Fluorescence lifetime imaging microscopy of Chlamydomonas reinhardtii: non-photochemical quenching mutants and the effect of photosynthetic inhibitors on the slow chlorophyll fluorescence transient

Revelations on photosystem II, thermoluminescence, and artificial photosynthesis: a retrospective of Govindjee from fundamentals to applications

Photosynthesis, as one of the most important chemical reactions, has powered our planet for over four billion years on a massive scale. This review summarizes and highlights the major contributions of Govindjee from fundamentals to applications in photosynthesis. His research included primary photochemistry measurements, in the picosecond time scale, in both Photosystem I and II and electron transport leading to NADP reduction, using two light reactions. He was the first to suggest the existence of P680, the reaction center of PSII, and to prove that it was not an artefact of Chlorophyll a fluorescence. For most photobiologists, Govindjee is best known for successfully exploiting Chlorophyll a fluorescence to understand the various steps in photosynthesis as well as to predict plant productivity. His contribution in resolving the controversy on minimum number of quanta in favor of 8-12 vs 3-4, needed for the evolution of one molecule of oxygen, is a milestone in the area of photosynthesis research. Furthermore, together with Don DeVault, he is the first to provide the correct theory of thermoluminescence in photosynthetic systems. His research productivity is very high: ~ 600 published articles and total citations above 27,000 with an h-index of 82. He is a recipient of numerous awards and honors including a 2022: Lifetime Achievement Award of the International Society of Photosynthesis Research. We hope that the retrospective of Govindjee described in this work will inspire and stimulate the readers to continue probing the photosynthetic apparatuses with new discoveries and breakthroughs.

Comparative Phenotyping of Two Commonly Used Chlamydomonas reinhardtii Background Strains: CC-1690 (21gr) and CC-5325 (The CLiP Mutant Library Background)

The unicellular green alga Chlamydomonas reinhardtii is an excellent model organism to investigate many essential cellular processes in photosynthetic eukaryotes. Two commonly used background strains of Chlamydomonas are CC-1690 and CC-5325. CC-1690, also called 21gr, has been used for the Chlamydomonas genome project and several transcriptome analyses. CC-5325 is the background strain for the Chlamydomonas Library Project (CLiP). Photosynthetic performance in CC-5325 has not been evaluated in comparison with CC-1690. Additionally, CC-5325 is often considered to be cell-wall deficient, although detailed analysis is missing. The circadian rhythms in CC-5325 are also unclear. To fill these knowledge gaps and facilitate the use of the CLiP mutant library for various screens, we performed phenotypic comparisons between CC-1690 and CC-5325. Our results showed that CC-5325 grew faster heterotrophically in dark and equally well in mixotrophic liquid medium as compared to CC-1690. CC-5325 had lower photosynthetic efficiency and was more heat-sensitive than CC-1690. Furthermore, CC-5325 had an intact cell wall which had comparable integrity to that in CC-1690 but appeared to have reduced thickness. Additionally, CC-5325 could perform phototaxis, but could not maintain a sustained circadian rhythm of phototaxis as CC1690 did. Finally, in comparison to CC-1690, CC-5325 had longer cilia in the medium with acetate but slower swimming speed in the medium without nitrogen and acetate. Our results will be useful for researchers in the Chlamydomonas community to choose suitable background strains for mutant analysis and employ the CLiP mutant library for genome-wide mutant screens under appropriate conditions, especially in the areas of photosynthesis, thermotolerance, cell wall, and circadian rhythms.

Photoacclimation to high-light stress in Chlamydomonas reinhardtii during conditional senescence relies on generating pH-dependent, high-quenching centres

Microalgae can respond to long-term increases in light intensity by altering the concentration of photosynthetic complexes. Under active growth, the ability of Chlamydomonas reinhardtii to acclimate to excess light is dependent on cell division to reduce the concentration of photosynthetic complexes. But, in batch culture, cells eventually reach stationary phase where their ability to divide is limited; this should impact their capacity to undergo photoacclimation. Our goal is to dissect excess-light responses as cells approach stationary phase and to determine how the strategies of photoacclimation differ compared to cells in the exponential-growth phase. In this study, cultures exited exponential growth and transitioned into a declining growth phase (DGP), where cells continued a slow rate of growth for the next seven days in both low (LL) and high-light (HL). During this period, both cultures experience a conditional senescence-related decline in chlorophyll levels. Under HL, however, the senescing cultures have a rapid decline in PSII reaction centres, maintain a stable concentration of LHCII antenna, rapidly increase LHCSR levels, and have a sustained increase in Fo/Fm. Collectively this implies that the remaining antenna act as pH-dependent, quenching centres, presumably to protect the senescing chloroplast against HL. We discovered that acclimating to HL post-exponential phase involves active degradation that is intertwined with the normal senescence process that allowed for a limited rate of cell division.

Functional characterization of the corticular photosynthetic apparatus in grapevine

A comparative analysis of functional characteristics of the grapevine leaf photosynthetic apparatus (LPA) and corticular photosynthetic apparatus (CPA) in chlorenchyma tissues of first-year lignified vine was performed. Obtained results demonstrate significant differences between the functional properties of the CPA and the LPA. CPA contains an increased proportion (about 2/3) of Q_B-non-reducing centers of photosystem II (PSII) that is confirmed by elevated O-J phase in fluorescence kinetics, high PSIIβ content, and slower Q_A^-• reoxidation. CPA and LPA use different strategies to utilize absorbed light energy and to protect itself against excessive light. CPA dissipates a significant proportion of absorbed light energy as heat (regulated and non-regulated dissipation), and only a smaller part of the excitation energy is used in the dark stages of photosynthesis. The rate constant of photoinhibition and fluorescence quenching due to photoinhibition in CPA is almost three times higher than in LPA, while high-energy state fluorescence quenching value is twice lower. The saturation of vine chlorenchyma tissue with water increases the CPA tolerance to photoinhibition and promotes the ability to restore the photosynthetic activity after photoinhibition. The electron microscopy analysis confirmed the presence of intact plastids in vine chlorenchyma tissue, the interior space of plastids is filled with large starch grains while bands of stacked thylakoid membranes are mainly localized on the periphery. Analyzes showed that corticular plastids are specialized organelles combining features of chloroplasts, amyloplasts and gerontoplasts. Distinct structural organization of photosynthetic membranes and microenvironment predetermine distinctive functional properties of CPA.

Autofluorescence in Plants

Plants contain abundant autofluorescent molecules that can be used for biochemical, physiological, or imaging studies. The two most studied molecules are chlorophyll (orange/red fluorescence) and lignin (blue/green fluorescence). Chlorophyll fluorescence is used to measure the physiological state of plants using handheld devices that can measure photosynthesis, linear electron flux, and CO2 assimilation by directly scanning leaves, or by using reconnaissance imaging from a drone, an aircraft or a satellite. Lignin fluorescence can be used in imaging studies of wood for phenotyping of genetic variants in order to evaluate reaction wood formation, assess chemical modification of wood, and study fundamental cell wall properties using Förster Resonant Energy Transfer (FRET) and other methods. Many other fluorescent molecules have been characterized both within the protoplast and as components of cell walls. Such molecules have fluorescence emissions across the visible spectrum and can potentially be differentiated by spectral imaging or by evaluating their response to change in pH (ferulates) or chemicals such as Naturstoff reagent (flavonoids). Induced autofluorescence using glutaraldehyde fixation has been used to enable imaging of proteins/organelles in the cell protoplast and to allow fluorescence imaging of fungal mycelium.

Quantum yield alterations due to the static magnetic fields action on Arthrospira platensis SAG 21.99: Evaluation of photosystem activity

Static magnetic fields (SMF) influence the metabolism of microorganisms, however, there is no knowledge explaining how SMF act in cells. This study aimed at evaluating the SMF (30 mT) effect on photosynthetic performance, growth and biomass composition of the cyanobacterium Arthrospira platensis SAG 21.99. A. platensis was cultivated under 30 mT applied for 1 h d^-1 and 24 h for 10 d in glass bottles. SMF in both conditions increased cellular growth, achieving a 30% higher biomass concentration. SMF applied for 1 h d^-1 increased the pigments and carbohydrate content. The quantum yield was used as an indicator of the photosystem II (PSII) activity and was shown to have been positively affected. SMF for 1 h d^-1 had a significant effect on the OJIP curves. This is the first study that evaluated the photosynthetic activity in cyanobacteria cultures under SMF action.

Leaves of isoprene-emitting tobacco plants maintain PSII stability at high temperatures

At high temperatures, isoprene-emitting plants display a higher photosynthetic rate and a lower nonphotochemical quenching (NPQ) compared with nonemitting plants. The mechanism of this phenomenon, which may be very important under current climate warming, is still elusive. NPQ was dissected into its components, and chlorophyll fluorescence lifetime imaging microscopy (FLIM) was used to analyse the dynamics of excited chlorophyll relaxation in isoprene-emitting and nonemitting plants. Thylakoid membrane stiffness was also measured using atomic force microscope (AFM) to identify a possible mode of action of isoprene in improving photochemical efficiency and photosynthetic stability. We show that, when compared with nonemitters, isoprene-emitting tobacco plants exposed at high temperatures display a reduced increase of the NPQ energy-dependent component (qE) and stable (1) chlorophyll fluorescence lifetime; (2) amplitude of the fluorescence decay components; and (3) thylakoid membrane stiffness. Our study shows for the first time that isoprene maintains PSII stability at high temperatures by preventing the modifications of the surrounding environment, namely providing a more steady and homogeneous distribution of the light-absorbing centres and a stable thylakoid membrane stiffness. Isoprene photoprotects leaves with a mechanism alternative to NPQ, enabling plants to maintain a high photosynthetic rate at rising temperatures.

A sixty-year tryst with photosynthesis and related processes: an informal personal perspective

After briefly describing my early collaborative work at the University of Allahabad, that had laid the foundation of my research life, I present here some of our research on photosynthesis at the University of Illinois at Urbana-Champaign, randomly selected from light absorption to NADP⁺ reduction in plants, algae, and cyanobacteria. These include the fact that (i) both the light reactions I and II are powered by light absorbed by chlorophyll (Chl) a of different spectral forms; (ii) light emission (fluorescence, delayed fluorescence, and thermoluminescence) by plants, algae, and cyanobacteria provides detailed information on these reactions and beyond; (iii) primary photochemistry in both the photosystems I (PS I) and II (PS II) occurs within a few picoseconds; and (iv) most importantly, bicarbonate plays a unique role on the electron acceptor side of PS II, specifically at the two-electron gate of PS II. Currently, the ongoing research around the world is, and should be, directed towards making photosynthesis better able to deal with the global issues (such as increasing population, dwindling resources, and rising temperature) particularly through genetic modification. However, basic research is necessary to continue to provide us with an understanding of the molecular mechanism of the process and to guide us in reaching our goals of increasing food production and other chemicals we need for our lives.

Low temperature induced modulation of photosynthetic induction in non-acclimated and cold-acclimated Arabidopsis thaliana: chlorophyll a fluorescence and gas-exchange measurements

Cold acclimation modifies the photosynthetic machinery and enables plants to survive at sub-zero temperatures, whereas in warm habitats, many species suffer even at non-freezing temperatures. We have measured chlorophyll a fluorescence (ChlF) and CO₂ assimilation to investigate the effects of cold acclimation, and of low temperatures, on a cold-sensitive Arabidopsis thaliana accession C24. Upon excitation with low intensity (40 µmol photons m^- 2 s^- 1) ~ 620 nm light, slow (minute range) ChlF transients, at ~ 22 °C, showed two waves in the SMT phase (S, semi steady-state; M, maximum; T, terminal steady-state), whereas CO₂ assimilation showed a linear increase with time. Low-temperature treatment (down to - 1.5 °C) strongly modulated the SMT phase and stimulated a peak in the CO₂ assimilation induction curve. We show that the SMT phase, at ~ 22 °C, was abolished when measured under high actinic irradiance, or when 3-(3, 4-dichlorophenyl)-1, 1- dimethylurea (DCMU, an inhibitor of electron flow) or methyl viologen (MV, a Photosystem I (PSI) electron acceptor) was added to the system. Our data suggest that stimulation of the SMT wave, at low temperatures, has multiple reasons, which may include changes in both photochemical and biochemical reactions leading to modulations in non-photochemical quenching (NPQ) of the excited state of Chl, "state transitions," as well as changes in the rate of cyclic electron flow through PSI. Further, we suggest that cold acclimation, in accession C24, promotes "state transition" and protects photosystems by preventing high excitation pressure during low-temperature exposure.

On the origin of the slow M-T chlorophyll a fluorescence decline in cyanobacteria: interplay of short-term light-responses

The slow kinetic phases of the chlorophyll a fluorescence transient (induction) are valuable tools in studying dynamic regulation of light harvesting, light energy distribution between photosystems, and heat dissipation in photosynthetic organisms. However, the origin of these phases are not yet fully understood. This is especially true in the case of prokaryotic oxygenic photoautotrophs, the cyanobacteria. To understand the origin of the slowest (tens of minutes) kinetic phase, the M-T fluorescence decline, in the context of light acclimation of these globally important microorganisms, we have compared spectrally resolved fluorescence induction data from the wild type Synechocystis sp. PCC 6803 cells, using orange (λ = 593 nm) actinic light, with those of mutants, ΔapcD and ΔOCP, that are unable to perform either state transition or fluorescence quenching by orange carotenoid protein (OCP), respectively. Our results suggest a multiple origin of the M-T decline and reveal a complex interplay of various known regulatory processes in maintaining the redox homeostasis of a cyanobacterial cell. In addition, they lead us to suggest that a new type of regulatory process, operating on the timescale of minutes to hours, is involved in dissipating excess light energy in cyanobacteria.

In vivo label-free mapping of the effect of a photosystem II inhibiting herbicide in plants using chlorophyll fluorescence lifetime

BACKGROUND

In order to better understand and improve the mode of action of agrochemicals, it is useful to be able to visualize their uptake and distribution in vivo, non-invasively and, ideally, in the field. Here we explore the potential of plant autofluorescence (specifically chlorophyll fluorescence) to provide a readout of herbicide action across the scales utilising multiphoton-excited fluorescence lifetime imaging, wide-field single-photon excited fluorescence lifetime imaging and single point fluorescence lifetime measurements via a fibre-optic probe.

RESULTS

Our studies indicate that changes in chlorophyll fluorescence lifetime can be utilised as an indirect readout of a photosystem II inhibiting herbicide activity in living plant leaves at three different scales: cellular (~μm), single point (~1 mm²) and macroscopic (~8 × 6 mm² of a leaf). Multiphoton excited fluorescence lifetime imaging of Triticum aestivum leaves indicated that there is an increase in the spatially averaged chlorophyll fluorescence lifetime of leaves treated with Flagon EC-a photosystem II inhibiting herbicide. The untreated leaf exhibited an average lifetime of 560 ± 30 ps while the leaf imaged 2 h post treatment exhibited an increased lifetime of 2000 ± 440 ps in different fields of view. The results from in vivo wide-field single-photon excited fluorescence lifetime imaging excited at 440 nm indicated an increase in chlorophyll fluorescence lifetime from 521 ps in an untreated leaf to 1000 ps, just 3 min after treating the same leaf with Flagon EC, and to 2150 ps after 27 min. In vivo single point fluorescence lifetime measurements demonstrated a similar increase in chlorophyll fluorescence lifetime. Untreated leaf presented a fluorescence lifetime of 435 ps in the 440 nm excited chlorophyll channel, CH4 (620-710 nm). In the first 5 min after treatment, mean fluorescence lifetime is observed to have increased to 1 ns and then to 1.3 ns after 60 min. For all these in vivo plant autofluorescence lifetime measurements, the plants were not dark-adapted.

CONCLUSIONS

We demonstrate that the local impact of a photosystem II herbicide on living plant leaves can be conveniently mapped in space and time via changes in autofluorescence lifetime, which we attribute to changes in chlorophyll fluorescence. Using portable fibre-optic probe instrumentation originally designed for label-free biomedical applications, this capability could be deployed outside the laboratory for monitoring the distribution of herbicides in growing plants.

Time-resolved spectroscopy at 19,000 lines per second using a CMOS SPAD line array enables advanced biophotonics applications

A SPAD-based line sensor fabricated in 130 nm CMOS technology capable of acquiring time-resolved fluorescence spectra (TRFS) in 8.3 milliseconds is presented. To the best of our knowledge, this is the fastest time correlated single photon counting (TCSPC) TRFS acquisition reported to date. The line sensor is an upgrade to our prior work and incorporates: i) parallelized interface from sensor to surrounding circuitry enabling high line rate to the PC (19,000 lines/s) and ii) novel time-gating architecture where detected photons in the OFF region are rejected digitally after the output stage of the SPAD. The time-gating architecture was chosen to avoid electrical transients on the SPAD high voltage supplies when gating is achieved by excess bias modulation. The time-gate has an adjustable location and time window width allowing the user to focus on time-events of interest. On-chip integrated center-of-mass (CMM) calculations provide efficient acquisition of photon arrivals and direct lifetime estimation of fluorescence decays. Furthermore, any of the SPC, TCSPC and on-chip CMM modes can be used in conjunction with the time-gating. The higher readout rate and versatile architecture greatly empower the user and will allow widespread applications across many techniques and disciplines. Here we focused on 3 examples of TRFS and time-gated Raman spectroscopy: i) kinetics of chlorophyll A fluorescence from an intact leaf; ii) kinetics of a thrombin biosensor FRET probe from quenched to fluorescence states; iii) ex vivo mouse lung tissue autofluorescence TRFS; iv) time-gated Raman spectroscopy of toluene at 3056 cm^-1 peak. To the best of our knowledge, we detect spectrally for the first time the fast rise in fluorescence lifetime of chlorophyll A in a measurement over single fluorescent transient.

Chlorophyll fluorescence lifetime imaging provides new insight into the chlorosis induced by plant virus infection

KEY MESSAGE

Leaf chlorosis induced by plant virus infection has a short fluorescence lifetime, which reflects damaged photosynthetic complexes and degraded chloroplasts. Plant viruses often induce chlorosis and necrosis, which are intimately related to photosynthetic functions. Chlorophyll fluorescence lifetime measurement is a valuable noninvasive tool for analyzing photosynthetic processes and is a sensitive indicator of the environment surrounding the fluorescent molecules. In this study, our central goal was to explore the effect of viral infection on photosynthesis by employing chlorophyll fluorescence lifetime imaging (FLIM), steady-state fluorescence, non-photochemical quenching (NPQ), transmission electron microscopy (TEM), and pigment analysis. The data indicated that the chlorophyll fluorescence lifetime of chlorotic leaves was significantly shorter than that of healthy control leaves, and the fitted short lifetime component of chlorophyll fluorescence of chlorotic leaves was dominant. This dominant short lifetime component may result from damage to the structure of thylakoid, which was confirmed by TEM. The NPQ value of chlorotic leaves was slightly higher than that of healthy green leaves, which can be explained by increased neoxanthin, lutein and violaxanthin content relative to chlorophyll a. The difference in NPQ is slight, but FLIM can provide simple and direct characterization of PSII structure and photosynthetic function. Therefore, this technique shows great potential as a simple and rapid method for studying mechanisms of plant virus infection.

Protein and lipid dynamics in photosynthetic thylakoid membranes investigated by in-situ solid-state NMR

Photosynthetic thylakoid membranes contain the protein machinery to convert sunlight in chemical energy and regulate this process in changing environmental conditions via interplay between lipid, protein and xanthophyll molecular constituents. This work addresses the molecular effects of zeaxanthin accumulation in thylakoids, which occurs in native systems under high light conditions through the conversion of the xanthophyll violaxanthin into zeaxanthin via the so called xanthophyll cycle. We applied biosynthetic isotope labeling and ¹³C solid-state NMR spectroscopy to simultaneously probe the conformational dynamics of protein, lipid and xanthophyll constituents of thylakoids isolated from wild type (cw15) and npq2 mutant of the green alga Chlamydomonas reinhardtii, that accumulates zeaxanthin constitutively. Results show differential dynamics of wild type and npq2 thylakoids. Ordered-phase lipids have reduced mobility and mobile-phase lipids have enlarged dynamics in npq2 membranes, together spanning a broader dynamical range. The fraction of ordered lipids is much larger than the fraction of mobile lipids, which explains why zeaxanthin appears to cause overall reduction of thylakoid membrane fluidity. In addition to the ordered lipids, also the xanthophylls and a subset of protein sites in npq2 thylakoids have reduced conformational dynamics. Our work demonstrates the applicability of solid-state NMR spectroscopy for obtaining a microscopic picture of different membrane constituents simultaneously, inside native, heterogeneous membranes.

Characterization of thylakoid membrane in a heterocystous cyanobacterium and green alga with dual-detector fluorescence lifetime imaging microscopy with a systematic change of incident laser power

Fluorescence Lifetime Imaging Microscopy (FLIM) has been applied to plants, algae and cyanobacteria, in which excitation laser conditions affect the chlorophyll fluorescence lifetime due to several mechanisms. However, the dependence of FLIM data on input laser power has not been quantitatively explained by absolute excitation probabilities under actual imaging conditions. In an effort to distinguish between photosystem I and photosystem II (PSI and PSII) in microscopic images, we have obtained dependence of FLIM data on input laser power from a filamentous cyanobacterium Anabaena variabilis and single cellular green alga Parachlorella kessleri. Nitrogen-fixing cells in A. variabilis, heterocysts, are mostly visualized as cells in which short-lived fluorescence (≤0.1 ns) characteristic of PSI is predominant. The other cells in A. variabilis (vegetative cells) and P. kessleri cells show a transition in the status of PSII from an open state with the maximal charge separation rate at a weak excitation limit to a closed state in which charge separation is temporarily prohibited by previous excitation(s) at a relatively high laser power. This transition is successfully reproduced by a computer simulation with a high fidelity to the actual imaging conditions. More details in the fluorescence from heterocysts were examined to assess possible functions of PSII in the anaerobic environment inside the heterocysts for the nitrogen-fixing enzyme, nitrogenase. Photochemically active PSII:PSI ratio in heterocysts is tentatively estimated to be typically below our detection limit or at most about 5% in limited heterocysts in comparison with that in vegetative cells.

The slow S to M rise of chlorophyll a fluorescence reflects transition from state 2 to state 1 in the green alga Chlamydomonas reinhardtii

The green alga Chlamydomonas (C.) reinhardtii is a model organism for photosynthesis research. State transitions regulate redistribution of excitation energy between photosystem I (PS I) and photosystem II (PS II) to provide balanced photosynthesis. Chlorophyll (Chl) a fluorescence induction (the so-called OJIPSMT transient) is a signature of several photosynthetic reactions. Here, we show that the slow (seconds to minutes) S to M fluorescence rise is reduced or absent in the stt7 mutant (which is locked in state 1) in C. reinhardtii. This suggests that the SM rise in wild type C. reinhardtii may be due to state 2 (low fluorescence state; larger antenna in PS I) to state 1 (high fluorescence state; larger antenna in PS II) transition, and thus, it can be used as an efficient and quick method to monitor state transitions in algae, as has already been shown in cyanobacteria (Papageorgiou et al. 1999, 2007; Kaňa et al. 2012). We also discuss our results on the effects of (1) 3-(3,4-dichlorophenyl)-1,4-dimethyl urea, an inhibitor of electron transport; (2) n-propyl gallate, an inhibitor of alternative oxidase (AOX) in mitochondria and of plastid terminal oxidase in chloroplasts; (3) salicylhydroxamic acid, an inhibitor of AOX in mitochondria; and (4) carbonyl cyanide p-trifluoromethoxyphenylhydrazone, an uncoupler of phosphorylation, which dissipates proton gradient across membranes. Based on the data presented in this paper, we conclude that the slow PSMT fluorescence transient in C. reinhardtii is due to the superimposition of, at least, two phenomena: qE dependent non-photochemical quenching of the excited state of Chl, and state transitions.

Light stress and photoprotection in Chlamydomonas reinhardtii

Plants and algae require light for photosynthesis, but absorption of too much light can lead to photo-oxidative damage to the photosynthetic apparatus and sustained decreases in the efficiency and rate of photosynthesis (photoinhibition). Light stress can adversely affect growth and viability, necessitating that photosynthetic organisms acclimate to different environmental conditions in order to alleviate the detrimental effects of excess light. The model unicellular green alga, Chlamydomonas reinhardtii, employs diverse strategies of regulation and photoprotection to avoid, minimize, and repair photo-oxidative damage in stressful light conditions, allowing for acclimation to different and changing environments.

Modeling chlorophyll a fluorescence transient: relation to photosynthesis

To honor Academician Alexander Abramovitch Krasnovsky, we present here an educational review on the relation of chlorophyll a fluorescence transient to various processes in photosynthesis. The initial event in oxygenic photosynthesis is light absorption by chlorophylls (Chls), carotenoids, and, in some cases, phycobilins; these pigments form the antenna. Most of the energy is transferred to reaction centers where it is used for charge separation. The small part of energy that is not used in photochemistry is dissipated as heat or re-emitted as fluorescence. When a photosynthetic sample is transferred from dark to light, Chl a fluorescence (ChlF) intensity shows characteristic changes in time called fluorescence transient, the OJIPSMT transient, where O (the origin) is for the first measured minimum fluorescence level; J and I for intermediate inflections; P for peak; S for semi-steady state level; M for maximum; and T for terminal steady state level. This transient is a real signature of photosynthesis, since diverse events can be related to it, such as: changes in redox states of components of the linear electron transport flow, involvement of alternative electron routes, the build-up of a transmembrane pH gradient and membrane potential, activation of different nonphotochemical quenching processes, activation of the Calvin-Benson cycle, and other processes. In this review, we present our views on how different segments of the OJIPSMT transient are influenced by various photosynthetic processes, and discuss a number of studies involving mathematical modeling and simulation of the ChlF transient. A special emphasis is given to the slower PSMT phase, for which many studies have been recently published, but they are less known than on the faster OJIP phase.

Potential of fluorescence imaging techniques to monitor mutagenic PAH uptake by microalga

Benzo[a]pyrene (BaP), a polycyclic aromatic hydrocarbon (PAH), is one of the major environmental pollutants that causes mutagenesis and cancer. BaP has been shown to accumulate in phytoplankton and zooplankton. We have studied the localization and aggregation of BaP in Chlorella sp., a microalga that is one of the primary producers in the food chain, using fluorescence confocal microscopy and fluorescence lifetime imaging microscopy with the phasor approach to characterize the location and the aggregation of BaP in the cell. Our results show that BaP accumulates in the lipid bodies of Chlorella sp. and that there is Förster resonance energy transfer between BaP and photosystems of Chlorella sp., indicating the close proximity of the two molecular systems. The lifetime of BaP fluorescence was measured to be 14 ns in N,N-dimethylformamide, an average of 7 ns in Bold's basal medium, and 8 ns in Chlorella cells. Number and brightness analysis suggests that BaP does not aggregate inside Chlorella sp. (average brightness = 5.330), while it aggregates in the supernatant. In Chlorella grown in sediments spiked with BaP, in 12 h the BaP uptake could be visualized using fluorescence microscopy.

Chlorophyll a and NADPH fluorescence lifetimes in the microalgae Haematococcus pluvialis (Chlorophyceae) under normal and astaxanthin-accumulating conditions

In vivo fluorescence lifetimes of chlorophyll-a (chl-a) and nicotinamide adenine dinucleotide phosphate (NADPH) were obtained from the green microalgae Haematococcus pluvialis under normal and nutrient-stressed conditions (green stage and red stage, respectively), using two-photon excitation provided by a laser generating pulses in the femtosecond range, and a Leica microscope setup. Analysis of the fluorescence lifetime decay curve revealed two separate lifetime components in all our measurements. A short-lifetime component for chl-a of ~250 ps was completely dominant, contributing more than 90% of overall intensity in both green-stage and red-stage cells. Green-stage cells inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU) displayed a significant chl-a lifetime increase for the short component. However, this was not the case for red-stage cells, in which DCMU inhibition did not significantly affect the lifetime. For green-stage cells, we found a short NADPH (free) lifetime component at ~150 ps to be completely dominating, but for red-stage cells, a longer component (protein bound) at ~3 ns contributed as much as 35% of the total intensity. We hypothesize that the long lifetime component of NADPH is connected to photoprotection in the cells and coupled to production of astaxanthin. DCMU does not seem to affect the fluorescence lifetimes of NADPH.

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.

Fluorescence lifetime imaging comes of age how to do it and how to interpret it

Fluorescence lifetime imaging (FLI) has been used widely for measuring biomedical samples. Practical guidelines on taking successful FLI data are provided to avoid common errors that arise during the measurement. Several methods for analyzing and interpreting FLI results are also introduced; e.g., a model-free data analysis method called the polar plot allows visualization and analysis of FLI data without iterative fitting, and an image denoising algorithm called variance-stabilizing-transform TI Haar helps to elucidate the information of a complex biomedical sample. The instrument considerations and data analysis of Spectral-FLI are also discussed.

Acclimation of Chlamydomonas reinhardtii to different growth irradiances

We report on the changes the photosynthetic apparatus of Chlamydomonas reinhardtii undergoes upon acclimation to different light intensity. When grown in high light, cells had a faster growth rate and higher biomass production compared with low and control light conditions. However, cells acclimated to low light intensity are indeed able to produce more biomass per photon available as compared with high light-acclimated cells, which dissipate as heat a large part of light absorbed, thus reducing their photosynthetic efficiency. This dissipative state is strictly dependent on the accumulation of LhcSR3, a protein related to light-harvesting complexes, responsible for nonphotochemical quenching in microalgae. Other changes induced in the composition of the photosynthetic apparatus upon high light acclimation consist of an increase of carotenoid content on a chlorophyll basis, particularly zeaxanthin, and a major down-regulation of light absorption capacity by decreasing the chlorophyll content per cell. Surprisingly, the antenna size of both photosystem I and II is not modulated by acclimation; rather, the regulation affects the PSI/PSII ratio. Major effects of the acclimation to low light consist of increased activity of state 1 and 2 transitions and increased contributions of cyclic electron flow.

Photosystem II fluorescence: slow changes--scaling from the past

With the advent of photoelectric devices (photocells, photomultipliers) in the 1930s, fluorometry of chlorophyll (Chl) a in vivo emerged as a major method in the science of photosynthesis. Early researchers employed fluorometry primarily for two tasks: to elucidate the role in photosynthesis, if any, of other plant pigments, such as Chl b, Chl c, carotenoids and phycobilins; and to use it as a convenient inverse measure of photosynthetic activity. In pursuing the latter task, it became apparent that Chl a fluorescence emission is influenced (i) by redox active Chl a molecules in the reaction center of photosystem (PS) II (photochemical quenching); (ii) by an electrochemical imbalance across the thylakoid membrane (high energy quenching); and (iii) by the size of the peripheral antennae of weakly fluorescent PSI and strongly fluorescent PSII in response to changes in the ambient light (state transitions). In this perspective we trace the historical evolution of our awareness of these concepts, particularly of the so-called 'State Transitions'.

Photosystem II fluorescence lifetime imaging in avocado leaves: contributions of the lutein-epoxide and violaxanthin cycles to fluorescence quenching

Lifetime-resolved imaging measurements of chlorophyll a fluorescence were made on leaves of avocado plants to study whether rapidly reversible ΔpH-dependent (transthylakoid H(+) concentration gradient) thermal energy dissipation (qE) and slowly reversible ΔpH-independent fluorescence quenching (qI) are modulated by lutein-epoxide and violaxanthin cycles operating in parallel. Under normal conditions (without inhibitors), analysis of the chlorophyll a fluorescence lifetime data revealed two major lifetime pools (1.5 and 0.5 ns) for photosystem II during the ΔpH build-up under illumination. Formation of the 0.5-ns pool upon illumination was correlated with dark-retention of antheraxanthin and photo-converted lutein in leaves. Interconversion between the 1.5- and 0.5-ns lifetime pools took place during the slow part of the chlorophyll a fluorescence transient: first from 1.5 ns to 0.5 ns in the P-to-S phase, then back from 0.5 ns to 1.5 ns in the S-to-M phase. When linear electron transport and the resulting ΔpH build-up were inhibited by treatment with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the major fluorescence intensity was due to a 2.2-ns lifetime pool with a minor faster contribution of approximately 0.7 ns. In the presence of DCMU, neither the intensity nor the lifetimes of fluorescence were affected by antheraxanthin and photo-converted lutein. Thus, we conclude that both antheraxanthin and photo-converted lutein are able to enhance ΔpH-dependent qE processes that are associated with the 0.5-ns lifetime pool. However, unlike zeaxanthin, retention of antheraxanthin and photo-converted lutein may not by itself stabilize quenching or cause qI.

Wide-field photon counting fluorescence lifetime imaging microscopy: application to photosynthesizing systems

Fluorescence lifetime imaging microscopy (FLIM) is a technique that visualizes the excited state kinetics of fluorescence molecules with the spatial resolution of a fluorescence microscope. We present a scanningless implementation of FLIM based on a time- and spacecorrelated single photon counting (TSCSPC) method employing a position-sensitive quadrant anode detector and wide-field illumination. The standard time-correlated photon counting approach leads to picosecond temporal resolution, making it possible to resolve complex fluorescence decays. This allows parallel acquisition of time-resolved images of biological samples under minimally invasive low-excitation conditions (<10 mW/cm(2)). In this way unwanted photochemical reactions induced by high excitation intensities and distorting the decay kinetics are avoided. Comparably low excitation intensities are practically impossible to achieve with a conventional laser scanning microscope, where focusing of the excitation beam into a tight spot is required. Therefore, wide-field FLIM permits to study Photosystem II (PS II) in a way so far not possible with a laser scanning microscope. The potential of the wide-field TSCSPC method is demonstrated by presenting FLIM measurements of the fluorescence dynamics of photosynthetic systems in living cells of the chlorophyll d-containing cyanobacterium Acaryochloris marina.

Fluorescence lifetime-resolved imaging

This is a short account of fluorescence lifetime-resolved imaging, in order to acquaint readers who are not experts with the basic methods for measuring lifetime-resolved signals throughout an image. We present the early FLI (fluorescence lifetime imaging) history, review shortly the instrumentation and experimental design, discuss briefly the fundamentals of the measured fluorescence response, and introduce the basic measurement methodologies. We also emphasize the complex nature of the fluorescence response in FLI signals, and introduce certain analysis methods that are appropriate and informative for complex fluorescence decays. The advantages of model independent analyses are discussed and examples given.

Optical microscopy in photosynthesis

Emerging as well as the most frequently used optical microscopy techniques are reviewed and image contrast generation methods in a microscope are presented, focusing on the nonlinear contrasts such as harmonic generation and multiphoton excitation fluorescence. Nonlinear microscopy presents numerous advantages over linear microscopy techniques including improved deep tissue imaging, optical sectioning, and imaging of live unstained samples. Nonetheless, with the exception of multiphoton excitation fluorescence, nonlinear microscopy is in its infancy, lacking protocols, users and applications; hence, this review focuses on the potential of nonlinear microscopy for studying photosynthetic organisms. Examples of nonlinear microscopic imaging are presented including isolated light-harvesting antenna complexes from higher plants, starch granules, chloroplasts, unicellular alga Chlamydomonas reinhardtii, and cyanobacteria Leptolyngbya sp. and Anabaena sp. While focusing on nonlinear microscopy techniques, second and third harmonic generation and multiphoton excitation fluorescence microscopy, other emerging nonlinear imaging modalities are described and several linear optical microscopy techniques are reviewed in order to clearly describe their capabilities and to highlight the advantages of nonlinear microscopy.

Fluorescence lifetimes: fundamentals and interpretations

Fluorescence measurements have been an established mainstay of photosynthesis experiments for many decades. Because in the photosynthesis literature the basics of excited states and their fates are not usually described, we have presented here an easily understandable text for biology students in the style of a chapter in a text book. In this review we give an educational overview of fundamental physical principles of fluorescence, with emphasis on the temporal response of emission. Escape from the excited state of a molecule is a dynamic event, and the fluorescence emission is in direct kinetic competition with several other pathways of de-excitation. It is essentially through a kinetic competition between all the pathways of de-excitation that we gain information about the fluorescent sample on the molecular scale. A simple probability allegory is presented that illustrates the basic ideas that are important for understanding and interpreting most fluorescence experiments. We also briefly point out challenges that confront the experimenter when interpreting time-resolved fluorescence responses.

Rapid frequency-domain FLIM spinning disk confocal microscope: lifetime resolution, image improvement and wavelet analysis

A spinning disk confocal attachment is added to a full-field real-time frequency-domain fluorescence lifetime-resolved imaging microscope (FLIM). This provides confocal 3-D imaging while retaining all the characteristics of the normal 2-D FLIM. The spinning disk arrangement allows us to retain the speed of the normal 2-D full field FLIM while gaining true 3-D resolution. We also introduce the use of wavelet image transformations into the FLIM analysis. Wavelets prove useful for selecting objects according to their morphology, denoising and background subtraction. The performance of the instrument and the analysis routines are tested with quantitative physical samples and examples are presented with complex biological samples.

Annual variation of the steady-state chlorophyll fluorescence emission of evergreen plants in temperate zone

Remotely sensed passive chlorophyll fluorescence emission has a potential to become one of the major global-scale reporter signals on vegetation performance and stress. In contrast to the actively probed parameters such as maximal (F_M') or minimal (F₀') emission, the steady-state chlorophyll fluorescence, Chl-F_S, (F_M' > Chl-F_S > F₀') has not been adequately studied. Using fluorescence imaging of leaves, we explored the modulation of Chl-F_S by actinic irradiance and by temperature in laboratory, as well as the changes that occurred in three coniferous and broadleaf plant species grown in field. The experiments revealed that Chl-F_S is largely insensitive to the incident irradiance once this is above early morning or late evening levels. The characteristic, pre-noon measured Chl-F_S correlated positively with the CO₂ assimilation rate when measured in field during the year. It was low and stable in the cold winter months and steeply increased with the spring onset. The high values of the characteristic Chl-F_S persisted throughout the vegetation season and rapidly decreased in the fall. The seasonal Chl-F_S transitions coincided with the last spring frosts or the first fall frosts that persisted for several consecutive nights. The transitions were marked by an elevated variability of the Chl-F_S signal. We propose that the signal variability occurring during the transition periods can be used to detect from satellites the beginning and the end of the photosynthetic activity in evergreen canopies of the temperate zone.

The fast and slow kinetics of chlorophyll a fluorescence induction in plants, algae and cyanobacteria: a viewpoint

The light-induced/dark-reversible changes in the chlorophyll (Chl) a fluorescence of photosynthetic cells and membranes in the mus-to-several min time window (fluorescence induction, FI; or Kautsky transient) reflect quantum yield changes (quenching/de-quenching) as well as changes in the number of Chls a in photosystem II (PS II; state transitions). Both relate to excitation trapping in PS II and the ensuing photosynthetic electron transport (PSET), and to secondary PSET effects, such as ion translocation across thylakoid membranes and filling or depletion of post-PS II and post-PS I pools of metabolites. In addition, high actinic light doses may depress Chl a fluorescence irreversibly (photoinhibitory lowering; q(I)). FI has been studied quite extensively in plants an algae (less so in cyanobacteria) as it affords a low resolution panoramic view of the photosynthesis process. Total FI comprises two transients, a fast initial (OPS; for Origin, Peak, Steady state) and a second slower transient (SMT; for Steady state, Maximum, Terminal state), whose details are characteristically different in eukaryotic (plants and algae) and prokaryotic (cyanobacteria) oxygenic photosynthetic organisms. In the former, maximal fluorescence output occurs at peak P, with peak M lying much lower or being absent, in which case the PSMT phases are replaced by a monotonous PT fluorescence decay. In contrast, in phycobilisome (PBS)-containing cyanobacteria maximal fluorescence occurs at M which lies much higher than peak P. It will be argued that this difference is caused by a fluorescence lowering trend (state 1 --> 2 transition) that dominates the FI pattern of plants and algae, and correspondingly by a fluorescence increasing trend (state 2 --> 1 transition) that dominates the FI of PBS-containing cyanobacteria. Characteristically, however, the FI pattern of the PBS-minus cyanobacterium Acaryochloris marina resembles the FI patterns of algae and plants and not of the PBS-containing cyanobacteria.