Chlorophyll fluorescence induction in anaerobic Scenedesmus obliquus

Induction of photosynthesis under anoxic condition in Thalassiosira pseudonana and Euglena gracilis: interactions between fermentation and photosynthesis

INTRODUCTION

In their natural environment, microalgae can be transiently exposed to hypoxic or anoxic environments. Whereas fermentative pathways and their interactions with photosynthesis are relatively well characterized in the green alga model Chlamydomonas reinhardtii, little information is available in other groups of photosynthetic micro-eukaryotes. In C. reinhardtii cyclic electron flow (CEF) around photosystem (PS) I, and light-dependent oxygen-sensitive hydrogenase activity both contribute to restoring photosynthetic linear electron flow (LEF) in anoxic conditions.

METHODS

Here we analyzed photosynthetic electron transfer after incubation in dark anoxic conditions (up to 24 h) in two secondary microalgae: the marine diatom Thalassiosira pseudonana and the excavate Euglena gracilis.

RESULTS

Both species showed sustained abilities to prevent over-reduction of photosynthetic electron carriers and to restore LEF. A high and transient CEF around PSI was also observed specifically in anoxic conditions at light onset in both species. In contrast, at variance with C. reinhardtii, no sustained hydrogenase activity was detected in anoxic conditions in both species.

DISCUSSION

Altogether our results suggest that another fermentative pathway might contribute, along with CEF around PSI, to restore photosynthetic activity in anoxic conditions in E. gracilis and T. pseudonana. We discuss the possible implication of the dissimilatory nitrate reduction to ammonium (DNRA) in T. pseudonana and the wax ester fermentation in E. gracilis.

High light-induced changes in whole-cell proteomic profile and its correlation with the organization of thylakoid super-complex in cyclic electron transport mutants of Chlamydomonas reinhardtii

Light and nutrients are essential components of photosynthesis. Activating the signaling cascades is critical in starting adaptive processes in response to high light. In this study, we have used wild-type (WT), cyclic electron transport (CET) mutants like Proton Gradient Regulation (PGR) (PGRL1), and PGR5 to elucidate the actual role in regulation and assembly of photosynthetic pigment-protein complexes under high light. Here, we have correlated the biophysical, biochemical, and proteomic approaches to understand the targeted proteins and the organization of thylakoid pigment-protein complexes in the photoacclimation. The proteomic analysis showed that 320 proteins were significantly affected under high light compared to the control and are mainly involved in the photosynthetic electron transport chain, protein synthesis, metabolic process, glycolysis, and proteins involved in cytoskeleton assembly. Additionally, we observed that the cytochrome (Cyt) b₆ expression is increased in the pgr5 mutant to regulate proton motive force and ATPase across the thylakoid membrane. The increased Cyt b₆ function in pgr5 could be due to the compromised function of chloroplast (cp) ATP synthase subunits for energy generation and photoprotection under high light. Moreover, our proteome data show that the photosystem subunit II (PSBS) protein isoforms (PSBS1 and PSBS2) expressed more than the Light-Harvesting Complex Stress-Related (LHCSR) protein in pgr5 compared to WT and pgrl1 under high light. The immunoblot data shows the photosystem II proteins D1 and D2 accumulated more in pgrl1 and pgr5 than WT under high light. In high light, CP43 and CP47 showed a reduced amount in pgr5 under high light due to changes in chlorophyll and carotenoid content around the PSII protein, which coordinates as a cofactor for efficient energy transfer from the light-harvesting antenna to the photosystem core. BN-PAGE and circular dichroism studies indicate changes in macromolecular assembly and thylakoid super-complexes destacking in pgrl1 and pgr5 due to changes in the pigment-protein complexes under high light. Based on this study, we emphasize that this is an excellent aid in understanding the role of CET mutants in thylakoid protein abundances and super-complex organization under high light.

Rapidly reversible chlorophyll fluorescence quenching induced by pulses of supersaturating light in vivo

The saturation pulse method provides a means to distinguish between photochemical and non-photochemical quenching, based on the assumption that the former is suppressed by a saturating pulse of light (SP) and that the latter is not affected by the SP. Various types of non-photochemical quenching have been distinguished by their rates of dark relaxation in the time ranges of seconds, minutes, and hours. Here we report on a special type of non-photochemical quenching, which is rapidly induced by a pulse of high-intensity light, when PS II reaction centers are closed, and rapidly relaxes again after the pulse. This high-intensity quenching, HIQ, can be quantified by pulse-amplitude-modulation (PAM) fluorimetry (MULTI-COLOR-PAM, high sensitivity combined with high time resolution) via the quasi-instantaneous post-pulse fluorescence increase that precedes recovery of photochemical quenching in the 100-400-µs range. The HIQ amplitude increases linearly with the effective rate of quantum absorption by photosystem II, reaching about 8% of maximal fluorescence yield. It is not affected by DCMU, is stimulated by anoxic conditions, and is suppressed by energy-dependent non-photochemical quenching (NPQ). The HIQ amplitude is close to proportional to the square of maximal fluorescence yield, Fm', induced by an SP and varied by NPQ. These properties are in line with the working hypothesis of HIQ being caused by the annihilation of singlet excited chlorophyll a by triplet excited carotenoid. Significant underestimation of maximal fluorescence yield and photosystem II quantum yield in dark-acclimated samples can be avoided by use of moderate SP intensities. In physiologically healthy illuminated samples, NPQ prevents significant lowering of effective photosystem II quantum yield by HIQ, if excessive SP intensities are avoided.

Unique photosynthetic electron transport tuning and excitation distribution in heterokont algae

Heterokont algae are significant contributors to marine primary productivity. These algae have a photosynthetic machinery that shares many common features with that of Viridiplantae (green algae and land plants). Here we demonstrate, however, that the photosynthetic machinery of heterokont algae responds to light fundamentally differently than that of Viridiplantae. While exposure to high light leads to electron accumulation within the photosynthetic electron transport chain in Viridiplantae, this is not the case in heterokont algae. We use this insight to manipulate the photosynthetic electron transport chain and demonstrate that heterokont algae can dynamically distribute excitation energy between the two types of photosystems. We suggest that the reported electron transport and excitation distribution features are adaptations to the marine light environment.

William E. Vidaver (1921-2017): an innovator, enthusiastic scientist, inspiring teacher and a wonderful friend

William (Bill) E. Vidaver (February 2, 1921-August 31, 2017), who did his Ph.D. with Laurence (Larry) R. Blinks at Stanford (1964) and a postdoc with C. Stacy French (1965), taught and did research at Simon Fraser University (SFU) for almost 30 years. Here he published over 80 papers in photosynthesis-related areas co-authored by his graduate students, postdocs, visiting professors and SFU colleagues. He developed a unique high-pressure cuvette for the study of oxygen exchange and studied high-pressure effects in photosynthesis. Ulrich (Uli) Schreiber, as a postdoctoral fellow from Germany, introduced measurements on chlorophyll (Chl) a fluorescence to Bill's lab, leading to the discovery of reversible inhibition of excitation energy transfer between photosynthetic pigments and of a pivotal role of O₂ in the oxidation of the electron transport chain between Photosystem II (PS II) and PS I. Bill's and Uli's work led to a patent of a portable chlorophyll fluorometer, the first available commercially, which was later modified to measure whole plantlets. The latter was used in pioneering measurement of the health of forest and crop plants undergoing in vitro clonal micropropagation. With several other researchers (including Doug Bruce, the late Radovan Popovic, and Sarah Swenson), he localized the quenching site of O₂ and showed a dampening effect on measurements of the four-step process of O₂ production by endogenous oxygen uptake. Bill is remembered as a hard-working but fun-loving person with a keen mind and strong sense of social justice.

Redox changes of ferredoxin, P700, and plastocyanin measured simultaneously in intact leaves

Properties and performance of the recently introduced Dual/KLAS-NIR spectrophotometer for simultaneous measurements of ferredoxin (Fd), P700, and plastocyanin (PC) redox changes, together with whole leaf chlorophyll a (Chl) fluorescence (emission >760, 540 nm excitation) are outlined. Spectral information on in vivo Fd, P700, and PC in the near-infrared region (NIR, 780-1000 nm) is presented, on which the new approach is based. Examples of application focus on dark-light and light-dark transitions, where maximal redox changes of Fd occur. After dark-adaptation, Fd reduction induced by moderate light parallels the Kautsky effect of Chl fluorescence induction. Both signals are affected analogously by removal of O₂. A rapid type of Fd reoxidation, observed after a short pulse of light before light activation of linear electron transport (LET), is more pronounced in C4 compared to C3 leaves and interpreted to reflect cyclic PS I (CET). Light activation of LET, as assessed via the rate of Fd reoxidation after short light pulses, occurs at very low intensities and is slowly reversed (half-time ca. 20 min). Illumination with strong far-red light (FR, 740 nm) reveals two fractions of PS I, PS I (LET), and PS I (CET), differing in the rates of Fd reoxidation upon FR-off and the apparent equilibrium constants between P700 and PC. Parallel information on oxidation of Fd and reduction of P700 plus PC proves essential for identification of CET. Comparison of maize (C4) with sunflower and ivy (C3) responses leads to the conclusion that segregation of two types of PS I may not only exist in C4 (mesophyll and bundle sheath cells), but also in C3 photosynthesis (grana margins plus end membranes and stroma lamellae).

Analysis of Photosystem I Donor and Acceptor Sides with a New Type of Online-Deconvoluting Kinetic LED-Array Spectrophotometer

The newly developed Dual/KLAS-NIR spectrophotometer, technical details of which were reported very recently, is used in measuring redox changes of P700, plastocyanin (PC) and ferredoxin (Fd) in intact leaves of Hedera helix, Taxus baccata and Brassica napus An overview of various light-/dark-induced changes of deconvoluted P700⁺, PC⁺ and Fd^- signals is presented demonstrating the wealth of novel information and the consistency of the obtained results. Fd^- changes are particularly large after dark adaptation. PC oxidation precedes P700 oxidation during dark-light induction and in steady-state light response curves. Fd reoxidation during induction correlates with the secondary decline of simultaneously measured fluorescence yield, both of which are eliminated by removal of O₂ By determination of 100% redox changes, relative contents of PC/P700 and Fd/P700 can be assessed, which show considerable variations between different leaves, with a trend to higher values in sun leaves. Based on deconvoluted P700⁺ signals, the complementary quantum yields of PSI, Y(I) (photochemical energy use), Y(ND) (non-photochemical loss due to oxidized primary donor) and Y(NA) (non-photochemical loss due to reduced acceptor) are determined as a function of light intensity and compared with the corresponding complementary quantum yields of PSII, Y(II) (photochemical energy use), Y(NPQ) (regulated non-photochemical loss) and Y(NO) (non-regulated non-photochemical loss). The ratio Y(I)/Y(II) increases with increasing intensities. In the low intensity range, a two-step increase of PC⁺ is indicative of heterogeneous PC pools.

Variations between the photosynthetic properties of elite and landrace Chinese rice cultivars revealed by simultaneous measurements of 820 nm transmission signal and chlorophyll a fluorescence induction

The difference between the photosynthetic properties of elite and landrace Chinese rice cultivars was studied, using chlorophyll a fluorescence induction (mostly a monitor of Photosystem II activity) and I820 transmission signal (mostly a monitor of Photosystem I activity) to identify potential photosynthetic features differentiating these two groups, which show different degrees of artificial selection and grain yields. A higher fluorescence (related to PSII) IP rise phase and a lower P700(+) (related to PSI) accumulation were observed in the elite cultivars as compared to the landraces. Using these data, together with simulation data from a kinetic model of fluorescence induction, we show that the high IP rise phase and the low P700(+) accumulation can be a result of transient block on electron transfer and traffic jam on the electron acceptor side of PSI under a high [NADPH]/[NADP(+)] ratio. Considering that the ferredoxin NADP(+) reductase (FNR) transcript levels of XS134 (a representative elite cultivars) remains unaffected during the first few minutes of light/dark transition compared to Q4145 (a representative landrace cultivars), which shows a strong decline during the same time range, we propose that the FNR of elite cultivars may take more time to be inactivated in darkness. During this time the FNR enzyme can continue to reduce NADP(+) molecules, leading to initially high [NADPH]/[NADP(+)] ratio during OJIP transient. These data suggested a potential artificial selection of FNR during the breeding process of these examined elite rice cultivars.

Function of the chloroplast hydrogenase in the microalga Chlamydomonas: the role of hydrogenase and state transitions during photosynthetic activation in anaerobiosis

Like a majority of photosynthetic microorganisms, the green unicellular alga Chlamydomonas reinhardtii may encounter O₂ deprived conditions on a regular basis. In response to anaerobiosis or in a respiration defective context, the photosynthetic electron transport chain of Chlamydomonas is remodeled by a state transition process to a conformation that favours the photoproduction of ATP at the expense of reductant synthesis. In some unicellular green algae including Chlamydomonas, anoxia also triggers the induction of a chloroplast-located, oxygen sensitive hydrogenase, which accepts electrons from reduced ferredoxin to convert protons into molecular hydrogen. Although microalgal hydrogen evolution has received much interest for its biotechnological potential, its physiological role remains unclear. By using specific Chlamydomonas mutants, we demonstrate that the state transition ability and the hydrogenase function are both critical for induction of photosynthesis in anoxia. These two processes are thus important for survival of the cells when they are transiently placed in an anaerobic environment.

Central carbon metabolism and electron transport in Chlamydomonas reinhardtii: metabolic constraints for carbon partitioning between oil and starch

The metabolism of microalgae is so flexible that it is not an easy task to give a comprehensive description of the interplay between the various metabolic pathways. There are, however, constraints that govern central carbon metabolism in Chlamydomonas reinhardtii that are revealed by the compartmentalization and regulation of the pathways and their relation to key cellular processes such as cell motility, division, carbon uptake and partitioning, external and internal rhythms, and nutrient stress. Both photosynthetic and mitochondrial electron transfer provide energy for metabolic processes and how energy transfer impacts metabolism and vice versa is a means of exploring the regulation and function of these pathways. A key example is the specific chloroplast localization of glycolysis/gluconeogenesis and how it impacts the redox poise and ATP budget of the plastid in the dark. To compare starch and lipids as carbon reserves, their value can be calculated in terms of NAD(P)H and ATP. As microalgae are now considered a potential renewable feedstock, we examine current work on the subject and also explore the possibility of rerouting metabolism toward lipid production.

Anaerobiosis induced state transition: a non photochemical reduction of PQ pool mediated by NDH in Arabidopsis thaliana

Background

Non photochemical reduction of PQ pool and mobilization of LHCII between PSII and PSI are found to be linked under abiotic stress conditions. The interaction of non photochemical reduction of PQ pool and state transitions associated physiological changes are critically important under anaerobic condition in higher plants.

Methodology/Findings

The present study focused on the effect of anaerobiosis on non-photochemical reduction of PQ pool which trigger state II transition in Arabidopsis thaliana. Upon exposure to dark-anaerobic condition the shape of the OJIP transient rise is completely altered where as in aerobic treated leaves the rise is unaltered. Rise in F_o and F_J was due to the loss of oxidized PQ pool as the PQ pool becomes more reduced. The increase in F_o′ was due to the non photochemical reduction of PQ pool which activated STN7 kinase and induced LHCII phosphorylation under anaerobic condition. Further, it was observed that the phosphorylated LHCII is migrated and associated with PSI supercomplex increasing its absorption cross-section. Furthermore, evidences from crr2-2 (NDH mutant) and pgr5 mutants (deficient in non NDH pathway of cyclic electron transport) have indicated that NDH is responsible for non photochemical reduction of the PQ pool. We propose that dark anaerobic condition accelerates production of reducing equivalents (such as NADPH by various metabolic pathways) which reduce PQ pool and is mediated by NDH leading to state II transition.

Conclusions/Significance

Anaerobic condition triggers non photochemical reduction of PQ pool mediated by NDH complex. The reduced PQ pool activates STN7 kinase leading to state II transition in A. thaliana.

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J-I-P rise

The fast (up to 1 s) chlorophyll (Chl) a fluorescence induction (FI) curve, measured under saturating continuous light, has a photochemical phase, the O-J rise, related mainly to the reduction of Q(A), the primary electron acceptor plastoquinone of Photosystem II (PSII); here, the fluorescence rise depends strongly on the number of photons absorbed. This is followed by a thermal phase, the J-I-P rise, which disappears at subfreezing temperatures. According to the mainstream interpretation of the fast FI, the variable fluorescence originates from PSII antenna, and the oxidized Q(A) is the most important quencher influencing the O-J-I-P curve. As the reaction centers of PSII are gradually closed by the photochemical reduction of Q(A), Chl fluorescence, F, rises from the O level (the minimal level) to the P level (the peak); yet, the relationship between F and [Q(A) (-)] is not linear, due to the presence of other quenchers and modifiers. Several alternative theories have been proposed, which give different interpretations of the O-J-I-P transient. The main idea in these alternative theories is that in saturating light, Q(A) is almost completely reduced already at the end of the photochemical phase O-J, but the fluorescence yield is lower than its maximum value due to the presence of either a second quencher besides Q(A), or there is an another process quenching the fluorescence; in the second quencher hypothesis, this quencher is consumed (or the process of quenching the fluorescence is reversed) during the thermal phase J-I-P. In this review, we discuss these theories. Based on our critical examination, that includes pros and cons of each theory, as well mathematical modeling, we conclude that the mainstream interpretation of the O-J-I-P transient is the most credible one, as none of the alternative ideas provide adequate explanation or experimental proof for the almost complete reduction of Q(A) at the end of the O-J phase, and for the origin of the fluorescence rise during the thermal phase. However, we suggest that some of the factors influencing the fluorescence yield that have been proposed in these newer theories, as e.g., the membrane potential ΔΨ, as suggested by Vredenberg and his associates, can potentially contribute to modulate the O-J-I-P transient in parallel with the reduction of Q(A), through changes at the PSII antenna and/or at the reaction center, or, possibly, through the control of the oxidation-reduction of the PQ-pool, including proton transfer into the lumen, as suggested by Rubin and his associates. We present in this review our personal perspective mainly on our understanding of the thermal phase, the J-I-P rise during Chl a FI in plants and algae.

The redox state of the plastoquinone pool directly modulates minimum chlorophyll fluorescence yield in Chlamydomonas reinhardtii

The effect of the plastoquionone (PQ) pool oxidation state on minimum chlorophyll fluorescence was studied in the green alga Chlamydomonas reinhardtii. In wild type and a mutant strain that lacks both photosystems but retains light harvesting complexes, oxygen depletion induced a rise in minimum chlorophyll fluorescence. An increase in minimum fluorescence yield is also observed when the PQ pool becomes reduced in the presence of oxygen and after application of an ionophore that collapses the transmembrane proton gradient. Together these results indicate that minimum chlorophyll fluorescence is modulated by the PQ oxidation state.

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of FV/FM on excitation wavelength

The F(0) and F(M) level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F(V)/F(M) (=(F(M) - F(0))/F (M)) have been derived: within each of the three spectra of F(V)/F(M), statistically meaningful variations were detected. Also, at distinct wavelength regions, the (V)/F(M) differed significantly between leaf types. From spectra of F(V)/F (M), excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F(V)/F(M) between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F(V)/F(M) at these wavelengths. All three leaves exhibited low values of F(V)/F(M) around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550-650 nm region, the F(V)/F(M) in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C(4) species, than in barley, a C(3) species. Finally, low values of F(V)/F(M) at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F(V)/F(M) ratio to derive information on spectral absorption properties of PS I and PS II is discussed.

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.

A non-invasive assay of the plastoquinone pool redox state based on the OJIP-transient

The plastoquinone (PQ) pool of the photosynthetic electron transport chain becomes reduced under anaerobic conditions. Here, anaerobiosis was used as a tool to manipulate the PQ-pool redox state in darkness and to study the effects of the PQ-redox state on the Chl-a fluorescence (OJIP) kinetics in pea leaves (Pisum sativum L.). It is shown that the F(J) (fluorescence intensity at 3 ms) is linearly related to the area above the OJ-phase (first 3 ms) representing the reduction of the acceptor side of photosystem II (PSII) and F(J) is also linearly related to the area above the JI-phase (3-30 ms) that parallels the reduction of the PQ-pool. This means that F(J) depends on the availability of oxidized PQ-molecules bound to the Q(B)-site. The linear relationships between F(J) and the two areas indicate that F(J) is not sensitive to energy transfer between PSII-antennae (connectivity). It is further shown that a approximately 94% reduced PQ-pool is in equilibrium with a approximately 19% reduction of Q(A) (primary quinone acceptor of PSII). The non-linear relationship between the initial fluorescence value (F(20 micros)) and the area above the OJ-phase supports the idea that F(20 mus )is sensitive to connectivity. This is reinforced by the observation that this non-linearity can be overcome by transforming the F(20 micros)-values into [Q(A) (-)]-values. Based on the F(J)-value of the OJIP-transient, a simple method for the quantification of the redox state of the PQ-pool is proposed.

Photosynthesis research in Canada from 1945 to the early 1970s

This history traces the development of photosynthesis research in Canada from 1945 to 1975, starting with the work of Gleb(1) Krotkov and his students, Paul Vittorio, Tony(1) Bidwell, Don(1) Nelson, Jim(1) Craigie, Bruce Tregunna, Andreas Hauschild, Geoff Lister and others in the Department of Biology at Queen's University, Kingston, Ontario. They focused on the influence of taxonomy and light quality on the path of carbon into early products, photorespiration and photosynthesis in young trees. During the same period, Ken(1) Clendenning and one of the authors (PRG) at the National Research Council of Canada (NRCC) laboratory in Ottawa began studies of chloroplast photoreduction and leaf carboxylases. They were joined by Don(1) Mortimer, who showed that the path of carbon varies with species of plant and by Morris Kates, who studied phospholipid enzymology in chloroplasts and leaves. Stan(1) Holt researched the chemistry and distribution of chlorophylls in different taxa. In 1952, Ralph Lewin joined NRCC's new Atlantic Regional Laboratory in Halifax, Nova Scotia, followed by Jim Craigie, Jack McLachlan and Tony Bidwell who mainly investigated the products of photosynthesis in marine algae. Tony Bidwell continued these studies in 1959 at the Department of Botany, University of Toronto. Dave(1) Canvin joined the staff at Queen's in 1965 and became involved in solving the mystery of photorespiration. Tony Bidwell returned to Queen's in 1969 and studied photosynthesis of algal chloroplasts using an 'artificial leaf.' In 1965, Don Nelson established a group at Simon Fraser University in Burnaby, British Columbia that included his former student, Geoff Lister who produced the first photosynthetic action spectra for trees, and Bill(1) Vidaver, who showed the useful relation between chlorophyll fluorescence and photosynthetic activity. In 1970, Mário Fragata headed a group at the Université du Québec à Trois Rivières, Québec, that began with studies of Photosystem II in chloroplasts and particles.

Methylviologen and dibromothymoquinone treatments of pea leaves reveal the role of photosystem I in the Chl a fluorescence rise OJIP

The effects of dibromothymoquinone (DBMIB) and methylviologen (MV) on the Chl a fluorescence induction transient (OJIP) were studied in vivo. Simultaneously measured 820-nm transmission kinetics were used to monitor electron flow through photosystem I (PSI). DBMIB inhibits the reoxidation of plastoquinol by binding to the cytochrome b(6)/f complex. MV accepts electrons from the FeS clusters of PSI and it allows electrons to bypass the block that is transiently imposed by ferredoxin-NADP(+)-reductase (FNR) (inactive in dark-adapted leaves). We show that the IP phase of the OJIP transient disappears in the presence of DBMIB without affecting F(m). MV suppresses the IP phase by lowering the P level compared to untreated leaves. These observations indicate that PSI activity plays an important role in the kinetics of the OJIP transient. Two requirements for the IP phase are electron transfer beyond the cytochrome b(6)/f complex (blocked by DBMIB) and a transient block at the acceptor side of PSI (bypassed by MV). It is also observed that in leaves, just like in thylakoid membranes, DBMIB can bypass its own block at the cytochrome b(6)/f complex and donate electrons directly to PC(+) and P700(+) with a donation time tau of 4.3 s. Further, alternative explanations of the IP phase that have been proposed in the literature are discussed.

Effect of partial shading on photoproduction of hydrogen by Chlorella

The photoproduction of hydrogen by the green alga Chlorella pyrenoidosa was studied in 650-ml bubble columns made of glass with 7 cm diameter. Hydrogen was produced by adding sodium dithionite as an oxygen scavenger directly to an algal suspension. When a part of the wall of the bubble column was shaded with a sheet of plastic film impervious to light, the production rate and total volume of hydrogen increased as compared to those in the columns without partial shading. This relationship between the hydrogen photoevolution rate and the proportion of lighted region is contrary to normal photochemical reactions. This phenomenon is considered to be related to the regulation by light of the activity of the enzymes and/or the photosynthetic electron transport systems, which was examined by measuring fluorescence induction curves of dark-adapted Chlorella cells and also the distribution of light intensity within the bubble column bioreactors.

A plastid terminal oxidase comes to light: implications for carotenoid biosynthesis and chlororespiration

Inactivation of a plastid located quinone-oxygen oxidoreductase gene in the immutans Arabidopsis mutant leads to a photobleached phenotype because of a lack of photoprotective carotenoids. Inactivation of the corresponding gene in the ghost tomato mutant leads to a similar phenotype in leaves and to carotenoid deficiency in petals and ripe fruits. This plastid terminal oxidase (the first to be cloned and biochemically characterized) resembles the mitochondrial cyanide-insensitive alternative oxidase. Here, we propose a model integrating this novel oxidase as a component of an electron transport chain associated to carotenoid desaturation, as well as to a respiratory activity within plastids.

Linear analysis applied to the comparative study of the I-D-P phase of chlorophyll fluorescence as induced by actinic PS-II light, PS-I light and changes in CO2-concentration

The investigation of the kinetics of chlorophyll-fluorescence under continuous background light enables the application of linearizing conditions. This approach, which provides a quantitative evaluation by means of curve-fitting routines, is applied to the investigation of the linear kinetics of the I-D-P phase. Using changes in PS II-light, PS I-light and in CO2-concentration as input signals showed that a pool at the acceptor side of PS I, in addition to the plastoquinone pool, plays an essential role in the generation of the dip. The occurrence of the dip is related to the sign of the faster one of the two components related to the I-D and the D-P phase. This sign can be inverted by the ratio of PS I and PS II light. However, model calculations show that the change of this sign does not allow a decision which one of the two components is related to which one of the two pools. The dependence of the sign of the faster component on light conditions can generate different types of I-D-P transitions, namely nearly monophasic increases, sigmoid responses or dips. As these phenomena are already created by the linear responses, non-linear effects or additional loops between PS II and PS I are not required for the explanation of the basic features.

Chlorophyll a fluorescence transient as an indicator of active and inactive Photosystem II in thylakoid membranes

Upon illumination, a dark-adapted photosynthetic sample shows time-dependent changes in chlorophyll (Chl) a fluorescence yield, known as the Kautsky phenomenon or the OIDPS transient. Based on the differential effects of electron acceptors such as 2,5-dimethyl-p-benzoquinone (DMQ) and 2,6-dichloro-p-benzoquinone (DCBQ) on Chl a fluorescence transients of spinach thylakoids, we suggest that the OID phase reflects the reduction of the electron acceptor QA to QA- in the inactive PS II (see Graan, T. and Ort, D. (1986) Diochim. Biophys. Acta 852, 320-330). In spinach thylakoids, heat-induced increase of the Chl a fluorescence yield is also differentially sensitive to the addition of DMQ and DCBQ suggesting that this increase is mainly on the 'I' level, and thus heating is suggested to convert active PS II to inactive PS II centers. The kinetics of decay of QA-, calculated from variable Chl a fluorescence, was analyzed into three exponential components (365-395 microseconds; 6-7 ms; and 1.4-1.7 s). In heated samples, the decay rate of variable Chl a fluorescence is slower than the normal back-reaction rate; there is a preponderance of the slow component that may be due, partly, to the active centers undergoing slow back reaction between QA- and the S2 state of the oxygen-evolving complex.

The relationship between the redox state of Q A and photosynthesis in leaves at various carbon-dioxide, oxygen and light regimes

The response of chlorophyll fluorescence elicited by a low-fluence-rate modulated measuring beam to actinic light and to superimposed 1-s pulses from a high-fluence-rate light source was used to measure the redox state of the primary acceptor Q A of photosystem II in leaves which were photosynthesizing under steady-state conditions. The leaves were exposed to various O2 and CO2 concentrations and to different energy fluence rates of actinic light to assess the relationship between rates of photosynthesis and the redox state of Q A. Both at low and high fluence rates, the redox state of Q A was little altered when the CO2 concentration was reduced from saturation to about 600 μl·l(-1) although photosynthesis was decreased particularly at high fluence rates. Upon further reduction in CO2 content the amount of reduced Q A increased appreciably even at low fluence rates where light limited CO2 reduction. Both in the presence and in the absence of CO2, a more reduced Q A was observed when the O2 concentration was below 2%. Q A was almost fully reduced when leaves were exposed to high fluence rates under nitrogen. Even at low fluence rates, Q A was more reduced in shade leaves of Asarum europaeum and Fagus sylvatica than in leaves of Helianthus annuus and Fagus sylvatica grown under high light. Also, in shade leaves the redox state of Q A changed more during a transition from air containing 350 μl·l(-1) CO2 to CO2-free air than in sun leaves. The results are discussed with respect to the energy status and the CO2-fixation rate of the leaves.

The interaction of nitrite with photosynthetic electrom transport under anaerobic conditions

Chlorella cells were examined in a modulated oxygen polarograph under aerobic and anaerobic conditions. At light intensities below about 600 ergs . cm-2 . s-1 of 650 nm light, the oxygen yield and phase lag are lower under anaerobic conditions. Addition of 25 mM sodium nitrite increases both these parameters to values close to those found in the presence of oxygen. It is proposed that nitrite is reduced by Photosystem I thus diverting electrons from the cyclic electron transport pathway. The intersystem electron transport chain becomes more oxidized and this suppresses a backflow of electrons to the oxidizing side of Photosystem II, hence increasing the oxygen yield and the phase lag. The removal of oxygen from the bathing medium also alters the response of dark adapted Chlorella to a series of saturating light flashes. In terms of the Kok model of Photosystem II (Kok, B., Forbush, B. and McGloin, M. (1970) Photochem. Photobiol. 11, 457-475) there is a large increase in the parameter alpha. Addition of nitrite reverses this change and virtually restores the response seen in the presence of oxygen. The deactivation of the S2 state is greatly speeded up in the absence of oxygen but the addition of nitrite again reverses this.

A backflow of electrons around photosystem II in Chlorella cells

A study was made with a modulated oxygen electrode of the effect of variations of oxygen concentration on photosynthetic oxygen evolution from algal cells. When Chlorella vulgaris is examined with a modulated 650 nm light at 22 degrees C, both the oxygen yield and the phase lag between the modulated oxygen signal and the light modulations have virtually constant values between 800 and 120 ergs . cm-1 . s-1 if the bathing medium is in equilibrium with the air. Similar results are obtained at 32 degrees C between 1600 and 120 ergs . cm-2 . s-1. Under anaerobic conditions both the oxygen yield and the phase lag decrease if the light intensity is lowered below about 500 ergs . cm-2 . s-1 at 22 degrees C or about 1000 ergs . cm-2 . s-1 at 32 degrees C. A modulated 706 nm beam also gives rise to these phenomena but only at significantly lower rates of oxygen evolution. The cells of Anacystis nidulans and Porphyridium cruentum appear to react in the same way to anaerobic conditions as C. vulgaris. An examination of possible mechanisms to explain these results was performed using a computer simulation of photosynthetic electron transport. The simulation suggests that a backflow of electrons from a redox pool between the Photosystems to the rate-limiting reaction between Photosystem II and the water-splitting act can cause a decrease in oxygen yield and phase lag. If the pool between the Photosystems is in a very reduced state a significant cyclic flow is expected, whereas if the pool is largely oxidized little or no cyclic flow should occur. It is shown that the effects of 706 nm illumination and removal of oxygen can be interpreted in accordance with these proposals. Since a partial inhibition of oxygen evolution by 3-(3.4-dichlorophenyl)-1,1-dimethylurea (10(-8) M) magnifies the decreases in oxygen yield and phase lag, it is proposed that the pool which cycles back electrons is in front of the site of 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibition and is probably the initial electron acceptor pool after Photosystem II.

Requirement of the light-harvesting pigment.protein complex for magnesium ion regulation of excitation energy distribution in chloroplasts

Cation regulation of excitation energy distribution was examined in chloroplasts isolated from (a) pea seedlings, grown in intermittent illumination, which contain no light-harvesting complex, (b) a barley mutant which is deficient in the major polypeptide component of the light-harvesting complex, and (c) a soybean mutant which contains a reduced amount of light-harvesting complex. It was found that: (1) Mg2+-induced increase in Photosystem II fluorescence at room temperature is small in the chloroplasts of the soybean mutant, smaller in the barley mutant, and almost absent in the light-harvesting complex-less chloroplasts of pea as compared to their respective controls. (2) Mg2+-induced increase in the F685/F730 emission peak ratio at 77 K is not detected in the isolated chloroplasts of the intermittent light-grown pea and the barley mutant. (3) Pre-illumination induced State 1-State 2 and adaptation in vivo is absent in the barley mutant and is less pronounced in the soybean mutant as compared to their respective controls. (4) Increase of slow fluorescence decay upon addition of Mg2+ observed in control chloroplasts was not detected in chloroplasts of intermittent-light grown peas. These results confirm earlier conclusions (Armond, P.A., Arntzen, C.J., Briantais, J.M. and Vernotte, C. (1976) Arch. Biochem. Biophys. 175, 54--63; Davis, D.J., Armond, P.A., Gross, E.L. and Arntzen, C.J. (1976) Arch. Biochem. Biophys. 175, 64--70) that light-harvesting complex is required for the Mg2+-induced regulation of the excitation energy distribution between Photosystems I and II. The characteristic P-S decay and I-D dip of the in vivo fluorescence inductions (Kautsky effect) were not significantly altered in the light-harvesting complex-less and the light-harvesting complex-deficient chloroplasts as compared to their respective controls. These results indicate that light-harvesting complex is not obligatorily required to observe the P-S decay or the I-D dip.

Heat-induced changes of chlorophyll fluorescence in isolated chloroplasts and related heat-damage at the pigment level

The heat-induced changes of chlorophyll fluorescence excitation and emission properties were studied in isolated chloroplasts of Larrea divaricata Cav. An analysis of the temperature dependency of fluorescence, under Fo and Fmax conditions, of temperature-jump fluorescence induction kinetics, and of 77 degrees K emission spectra of preheated chloroplasts revealed two major components in the heat-induced fluorescence changes: (1) a fluorescence rise, reflecting the block of Photosystem II reaction centers; and (2) a fluorescence decrease, caused by the functional separation of light-harvesting pigment protein complex from the rest of the pigment system. Preferential excitation of chlorophyll a around 420 nm, produced a predominant fluorescence rise. Preferential excitation of chlorophyll b, at 480 nm, gives a predominant fluorescence decrease. It is proposed that the overlapping of the fluorescence decrease on the somewhat faster fluorescence rise, results in the biphasic fluorescence rise kinetics observed in isolated chloroplasts. Both the rise component and the decay component are affected by the thermal stability of the chloroplasts, acquired during growth of the plants in different thermal environments. Mg2+ enhances the stability against heat-damage expressed in the decrease component, but has no effect on the rise component. Heat pretreatment leads to a decrease of the variable fluorescence in the light-induced 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) rise curve, but no change in half-rise time is observed. It is concluded that the block of Photosystem II reaction centers precedes the loss of the light-harvesting pigment protein complex. However, the approximately antiparallel heat-induced Fmax decrease and Fo increase suggest a common cause for the two events. A heat-induced perturbation of the thylakoid membrane is discussed.

Heat-induced changes of chlorophyll fluorescence in intact leaves correlated with damage of the photosynthetic apparatus

Methods were developed to measure chlorophyll fluorescence yield of intact leaf tissue during heat treatment under varying conditions of light intensity and photosynthetic activity. Fluorescence yield of a dark-adapted leaf increases by 2- to 3-fold with an increase of temperature into the region where heat-damage occurs. The temperatures of the fluorescence transition correlate well with the temperatures where quantum yield of CO2 fixation is irreversibly depressed. Fluorescence-temperature (F-T) curves allow ranking of different species according to their heat sensitivity. Within a single species acclimation to different growth temperatures is reflected by shifts of the transition temperatures in the F-T curves. When F-T curves are recorded in the steady light states at increasing light intensities, substantial shifts (up to 6°C) of transition temperatures to higher values are observed. Quantum yield measurements of CO2 fixation confirm that hight-light conditions protect from heat-damage. It is suggested that chlorophyll acts as an intrinsic fluorescence probe of the thylakoid membrane and responds to the same changes which cause irreversible denaturation of photosynthetic enzymes.

The I-D fluorescence transient. An indicator of rapid energy distribution changes in photosynthesis

The I-D transient in the chlorophyll fluorescence induction curve (kautsky effect) is investigated in the view of recently discovered rapid changes in energy distribution between the two photosystems (Schreiber, U. and Vidaver, W., FEBS Lett., in the press). Fluorescence induction curves differ appreciably depending on whether measured at lambda less than 690 nm, originating in pigment system II, or at lambda greater than 715 nm, which is in part from pigment system I. The differences occur as well in the rapid part of the induction curve (O-I-D-P) as in the slower P-S decay. Most significant changes in energy distribution are indicated in the region of the I-D dip, being induced by appropriate preillumination. The effect is studied by (a) comparing the individual fluorescence time courses at lambda less than 690 nm and lambda greather than 715, (b) plotting F less than 690 vs. F greater than 715 and (c) recording time courses of F less than 690/F greater than 715 ratios. In (a) the I and D characteristics are delayed at F greater than 715 relative to F less than 690, which is accompanied by periods close to I and D, where the two emissions follow inverse courses. In (b) the I-D dip corresponds to a loop. And in (c) it is shown that a rapid ratio decay, reflecting increasing excitation of System I pigments, is initiated before the I-D dip. These data indicate that the I-D transient is caused by a rapid switch of energy distribution in favor of System I and resulting stimulation of Q reoxidation via the electron transport chain. It is suggested that as with the slow fluorescence transients the rapid also can be understood as a composite of two different changes, (1) direct changes resulting from a switch in energy distribution, which are inverse for F less than 690 and F greater than 715, and (2) indirect changes due to stimulated Q reduction or Q oxidation, which are parallel for both emissions. The rapid ratio decay, correlated to I-D, persists and is even stimulated in the presence of electron transport inhibitors. This and the speed of the phenomenon make it improbable that the rapid energy distribution changes are affected by an ion flux-induced mechanism. It is proposed that the electrical field across the thylakoid membrane is involved in the energy switch mechanism.

Analysis of temperature-jump chlorophyll fluorescence induction in plants

A newly observed general chlorophyll fluorescence induction effect in plants is described. Fluorescence yield can rise through as many as four different phases (alpha, beta, gamma, ) in the dark, when intact cells or leaves are rapidly heated (within approx. 2.5 s) from 20 to 40-50 degrees C. An analysis of this temperature-jump fluorescence induction in Scenedesmus obliquus leads to the following: 1. Phase alpha is due to removal of S-quenching and appears to be related to heat deactivation of the water-splitting enzyme system. With prolonged heating, irreversibility of alpha upon recooling reflects irreversible damage to the water-splitting enzyme system. 2. beta is independent of the S-states and of the redox state of primary System II acceptor Q. It is suggested that beta parallels functional separation of Q from the System II trapping centre. This effect is highly reversible. 3. gamma and beta reflect reduction of primary System II acceptor Q by a heat-induced endogenous reductant, which is probably identical to hydrogenase. Critical temperatures for pronounced alpha and beta phases differ markedly in different plants. Possible correlations between temperature-jump fluorescence inductio, thylakoid membrane lipid composition, lipid phase transition and lipid-protein interactions are discussed.

Fluorescence induction in intact spinach chloroplasts

Under conditions in which the Photosystem II quencher is rapidly reduced upon illumination, either after a preillumination or following treatment with dithionite, the fluorescence-induction curve of intact spinich chloroplasts (class I type) displays a pronounced dip. This dip is probably identical with that observed after prolonged anaerobic incubation of whole algal cells ("I-D dip"). It is inhibited by 3(3,4-dichlorophenyl)-1,1-dimethylurea and occurs in the presence of dithionite, sufficient to reduce the plastoquinone pool. It is influenced by far red light, methylviologen, anaerobiosis and uncouplers in a manner consistent with the interpretation that it represents a photochemical quenching of fluorescence by an electron transport component situated between the Photosystem II quencher and plastoquinone. Glutaraldehyde inhibition may indicate that protein structural changes are involved.

Analysis of anaerobic fluorescence decay in Scenedesmus obliquus

With reduction of System II acceptors during dark anaerobic adaptation in Scenedesmus obliquus fluorescence yield rises to a maximum value in two distinct transitions. Subsequent illumination results in a decay of fluorescence yield with the following characteristics: 1. In low intensity light it is independent of temperature and is an expression of light reaction I. 2. In high intestity light it reflects the dark limiting step in the reoxidation mechanism of System II primary acceptors. 3. There is strong inhibition by agents known to block electron transport between the two systems. 4. At light limiting conditions decay kinetics include an initial delay phase and thereafter close to second order behaviour. 5. Following a single brief saturating flash a maximum of 80% quenching is restored and a second flash yields approx. 95% restoration. Comparison with the fluorescence rise in the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea suggests that the decay reflects transfer of one positive charge from System I to the primary System II acceptor with the intermediary carrier pool remaining reduced.