Modeling the light-induced electric potential difference (ΔΨ), the pH difference (ΔpH) and the proton motive force across the thylakoid membrane in C3 leaves

A mathematical model to simulate the dynamics of photosynthetic light reactions under harmonically oscillating light

Alternating electric current and alternating electromagnetic fields revolutionized physics and engineering and led to many technologies that shape modern life. Despite these undisputable achievements that have been reached using stimulation by harmonic oscillations over centuries, applications in biology remain rare. Photosynthesis research is uniquely suited to unleash this potential because light can be modulated as a harmonic function, here sinus. Understanding the response of photosynthetic organisms to sinusoidal light is hindered by the complexity of dynamics that such light elicits, and by the mathematical apparatus required for understanding the signals in the frequency domain which, although well-established and simple, is outside typical curricula in biology. Here, we approach these challenges by presenting a mathematical model that was designed specifically to simulate the response of photosynthetic light reactions to light which oscillates with periods that often occur in nature. The independent variables of the model are the plastoquinone pool, the photosystem I donors, lumen pH, ATP, and the chlorophyll fluorescence (ChlF) quencher that is responsible for the qE non-photochemical quenching. Dynamics of ChlF emission, rate of oxygen evolution, and non-photochemical quenching are approximated by dependent model variables. The model is used to explain the essentials of the frequency-domain approaches up to the level of presenting Bode plots of frequency-dependence of ChlF. The model simulations were found satisfactory when compared with the Bode plots of ChlF response of the green alga Chlamydomonas reinhardtii to light that was oscillating with a small amplitude and frequencies between 7.8 mHz and 64 Hz.

Assessing key parameters in simultaneous simulation of rapid kinetics of chlorophyll a fluorescence and trans-thylakoid electric potential difference

Our study attempts to address the following questions: among numerous photosynthetic modules, which parameters notably influence the rapid chlorophyll fluorescence (ChlF) rise, the so-called O-J-I-P transient, in conjunction with the P515 signal, as these two records are easily obtained and widely used in photosynthesis research, and how are these parameters ranked in terms of their importance? These questions might be difficult to answer solely through experimental assays. Therefore, we employed an established photosynthesis model. Firstly, we utilized the model to simulate the measured rapid ChlF rise and P515 kinetics simultaneously. Secondly, we employed the sensitivity analysis (SA) tool by randomly altering model parameters to observe their effects on model output variables. Thirdly, we systematically identified significant parameters for both or one of the kinetics across various scenarios. A novel aspect of our study is the application of the Morris method, a global SA tool, to simultaneously assess the significance of model parameters in shaping both or one of the kinetics. The Morris SA technique enables the quantification of how much a specific parameter affects O-J-I-P transient during particular time intervals (e.g., J, I, and P steps). This allowed us to theoretically analyze which step is more significantly influenced by the parameter. In summary, our study contributes to the field by providing a comprehensive analysis of photosynthesis kinetics and emphasizing the importance of parameter selection in modelling this process. These findings can inform future research efforts aimed at improving photosynthesis models and advancing our understanding of photosynthetic processes.

From leaf to multiscale models of photosynthesis: applications and challenges for crop improvement

To keep up with the growth of human population and to circumvent deleterious effects of global climate change, it is essential to enhance crop yield to achieve higher production. Here we review mathematical models of oxygenic photosynthesis that are extensively used, and discuss in depth a subset that accounts for diverse approaches providing solutions to our objective. These include models (1) to study different ways to enhance photosynthesis, such as fine-tuning antenna size, photoprotection and electron transport; (2) to bioengineer carbon metabolism; and (3) to evaluate the interactions between the process of photosynthesis and the seasonal crop dynamics, or those that have included statistical whole-genome prediction methods to quantify the impact of photosynthesis traits on the improvement of crop yield. We conclude by emphasizing that the results obtained in these studies clearly demonstrate that mathematical modelling is a key tool to examine different approaches to improve photosynthesis for better productivity, while effective multiscale crop models, especially those that also include remote sensing data, are indispensable to verify different strategies to obtain maximized crop yields.

Screening rate constants in the simulation of rapid kinetics of chlorophyll a fluorescence using the Morris method

Chlorophyll a fluorescence, a sensitive and cost-effective probe, is widely used in photosynthetic research. Its rapid phase, occurring within 1 second under intense illumination, displays complex O-J-I-P transients, providing valuable insights into various aspects of photosynthesis. In addition to employing experimental approaches to measure the rapid Fluorescence Induction (FI) kinetics, mathematical modeling serves as a crucial tool for understanding the underlying mechanisms that drive FI dynamics. However, the significant uncertainty and arbitrary nature of selecting model parameters amplify concerns about the effectiveness of modeling tools in aiding photosynthesis research. Therefore, there is a need to gain a deeper understanding of how these models operate and how arbitrary parameter choices may influence their outcomes. In this study, we employed the Morris method, a global Sensitivity Analysis (SA) tool, to assess the significance of rate constants employed in an existing fluorescence model, particularly those linked to the entire electron transport chain, in shaping the rapid FI dynamics. In summary, utilizing the insights gained from the Morris SA allows for targeted refinement of the photosynthesis model, thereby improving our understanding of the complex processes inherent in photosynthetic systems.

Dynamic modulation of transthylakoid electric potential by chloroplast ATP synthases

The light-induced transthylakoid membrane potential (ΔΨm) can function as a driving force to help catalyzing the formation of ATP molecules, proving a tight connection between ΔΨm and the ATP synthase. Naturally, a question can be raised on the effects of altered functioning of ATP synthases on regulating ΔΨm, which is attractive in the area of photosynthetic research. Lots of findings, when making efforts of solving this difficulty, can offer an in-depth understanding into the mechanism behind. However, the functional network on modulating ΔΨm is highly interdependent. It is difficult to comprehend the consequences of altered activity of ATP synthases on adjusting ΔΨm because parameters that have influences on ΔΨm would themselves be affected by ΔΨm. In this work, a computer model was applied to check the kinetic changes in polarization/depolarization across the thylakoid membrane (TM) regulated by the modified action of ATP synthases. The computing data revealed that under the extreme condition by numerically "switching off" the action of the ATP synthase, the complete inactivation of ATP synthase would markedly impede proton translocation at the cytb6f complex. Concurrently, the KEA3 (CLCe) porter, actively pumping protons into the stroma, further contributes to achieving a sustained low level of ΔΨm. Besides, the quantitative consequences on every particular component of ΔΨm adjusted by the modified functioning of ATP synthases were also explored. By employing the model, we bring evidence from the theoretical perspective that the ATP synthase is a key factor in forming a transmembrane proton loop thereby maintaining a propriate steady-state ΔΨm to meet variable environmental conditions.

Solar energy conversion by photosystem II: principles and structures

Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.

Effect of ion fluxes on regulating the light-induced transthylakoid electric potential difference

The light-induced transthylakoid membrane potential (ΔΨ) can not only drive the ATP synthesis through the ATP-synthase in chloroplasts but serve as an essential modifier in the acclimation of photosynthesis to fluctuating light conditions. It has been manifested that during photosynthesis, the light-induced ΔΨ is responsive to multiple factors among which the ion channels/transporters (e.g., V-K⁺, VCCN1, and KEA3) are key to adjust the ion distribution on the two sides of the thylakoid membrane and hence shape the kinetics of ΔΨ. However, an in-depth mechanistic understanding of ion fluxes on adjusting the transthylakoid electric potentials is still unclear. This lack of a mechanistic understanding is due to the experimental difficulty of closely observing ion fluxes in vivo and also hacking the evolution of parameters in a highly intertwined photosynthetic network. In this work, a computer model was applied to investigate the roles of ion fluxes on adjusting transthylakoid electric potentials upon a temporal cycle of a period of high illumination followed by a dark-adapted phase. The computing data revealed that, firstly, upon illumination, the dissipation of the steady-ΔΨ by ∼10 mV is contributed from the V-K⁺-driven K⁺ flux whilst ∼8 mV of the steady-ΔΨ is dissipated by the VCCN1-pumped Cl^- flux, but there were no appreciable KEA3-evoked variations on ΔΨ; secondly, on transition from high light to darkness, V-K⁺ and KEA3 are serving as major contributors whereas VCCN1 taking a counterbalancing part in shaping a standard trace of ECS (electrochromic shift), which commonly shows a sharp fall to a minimum before returning to the baseline in darkness. Besides, the functional consequences on components of ΔΨ adjusted by every particular ion channel/transporter were also explored. By employing the model, we bring evidence that particular thylakoid-harbored proteins, namely V-K⁺, VCCN1, and KEA3, function by distinct mechanisms in the dynamic adjustment of electric potential, which might be mainly importnat under fluctuating light conditions.

Analyzing the effect of ion binding to the membrane-surface on regulating the light-induced transthylakoid electric potential (ΔΨm)

The transthylakoid membrane potential (ΔΨm) is essential because it can drive the ATP synthesis through the CF0-CF1 type of ATP-synthase in chloroplasts as an energetic equivalent similar to ΔpH. In addition, a high fraction of proton motive force (PMF) stored as the ΔΨm component is physiologically important in the acclimation of photosynthesis to environmental stresses. It has been shown that ΔΨm is the sum of the Donnan potential difference (ΔΨdn) and the diffusion potential difference (ΔΨd). Specifically, ΔΨdn, ΔΨd, and ΔΨm are strongly associated with the ionic activities near the membrane surface, particularly, the extent of ion binding to the charged/neutral sites adjacent to the membrane surface. However, an in-depth analysis of the effect of altered cationic binding to the membrane surface on adjusting the transthylakoid electric potentials (ΔΨdn, ΔΨd, and ΔΨm) is still missing. This lack of a mechanistic understanding is due to the experimental difficulty of closely observing cations binding to the membrane surface in vivo. In this work, a computer model was proposed to investigate the transthylakoid electric phenomena in the chloroplast focusing on the interaction between cations and the negative charges close to the membrane surface. By employing the model, we simulated the membrane potential and consequently, the measured ECS traces, proxing the ΔΨm, were well described by the computing results on continuous illumination followed by a dark-adapted period. Moreover, the computing data clarified the components of transthylakoid membrane potential, unraveled the functional consequences of altered cationic attachment to the membrane surface on adjusting the transthylakoid electric potential, and further revealed the key role played by Donnan potential in regulating the energization of the thylakoid membrane. The current model for calculating electric potentials can function as a preliminary network for the further development into a more detailed theoretical model by which multiple important variables involved in photosynthesis can be explored.

Photosynthesis dynamics and regulation sensed in the frequency domain

Foundations of photosynthesis research have been established mainly by studying the response of plants to changing light, typically to sudden exposure to a constant light intensity after dark acclimation or light flashes. This approach remains valid and powerful, but can be limited by requiring dark acclimation before time-domain measurements and often assumes that rate constants determining the photosynthetic response do not change between dark and light acclimation. We show that these limits can be overcome by measuring plant responses to sinusoidally modulated light of varying frequency. By its nature, such frequency-domain characterization is performed in light-acclimated plants with no need for prior dark acclimation. Amplitudes, phase shifts, and upper harmonic modulation extracted from the data for a wide range of frequencies can target different kinetic domains and regulatory feedbacks. The occurrence of upper harmonic modulation reflects nonlinear phenomena, including photosynthetic regulation. To support these claims, we measured chlorophyll fluorescence emission of the green alga Chlorella sorokiniana in light that was sinusoidally modulated in the frequency range 1000-0.001 Hz. Based on these experimental data and numerical as well as analytical mathematical models, we propose that frequency-domain measurements can become a versatile tool in plant sensing.

Proton motive force in plant photosynthesis dominated by ΔpH in both low and high light

The proton motive force (pmf) across the thylakoid membrane couples photosynthetic electron transport and ATP synthesis. In recent years, the electrochromic carotenoid and chlorophyll absorption band shift (ECS), peaking ∼515 nm, has become a widely used probe to measure pmf in leaves. However, the use of this technique to calculate the parsing of the pmf between the proton gradient (ΔpH) and electric potential (Δψ) components remains controversial. Interpretation of the ECS signal is complicated by overlapping absorption changes associated with violaxanthin de-epoxidation to zeaxanthin (ΔA505) and energy-dependent nonphotochemical quenching (qE; ΔA535). In this study, we used Arabidopsis (Arabidopsis thaliana) plants with altered xanthophyll cycle activity and photosystem II subunit S (PsbS) content to disentangle these overlapping contributions. In plants where overlap among ΔA505, ΔA535, and ECS is diminished, such as npq4 (lacking ΔA535) and npq1npq4 (also lacking ΔA505), the parsing method implies the Δψ contribution is virtually absent and pmf is solely composed of ΔpH. Conversely, in plants where ΔA535 and ECS overlap is enhanced, such as L17 (a PsbS overexpressor) and npq1 (where ΔA535 is blue-shifted to 525 nm) the parsing method implies a dominant contribution of Δψ to the total pmf. These results demonstrate the vast majority of the pmf attributed by the ECS parsing method to Δψ is caused by ΔA505 and ΔA535 overlap, confirming pmf is dominated by ΔpH following the first 60 s of continuous illumination under both low and high light conditions. Further implications of these findings for the regulation of photosynthesis are discussed.

The role of Cytochrome b6f in the control of steady-state photosynthesis: a conceptual and quantitative model

Here, we present a conceptual and quantitative model to describe the role of the Cytochrome [Formula: see text] complex in controlling steady-state electron transport in [Formula: see text] leaves. The model is based on new experimental methods to diagnose the maximum activity of Cyt [Formula: see text] in vivo, and to identify conditions under which photosynthetic control of Cyt [Formula: see text] is active or relaxed. With these approaches, we demonstrate that Cyt [Formula: see text] controls the trade-off between the speed and efficiency of electron transport under limiting light, and functions as a metabolic switch that transfers control to carbon metabolism under saturating light. We also present evidence that the onset of photosynthetic control of Cyt [Formula: see text] occurs within milliseconds of exposure to saturating light, much more quickly than the induction of non-photochemical quenching. We propose that photosynthetic control is the primary means of photoprotection and functions to manage excitation pressure, whereas non-photochemical quenching functions to manage excitation balance. We use these findings to extend the Farquhar et al. (Planta 149:78-90, 1980) model of [Formula: see text] photosynthesis to include a mechanistic description of the electron transport system. This framework relates the light captured by PS I and PS II to the energy and mass fluxes linking the photoacts with Cyt [Formula: see text], the ATP synthase, and Rubisco. It enables quantitative interpretation of pulse-amplitude modulated fluorometry and gas-exchange measurements, providing a new basis for analyzing how the electron transport system coordinates the supply of Fd, NADPH, and ATP with the dynamic demands of carbon metabolism, how efficient use of light is achieved under limiting light, and how photoprotection is achieved under saturating light. The model is designed to support forward as well as inverse applications. It can either be used in a stand-alone mode at the leaf-level or coupled to other models that resolve finer-scale or coarser-scale phenomena.

Model quantification of the light-induced thylakoid membrane processes in Synechocystis sp. PCC 6803 in vivo and after exposure to radioactive irradiation

Measurements of OJIP-SMT patterns of fluorescence induction (FI) in Synechocystis sp. PCC 6803 (Synechocystis) cells on a time scale up to several minutes were mathematically treated within the framework of thylakoid membrane (T-M) model (Belyaeva et al., Photosynth Res 140:1-19, 2019) that was renewed to account for the state transitions effects. Principles of describing electron transfer in reaction centers of photosystems II and I (PSII and PSI) and cytochrome b₆f complex remained unchanged, whereas parameters for dissipative reactions of non-radiative charge recombination were altered depending on the oxidation state of Q_B-site (neutral, reduced by one electron, empty, reduced by two electrons). According to our calculations, the initial content of plastoquinol (PQH₂) in the total quinone pool of Synechocystis cells adapted to darkness for 10 min ranged between 20 and 40%. The results imply that the PQ pool mediates photosynthetic and respiratory charge flows. The redistribution of PBS antenna units responsible for the increase of Chl fluorescence in cyanobacteria (qT_{2 → 1}) upon state 2 → 1 transition or the fluorescence lowering (qT_{1 → 2}) due to state 1 → 2 transition were described in the model by exponential functions. Parameters of dynamically changed effective cross section were found by means of simulations of OJIP-SMT patterns observed on Synechocystis cells upon strong (3000 μmol photons m^-2s^-1) and moderate (1000 μmol photons m^-2s^-1) actinic light intensities. The corresponding light constant values kL_ΣAnt = 1.2 ms^-1 and 0.4 ms^-1 define the excitation of total antenna pool dynamically redistributed between PSII and PSI reaction centers. Although the OCP-induced quenching of antenna excitation is not involved in the model, the main features of the induction signals have been satisfactorily explained. In the case of strong illumination, the effective cross section decreases by approximately 33% for irradiated Synechocystis cells as compared to untreated cells. Under moderate light, the irradiated Synechocystis cells showed in simulations the same cross section as the untreated cells. The thylakoid model renewed with state transitions description allowed simulation of fluorescence induction OJIP-SMT curves detected on time scale from microseconds to minutes.

Photosynthesis: basics, history and modelling

BACKGROUND

With limited agricultural land and increasing human population, it is essential to enhance overall photosynthesis and thus productivity. Oxygenic photosynthesis begins with light absorption, followed by excitation energy transfer to the reaction centres, primary photochemistry, electron and proton transport, NADPH and ATP synthesis, and then CO2 fixation (Calvin-Benson cycle, as well as Hatch-Slack cycle). Here we cover some of the discoveries related to this process, such as the existence of two light reactions and two photosystems connected by an electron transport 'chain' (the Z-scheme), chemiosmotic hypothesis for ATP synthesis, water oxidation clock for oxygen evolution, steps for carbon fixation, and finally the diverse mechanisms of regulatory processes, such as 'state transitions' and 'non-photochemical quenching' of the excited state of chlorophyll a.

SCOPE

In this review, we emphasize that mathematical modelling is a highly valuable tool in understanding and making predictions regarding photosynthesis. Different mathematical models have been used to examine current theories on diverse photosynthetic processes; these have been validated through simulation(s) of available experimental data, such as chlorophyll a fluorescence induction, measured with fluorometers using continuous (or modulated) exciting light, and absorbance changes at 820 nm (ΔA820) related to redox changes in P700, the reaction centre of photosystem I.

CONCLUSIONS

We highlight here the important role of modelling in deciphering and untangling complex photosynthesis processes taking place simultaneously, as well as in predicting possible ways to obtain higher biomass and productivity in plants, algae and cyanobacteria.

Chloride involvement in the synthesis, functioning and repair of the photosynthetic apparatus in vivo

Cl^- has long been known as a micronutrient for oxygenic photosynthetic resulting from its role an essential cofactor for photosystem II (PSII). Evidence on the in vivo Cl^- distribution in Spinacia oleracea leaves and chloroplasts shows that sufficient Cl^- is present for the involvement in PSII function, as indicated by in vitro studies on, among other organisms, S. oleracea PsII. There is also sufficient Cl^- to function, with K⁺ , in parsing the H⁺ electrochemical potential difference (proton motive force) across the illuminated thylakoid membrane into electrical potential difference and pH difference components. However, recent in vitro work on PSII from S. oleracea shows that oxygen evolving complex (OEC) synthesis, and resynthesis after photodamage, requires significantly higher Cl^- concentrations than would satisfy the function of assembled PSII O₂ evolution of the synthesised PSII with the OEC. The low Cl^- affinity of OEC (re-)assembly could be a component limiting the rate of OEC (re-)assembly.

Photosynthetic Response Mechanism of Soil Salinity-Induced Cross-Tolerance to Subsequent Drought Stress in Tomato Plants

Soil salinization and water shortage cause ion imbalance and hyperosmoticity in plant cells, adversely impairing photosynthesis efficiency. How soil salinity-induced photosynthetic acclimation influences the cross-tolerance to drought conditions represents a promising research topic. This study was to reveal the photosynthetic mechanism of soil salinity-induced resistance to the subsequent drought stress in tomato leaves through comprehensive photosynthesis-related spectroscopy analysis. We conducted soil salinity pretreatment and subsequent drought stress experiments, including irrigation with 100 mL water, 100 mL 100 mM NaCl solution (NaCl100), 50 mL water, and 50 mL 100 mM NaCl solution (NaCl50) for five days, followed by five-day drought stress. The results showed that soil salinity treatment by NaCl decreased the rate of photosynthetic gas exchange but enhanced CO₂ assimilation, along with photosystem II [PS(II)] and photosystem I [PS(I)] activity and photochemical efficiency in tomato plants compared with water pretreatment after subsequent drought stress. NaCl100 and NaCl50 had the capacity to maintain non-photochemical quenching (NPQ) of chlorophyll fluorescence and the cyclic electron (CEF) flow around PSI in tomato leaves after being subjected to subsequent drought stress, thus avoiding the decrease of photosynthetic efficiency under drought conditions. NaCl100 and NaCl50 pretreatment induced a higher proton motive force (pmf) and also alleviated the damage to the thylakoid membrane and adenosine triphosphate (ATP) synthase of tomato leaves caused by subsequent drought stress. Overall, soil salinity treatment could enhance drought resistance in tomato plants by inducing NPQ, maintaining CEF and pmf that tradeoff between photoprotection and photochemistry reactions. This study also provides a photosynthetic perspective for salt and drought cross-tolerance.

Variable fluorescence of closed photochemical reaction centers

Chlorophyll fluorescence induction during 0.4 to 200 ms multiple-turnover pulses (MTP) was measured in parallel with O₂ evolution induced by the MTP light. Additionally, a saturating single-turnover flash (STF) was applied at the end of each MTP and the total MTP +STF O₂ evolution was measured. Quantum yield of O₂ evolution during the MTP transients was calculated and related to the number of open PSII centers, found from the STF O₂ evolution. Proportionality between the number of open PSII and their running photochemical activity showed the quantum yield of open PSII remained constant independent of the closure of adjacent centers. During the induction, total fluorescence was partitioned between F_o of all the open centers and F_c of all the closed centers. The fluorescence yield of a closed center was 0.55 of the final F_m while less than a half of the centers were closed, but later increased, approaching F_m to the end of the induction. In the framework of the antenna/radical pair equilibrium model, the collective rise of the fluorescence of centers closed earlier during the induction is explained by an electric field, facilitating return of excitation energy from the Pheo^- P680⁺ radical pair to the antenna.

Oxidation of P700 Induces Alternative Electron Flow in Photosystem I in Wheat Leaves

Oxygen (O₂)-evolving photosynthetic organisms oxidize the reaction center chlorophyll, P700, in photosystem I (PSI) to suppress the production of reactive oxygen species. The oxidation of P700 is accompanied by alternative electron flow in PSI (AEF-I), which is not required for photosynthetic linear electron flow (LEF). To characterize AEF-I, we compared the redox reactions of P700 and ferredoxin (Fd) during the induction of carbon dioxide (CO₂) assimilation in wheat leaves, using dark-interval relaxation kinetics analysis. Switching on an actinic light (1000 μmol photons m⁻² s⁻¹) at ambient CO₂ partial pressure of 40 Pa and ambient O₂ partial pressure of 21 kPa gradually oxidized P700 (P700⁺) and enhanced the reduction rate of P700⁺ (vP700) and oxidation rate of reduced Fd (vFd). The vFd showed a positive linear relationship with an apparent photosynthetic quantum yield of PSII (Y[II]) originating at point zero; the redox turnover of Fd is regulated by LEF via CO₂ assimilation and photorespiration. The vP700 also showed a positive linear relationship with Y(II), but the intercept was positive, not zero. That is, the electron flux in PSI included the electron flux in AEF-I in addition to that in LEF. This indicates that the oxidation of P700 induces AEF-I. We propose a possible mechanism underlying AEF-I and its physiological role in the mitigation of oxidative damage.

Analyzing both the fast and the slow phases of chlorophyll a fluorescence and P700 absorbance changes in dark-adapted and preilluminated pea leaves using a Thylakoid Membrane model

The dark-to-light transitions enable energization of the thylakoid membrane (TM), which is reflected in fast and slow (OJIPSMT or OABCDE) stages of fluorescence induction (FI) and P700 oxidoreduction changes (ΔA810). A Thylakoid Membrane model (T-M model), in which special emphasis has been placed on ferredoxin-NADP+-oxidoreductase (FNR) activation and energy-dependent qE quenching, was applied for quantifying the kinetics of FI and ΔA810. Pea leaves were kept in darkness for 15 min and then the FI and ΔA810 signals were measured upon actinic illumination, applied either directly or after a 10-s light pulse coupled with a subsequent 10-s dark interval. On the time scale from 40 µs to 30 s, the parallel T-M model fittings to both FI and ΔA810 signals were obtained. The parameters of FNR activation and the buildup of qE quenching were found to differ for dark-adapted and preilluminated leaves. At the onset of actinic light, photosystem II (PSII) acceptors were oxidized (neutral) after dark adaptation, while the redox states with closed and/or semiquinone QA(-)QB(-) forms were supposedly generated after preillumination, and did not relax within the 10 s dark interval. In qE simulations, a pH-dependent Hill relationship was used. The rate constant of heat losses in PSII antenna kD(t) was found to increase from the basic value kDconst, at the onset of illumination, to its maximal level kDvar due to lumenal acidification. In dark-adapted leaves, a low value of kDconst of ∼ 2 × 106 s-1 was found. Simulations on the microsecond to 30 s time scale revealed that the slow P-S-M-T phases of the fluorescence induction were sensitive to light-induced FNR activation and high-energy qE quenching. Thus, the corresponding time-dependent rate constants kD(t) and kFNR(t) change substantially upon the release of electron transport on the acceptor side of PSI and during the NPQ development. The transitions between the cyclic and linear electron transport modes have also been quantified in this paper.

Genetic characterization of a flap1 null mutation in Arabidopsis npq4 and pgr5 plants suggests that the regulatory role of FLAP1 involves the control of proton homeostasis in chloroplasts

Precise control of the proton concentration gradient across thylakoid membranes (ΔpH) is essential for photosynthesis and its regulation because the gradient contributes to the generation of the proton motive force used for ATP synthesis and also for the fast and reversible induction of non-photochemical quenching (NPQ) to avoid photoinhibition and photodamage. However, the regulatory mechanism(s) controlling ΔpH in response to fluctuating light has not been fully elucidated. We previously described a new NPQ-regulatory chloroplastic protein, Fluctuating-Light-Acclimation Protein1 (FLAP1), which is important for plant growth and modulation of ΔpH under fluctuating light conditions. For this report, we further characterized FLAP1 activity by individually crossing an Arabidopsis flap1 mutant with npq4 and pgr5 plants; npq4 is defective in PsbS-dependent NPQ, and pgr5 is defective in induction of steady-state proton motive force (pmf) and energy-dependent quenching (qE). Both npq4 and npq4 flap1 exhibited similar NPQ kinetics and other photosynthetic parameters under constant or fluctuating actinic light. Conversely, pgr5 flap1 had recovered NPQ, photosystem II quantum yield and growth under fluctuating light, each of which was impaired in pgr5. Together with other data, we propose that FLAP1 activity controls proton homeostasis under steady-state photosynthesis to manipulate luminal acidification levels appropriately to balance photoprotection and photochemical processes.

Formation of Proton Motive Force Under Low-Aeration Alkaline Conditions in Alkaliphilic Bacteria

In Mitchell’s chemiosmotic theory, a proton (H⁺) motive force across the membrane (Δp), generated by the respiratory chain, drives F₁F_o-ATPase for ATP production in various organisms. The bulk-base chemiosmotic theory cannot account for ATP production in alkaliphilic bacteria. However, alkaliphiles thrive in environments with a H⁺ concentrations that are one-thousandth (ca. pH 10) the concentration required by neutralophiles. This situation is similar to the production of electricity by hydroelectric turbines under conditions of very limited water. Alkaliphiles manage their metabolism via various strategies involving the cell wall structure, solute transport systems and molecular mechanisms on the outer surface membrane. Our experimental results indicate that efficient ATP production in alkaliphilic Bacillus spp. is attributable to a high membrane electrical potential (ΔΨ) generated for an attractive force for H⁺ on the outer surface membrane. In addition, the enhanced F₁F_o-ATPase driving force per H⁺ is derived from the high ΔΨ. However, it is difficult to explain the reasons for high ΔΨ formation based on the respiratory rate. The Donnan effect (which is observed when charged particles that are unable to pass through a semipermeable membrane create an uneven electrical charge) likely contributes to the formation of the high ΔΨ because the intracellular negative ion capacities of alkaliphiles are much higher than those of neutralophiles. There are several variations in the adaptation to alkaline environments by bacteria. However, it could be difficult to utilize high ΔΨ in the low aeration condition due to the low activity of respiration. To explain the efficient ATP production occurring in H⁺-less and air-limited environments in alkaliphilic bacteria, we propose a cytochrome c-associated “H⁺ capacitor mechanism” as an alkaline adaptation strategy. As an outer surface protein, cytochrome c-550 from Bacillusclarkii possesses an extra Asn-rich segment between the region anchored to the membrane and the main body of the cytochrome c. This structure may contribute to the formation of the proton-binding network to transfer H⁺ at the outer surface membrane in obligate alkaliphiles. The H⁺ capacitor mechanism is further enhanced under low-aeration conditions in both alkaliphilic Bacillus spp. and the Gram-negative alkaliphile Pseudomonas alcaliphila.

Effect of carbon limitation on photosynthetic electron transport in Nannochloropsis oculata

This study describes the impacts of inorganic carbon limitation on the photosynthetic efficiency and operation of photosynthetic electron transport pathways in the biofuel-candidate microalga Nannochloropsis oculata. Using a combination of highly-controlled cultivation setup (photobioreactor), variable chlorophyll a fluorescence and transient spectroscopy methods (electrochromic shift (ECS) and P₇₀₀ redox kinetics), we showed that net photosynthesis and effective quantum yield of Photosystem II (PSII) decreased in N. oculata under carbon limitation. This was accompanied by a transient increase in total proton motive force and energy-dependent non-photochemical quenching as well as slightly elevated respiration. On the other hand, under carbon limitation the rapid increase in proton motive force (PMF, estimated from the total ECS signal) was also accompanied by reduced conductivity of ATP synthase to protons (estimated from the rate of ECS decay in dark after actinic illumination). This indicates that the slow operation of ATP synthase results in the transient build-up of PMF, which leads to the activation of fast energy dissipation mechanisms such as energy-dependent non-photochemical quenching. N. oculata also increased content of lipids under carbon limitation, which compensated for reduced NAPDH consumption during decreased CO₂ fixation. The integrated knowledge of the underlying energetic regulation of photosynthetic processes attained with a combination of biophysical methods may be used to identify photo-physiological signatures of the onset of carbon limitation in microalgal cultivation systems, as well as to potentially identify microalgal strains that can better acclimate to carbon limitation.

Modeling the light-induced electric potential difference ΔΨ across the thylakoid membrane based on the transition state rate theory

In photosynthesis, electron transport-coupled proton movement initiates the formation of the light-induced electric potential difference, ΔΨ, across the thylakoid membrane (TM). Ions are transported across the TM to counterbalance the charge of protons accumulated in the lumen. The objective of this work is to construct range of mathematical models for simulation of ΔΨ, using the transition state rate theory (TSRT) for description of movement of ions through the channels. The TSRT considers either single-ion (TSRT-SI) or multi-ion occupancy (TSRT-MI) in the channels. Movement of ions through the channel pore is described by means of energy barriers and binding sites; ions move in and out of vacant sites with rate constants that depend on the barrier heights and well depths, as well as on the interionic repulsion in TSRT-MI model. Three energy motifs are used to describe the TSRT-SI model: two-barrier one-site (2B1S), three-barrier two-site (3B2S), and four-barrier three-site (4B3S). The 3B2S energy motif is used for the TSRT-MI model. The accumulation of cations due to the TM surface negative fixed charges is also taken into account. A model employing the electro-diffusion theory instead of the TSRT is constructed for comparison. The dual wavelength transmittance signal (ΔA515-560nm) measuring the electrochromic shift (ECS) provides a proxy for experimental light-induced ΔΨ. The simulated ΔΨ traces qualitatively agree with the measured ECS traces. The models can simulate different channel conducting regimes and assess their impact on ΔΨ. The ionic flux coupling in the TSRT-MI model suggests that an increase in the internal or external K⁺ concentration may block the outward or the inward Mg²⁺ current, respectively.

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.