Earth. Their photosynthetic capacity depends on efficiency of the light-harvesting system that differs in pigment composition from that of green plants and other groups of algae. Here we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the main light-harvesting complex of Xanthonema debile, denoted XLH, which contains four carotenoids--diadinoxanthin, heteroxanthin, diatoxanthin, and vaucheriaxanthin--and Chl-a. Overall carotenoid-to-chlorophyll energy transfer efficiency is about 60%, but energy transfer pathways are excitation wavelength dependent. Energy transfer from the carotenoid S(2) state is active after excitation at both 490 nm (maximum of carotenoid absorption) and 510 nm (red edge of carotenoid absorption), but this channel is significantly more efficient after 510 nm excitation. Concerning the energy transfer pathway from the S(1) state, XLH contains two groups of carotenoids: those that have the S(1) route active (~25%) and those having the S(1) pathway silent. For a fraction of carotenoids that transfer energy via the S(1) channel, energy transfer is observed after both excitation wavelengths, though energy transfer times are different, yielding 3.4 ps (490 nm excitation) and 1.5 ps (510 nm excitation). This corresponds to efficiencies of the S(1) channel of ~85% that is rather unusual for a donor-acceptor pair consisting of a noncarbonyl carotenoid and Chl-a. Moreover, major carotenoids in XLH, diadinoxanthin and diatoxanthin, have their S(1) energies in solution lower than the energy of the acceptor state, Q(y) state of Chl-a. Thus, binding of these carotenoids to XLH must tune their S(1) energy to allow for efficient energy transfer. Besides the light-harvesting function, carotenoids in XLH also have photoprotective role; they quench Chl-a triplets via triplet-triplet energy transfer from Chl-a to carotenoid.

The cost of photoinhibition

Photoinhibition is an inevitable consequence of oxygenic photosynthesis. However, the concept of a 'photoinhibition-proof' plant in which photosystem II (PSII) is immune to photodamage is useful as a benchmark for considering the performances of plants with varying mixes of mechanisms which limit the extent of photodamage and which repair photodamage. Some photodamage is bound to occur, and the energy costs of repair are the direct costs of repair plus the photosynthesis foregone during repair. One mechanism permitting partial avoidance of photodamage is restriction of the number of photons incident on the photosynthetic apparatus per unit time, achieved by phototactic movement of motile algae to places with lower incident photosynthetically active radiation (PAR), by phototactic movement of plastids within cells to positions that minimize the incident PAR and by photonastic relative movements of parts of photolithotrophs attached to a substrate. The other means of avoiding photodamage is dissipating excitation of photosynthetic pigments including state transitions, non-photochemical quenching by one of the xanthophyll cycles or some other process and photochemical quenching by increased electron flow through PSII involving CO₂ and other acceptors, including the engagement of additional electron transport pathways. These mechanisms inevitably have the potential to decrease the rate of growth. As well as the decreased photosynthetic rates as a result of photodamage and the restrictions on photosynthesis imposed by the repair, avoidance, quenching and scavenging mechanisms, there are also additional energy, nitrogen and phosphorus costs of producing and operating repair, avoidance, quenching and scavenging mechanisms. A comparison is also made between the costs of photoinhibition and those of other plant functions impeded by the occurrence of oxygenic photosynthesis, i.e. the competitive inhibition of the carboxylase activity of ribulose bisphosphate carboxylase-oxygenase by oxygen via the oxygenase activity, and oxygen damage to nitrogenase in diazotrophic organisms.

Regulation and function of xanthophyll cycle-dependent photoprotection in algae

The xanthophyll cycle represents one of the important photoprotection mechanisms in plant cells. In the present review, we summarize current knowledge about the violaxanthin cycle of vascular plants, green and brown algae, and the diadinoxanthin cycle of the algal classes Bacillariophyceae, Xanthophyceae, Haptophyceae, and Dinophyceae. We address the biochemistry of the xanthophyll cycle enzymes with a special focus on protein structure, co-substrate requirements and regulation of enzyme activity. We present recent ideas regarding the structural basis of xanthophyll cycle-dependent photoprotection, including different models for the mechanism of non-photochemical quenching of chlorophyll a fluorescence. In a dedicated chapter, we also describe the unique violaxanthin antheraxanthin cycle of the Prasinophyceae, together with its implication for the mechanism of xanthophyll cycle-dependent heat dissipation. The interaction between the diadinoxanthin cycle and alternative electron flow pathways in the chloroplasts of diatoms is an additional topic of this review, and in the last chapter we cover aspects of the importance of xanthophyll cycle-dependent photoprotection for different algal species in their natural environments.

Energies of low-lying excited states of linear polyenes

Room temperature absorption and emission spectra of the all-trans isomers of decatetraene, dodecapentaene, tetradecahexaene, and hexadecaheptaene have been obtained in a series of nonpolar solvents. The resolved vibronic features in the optical spectra of these model systems allow the accurate determination of S(0) (1(1)A(g)(-)) --> S(2) (1(1)B(u)(+)) and S(1) (2(1)A(g)(-)) --> S(0) (1(1)A(g)(-)) electronic origins as a function of solvent polarizability. These data can be extrapolated to predict the transition energies in the absence of solvent perturbations. The effects of the terminal methyl substituents on the transition energies also can be estimated. Franck-Condon maxima in the absorption and emission spectra were used to estimate differences between S(0) (1(1)A(g)(-)) --> S(1) (2(1)A(g)(-)) and S(0) (1(1)A(g)(-)) --> S(2) (1(1)B(u)(+)) electronic origins and "vertical" transition energies. Experimental estimates of the vertical transition energies of unsubstituted, all-trans polyenes in vacuum as a function of conjugation length are compared with long-standing multireference configuration interaction (MRCI) treatments and with more recent ab initio calculations of the energies of the 2(1)A(g)(-) (S(1)) and 1(1)B(u)(+) (S(2)) states.

Energy Transfer, Excited-State Deactivation, and Exciplex Formation in Artificial Caroteno-Phthalocyanine Light-Harvesting Antennas

We present results from transient absorption spectroscopy on a series of artificial light-harvesting dyads made up of a zinc phthalocyanine (Pc) covalently linked to carotenoids with 9, 10, or 11 conjugated carbon-carbon double bonds, referred to as dyads 1, 2, and 3, respectively. We assessed the energy transfer and excited-state deactivation pathways following excitation of the strongly allowed carotenoid S2 state as a function of the conjugation length. The S2 state rapidly relaxes to the S* and S1 states. In all systems we detected a new pathway of energy deactivation within the carotenoid manifold in which the S* state acts as an intermediate state in the S2-->S1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type of collective carotenoid-Pc electronic state is observed that may correspond to a carotenoid excited state(s)-Pc Q exciplex. The exciplex is only observed upon direct carotenoid excitation and is nonfluorescent. In dyad 1, two carotenoid singlet excited states, S2 and S1, contribute to singlet-singlet energy transfer to Pc, making the process very efficient (>90%) while for dyads 2 and 3 the S1 energy transfer channel is precluded and only S2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singlet excited state of Pc is dramatically shortened compared to the 9 double-bond dyad and model Pc, indicating that the carotenoid acts as a strong quencher of the phthalocyanine excited-state energy.

Supplementary ultraviolet-B radiation induces a rapid reversal of the diadinoxanthin cycle in the strong light-exposed diatom Phaeodactylum tricornutum

A treatment of the diatom Phaeodactylum tricornutum with high light (HL) in the visible range led to the conversion of diadinoxanthin (Dd) to diatoxanthin (Dt). In a following treatment with HL plus supplementary ultraviolet (UV)-B, the Dt was rapidly epoxidized to Dd. Photosynthesis of the cells was inhibited under HL + UV-B. This is accounted for by direct damage by UV-B and damage because of the UV-B-induced reversal of the Dd cycle and the associated loss of photoprotection. The reversal of the Dd cycle by UV-B was faster in the presence of dithiothreitol, an inhibitor of the Dd de-epoxidase. Our results imply that the reversal of the Dd cycle by HL + UV-B was caused by an increase in the rate of gross Dt epoxidation, whereas the de-epoxidation of Dd was unaffected by UV-B. This is further supported by our finding that the in vitro de-epoxidation activity and the affinity toward the cosubstrate ascorbic acid of the Dd de-epoxidase were both unaffected by UV-B pretreatment of intact cells. We provide evidence that Dt epoxidation is normally down-regulated by a high pH gradient under HL. It is proposed that supplementary UV-B affected the pH gradient across the thylakoid membrane, which disrupted the down-regulation of Dt epoxidation and led to the observed increase in the rate of Dt epoxidation.

The design and synthesis of artificial photosynthetic antennas, reaction centres and membranes

Artificial antenna systems and reaction centres synthesized in our laboratory are used to illustrate that structural and thermodynamic factors controlling energy and electron transfer in these constructs can be modified to optimize performance. Artificial reaction centres have been incorporated into liposomal membranes where they convert light energy to vectorial redox potential. This redox potential drives a Mitchellian, quinone-based, proton-transporting redox loop that generates a Deltamu H(+) of ca. 4.4 kcal mol(-1) comprising DeltapH ca. 2.1 and Deltapsi ca. 70 mV. In liposomes containing CF(0)F(1)-ATP synthase, this system drives ATP synthesis against an ATP chemical potential similar to that observed in natural systems.

In diatoms, a transthylakoid proton gradient alone is not sufficient to induce a non-photochemical fluorescence quenching

Non-photochemical fluorescence quenching (NPQ) in diatoms is associated with a xanthophyll cycle involving diadinoxanthin (DD) and its de-epoxidized form, diatoxanthin (DT). In higher plants, an obligatory role of de-epoxidized xanthophylls in NPQ remains controversial and the presence of a transthylakoid proton gradient (DeltapH) alone may induce NPQ. We used inhibitors to alter the amplitude of DeltapH and/or DD de-epoxidation, and coupled NPQ. No DeltapH-dependent quenching was detected in the absence of DT. In diatoms, both DeltapH and DT are required for NPQ. The binding of DT to protonated antenna sites could be obligatory for energy dissipation.

Algae displaying the diadinoxanthin cycle also possess the violaxanthin cycle

According to general agreement, all photosynthetic organisms using xanthophyll cycling for photoprotection contain either the violaxanthin (Vx) cycle or the diadinoxanthin (Ddx) cycle instead. Here, we report the temporal accumulation of substantial amounts of pigments of the Vx cycle under prolonged high-light stress in several microalgae thought to possess only the Ddx cycle. In the diatom Phaeodactylum tricornutum, used as a model organism, these pigments also participate in xanthophyll cycling, and their accumulation depends on de novo synthesis of carotenoids and on deepoxidase activity. Furthermore, our data strongly suggest a biosynthetic sequence from Vx via Ddx to fucoxanthin in P. tricornutum. This gives experimental support to the long-stated hypothesis that Vx is a common precursor of all carotenoids with an allenic or acetylenic group, including the main light-harvesting carotenoids in most chlorophyll a/c-containing algae. Thus, another important function for xanthophyll cycling may be to optimize the biosynthesis of light-harvesting xanthophylls under fluctuating light conditions.