Photosynthesis and negative entropy production

The widely held view that the maximum efficiency of a photosynthetic pigment system is given by the Carnot cycle expression (1 − T/T_r) for energy transfer from a hot bath (radiation at temperature T_r) to a cold bath (pigment system at temperature T) is critically examined and demonstrated to be inaccurate when the entropy changes associated with the microscopic process of photon absorption and photochemistry at the level of single photosystems are considered. This is because entropy losses due to excited state generation and relaxation are extremely small (ΔS ? T/T_r) and are essentially associated with the absorption-fluorescence Stokes shift. Total entropy changes associated with primary photochemistry for single photosystems are shown to depend critically on the thermodynamic efficiency of the process. This principle is applied to the case of primary photochemistry of the isolated core of higher plant photosystem I and photosystem II, which are demonstrated to have maximal thermodynamic efficiencies of ξ > 0.98 and ξ > 0.92 respectively, and which, in principle, function with negative entropy production. It is demonstrated that for the case of ξ > (1 − T/T_r) entropy production is always negative and only becomes positive when ξ < (1 − T/T_r).     

Reply to “Commentary on Photosynthesis and Negative Entropy Production by Jennings and coworkers” by J. Lavergne

It is argued that the chemical potential analogy does not provide useful information on the thermodynamics of photosystems, as the thermodynamic efficiency of an absorbed quantum is not considered. Instead, the approach based on either entropy balance or entropy flux considerations does provide this information. At high thermodynamic efficiencies, primary photochemistry can, in principle, violate the Second Law of Thermodynamics.     

Entropy consumption in primary photosynthesis

Knox and Parson have objected to our previous conclusion on possible negative entropy production during primary photochemistry, i.e., from photon absorption to primary charge separation, by considering a pigment system in which primary photochemistry is not specifically considered. This approach does not address our proposal. They suggest that when a pigment absorbs light and passes to an excited state, its entropy increases by hν/T. This point is discussed in two ways: (i) from considerations based on the energy gap law for excited state relaxation; (ii) using classical thermodynamics, in which free energy is introduced into the pigment (antenna) system by photon absorption. Both approaches lead us to conclude that the excited state and the ground state are isoentropic, in disagreement with Knox and Parson. A discussion on total entropy changes specifically during the charge separation process itself indicates that this process may be almost isoentropic and thus our conclusions on possible negentropy production associated with the sequence of reactions which go from light absorption to the first primary charge separation event, due to its very high thermodynamic efficiency, remain unchanged.     

The Carnot efficiency and plant photosystems

The concept that the Carnot efficiency places an upper limit of 0.60–0.75 on the thermodynamic efficiency of photosynthetic primary photochemistry is examined using a PSI-LHCI preparation. The maximal quantum efficiency was determined ≈0.99 which yielded a thermodynamic efficiency of at least 0.96, a value far above that predicted on the basis of the Carnot efficiency. The commonly presented reasoning leading to the Carnot efficiency idea was therefore critically examined. It is concluded that the conventional assumption that the excited/ground state pigments are ergodic is incorrect, as is the assumption that the pigment system, under illumination, is in equilibrium with the incident light field, at a black body temperature of T _r. It is concluded that the classical reasoning used to describe the thermodynamics of heat systems is not applicable to “photonic” systems such as plant photosystems.     

Entropy consumption in primary photosynthesis

Knox and Parson have objected to our previous conclusion on possible negative entropy production during primary photochemistry, i.e., from photon absorption to primary charge separation, by considering a pigment system in which primary photochemistry is not specifically considered. This approach does not address our proposal. They suggest that when a pigment absorbs light and passes to an excited state, its entropy increases by hnu/T. This point is discussed in two ways: (i) from considerations based on the energy gap law for excited state relaxation; (ii) using classical thermodynamics, in which free energy is introduced into the pigment (antenna) system by photon absorption. Both approaches lead us to conclude that the excited state and the ground state are isoentropic, in disagreement with Knox and Parson. A discussion on total entropy changes specifically during the charge separation process itself indicates that this process may be almost isoentropic and thus our conclusions on possible negentropy production associated with the sequence of reactions which go from light absorption to the first primary charge separation event, due to its very high thermodynamic efficiency, remain unchanged.     

Light Harvesting and State Transitions in Cyanobacteria

Cyanobacteria are oxygenic phototrophic prokaryotes and are considered to be the ancestors of chloroplasts. Their photosynthetic machinery is functionally equivalent in terms of primary photochemistry and photosynthetic electron transport. Fluorescence measurements and other techniques indicate that cyanobacteria, like plants, are capable of redirecting pathways of excitation energy transfer from light harvesting antennae to both photosystems. Cyanobacterial cells can reach two energetically different states, which are defined as “State 1” (obtained after preferential excitation of photosystem I) and “State 2” (preferential excitation of photosystem II). These states can be distinguished by static and time resolved fluorescence techniques. One of the most important conclusions reached so far is that the presence of both photosystems, as well as certain antenna components, are necessary for state transitions to occur. Spectroscopic evidence suggests that changes in the coupling state of the light harvesting antenna complexes (the phycobilisomes) to both photosystems occur during state transitions. The finding that the phycobilisome complexes are highly mobile on the surface of the thylakoid membrane (the mode of interaction with the thylakoid membrane is essentially unknown), has led to the proposal that they are in dynamic equilibrium with both photosystems and regulation of energy transfer is mediated by changes in affinity for either photosystem.     

The relationship between non-photochemical quenching of chlorophyll fluorescence and the rate of photosystem 2 photochemistry in leaves

It has been suggested previously that non-photochemical quenching of chlorophyll fluorescence is associated with a decrease in the rate of photosystem 2 (PS 2) photochemistry. In this study analyses of fluorescence yield changes, induced by flashes in leaves exhibiting different amounts of non-photochemical quenching of fluorescence, are made to determine the effect of non-photochemical excitation energy quenching processes on the rate of PS 2 photochemistry. It is demonstrated that both the high-energy state and the more slowly relaxing components of non-photochemical quenching reduce the rate of PS 2 photochemistry. Flash dosage response curves for fluorescence yield show that non-photochemical quenching processes effectively decrease the relative effective absorption cross-section for PS 2 photochemistry. It is suggested that non-photochemical quenching processes exert an effect on the rate of PS 2 photochemistry by increasing the dissipation of excitation energy by non-radiative processes in the pigment matrices of PS 2, which consequently results in a decrease in the efficiency of delivery of excitation energy for PS 2 photochemistry.     

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: a pulsed photoacoustic study of electron transfer in photosystem I reveals a similarity to bacterial reaction centers in both volume change and entropy

The thermodynamic properties of electron transfer in biological systems are far less known in comparison with that of their kinetics. In this paper the enthalpy and entropy of electron transfer in the purified photosystem I trimer complexes from Synechocystis sp. PCC 6803 have been studied, using pulsed time-resolved photoacoustics on the 1 micros time scale. The volume contraction of reaction centers of photosystem I, which results directly from the light-induced charge separation forming P(700+F(A)/F(B-) from the excited-state P700*, is determined to be -26 +/- 2 A3. The enthalpy of the above electron-transfer reaction is found to be -0.39 +/- 0.1 eV. Photoacoustic estimation of the quantum yield of photochemistry in the purified photosystem I trimer complex showed it to be close to unity. Taking the free energy of the above reaction as the difference of their redox potentials in situ allows us to calculate an apparent entropy change (TDeltaS) of +0.35 +/- 0.1 eV. These values of DeltaV and TDeltaS are similar to those of bacterial reaction centers. The unexpected sign of entropy of electron transfer is tentatively assigned, as in the bacterial case, to the escape of counterions from the surface of the particles. The apparent entropy change of electron transfer in biological system is significant and cannot be neglected.     

Limited sensitivity of pigment photo-oxidation in isolated thylakoids to singlet excited state quenching in photosystem II antenna

Light-induced pigment oxidation and its relation to excited state quenching in photosystems antennae have been investigated in isolated thylakoids. The results indicate that (i) chlorophyll oxidation takes place in two sequential steps. A slow initial phase is followed by a steep increase in the bleaching rate when more than one quarter of the chromophores are oxidised. (ii) During the initial slow phase, the carotenoid pool is bleached with an apparent rate which is about three times faster than that found for chlorophyll a and more than six times faster than that of chlorophyll b. (iii) Pigment bleaching has been observed both in photosystem I and photosystem II, and it has been possible to estimate a similar carotenoid bleaching rate in the two photosystems. (iv) The protection conferred by singlet state quenchers in the initial slow phase of pigment oxidation is modest. Taking into consideration that both the photosystems are subjected to the oxidative treatment, a somewhat larger protective effect than those estimated for photo-inhibition in thylakoids [S. Santabarbara, F.M. Garlaschi, G. Zucchelli, R.C. Jennings, Biochim. Biophys. Acta 1409 (1999) 165-170] can be computed, although it is less than 50% of the expected level on the basis of the observed reciprocity to the number of incident photons. (v) Pigment oxidation is associated with the loss of membrane ultra-structure, which is interpreted as originating from a decrease in grana stacking. The dynamics of loss of membrane ultra-structure parallel the phases observed for chlorophyll photo-bleaching.     

Cyanobacterial Acclimation to Photosystem I or Photosystem II Light

The organization and function of the photochemical apparatus of Synechococcus 6301 was investigated in cells grown under yellow and red light regimes. Broadband yellow illumination is absorbed preferentially by the phycobilisome (PBS) whereas red light is absorbed primarily by the chlorophyll (Chl) pigment beds. Since PBSs are associated exclusively with photosystem II (PSII) and most of the Chl with photosystem I (PSI), it follows that yellow and red light regimes will create an imbalance of light absorption by the two photosystems. The cause and effect relationship between light quality and photosystem stoichiometry in Synechococcus was investigated. Cells grown under red light compensated for the excitation imbalance by synthesis/assembly of more PBS-PSII complexes resulting in high PSII/PSI = 0.71 and high bilin/Chl = 1.30. The adjustment of the photosystem stoichiometry in red light-grown cells was necessary and sufficient to establish an overall balanced absorption of red light by PSII and PSI. Cells grown under yellow light compensated for this excitation imbalance by assembly of more PSI complexes, resulting in low PSII/PSI = 0.27 and low bilin/Chl = 0.42. This adjustment of the photosystem stoichiometry in yellow light-grown cells was necessary but not quite sufficient to balance the absorption of yellow light by the PBS and the Chl pigment beds. A novel excitation quenching process was identified in yellow light-grown cells which dissipated approximately 40% of the PBS excitation, thus preventing over-excitation of PSII under yellow light conditions. It is hypothesized that State transitions in O₂ evolving photosynthetic organisms may serve as the signal for change in the stoichiometry of photochemical complexes in response to light quality conditions.     

Temperature dependence of the photoresponse ofHalobacterium halobium

The temperature dependence of the phototactic behavior of bacteriorhodopsin-pigmentedHalobacterium halobium was measured separately toward attractive yellow-green and repellent violet light. A switchoff phenomenon was found for the former below ca. 27°C, while the response to the latter was constant down to about 7°C. This agreement between the transition temperature for the attractive photosystem with that observed in vitro for several properties of purple membrane, of which bacteriorhodopsin is the sole protein pigment, supports the idea that the attractive photosystem is associated with bacteriorhodopsin. Conversely, the absence of such a transition for the photosystem controlling photorepulsion confirms the separate identity of the two photosystems.     

He–Ne laser illumination ameliorates photochemical impairment in ultraviolet-B stressed-wheat seedlings via detoxifying ROS cytotoxicity

The protective effect and physiochemical mechanism of He-Ne laser illumination on photochemical impairment were evaluated by investigating chlorophyll fluorescence characteristics, photochemical activities of two photosystems, reactive oxygen species (ROS) levels and antioxidant enzyme activities in UV-B stressed-wheat (Triticum aestivum L.) seedlings. The results showed that enhanced UV-B stress significantly inhibited plant growth, reduced photosynthetic pigment content and antioxidant enzyme activities, while increased intracellular ROS levels. Meanwhile, UV-B stress also altered chlorophyll fluorescence characteristics and photochemical activities of seedlings. However, He-Ne laser illumination markedly improved photochemical activities and photosynthetic efficiency of two photosystems through detoxifying excessive ROS productions. Illumination with white fluorescent lamps (W), red light (R), or red light, then far-red light (R + FR) had not alleviated the inhibitory effect of UV-B stress on plant growth, suggesting that He-Ne laser illumination might be responsible for UV-B-stressed seedlings due to its regulation for intracellular ROS levels and plant oxidant/antioxidant balance. Furthermore, the laser alone also showed a positive impact on photochemical activities of photosystem I and photosystem II in plants.     

Optimization and evolution of light harvesting in photosynthesis: the role of antenna chlorophyll conserved between photosystem II and photosystem I

The efficiency of oxygenic photosynthesis depends on the presence of core antenna chlorophyll closely associated with the photochemical reaction centers of both photosystem II (PSII) and photosystem I (PSI). Although the number and overall arrangement of these chlorophylls in PSII and PSI differ, structural comparison reveals a cluster of 26 conserved chlorophylls in nearly identical positions and orientations. To explore the role of these conserved chlorophylls within PSII and PSI we studied the influence of their orientation on the efficiency of photochemistry in computer simulations. We found that the native orientations of the conserved chlorophylls were not optimal for light harvesting in either photosystem. However, PSII and PSI each contain two highly orientationally optimized antenna chlorophylls, located close to their respective reaction centers, in positions unique to each photosystem. In both photosystems the orientation of these optimized bridging chlorophylls had a much larger impact on photochemical efficiency than the orientation of any of the conserved chlorophylls. The differential optimization of antenna chlorophyll is discussed in the context of competing selection pressures for the evolution of light harvesting in photosynthesis.     

The role of light-harvesting complex I in excitation energy transfer from LHCII to photosystem I in Arabidopsis

Photosynthesis powers nearly all life on Earth. Light absorbed by photosystems drives the conversion of water and carbon dioxide into sugars. In plants, photosystem I (PSI) and photosystem II (PSII) work in series to drive the electron transport from water to NADP⁺. As both photosystems largely work in series, a balanced excitation pressure is required for optimal photosynthetic performance. Both photosystems are composed of a core and light-harvesting complexes (LHCI) for PSI and LHCII for PSII. When the light conditions favor the excitation of one photosystem over the other, a mobile pool of trimeric LHCII moves between both photosystems thus tuning their antenna cross-section in a process called state transitions. When PSII is overexcited multiple LHCIIs can associate with PSI. A trimeric LHCII binds to PSI at the PsaH/L/O site to form a well-characterized PSI–LHCI–LHCII supercomplex. The binding site(s) of the “additional” LHCII is still unclear, although a mediating role for LHCI has been proposed. In this work, we measured the PSI antenna size and trapping kinetics of photosynthetic membranes from Arabidopsis (Arabidopsis thaliana) plants. Membranes from wild-type (WT) plants were compared to those of the ΔLhca mutant that completely lacks the LHCI antenna. The results showed that “additional” LHCII complexes can transfer energy directly to the PSI core in the absence of LHCI. However, the transfer is about two times faster and therefore more efficient, when LHCI is present. This suggests LHCI mediates excitation energy transfer from loosely bound LHCII to PSI in WT plants.

The light-harvesting antennae of photosystem I facilitate energy transfer from trimeric light-harvesting complex II to photosystem I in the stroma lamellae membrane.     

Predicting Light Acclimation in Cyanobacteria from Nonphotochemical Quenching of Photosystem II Fluorescence,Which Reflects State Transitions in These Organisms

An important factor in photosynthetic ecophysiology is the light regime that a photobiont is acclimated to exploit. In a wide range of cyanobacteria and cyano-lichens, the easily measured fluorescence parameters, coefficient of nonphotochemical quenching of photosystem II variable fluorescence (qN) and nonphotochemical quenching, decline to a minimum near the acclimated growth light intensity. This characteristic pattern predicts the integrated light regime to which populations are acclimated, information that is particularly useful for cyanobacteria or cyano-lichens from habitats with highly variable light intensities. qN reflects processes that compete with photosystem II photochemistry for absorbed excitation energy. In cyanobacteria, we find no evidence for energy-dependent quenching mechanisms, which are the predominant components of qN in higher plants. Instead, in cyanobacteria, qN correlates closely with the excitation flow from the phycobilisome to photosystem I, indicating that qN reflects the state transition mechanism for equilibration of excitation from the phycobilisome to the two photosystems.     

On the nature of the photosynthetic energy storage monitored by photoacoustic spectroscopy

The photosynthetic energy storage yield of uncoupled thylakoid membranes was monitored by photoacoustic spectroscopy at various measuring beam intensities. The energy storage rate as evaluated by the half-saturation measuring beam intensity (i₅₀) was inhibited by 3-(3,4-dichlorophenyl)-1,1 dimethylurea, by heat inactivation or by artificial electron acceptors specific for photosystem I or photosystem II; and was activated by electron donors to photosystem I. The reactions involving both photosystems were all characterized by a similar maximal energy storage yield of 16±2 percent. The data could be interpreted if we assumed that the energy storage elicited by the photosystems at 35 Hz is detected at the level of the plastoquinone pool.Abbreviations PS photosystem - Tes N-Tris [hydroxymethl] methyl-2-aminoethanesulfonic acid - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCIP 2,6-dichlorophenolindophenol - FeCN potassium ferricyanide - DCBQ 2,5-dichlorobenzoquinone - TMPD N,N,N-tetramethyl-p-phenilenediamine     

Effects of foliar application of nitrogen on the photosynthetic performance and growth of two fescue cultivars under heat stress

The effects of nitrogen fertilization on the growth, photosynthetic pigment contents, gas exchange, and chlorophyll (Chl) fluorescence parameters in two tall fescue cultivars (Festuca arundinacea cv. Barlexas and Crossfire II) were investigated under heat stress at 38/30 °C (day/night) for two weeks. Shoot growth rate of two tall fescue cultivars declined significantly under heat stress, and N supply can improved the growth rates, especially for the Barlexas. Chl content, leaf net photosynthetic rate, stomatal conductance, water use efficiency, and the maximal efficiency of photosystem 2 photochemistry (F_v/F_m) also decreased less under heat stress by N supply, especially in Crossfire II. Moreover, cultivar variations in photosynthetic performance were associated with their different response to heat stress and nitrogen fertilization, which were evidenced by shoot growth rate and photosynthetic pigment contents.     

Association of fucoxanthin chlorophyll a/c-binding polypeptides with photosystems and phosphorylation in the centric diatom Cyclotella cryptica

Solubilization of thylakoid membranes of Cyclotella cryptica with dodecyl-beta maltoside followed by sucrose density gradient centrifugation or deriphate polyacrylamide gel electrophoresis resulted in the isolation of pigment protein complexes. These complexes were characterized by absorption and fluorescence spectroscopy, sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western immunoblotting using antisera against fucoxanthin chlorophyll a/c-binding proteins and the reaction center protein D2 of photosystem II. Sucrose density gradient centrifugation yielded four bands. Band 1 consisted of free pigments with minor amounts of fucoxanthin chlorophyll a/c-binding proteins. Bands 2, 3, and 4 represented a major fucoxanthin chlorophyll a/c-binding protein fraction, photosystem II, and photosystem I, respectively. Deriphate polyacrylamide gel electrophoresis gave rise to five bands, representing photosystem I, photosystem II, two fucoxanthin chlorophyll a/c-binding protein complexes, and a band mostly consisting of free pigments. In the Western immunoblotting experiments, the specific association of two fucoxanthin chlorophyll a/c-binding proteins, Fcp2 and Fcp4, to the photosystems could be demonstrated. In vivo experiments using antibodies against phosphothreonine residues and in vitro studies using [gamma-32P]ATP showed that fucoxanthin chlorophyll a/c binding-proteins of 22 kDa became phosphorylated.     

Increase in the rate of photosynthetic linear electron transport in cyanobacteruim <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 lacking phycobilisomes

Compensating changes in the pigment apparatus of photosynthesis that resulted from a complete loss of phycobilisomes (PBS) were investigated in the cells of a PAL mutant of cyanobacterium Synechocystis sp. PCC 6803. The ratio PBS/chlorophyll calculated on the basis of the intensity of bands in the action spectra of photosynthetic activity of two photosystems in the wild strain was 1: 70 for PSII and 1: 300 for PSI. Taking into consideration the number of chlorophyll molecules per reaction center in each photosystem, these ratios could be interpreted as association of PBS with dimers of PSII and trimers of PSI as well as greater dependence of PSII as compared with PSI on light absorption by PBS. The ratio PSI/PSII determined by photochemical cross-section of the reactions of two photosystems was 3.5: 1.0 for wild strain of Synechocystis sp. PCC 6803 and 0.7: 1.0 for the PAL mutant. A fivefold increase in the relative content of PSII in pigment apparatus corresponds to a 5-fold increase in the intensity of bands at 685 and 695 nm as related to the band of PSI at 726 nm recorded in low-temperature fluorescence spectrum of the PAL mutant. Inhibition of PSII with diuron resulted in a pronounced stimulation of chlorophyll fluorescence in the PAL mutant as compared to the wild strain of Synechocystis sp. PCC 6803; these data suggested an activation of electron transfer between PSII and PSI in the mutant cells. Thus, the lack of PBS in the mutant strain of Synechocystis sp. PCC 6803 was compensated for by the higher relative content of PSII in the pigment apparatus of photosynthesis and by a rise in the rate of linear electron transport.