The dependence of the shapes of fluorescence induction curves in chloroplasts on the duration of illumination pulses

Investigation of quinone reduction by microalgae using fluorescence - do "lake" and "puddle" mechanisms matter?

Photosynthesis is a fundamental process used by Nature to convert solar energy into chemical energy. For the last twenty years, many solutions have been explored to provide electrical power from the photosynthetic chain. In this context, the coupling between microalgae and exogenous quinones is an encouraging strategy because of the capability of quinones to be reduced by the photosynthetic chain. The ability of a quinone to be a good or bad electron acceptor can be evaluated by fluorescence measurements. Fluorescence analyses are thus a convenient tool helping to define a diverting parameter for some quinones. However, this parameter is implicitly designed on the basis of a particular light capture mechanism by algae. In this paper, we propose to revisit previous fluorescence experimental data by considering the two possible mechanisms (lake vs. puddle) and discussing their implication on the conclusions of the analysis. In particular, we show that the maximum extraction efficiency depends on the mechanism (in the case of 2,6-dichlorobenzoquinone - 2,6-DCBQ, (0.45 ± 0.02) vs (0.61 ± 0.03) for lake and puddle mechanisms respectively) but that the trends for different quinones remain correlated to the redox potentials independently of the mechanism.

Time- and reduction-dependent rise of photosystem II fluorescence during microseconds-long inductions in leaves

Lettuce (Lactuca sativa) and benth (Nicotiana benthamiana) leaves were illuminated with 720 nm background light to mix S-states and oxidize electron carriers. Green-filtered xenon flashes of different photon dose were applied and O2 evolution induced by a flash was measured. After light intensity gradient across the leaf was mathematically considered, the flash-induced PSII electron transport (= 4·O2 evolution) exponentially increased with the flash photon dose in any differential layer of the leaf optical density. This proved the absence of excitonic connectivity between PSII units. Time courses of flash light intensity and 680 nm chlorophyll fluorescence emission were recorded. While with connected PSII the sigmoidal fluorescence rise has been explained by quenching of excitation in closed PSII by its open neighbors, in the absence of connectivity the sigmoidicity indicates gradual rise of the fluorescence yield of an individual closed PSII during the induction. Two phases were discerned: the specific fluorescence yield immediately increased from Fo to 1.8Fo in a PSII, whose reaction center became closed; fluorescence yield of the closed PSII was keeping time-dependent rise from 1.8Fo to about 3Fo, approaching the flash fluorescence yield Ff = 0.6Fm during 40 μs. The time-dependent fluorescence rise was resolved from the quenching by 3Car triplets and related to protein conformational change. We suggest that QA reduction induces a conformational change, which by energetic or structural means closes the gate for excitation entrance into the central radical pair trap-efficiently when QB cannot accept the electron, but less efficiently when it can.

Rate-limiting steps in the dark-to-light transition of Photosystem II - revealed by chlorophyll-a fluorescence induction

Photosystem II (PSII) catalyses the photoinduced oxygen evolution and, by producing reducing equivalents drives, in concert with PSI, the conversion of carbon dioxide to sugars. Our knowledge about the architecture of the reaction centre (RC) complex and the mechanisms of charge separation and stabilisation is well advanced. However, our understanding of the processes associated with the functioning of RC is incomplete: the photochemical activity of PSII is routinely monitored by chlorophyll-a fluorescence induction but the presently available data are not free of controversy. In this work, we examined the nature of gradual fluorescence rise of PSII elicited by trains of single-turnover saturating flashes (STSFs) in the presence of a PSII inhibitor, permitting only one stable charge separation. We show that a substantial part of the fluorescence rise originates from light-induced processes that occur after the stabilisation of charge separation, induced by the first STSF; the temperature-dependent relaxation characteristics suggest the involvement of conformational changes in the additional rise. In experiments using double flashes with variable waiting times (∆τ) between them, we found that no rise could be induced with zero or short ∆τ, the value of which depended on the temperature - revealing a previously unknown rate-limiting step in PSII.

Rate-limiting steps in the dark-to-light transition of Photosystem II - revealed by chlorophyll-a fluorescence induction

Photosystem II (PSII) catalyses the photoinduced oxygen evolution and, by producing reducing equivalents drives, in concert with PSI, the conversion of carbon dioxide to sugars. Our knowledge about the architecture of the reaction centre (RC) complex and the mechanisms of charge separation and stabilisation is well advanced. However, our understanding of the processes associated with the functioning of RC is incomplete: the photochemical activity of PSII is routinely monitored by chlorophyll-a fluorescence induction but the presently available data are not free of controversy. In this work, we examined the nature of gradual fluorescence rise of PSII elicited by trains of single-turnover saturating flashes (STSFs) in the presence of a PSII inhibitor, permitting only one stable charge separation. We show that a substantial part of the fluorescence rise originates from light-induced processes that occur after the stabilisation of charge separation, induced by the first STSF; the temperature-dependent relaxation characteristics suggest the involvement of conformational changes in the additional rise. In experiments using double flashes with variable waiting times (∆τ) between them, we found that no rise could be induced with zero or short ∆τ, the value of which depended on the temperature - revealing a previously unknown rate-limiting step in PSII.

Excitonic connectivity between photosystem II units: what is it, and how to measure it?

In photosynthetic organisms, light energy is absorbed by a complex network of chromophores embedded in light-harvesting antenna complexes. In photosystem II (PSII), the excitation energy from the antenna is transferred very efficiently to an active reaction center (RC) (i.e., with oxidized primary quinone acceptor Q(A)), where the photochemistry begins, leading to O2 evolution, and reduction of plastoquinones. A very small part of the excitation energy is dissipated as fluorescence and heat. Measurements on chlorophyll (Chl) fluorescence and oxygen have shown that a nonlinear (hyperbolic) relationship exists between the fluorescence yield (Φ(F)) (or the oxygen emission yield, (Φ(O2)) and the fraction of closed PSII RCs (i.e., with reduced Q(A)). This nonlinearity is assumed to be related to the transfer of the excitation energy from a closed PSII RC to an open (active) PSII RC, a process called PSII excitonic connectivity by Joliot and Joliot (CR Acad Sci Paris 258: 4622-4625, 1964). Different theoretical approaches of the PSII excitonic connectivity, and experimental methods used to measure it, are discussed in this review. In addition, we present alternative explanations of the observed sigmoidicity of the fluorescence induction and oxygen evolution curves.

Excitonic connectivity between photosystem II units: what is it, and how to measure it?

In photosynthetic organisms, light energy is absorbed by a complex network of chromophores embedded in light-harvesting antenna complexes. In photosystem II (PSII), the excitation energy from the antenna is transferred very efficiently to an active reaction center (RC) (i.e., with oxidized primary quinone acceptor Q(A)), where the photochemistry begins, leading to O2 evolution, and reduction of plastoquinones. A very small part of the excitation energy is dissipated as fluorescence and heat. Measurements on chlorophyll (Chl) fluorescence and oxygen have shown that a nonlinear (hyperbolic) relationship exists between the fluorescence yield (Φ(F)) (or the oxygen emission yield, (Φ(O2)) and the fraction of closed PSII RCs (i.e., with reduced Q(A)). This nonlinearity is assumed to be related to the transfer of the excitation energy from a closed PSII RC to an open (active) PSII RC, a process called PSII excitonic connectivity by Joliot and Joliot (CR Acad Sci Paris 258: 4622-4625, 1964). Different theoretical approaches of the PSII excitonic connectivity, and experimental methods used to measure it, are discussed in this review. In addition, we present alternative explanations of the observed sigmoidicity of the fluorescence induction and oxygen evolution curves.

Theoretical fluorescence induction curves derived from coupled differential equations describing the primary photochemistry of photosystem II by an exciton-radical pair equilibrium

Fluorescence induction curves were calculated from a molecular model for the primary photophysical and photochemical processes of photosystem II that includes reversible exciton trapping by open (PHQ(A)) and closed (PHQ(-) (A)) reaction centers (RCs), charge stabilization as well as quenching by oxidized (P(+)HQ((-)) (A)) RCs. For the limiting case of perfectly connected photosynthetic units ("lake model") and thermal equilibrium between the primary radical pair (P(+)H(-)) and the excited singlet state, the primary reactions can be mathematically formulated by a set of coupled ordinary differential equations (ODE). These were numerically solved for weak flashes in a recursive way to simulate experiments with continuous illumination. Using recently published values for the molecular rate constants, this procedure yielded the time dependence of closed RCs as well as of the fluorescence yield (= fluorescence induction curves). The theoretical curves displayed the same sigmoidal shapes as experimental fluorescence induction curves. From the time development of closed RCs and the fluorescence yield, it was possible to check currently assumed proportionalities between the fraction of closed RCs and either (a) the variable fluorescence, (b) the complementary area above the fluorescence induction curve, or (c) the complementary area normalized to the variable fluorescence. By changing selected molecular rate constants, it is shown that, in contrast to current beliefs, none of these correlations obeys simple laws. The time dependence of these quantities is strongly nonexponential. In the presence of substances that quench the excited state, the model predicts straight lines in Stern-Volmer plots. We further conclude that it is impossible to estimate the degree of physical interunit energy transfer from the sigmoidicity of the fluorescence induction curve or from the curvature of the variable fluorescence plotted versus the fraction of closed RCs.

Observation of nanosecond laser induced fluorescence of in vitro seawater phytoplankton

Seawater has been irradiated using a train of 70 ns flashes from a 440 nm laser source. This wavelength is on resonance with the blue absorption peak of Chlorophyll pigment associated with the photosystem of in vitro phytoplankton. The resulting fluorescence at 685 nm is instantaneously recorded during each laser pulse using a streak camera. Delayed fluorescence is observed, yielding clues about initiation of the photosynthetic process on a nanosecond time scale. Further data processing allows for determination of the functional absorption cross section, found to be 0.0095 A(2), which is the first reporting of this number for in vitro phytoplankton. Unlike other flash-pump studies of Chlorophyll, using a LED or flashlamp-based sources, the short laser pulse used here does not reveal any pulse-to-pulse hysteresis (i.e., variable fluorescence), indicating that the laser pulses used here are not able to drive the photosynthetic process to completion. This is attributed to competition from a back reaction between the photoexcited photosystem II and the intermediate electron acceptor. The significance of this work as a new type of deployable ocean fluorimeter is discussed, and it is believed the apparatus will have applications in thin-layer phytoplankton research.

On the relationship between the non-photochemical quenching of the chlorophyll fluorescence and the Photosystem II light harvesting efficiency. A repetitive flash fluorescence induction study

Plants respond to excess light by a photoprotective reduction of the light harvesting efficiency. The notion that the non-photochemical quenching of chlorophyll fluorescence can be reliably used as an indicator of the photoprotection is put to a test here. The technique of the repetitive flash fluorescence induction is employed to measure in parallel the non-photochemical quenching of the maximum fluorescence and the functional cross-section (sigma(PS II)) which is a product of the photosystem II optical cross-section a(PS II) and of its photochemical yield Phi(PS II) (sigma (PS II) = a(PS II) Phi(PS II)). The quenching is measured for both, the maximum fluorescence found in a single-turnover flash (F(M) (ST)) and in a multiple turnover light pulse (F(M) (MT)). The experiment with the diatom Phaeodactylum tricornutum confirmed that, in line with the prevalent model, the PS II functional cross-section sigma (PS II) is reduced in high light and restored in the dark with kinetics and amplitude that are closely matching the changes of the F(M) (ST) and F(M) (MT) quenching. In contrast, a poor correlation between the light-induced changes in the PS II functional cross-section sigma (PS II) and the quenching of the multiple-turnover F(M) (MT) fluorescence was found in the green alga Scenedesmus quadricauda. The non-photochemical quenching in Scenedesmus quadricauda was further investigated using series of single-turnover flashes given with different frequencies. Several mechanisms that modulate the fluorescence emission in parallel to the Q(A) redox state and to the membrane energization were resolved and classified in relation to the light harvesting capacity of Photosystem II.

Short-pulse pump-and-probe technique for airborne laser assessment of Photosystem II photochemical characteristics

The development of a technique for laser measurement of fPhotosystem II (PS II) photochemical characteristics of phytoplankton and terrestrial vegetation from an airborne platform is described. Results of theoretical analysis and experimental study of pump-and-probe measurement of the PS II functional absorption cross-section and photochemical quantum yield are presented. The use of 10 ns probe pulses of PS II sub-saturating intensity provides a significant, up to 150-fold, increase in the fluorescence signal compared to conventional 'weak-probe' protocol. Little effect on the fluorescence yield from the probe-induced closure of PS II reaction centers is expected over the short pulse duration, and thus a relatively intense probe pulse can be used. On the other hand, a correction must be made for the probe-induced carotenoid triplet quenching and singlet-singlet annihilation. A Stern-Volmer model developed for this correction assumes a linear dependence of the quenching rate on the laser pulse fluence, which was experimentally validated. The PS II saturating pump pulse fluence (532 nm excitation) was found to be 10 and 40 mumol quanta m(-2) for phytoplankton samples and leaves of higher plants, respectively. Thirty mus was determined as the optimal delay in the pump-probe pair. Our results indicate that the short-pulse pump-and-probe measurement of PS II photochemical characteristics can be implemented from an airborne platform using existing laser and LIDAR technologies.

Nonphotochemical quenching of excitation energy in photosystem II. A picosecond time-resolved study of the low yield of chlorophyll a fluorescence induced by single-turnover flash in isolated spinach thylakoids

Chlorophyll a fluorescence emission is widely used as a noninvasive measure of a number of parameters related to photosynthetic efficiency in oxygenic photosynthetic organisms. The most important component for the estimation of photochemistry is the relative increase in fluorescence yield between dark-adapted samples which have a maximal capacity for photochemistry and a minimal fluorescence yield (F0) and light-saturated samples where photochemistry is saturated and fluorescence yield is maximal (Fm). However, when photosynthesis is saturated with a short (less than 50 micro(s)) flash of light, which induces only one photochemical turnover of photosystem II, the maximal fluorescence yield is significantly lower (Fsat) than when saturation is achieved with a millisecond duration multiturnover flash (Fm). To investigate the origins of the difference in fluorescence yield between these two conditions, our time-resolved fluorescence apparatus was modified to allow collection of picosecond time-resolved decay kinetics over a short time window immediately following a saturating single-turnover flash (Fsat) as well as after a multiturnover saturating pulse (Fm). Our data were analyzed with a global kinetic model based on an exciton radical pair equilibrium model for photosystem II. The difference between Fm and Fsat was modeled well by changing only the rate constant for quenching of excitation energy in the antenna of photosystem II. An antenna-based origin for the quenching was verified experimentally by the observation that addition of the antenna quencher 5-hydroxy-1,4-naphthoquinone to thylakoids under Fm conditions resulted in decay kinetics and modeled kinetic parameters very similar to those observed under Fsat conditions in the absence of added quinone. Our data strongly support the origin of low fluorescence yield at Fsat to be an antenna-based nonphotochemical quenching of excitation energy in photosystem II which has not usually been considered explicitly in calculations of photochemical and nonphotochemical quenching parameters. The implications of our data with respect to kinetic models for the excited-state dynamics of photosystem II and the practical applications of the fluorescence yield parameters Fm and Fsat to calculations of photochemical yield are discussed.

The relationship between the capacity to evolve oxygen and the variable fluorescence of chlorophyll after microsecond illumination in Chlorella, pea leaves and pea thylakoids

The relationship between the capacity to evolve oxygen following a flash of a few microseconds duration and the variable fluorescence of chlorophyll was determined in Chlorella, leaf disks of peas, and thylakoids of pea leaves. It was found that this relationship is linear in intact plant material, while it is nonlinear in thylakoids. However, in thylakoids, the recovery of the capacity to evolve oxygen does not track the decay of the variable yield of Chl a fluorescence. This nonlinear relationship in thylakoids results from a reduced rate in the oxygen evolution reaction with no change in the decay of the variable yield of Chl a fluorescence. From a new application of the matrix analysis method to oxygen flash yields, the S1'-->S2, S2'-->S3 and S3'-->S4-->S0 transitions are found to be slowed or delayed in pea thylakoids and this underlies the slower rate of the recovery of the capacity to evolve oxygen in thylakoids. It is concluded that in intact cells the QA-QB(-)-->QA QB- reaction was rate-limiting for the recovery of the capacity to evolve oxygen, the equal energy-transfer hypothesis and the sequential double-hit model are valid in pea leaves and that the apparent invalidity in thylakoids is actually due to slowing of reaction steps in the oxygen evolution mechanism.