Kinetics of excitation trapping in intact Photosystem I of Chlamydomonas reinhardtii and Arabidopsis thaliana

Spectral diversity of photosystem I from flowering plants

Photosystem I and II (PSI and PSII) work together to convert solar energy into chemical energy. Whilst a lot of research has been done to unravel variability of PSII fluorescence in response to biotic and abiotic factors, the contribution of PSI to in vivo fluorescence measurements has often been neglected or considered to be constant. Furthermore, little is known about how the absorption and emission properties of PSI from different plant species differ. In this study, we have isolated PSI from five plant species and compared their characteristics using a combination of optical and biochemical techniques. Differences have been identified in the fluorescence emission spectra and at the protein level, whereas the absorption spectra were virtually the same in all cases. In addition, the emission spectrum of PSI depends on temperature over a physiologically relevant range from 280 to 298 K. Combined, our data show a critical comparison of the absorption and emission properties of PSI from various plant species.

Current state of the primary charge separation mechanism in photosystem I of cyanobacteria

This review analyzes new data on the mechanism of ultrafast reactions of primary charge separation in photosystem I (PS I) of cyanobacteria obtained in the last decade by methods of femtosecond absorption spectroscopy. Cyanobacterial PS I from many species harbours 96 chlorophyll a (Chl a) molecules, including six specialized Chls denoted Chl1A/Chl1B (dimer P700, or PAPB), Chl2A/Chl2B, and Chl3A/Chl3B arranged in two branches, which participate in electron transfer reactions. The current data indicate that the primary charge separation occurs in a symmetric exciplex, where the special pair P700 is electronically coupled to the symmetrically located monomers Chl2A and Chl2B, which can be considered together as a symmetric exciplex Chl2APAPBChl2B with the mixed excited (Chl2APAPBChl2B)* and two charge-transfer states P700 +Chl2A - and P700 +Chl2B -. The redistribution of electrons between the branches in favor of the A-branch occurs after reduction of the Chl2A and Chl2B monomers. The formation of charge-transfer states and the symmetry breaking mechanisms were clarified by measuring the electrochromic Stark shift of β-carotene and the absorption dynamics of PS I complexes with the genetically altered Chl 2B or Chl 2A monomers. The review gives a brief description of the main methods for analyzing data obtained using femtosecond absorption spectroscopy. The energy levels of excited and charge-transfer intermediates arising in the cyanobacterial PS I are critically analyzed.

High-light modification of excitation-energy-relaxation processes in the green flagellate Euglena gracilis

Photosynthetic organisms finely tune their photosynthetic machinery including pigment compositions and antenna systems to adapt to various light environments. However, it is poorly understood how the photosynthetic machinery in the green flagellate Euglena gracilis is modified under high-light conditions. In this study, we examined high-light modification of excitation-energy-relaxation processes in Euglena cells. Oxygen-evolving activity in the cells incubated at 300 µmol photons m^-2 s^-1 (HL cells) cannot be detected, reflecting severe photodamage to photosystem II (PSII) in vivo. Pigment compositions in the HL cells showed relative increases in 9'-cis-neoxanthin, diadinoxanthin, and chlorophyll b compared with the cells incubated at 30 µmol photons m^-2 s^-1 (LL cells). Absolute fluorescence spectra at 77 K exhibit smaller intensities of the PSII and photosystem I (PSI) fluorescence in the HL cells than in the LL cells. Absolute fluorescence decay-associated spectra at 77 K of the HL cells indicate suppression of excitation-energy transfer from light-harvesting complexes (LHCs) to both PSI and PSII with the time constant of 40 ps. Rapid energy quenching in LHCs and PSII in the HL cells is distinctly observed by averaged Chl-fluorescence lifetimes. These findings suggest that Euglena modifies excitation-energy-relaxation processes in addition to pigment compositions to deal with excess energy. These results provide insights into the photoprotection strategies of this alga under high-light conditions.

Effects of CO2 and temperature on photosynthetic performance in the diatom Chaetoceros gracilis

CO₂ concentration and temperature for growth of photosynthetic organisms are two important factors to ensure better photosynthetic performance. In this study, we investigated the effects of CO₂ concentration and temperature on the photosynthetic performance in a marine centric diatom Chaetoceros gracilis. Cells were grown under four different conditions, namely, at 25 °C with air bubbling, at 25 °C with a supplementation of 3% CO₂, at 30 °C with air bubbling, and at 30 °C with the CO₂ supplementation. It was found that the growth rate of cells at 30 °C with the CO₂ supplementation is faster than those at other three conditions. The pigment compositions of cells grown under the different conditions are altered, and fluorescence spectra measured at 77 K also showed different peak positions. A novel fucoxanthin chlorophyll a/c-binding protein complex is observed in the cells grown at 30 °C with the CO₂ supplementation but not in the other three types of cells. Since oxygen-evolving activities of the four types of cells are almost unchanged, it is suggested that the CO₂ supplementation and growth temperature are involved in the regulation of photosynthetic light-harvesting apparatus in C. gracilis at different degrees. Based on these observations, we discuss the favorable growth conditions for C. gracilis.

Complete mapping of energy transfer pathways in the plant light-harvesting complex Lhca4

The Lhca4 antenna complex of plant Photosystem I (PSI) is characterized by extremely red-shifted and broadened absorption and emission bands from its low-energy chlorophylls (Chls). The mixing of a charge-transfer (CT) state with the excited state manifold causing these so-called red forms results in highly complicated multi-component excited energy transfer (EET) kinetics within the complex. The two-dimensional electronic spectroscopy experiments presented here reveal that EET towards the CT state occurs on three timescales: fast from the red Chls (within 1 ps), slower (5-7 ps) from the stromal side Chls, and very slow (100-200 ps) from a newly discovered 690 nm luminal trap. The excellent agreement between the experimental data with the previously presented Redfield-Förster exciton model of Lhca4 strongly supports the equilibration scheme of the bulk excitations with the dynamically localized CT on the stromal side. Thus, a complete picture of the energy transfer pathways leading to the population of the CT final trap within the whole Lhca4 complex is presented. In view of the environmental sensitivity of the CT contribution to the Lhca4 energy landscape, we speculate that one role of the CT states is to regulate the EET from the peripheral antenna to the PSI core.

On the spectral properties and excitation dynamics of long-wavelength chlorophylls in higher-plant photosystem I

In higher-plant Photosystem I (PSI), the majority of "red" chlorophylls (absorbing at longer wavelengths than the reaction centre P700) are located in the peripheral antenna, but contradicting reports are given about red forms in the core complex. Here we attempt to clarify the spectroscopic characteristics and quantify the red forms in the PSI core complex, which have profound implication on understanding the energy transfer and charge separation dynamics. To this end we compare the steady-state absorption and fluorescence spectra and picosecond time-resolved fluorescence kinetics of isolated PSI core complex and PSI-LHCI supercomplex from Pisum sativum recorded at 77 K. Gaussian decomposition of the absorption spectra revealed a broad band at 705 nm in the core complex with an oscillator strength of three chlorophylls. Additional absorption at 703 nm and 711 nm in PSI-LHCI indicated up to five red chlorophylls in the peripheral antenna. Analysis of fluorescence emission spectra resolved states emitting at 705, 715 and 722 nm in the core and additional states around 705-710 nm and 733 nm in PSI-LHCI. The red states compete with P700 in trapping excitations in the bulk antenna, which occurs on a timescale of ~20 ps. The three red forms in the core have distinct decay kinetics, probably in part determined by the rate of quenching by the oxidized P700. These results affirm that the red chlorophylls in the core complex must not be neglected when interpreting kinetic experimental results of PSI.

Photoprotection mechanisms under different CO2 regimes during photosynthesis in a green alga Chlorella variabilis

Oxygenic photosynthesis converts light energy into chemical energy via electron transport and assimilates CO₂ in the Calvin-Benson cycle with the chemical energy. Thus, high light and low CO₂ conditions induce the accumulation of electrons in the photosynthetic electron transport system, resulting in the formation of reactive oxygen species. To prevent the accumulation of electrons, oxygenic photosynthetic organisms have developed photoprotection mechanisms, including non-photochemical quenching (NPQ) and alternative electron flow (AEF). There are diverse molecular mechanisms underlying NPQ and AEF, and the corresponding molecular actors have been identified and characterized using a model green alga Chlamydomonas reinhardtii. In contrast, detailed information about the photoprotection mechanisms is lacking for other green algal species. In the current study, we examined the photoprotection mechanisms responsive to CO₂ in the green alga Chlorella variabilis by combining the analyses of pulse-amplitude-modulated fluorescence, O₂ evolution, and the steady-state and time-resolved fluorescence spectra. Under the CO₂-limited condition, ΔpH-dependent NPQ occurred in photosystems I and II. Moreover, O₂-dependent AEF was also induced. Under the CO₂-limited condition with carbon supplementation, NPQ was relaxed and light-harvesting chlorophyll-protein complex II was isolated from both photosystems. In C. variabilis, the O₂-dependent AEF and the mechanisms that instantly convert the light-harvesting functions of both photosystems may be important for maintaining efficient photosynthetic activities under various CO₂ conditions.

Red-shifted light-harvesting system of freshwater eukaryotic alga Trachydiscus minutus (Eustigmatophyta, Stramenopila)

Survival of phototrophic organisms depends on their ability to collect and convert enough light energy to support their metabolism. Phototrophs can extend their absorption cross section by using diverse pigments and by tuning the properties of these pigments via pigment-pigment and pigment-protein interaction. It is well known that some cyanobacteria can grow in heavily shaded habitats by utilizing far-red light harvested with far-red-absorbing chlorophylls d and f. We describe a red-shifted light-harvesting system based on chlorophyll a from a freshwater eustigmatophyte alga Trachydiscus minutus (Eustigmatophyceae, Goniochloridales). A comprehensive characterization of the photosynthetic apparatus of T. minutus is presented. We show that thylakoid membranes of T. minutus contain light-harvesting complexes of several sizes differing in the relative amount of far-red chlorophyll a forms absorbing around 700 nm. The pigment arrangement of the major red-shifted light-harvesting complex is similar to that of the red-shifted antenna of a marine alveolate alga Chromera velia. Evolutionary aspects of the algal far-red light-harvesting complexes are discussed. The presence of these antennas in eustigmatophyte algae opens up new ways to modify organisms of this promising group for effective use of far-red light in mass cultures.

Effects of excess light energy on excitation-energy dynamics in a pennate diatom Phaeodactylum tricornutum

Controlling excitation energy flow is a fundamental ability of photosynthetic organisms to keep a better performance of photosynthesis. Among the organisms, diatoms have unique light-harvesting complexes, fucoxanthin chlorophyll (Chl) a/c-binding proteins. We have recently investigated light-adaptation mechanisms of a marine centric diatom, Chaetoceros gracilis, by spectroscopic techniques. However, it remains unclear how pennate diatoms regulate excitation energy under different growth light conditions. Here, we studied light-adaptation mechanisms in a marine pennate diatom Phaeodactylum tricornutum grown at 30 µmol photons m^-2 s^-1 and further incubated for 24 h either in the dark, or at 30 or 300 µmol photons m^-2 s^-1 light intensity, by time-resolved fluorescence (TRF) spectroscopy. The high-light incubated cells showed no detectable oxygen-evolving activity of photosystem II, indicating the occurrence of a severe photodamage. The photodamaged cells showed alterations of steady-state absorption and fluorescence spectra and TRF spectra compared with the dark and low-light adapted cells. In particular, excitation-energy quenching is significantly accelerated in the photodamaged cells as shown by mean lifetime analysis of the Chl fluorescence. These spectral changes by the high-light treatment may result from arrangements of pigment-protein complexes to maintain the photosynthetic performance under excess light illumination. These growth-light dependent spectral properties in P. tricornutum are largely different from those in C. gracilis, thus providing insights into the different light-adaptation mechanisms between the pennate and centric diatoms.

Biochemical characterization of photosystem I complexes having different subunit compositions of fucoxanthin chlorophyll a/c-binding proteins in the diatom Chaetoceros gracilis

Diatoms are dominant phytoplankton in aquatic environments and have unique light-harvesting apparatus, fucoxanthin chlorophyll a/c-binding protein (FCP). Diatom photosystem I (PSI) interacts with specific FCPs (FCPI); however, it remains unclear how PSI cores receive excitation energy from FCPI. To analyze the energy transfer dynamics, it is necessary to isolate both PSI cores and PSI-FCPI complexes. In this study, we prepared three PSI complexes, which are PSI-FCPI membrane fragments, detergent-solubilized PSI-FCPI supercomplexes and PSI core-like complexes, from the marine centric diatom, Chaetoceros gracilis, and examined their biochemical properties. Both the PSI-FCPI membrane fragments and supercomplexes showed similar subunit compositions including FCPI, whereas the PSI complexes were devoid of most FCPI subunits. The purity and homogeneity of the two detergent-solubilized PSI preparations were verified by clear-native PAGE and electron microscopy. The difference of pigment contents among the three PSI samples was shown by absorption spectra at 77 K. The intensity in the whole spectrum of PSI-FCPI membranes was much higher than those of the other two complexes, while the spectral shape of PSI complexes was similar to that of cyanobacterial PSI core complexes. 77-K fluorescence spectra of the three PSI preparations exhibited different spectral shapes, especially peak positions and band widths. Based on these observations, we discuss the merits of three PSI preparations for evaluating excitation energy dynamics in diatom PSI-FCPI complexes.

Adaptation of light-harvesting functions of unicellular green algae to different light qualities

Oxygenic photosynthetic organisms perform photosynthesis efficiently by distributing captured light energy to photosystems (PSs) at an appropriate balance. Maintaining photosynthetic efficiency under changing light conditions requires modification of light-harvesting and energy-transfer processes. In the current study, we examined how green algae regulate their light-harvesting functions in response to different light qualities. We measured low-temperature time-resolved fluorescence spectra of unicellular green algae Chlamydomonas reinhardtii and Chlorella variabilis cells grown under different light qualities. By observing the delayed fluorescence spectra, we demonstrated that both types of green algae primarily modified the associations between light-harvesting chlorophyll protein complexes (LHCs) and PSs (PSII and PSI). Under blue light, Chlamydomonas transferred more energy from LHC to chlorophyll (Chl) located far from the PSII reaction center, while energy was transferred from LHC to PSI via different energy-transfer pathways in Chlorella. Under green light, both green algae exhibited enhanced energy transfer from LHCs to both PSs. Red light induced fluorescence quenching within PSs in Chlamydomonas and LHCs in Chlorella. In Chlorella, energy transfer from PSII to PSI appears to play an important role in balancing excitation between PSII and PSI.

Uphill energy transfer in photosystem I from Chlamydomonas reinhardtii. Time-resolved fluorescence measurements at 77 K

Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI-LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI-LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI-LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI-LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~ 12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~ 675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.

An Integrated Response of Trichodesmium erythraeum IMS101 Growth and Photo-Physiology to Iron, CO2, and Light Intensity

We have assessed how varying CO₂ (180, 380, and 720 μatm) and growth light intensity (40 and 400 μmol photons m^-2 s^-1) affected Trichodesmium erythraeum IMS101 growth and photophysiology over free iron (Fe') concentrations between 20 and 9,600 pM. We found significant iron dependencies of growth rate and the initial slope and maximal relative PSII electron transport rates (rP_m). Under iron-limiting concentrations, high-light increased growth rates and rP_m; possibly indicating a lower allocation of resources to iron-containing photosynthetic proteins. Higher CO₂ increased growth rates across all iron concentrations, enabled growth to occur at lower Fe' concentrations, increased rP_m and lowered the iron half saturation constants for growth (K_m). We attribute these CO₂ responses to the operation of the CCM and the ATP spent/saved for CO₂ uptake and transport at low and high CO₂, respectively. It seems reasonable to conclude that T. erythraeum IMS101 can exhibit a high degree of phenotypic plasticity in response to CO₂, light intensity and iron-limitation. These results are important given predictions of increased dissolved CO₂ and water column stratification (i.e., higher light exposures) over the coming decades.

Excitation transfer and trapping kinetics in plant photosystem I probed by two-dimensional electronic spectroscopy

Photosystem I is a robust and highly efficient biological solar engine. Its capacity to utilize virtually every absorbed photon's energy in a photochemical reaction generates great interest in the kinetics and mechanisms of excitation energy transfer and charge separation. In this work, we have employed room-temperature coherent two-dimensional electronic spectroscopy and time-resolved fluorescence spectroscopy to follow exciton equilibration and excitation trapping in intact Photosystem I complexes as well as core complexes isolated from Pisum sativum. We performed two-dimensional electronic spectroscopy measurements with low excitation pulse energies to record excited-state kinetics free from singlet-singlet annihilation. Global lifetime analysis resolved energy transfer and trapping lifetimes closely matches the time-correlated single-photon counting data. Exciton energy equilibration in the core antenna occurred on a timescale of 0.5 ps. We further observed spectral equilibration component in the core complex with a 3-4 ps lifetime between the bulk Chl states and a state absorbing at 700 nm. Trapping in the core complex occurred with a 20 ps lifetime, which in the supercomplex split into two lifetimes, 16 ps and 67-75 ps. The experimental data could be modelled with two alternative models resulting in equally good fits-a transfer-to-trap-limited model and a trap-limited model. However, the former model is only possible if the 3-4 ps component is ascribed to equilibration with a "red" core antenna pool absorbing at 700 nm. Conversely, if these low-energy states are identified with the P700 reaction centre, the transfer-to-trap-model is ruled out in favour of a trap-limited model.

Zeaxanthin-dependent nonphotochemical quenching does not occur in photosystem I in the higher plant Arabidopsis thaliana

Nonphotochemical quenching (NPQ) is the process that protects the photosynthetic apparatus of plants and algae from photodamage by dissipating as heat the energy absorbed in excess. Studies on NPQ have almost exclusively focused on photosystem II (PSII), as it was believed that NPQ does not occur in photosystem I (PSI). Recently, Ballottari et al. [Ballottari M, et al. (2014) Proc Natl Acad Sci USA 111:E2431-E2438], analyzing PSI particles isolated from an Arabidopsis thaliana mutant that accumulates zeaxanthin constitutively, have reported that this xanthophyll can efficiently induce chlorophyll fluorescence quenching in PSI. In this work, we have checked the biological relevance of this finding by analyzing WT plants under high-light stress conditions. By performing time-resolved fluorescence measurements on PSI isolated from Arabidopsis thaliana WT in dark-adapted and high-light-stressed (NPQ) states, we find that the fluorescence kinetics of both PSI are nearly identical. To validate this result in vivo, we have measured the kinetics of PSI directly on leaves in unquenched and NPQ states; again, no differences were observed. It is concluded that PSI does not undergo NPQ in biologically relevant conditions in Arabidopsis thaliana The possible role of zeaxanthin in PSI photoprotection is discussed.

Multiple LHCII antennae can transfer energy efficiently to a single Photosystem I

Photosystems I and II (PSI and PSII) work in series to drive oxygenic photosynthesis. The two photosystems have different absorption spectra, therefore changes in light quality can lead to imbalanced excitation of the photosystems and a loss in photosynthetic efficiency. In a short-term adaptation response termed state transitions, excitation energy is directed to the light-limited photosystem. In higher plants a special pool of LHCII antennae, which can be associated with either PSI or PSII, participates in these state transitions. It is known that one LHCII antenna can associate with the PsaH site of PSI. However, membrane fractions were recently isolated in which multiple LHCII antennae appear to transfer energy to PSI. We have used time-resolved fluorescence-streak camera measurements to investigate the energy transfer rates and efficiency in these membrane fractions. Our data show that energy transfer from LHCII to PSI is relatively slow. Nevertheless, the trapping efficiency in supercomplexes of PSI with ~2.4 LHCIIs attached is 94%. The absorption cross section of PSI can thus be increased with ~65% without having significant loss in quantum efficiency. Comparison of the fluorescence dynamics of PSI-LHCII complexes, isolated in a detergent or located in their native membrane environment, indicates that the environment influences the excitation energy transfer rates in these complexes. This demonstrates the importance of studying membrane protein complexes in their natural environment.

Architecture of the light-harvesting apparatus of the eustigmatophyte alga Nannochloropsis oceanica

We present proteomic, spectroscopic, and phylogenetic analysis of light-harvesting protein (Lhc) function in oleaginous Nannochloropsis oceanica (Eustigmatophyta, Stramenopila). N. oceanica utilizes Lhcs of multiple classes: Lhcr-type proteins (related to red algae LHCI), Lhcv (VCP) proteins (violaxanthin-containing Lhcs related to Lhcf/FCP proteins of diatoms), Lhcx proteins (related to Lhcx/LhcSR of diatoms and green algae), and Lhc proteins related to Red-CLH of Chromera velia. Altogether, 17 Lhc-type proteins of the 21 known from genomic data were found in our proteomic analyses. Besides Lhcr-type antennas, a RedCAP protein and a member of the Lhcx protein subfamily were found in association with Photosystem I. The free antenna fraction is formed by trimers of a mixture of Lhcs of varied origins (Lhcv, Lhcr, Lhcx, and relatives of Red-CLH). Despite possessing several proteins of the Red-CLH-type Lhc clade, N. oceanica is not capable of chromatic adaptation under the same conditions as the diatom Phaeodactylum tricornutum or C. velia. In addition, a naming scheme of Nannochloropsis Lhcs is proposed to facilitate further work.

Visualizing heterogeneity of photosynthetic properties of plant leaves with two-photon fluorescence lifetime imaging microscopy

Two-photon fluorescence lifetime imaging microscopy (FLIM) was used to analyse the distribution and properties of Photosystem I (PSI) and Photosystem II (PSII) in palisade and spongy chloroplasts of leaves from the C3 plant Arabidopsis thaliana and the C4 plant Miscanthus x giganteus. This was achieved by separating the time-resolved fluorescence of PSI and PSII in the leaf. It is found that the PSII antenna size is larger on the abaxial side of A. thaliana leaves, presumably because chloroplasts in the spongy mesophyll are "shaded" by the palisade cells. The number of chlorophylls in PSI on the adaxial side of the A. thaliana leaf is slightly higher. The C4 plant M. x giganteus contains both mesophyll and bundle sheath cells, which have a different PSI/PSII ratio. It is shown that the time-resolved fluorescence of bundle sheath and mesophyll cells can be analysed separately. The relative number of chlorophylls, which belong to PSI (as compared to PSII) in the bundle sheath cells is at least 2.5 times higher than in mesophyll cells. FLIM is thus demonstrated to be a useful technique to study the PSI/PSII ratio and PSII antenna size in well-defined regions of plant leaves without having to isolate pigment-protein complexes.

Multiscale model of light harvesting by photosystem II in plants

The first step of photosynthesis in plants is the absorption of sunlight by pigments in the antenna complexes of photosystem II (PSII), followed by transfer of the nascent excitation energy to the reaction centers, where long-term storage as chemical energy is initiated. Quantum mechanical mechanisms must be invoked to explain the transport of excitation within individual antenna. However, it is unclear how these mechanisms influence transfer across assemblies of antenna and thus the photochemical yield at reaction centers in the functional thylakoid membrane. Here, we model light harvesting at the several-hundred-nanometer scale of the PSII membrane, while preserving the dominant quantum effects previously observed in individual complexes. We show that excitation moves diffusively through the antenna with a diffusion length of 50 nm until it reaches a reaction center, where charge separation serves as an energetic trap. The diffusion length is a single parameter that incorporates the enhancing effect of excited state delocalization on individual rates of energy transfer as well as the complex kinetics that arise due to energy transfer and loss by decay to the ground state. The diffusion length determines PSII's high quantum efficiency in ideal conditions, as well as how it is altered by the membrane morphology and the closure of reaction centers. We anticipate that the model will be useful in resolving the nonphotochemical quenching mechanisms that PSII employs in conditions of high light stress.

PSI-LHCI of Chlamydomonas reinhardtii: Increasing the absorption cross section without losing efficiency

Photosystem I (PSI) is an essential component of photosynthetic membranes. Despite the high sequence and structural homologies, its absorption properties differ substantially in algae, plants and cyanobacteria. In particular it is characterized by the presence of low-energy chlorophylls (red forms), the number and the energy of which vary in different organisms. The PSI-LHCI (PSI-light harvesting complex I) complex of the green alga Chlamydomonas reinhardtii (C.r.) is significantly larger than that of plants, containing five additional light-harvesting complexes (together binding≈65 chlorophylls), and contains red forms with higher energy than plants. To understand how these differences influence excitation energy transfer and trapping in the system, we studied two PSI-LHCI C.r. particles, differing in antenna size and red-form content, using time-resolved fluorescence and compared them to plant PSI-LHCI. The excited state kinetics in C.r. shows the same average lifetime (50 ps) as in plants suggesting that the effect of antenna enlargement is compensated by higher energy red forms. The system equilibrates very fast, indicating that all Lhcas are well-connected, despite their long distance to the core. The differences between C.r. PSI-LHCI with and without Lhca2 and Lhca9 show that these Lhcas bind red forms, although not the red-most. The red-most forms are in (or functionally close to) other Lhcas and slow down the trapping, but hardly affect the quantum efficiency, which remains as high as 97% even in a complex that contains 235 chlorophylls.

A comparison between plant photosystem I and photosystem II architecture and functioning

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Excitation dynamics in Photosystem I from Chlamydomonas reinhardtii. Comparative studies of isolated complexes and whole cells

Identical time-resolved fluorescence measurements with ~3.5-ps resolution were performed for three types of PSI preparations from the green alga, Chlamydomonas reinhardtii: isolated PSI cores, isolated PSI-LHCI complexes and PSI-LHCI complexes in whole living cells. Fluorescence decay in these types of PSI preparations has been previously investigated but never under the same experimental conditions. As a result we present consistent picture of excitation dynamics in algal PSI. Temporal evolution of fluorescence spectra can be generally described by three decay components with similar lifetimes in all samples (6-8ps, 25-30ps, 166-314ps). In the PSI cores, the fluorescence decay is dominated by the two fastest components (~90%), which can be assigned to excitation energy trapping in the reaction center by reversible primary charge separation. Excitation dynamics in the PSI-LHCI preparations is more complex because of the energy transfer between the LHCI antenna system and the core. The average trapping time of excitations created in the well coupled LHCI antenna system is about 12-15ps longer than excitations formed in the PSI core antenna. Excitation dynamics in PSI-LHCI complexes in whole living cells is very similar to that observed in isolated complexes. Our data support the view that chlorophylls responsible for the long-wavelength emission are located mostly in LHCI. We also compared in detail our results with the literature data obtained for plant PSI.

Modulation of the fluorescence yield in heliobacterial cells by induction of charge recombination in the photosynthetic reaction center

Heliobacteria contain a very simple photosynthetic apparatus, consisting of a homodimeric type I reaction center (RC) without a peripheral antenna system and using the unique pigment bacteriochlorophyll (BChl) g. They are thought to use a light-driven cyclic electron transport pathway to pump protons, and thereby phosphorylate ADP, although some of the details of this cycle are yet to be worked out. We previously reported that the fluorescence emission from the heliobacterial RC in vivo was increased by exposure to actinic light, although this variable fluorescence phenomenon exhibited very different characteristics to that in oxygenic phototrophs (Collins et al. 2010). Here, we describe the underlying mechanism behind the variable fluorescence in heliobacterial cells. We find that the ability to stably photobleach P800, the primary donor of the RC, using brief flashes is inversely correlated to the variable fluorescence. Using pump-probe spectroscopy in the nanosecond timescale, we found that illumination of cells with bright light for a few seconds put them in a state in which a significant fraction of the RCs underwent charge recombination from P800 (+)A0 (-) with a time constant of ~20 ns. The fraction of RCs in the rapidly back-reacting state correlated very well with the variable fluorescence, indicating that nearly all of the increase in fluorescence could be explained by charge recombination of P800 (+)A0 (-), some of which regenerated the singlet excited state. This hypothesis was tested directly by time-resolved fluorescence studies in the ps and ns timescales. The major decay component in whole cells had a 20-ps decay time, representing trapping by the RC. Treatment of cells with dithionite resulted in the appearance of a ~18-ns decay component, which accounted for ~0.6 % of the decay, but was almost undetectable in the untreated cells. We conclude that strong illumination of heliobacterial cells can result in saturation of the electron acceptor pool, leading to reduction of the acceptor side of the RC and the creation of a back-reacting RC state that gives rise to delayed fluorescence.

Wavelength dependence of the fluorescence emission under conditions of open and closed Photosystem II reaction centres in the green alga Chlorella sorokiniana

The fluorescence emission characteristics of the photosynthetic apparatus under conditions of open (F0) and closed (FM) Photosystem II reaction centres have been investigated under steady state conditions and by monitoring the decay lifetimes of the excited state, in vivo, in the green alga Chlorella sorokiniana. The results indicate a marked wavelength dependence of the ratio of the variable fluorescence, FV=FM-F0, over FM, a parameter that is often employed to estimate the maximal quantum efficiency of Photosystem II. The maximal value of the FV/FM ratio is observed between 660 and 680nm and the minimal in the 690-730nm region. It is possible to attribute the spectral variation of FV/FM principally to the contribution of Photosystem I fluorescence emission at room temperature. Moreover, the analysis of the excited state lifetime at F0 and FM indicates only a small wavelength dependence of Photosystem II trapping efficiency in vivo.

Novel type of red-shifted chlorophyll a antenna complex from Chromera velia: II. Biochemistry and spectroscopy

A novel chlorophyll a containing pigment-protein complex expressed by cells of Chromera velia adapted to growth under red/far-red illumination [1]. Purification of the complex was achieved by means of anion-exchange chromatography and gel-filtration. The antenna is shown to be an aggregate of ~20kDa proteins of the light-harvesting complex (LHC) family, unstable in the isolated form. The complex possesses an absorption maximum at 705nm at room temperature in addition to the main chlorophyll a maximum at 677nm producing the major emission band at 714nm at room temperature. The far-red absorption is shown to be the property of the isolated aggregate in the intact form and lost upon dissociation. The purified complex was further characterized by circular dichroism spectroscopy and fluorescence spectroscopy. This work thus identified the third different class of antenna complex in C. velia after the recently described FCP-like and LHCr-like antennas. Possible candidates for red antennas are identified in other taxonomic groups, such as eustigmatophytes and the relevance of the present results to other known examples of red-shifted antenna from other organisms is discussed. This work appears to be the first successful isolation of a chlorophyll a-based far-red antenna complex absorbing above 700nm unrelated to LHCI.

Light-harvesting in photosystem I

This review focuses on the light-harvesting properties of photosystem I (PSI) and its LHCI outer antenna. LHCI consists of different chlorophyll a/b binding proteins called Lhca's, surrounding the core of PSI. In total, the PSI-LHCI complex of higher plants contains 173 chlorophyll molecules, most of which are there to harvest sunlight energy and to transfer the created excitation energy to the reaction center (RC) where it is used for charge separation. The efficiency of the complex is based on the capacity to deliver this energy to the RC as fast as possible, to minimize energy losses. The performance of PSI in this respect is remarkable: on average it takes around 50 ps for the excitation to reach the RC in plants, without being quenched in the meantime. This means that the internal quantum efficiency is close to 100% which makes PSI the most efficient energy converter in nature. In this review, we describe the light-harvesting properties of the complex in relation to protein and pigment organization/composition, and we discuss the important parameters that assure its very high quantum efficiency. Excitation energy transfer and trapping in the core and/or Lhcas, as well as in the supercomplexes PSI-LHCI and PSI-LHCI-LHCII are described in detail with the aim of giving an overview of the functional behavior of these complexes.

Functional compartmental modeling of the photosystems in the thylakoid membrane at 77 K

Time-resolved fluorescence spectroscopy measurements at 77 K on thylakoid membrane preparations and isolated photosynthetic complexes thereof were investigated using target analysis with the aim of building functional compartmental models for the photosystems in the thylakoid membrane. Combining kinetic schemes with different spectral constraints enabled us to resolve the energy transfer pathways and decay characteristics of the different emissive species. We determined the spectral and energetic properties of the red Chl pools in both photosystems and quantified the formation of LHCII-LHCI-PSI supercomplexes in the transition from native to unstacked thylakoid membranes.

Quantum yield of charge separation in photosystem II: functional effect of changes in the antenna size upon light acclimation

We have studied thylakoid membranes of Arabidopsis thaliana acclimated to different light conditions and have related protein composition to excitation energy transfer and trapping kinetics in Photosystem II (PSII). In high light: the plants have reduced amounts of the antenna complexes LHCII and CP24, the overall trapping time of PSII is only ∼180 ps, and the quantum efficiency reaches a value of 91%. In low light: LHCII is upregulated, the PSII lifetime becomes ∼310 ps, and the efficiency decreases to 84%. This difference is largely caused by slower excitation energy migration to the reaction centers in low-light plants due to the LHCII trimers that are not part of the C2S2M2 supercomplex. This pool of "extra" LHCII normally transfers energy to both photosystems, whereas it transfers only to PSII upon far-red light treatment (state 1). It is shown that in high light the reduction of LHCII mainly concerns the LHCII-M trimers, while the pool of "extra" LHCII remains intact and state transitions continue to occur. The obtained values for the efficiency of PSII are compared with the values of Fv/Fm, a parameter that is widely used to indicate the PSII quantum efficiency, and the observed differences are discussed.

Fluorescence lifetime snapshots reveal two rapidly reversible mechanisms of photoprotection in live cells of Chlamydomonas reinhardtii

Photosynthetic organisms avoid photodamage to photosystem II (PSII) in variable light conditions via a suite of photoprotective mechanisms called nonphotochemical quenching (NPQ), in which excess absorbed light is dissipated harmlessly. To quantify the contributions of different quenching mechanisms to NPQ, we have devised a technique to measure the changes in chlorophyll fluorescence lifetime as photosynthetic organisms adapt to varying light conditions. We applied this technique to measure the fluorescence lifetimes responsible for the predominant, rapidly reversible component of NPQ, qE, in living cells of Chlamydomonas reinhardtii. Application of high light to dark-adapted cells of C. reinhardtii led to an increase in the amplitudes of 65 ps and 305 ps chlorophyll fluorescence lifetime components that was reversed after the high light was turned off. Removal of the pH gradient across the thylakoid membrane linked the changes in the amplitudes of the two components to qE quenching. The rise times of the amplitudes of the two components were significantly different, suggesting that the changes are due to two different qE mechanisms. We tentatively suggest that the changes in the 65 ps component are due to charge-transfer quenching in the minor light-harvesting complexes and that the changes in the 305 ps component are due to aggregated light-harvesting complex II trimers that have detached from PSII. We anticipate that this technique will be useful for resolving the various mechanisms of NPQ and for quantifying the timescales associated with these mechanisms.

PMS: photosystem I electron donor or fluorescence quencher

Light energy harvested by the pigments in Photosystem I (PSI) is used for charge separation in the reaction center (RC), after which the positive charge resides on a special chlorophyll dimer called P700. In studies on the PSI trapping kinetics, P700(+) is usually chemically reduced to re-open the RCs. So far, the information available about the reduction rate and possible chlorophyll fluorescence quenching effects of these reducing agents is limited. This information is indispensible to estimate the fraction of open RCs under known experimental conditions. Moreover, it would be important to understand if these reagents have a chlorophyll fluorescence quenching effects to avoid the introduction of exogenous singlet excitation quenching in the measurements. In this study, we investigated the effect of the commonly used reducing agent phenazine methosulfate (PMS) on the RC and fluorescence emission of higher plant PSI-LHCI. We measured the P700(+) reduction rate for different PMS concentrations, and show that we can give a reliable estimation on the fraction of closed RCs based on these rates. The data show that PMS is quenching chlorophyll fluorescence emission. Finally, we determined that the fluorescence quantum yield of PSI with closed RCs is 4% higher than if the RCs are open.

Photosystem I of Chlamydomonas reinhardtii contains nine light-harvesting complexes (Lhca) located on one side of the core

In this work we have purified the Photosystem I (PSI) complex of Chlamydomonas reinhardtii to homogeneity. Biochemical, proteomic, spectroscopic, and structural analyses reveal the main properties of this PSI-LHCI supercomplex. The data show that the largest purified complex is composed of one core complex and nine Lhca antennas and that it contains all Lhca gene products. A projection map at 15 Å resolution obtained by electron microscopy reveals that the Lhcas are organized on one side of the core in a double half-ring arrangement, in contrast with previous suggestions. A series of stable disassembled PSI-LHCI intermediates was purified. The analysis of these complexes suggests the sequence of the assembly/disassembly process. It is shown that PSI-LHCI of C. reinhardtii is larger but far less stable than the complex from higher plants. Lhca2 and Lhca9 (the red-most antenna complexes), although present in the largest complex in 1:1 ratio with the core, are only loosely associated with it. This can explain the large variation in antenna composition of PSI-LHCI from C. reinhardtii found in the literature. The analysis of several subcomplexes with reduced antenna size allows determination of the position of Lhca2 and Lhca9 and leads to a proposal for a model of the organization of the Lhcas within the PSI-LHCI supercomplex.

The role of the individual Lhcas in photosystem I excitation energy trapping

In this work, we have investigated the role of the individual antenna complexes and of the low-energy forms in excitation energy transfer and trapping in Photosystem I of higher plants. To this aim, a series of Photosystem I (sub)complexes with different antenna size/composition/absorption have been studied by picosecond fluorescence spectroscopy. The data show that Lhca3 and Lhca4, which harbor the most red forms, have similar emission spectra (λ(max) = 715-720 nm) and transfer excitation energy to the core with a relative slow rate of ∼25/ns. Differently, the energy transfer from Lhca1 and Lhca2, the "blue" antenna complexes, occurs about four times faster. In contrast to what is often assumed, it is shown that energy transfer from the Lhca1/4 and the Lhca2/3 dimer to the core occurs on a faster timescale than energy equilibration within these dimers. Furthermore, it is shown that all four monomers contribute almost equally to the transfer to the core and that the red forms slow down the overall trapping rate by about two times. Combining all the data allows the construction of a comprehensive picture of the excitation-energy transfer routes and rates in Photosystem I.

A retrieval algorithm to evaluate the Photosystem I and Photosystem II spectral contributions to leaf chlorophyll fluorescence at physiological temperatures

A new computational procedure to resolve the contribution of Photosystem I (PSI) and Photosystem II (PSII) to the leaf chlorophyll fluorescence emission spectra at room temperature has been developed. It is based on the Principal Component Analysis (PCA) of the leaf fluorescence emission spectra measured during the OI photochemical phase of fluorescence induction kinetics. During this phase, we can assume that only two spectral components are present, one of which is constant (PSI) and the other variable in intensity (PSII). Application of the PCA method to the measured fluorescence emission spectra of Ficus benjamina L. evidences that the temporal variation in the spectra can be ascribed to a single spectral component (the first principal component extracted by PCA), which can be considered to be a good approximation of the PSII fluorescence emission spectrum. The PSI fluorescence emission spectrum was deduced by difference between measured spectra and the first principal component. A single-band spectrum for the PSI fluorescence emission, peaked at about 735 nm, and a 2-band spectrum with maxima at 685 and 740 nm for the PSII were obtained. A linear combination of only these two spectral shapes produced a good fit for any measured emission spectrum of the leaf under investigation and can be used to obtain the fluorescence emission contributions of photosystems under different conditions. With the use of our approach, the dynamics of energy distribution between the two photosystems, such as state transition, can be monitored in vivo, directly at physiological temperatures. Separation of the PSI and PSII emission components can improve the understanding of the fluorescence signal changes induced by environmental factors or stress conditions on plants.

Excitation-energy transfer dynamics of higher plant photosystem I light-harvesting complexes

Photosystem I (PSI) plays a major role in the light reactions of photosynthesis. In higher plants, PSI is composed of a core complex and four outer antennas that are assembled as two dimers, Lhca1/4 and Lhca2/3. Time-resolved fluorescence measurements on the isolated dimers show very similar kinetics. The intermonomer transfer processes are resolved using target analysis. They occur at rates similar to those observed in transfer to the PSI core, suggesting competition between the two transfer pathways. It appears that each dimer is adopting various conformations that correspond to different lifetimes and emission spectra. A special feature of the Lhca complexes is the presence of an absorption band at low energy, originating from an excitonic state of a chlorophyll dimer, mixed with a charge-transfer state. These low-energy bands have high oscillator strengths and they are superradiant in both Lhca1/4 and Lhca2/3. This challenges the view that the low-energy charge-transfer state always functions as a quencher in plant Lhc's and it also challenges previous interpretations of PSI kinetics. The very similar properties of the low-energy states of both dimers indicate that the organization of the involved chlorophylls should also be similar, in disagreement with the available structural data.

Efficient light harvesting in a dark, hot, acidic environment: the structure and function of PSI-LHCI from Galdieria sulphuraria

Photosystem I-light harvesting complex I (PSI-LHCI) was isolated from the thermoacidophilic red alga Galdieria sulphuraria, and its structure, composition, and light-harvesting function were characterized by electron microscopy, mass spectrometry, and ultrafast optical spectroscopy. The results show that Galdieria PSI is a monomer with core features similar to those of PSI from green algae, but with significant differences in shape and size. A comparison with the crystal structure of higher plant (pea) PSI-LHCI indicates that Galdieria PSI binds seven to nine light-harvesting proteins. Results from ultrafast optical spectroscopy show that the functional coupling of the LHCI proteins to the PSI core is tighter than in other eukaryotic PSI-LHCI systems reported thus far. This tight coupling helps Galdieria perform efficient light harvesting under the low-light conditions present in its natural endolithic habitat.

The light-harvesting complexes of higher-plant Photosystem I: Lhca1/4 and Lhca2/3 form two red-emitting heterodimers

The outer antenna of higher-plant PSI (Photosystem I) is composed of four complexes [Lhc (light-harvesting complex) a1-Lhca4] belonging to the light-harvesting protein family. Difficulties in their purification have so far prevented the determination of their properties and most of the knowledge about Lhcas has been obtained from the study of the in vitro reconstituted antennas. In the present study we were able to purify the native complexes, showing that Lhca2/3 and Lhca1/4 form two functional heterodimers. Both dimers show red-fluorescence emission with maxima around 730 nm, as in the intact PSI complex. This indicates that the dimers are in their native state and that LHCI-680, which was previously assumed to be part of the PSI antenna, does not represent the native state of the system. The data show that the light-harvesting properties of the two dimers are functionally identical, concerning absorption, long-wavelength emission and fluorescence quantum yield, whereas they differ in their high-light response. Implications of the present study for the understanding of the energy transfer process in PSI are discussed. Finally, the comparison of the properties of the native dimers with those of the reconstituted complexes demonstrates that all of the major properties of the Lhcas are reproduced in the in vitro systems.

The structure and function of eukaryotic photosystem I

Eukaryotic photosystem I consists of two functional moieties: the photosystem I core, harboring the components for the light-driven charge separation and the subsequent electron transfer, and the peripheral light-harvesting complex (LHCI). While the photosystem I-core remained highly conserved throughout the evolution, with the exception of the oxidizing side of photosystem I, the LHCI complex shows a high degree of variability in size, subunits composition and bound pigments, which is due to the large variety of different habitats photosynthetic organisms dwell in. Besides summarizing the most current knowledge on the photosystem I-core structure, we will discuss the composition and structure of the LHCI complex from different eukaryotic organisms, both from the red and the green clade. Furthermore, mechanistic insights into electron transfer between the donor and acceptor side of photosystem I and its soluble electron transfer carrier proteins will be given. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Digalactosyl-diacylglycerol-deficiency lowers the thermal stability of thylakoid membranes

We investigated the effects of digalactosyl-diacylglycerol (DGDG) on the organization and thermal stability of thylakoid membranes, using wild-type Arabidopsis thaliana and the DGDG-deficient mutant, dgd1. Circular-dichroism measurements reveal that DGDG-deficiency hampers the formation of the chirally organized macrodomains containing the main chlorophyll a/b light-harvesting complexes. The mutation also brings about changes in the overall chlorophyll fluorescence lifetimes, measured in whole leaves as well as in isolated thylakoids. As shown by time-resolved measurements, using the lipophylic fluorescence probe Merocyanine 540 (MC540), the altered lipid composition affects the packing of lipids in the thylakoid membranes but, as revealed by flash-induced electrochromic absorbance changes, the membranes retain their ability for energization. Thermal stability measurements revealed more significant differences. The disassembly of the chiral macrodomains around 55°C, the thermal destabilization of photosystem I complex at 61°C as detected by green gel electrophoresis, as well as the sharp drop in the overall chlorophyll fluorescence lifetime above 45°C (values for the wild type-WT) occur at 4-7°C lower temperatures in dgd1. Similar differences are revealed in the temperature dependence of the lipid packing and the membrane permeability: at elevated temperatures MC540 appears to be extruded from the dgd1 membrane bilayer around 35°C, whereas in WT, it remains lipid-bound up to 45°C and dgd1 and WT membranes become leaky around 35 and 45°C, respectively. It is concluded that DGDG plays important roles in the overall organization of thylakoid membranes especially at elevated temperatures.

Effect of antenna-depletion in Photosystem II on excitation energy transfer in Arabidopsis thaliana

The role of individual photosynthetic antenna complexes of Photosystem II (PSII) both in membrane organization and excitation energy transfer have been investigated. Thylakoid membranes from wild-type Arabidopsis thaliana, and three mutants lacking light-harvesting complexes CP24, CP26, or CP29, respectively, were studied by picosecond-fluorescence spectroscopy. By using different excitation/detection wavelength combinations it was possible for the first time, to our knowledge, to separate PSI and PSII fluorescence kinetics. The sub-100 ps component, previously ascribed entirely to PSI, turns out to be due partly to PSII. Moreover, the migration time of excitations from antenna to PSII reaction center (RC) was determined for the first time, to our knowledge, for thylakoid membranes. It is four times longer than for PSII-only membranes, due to additional antenna complexes, which are less well connected to the RC. The results in the absence of CP26 are very similar to those of wild-type, demonstrating that the PSII organization is not disturbed. However, the kinetics in the absence of CP29 and, especially, of CP24 show that a large fraction of the light-harvesting complexes becomes badly connected to the RCs. Interestingly, the excited-state lifetimes of the disconnected light-harvesting complexes seem to be substantially quenched.

Live-cell imaging of photosystem II antenna dissociation during state transitions

Plants and green algae maintain efficient photosynthesis under changing light environments by adjusting their light-harvesting capacity. It has been suggested that energy redistribution is brought about by shuttling the light-harvesting antenna complex II (LHCII) between photosystem II (PSII) and photosystem I (PSI) (state transitions), but such molecular remodeling has never been demonstrated in vivo. Here, using chlorophyll fluorescence lifetime imaging microscopy, we visualized phospho-LHCII dissociation from PSII in live cells of the green alga Chlamydomonas reinhardtii. Induction of energy redistribution in wild-type cells led to an increase in, and spreading of, a 250-ps lifetime chlorophyll fluorescence component, which was not observed in the stt7 mutant incapable of state transitions. The 250-ps component was also the dominant component in a mutant containing the light-harvesting antenna complexes but no photosystems. The appearance of the 250-ps component was accompanied by activation of LHCII phosphorylation, supporting the visualization of phospho-LHCII dissociation. Possible implications of the unbound phospho-LHCII on energy dissipation are discussed.

Mechanism of strong quenching of photosystem II chlorophyll fluorescence under drought stress in a lichen, Physciella melanchla, studied by subpicosecond fluorescence spectroscopy

The mechanism of the severe quenching of chlorophyll (Chl) fluorescence under drought stress was studied in a lichen Physciella melanchla, which contains a photobiont green alga, Trebouxia sp., using a streak camera and a reflection-mode fluorescence up-conversion system. We detected a large 0.31 ps rise of fluorescence at 715 and 740 nm in the dry lichen suggesting the rapid energy influx to the 715-740 nm bands from the shorter-wavelength Chls with a small contribution from the internal conversion from Soret bands. The fluorescence, then, decayed with time constants of 23 and 112 ps, suggesting the rapid dissipation into heat through the quencher. The result confirms the accelerated 40 ps decay of fluorescence reported in another lichen (Veerman et al., 2007 [36]) and gives a direct evidence for the rapid energy transfer from bulk Chls to the longer-wavelength quencher. We simulated the entire PS II fluorescence kinetics by a global analysis and estimated the 20.2 ns(-1) or 55.0 ns(-1) energy transfer rate to the quencher that is connected either to the LHC II or to the PS II core antenna. The strong quenching with the 3-12 times higher rate compared to the reported NPQ rate, suggests the operation of a new type of quenching, such as the extreme case of Chl-aggregation in LHCII or a new type of quenching in PS II core antenna in dry lichens.

Fluorescence measurement by a streak camera in a single-photon-counting mode

We describe here a recently developed fluorescence measurement system that uses a streak camera to detect fluorescence decay in a single photon-counting mode. This system allows for easy measurements of various samples and provides 2D images of fluorescence in the wavelength and time domains. The great advantage of the system is that the data can be handled with ease; furthermore, the data are amenable to detailed analysis. We describe the picosecond kinetics of fluorescence in spinach Photosystem (PS) II particles at 4-77 K as a typical experimental example. Through the global analysis of the data, we have identified a new fluorescence band (F689) in addition to the already established F680, F685, and F695 emission bands. The blue shift of the steady-state fluorescence spectrum upon cooling below 77 K can be interpreted as an increase of the shorter-wavelength fluorescence, especially F689, due to the slowdown of the excitation energy transfer process. The F685 and F695 bands seem to be thermally equilibrated at 77 K but not at 4 K. The simple and efficient photon accumulation feature of the system allows us to measure fluorescence from leaves, solutions, single colonies, and even single cells. The 2D fluorescence images obtained by this system are presented for isolated spinach PS II particles, intact leaves of Arabidopsis thaliana, the PS I super-complex of a marine centric diatom, Chaetoceros gracilis, isolated membranes of a purple photosynthetic bacterium, Acidiphilium rubrum, which contains Zn-BChl a, and a coral that contains a green fluorescent protein and an algal endosymbiont, Zooxanthella.