Photoacoustic spectroscopy of Anacystis nidulans. III. Detection of photosynthetic activities

Cyanobacterial Bioenergetics in Relation to Cellular Growth and Productivity

Cyanobacteria, the evolutionary originators of oxygenic photosynthesis, have the capability to convert CO2, water, and minerals into biomass using solar energy. This process is driven by intricate bioenergetic mechanisms that consist of interconnected photosynthetic and respiratory electron transport chains coupled. Over the last few decades, advances in physiochemical analysis, molecular genetics, and structural analysis have enabled us to gain a more comprehensive understanding of cyanobacterial bioenergetics. This includes the molecular understanding of the primary energy conversion mechanisms as well as photoprotective and other dissipative mechanisms that prevent photodamage when the rates of photosynthetic output, primarily in the form of ATP and NADPH, exceed the rates that cellular assimilatory processes consume these photosynthetic outputs. Despite this progress, there is still much to learn about the systems integration and the regulatory circuits that control expression levels for optimal cellular abundance and activity of the photosynthetic complexes and the cellular components that convert their products into biomass. With an improved understanding of these regulatory principles and mechanisms, it should be possible to optimally modify cyanobacteria for enhanced biotechnological purposes.

Characterization of the structural changes and photochemical activity of photosystem I under Al(3+) effect

The photochemical activity of photosystem I (PSI) as affected by Al(3+) was investigated in thylakoid membranes and PSI submembrane fractions isolated from spinach. Biophysical and biochemical techniques such as oxygen uptake, light induced absorbance changes at 820nm, chlorophyll fluorescence emission, SDS-polyacrylamide gel electrophoresis, and FTIR spectroscopy have been used to analyze the sites and action modes of this cation on the PSI complex. Our results showed that Al(3+) above 3mM induces changes in the redox state of P700 reflected by an increase of P700 photooxidation phase and a delay of the slower rate of P700 re-reduction which reveals that Al(3+) exerted an inhibitory action at the donor side of PSI especially at plastocyanin (PC). Furthermore, results of P700 photooxidation monitored in the presence of DCMU with or without MV suggested that the same range of Al(3+) concentrations impairs the photochemical reaction centers (RC) of PSI, as shown by the decline in the amount of active population of P700, and disrupts the charge separation between P700 and the primary electron acceptor A0 leading to the inhibition of electron transfer at the acceptor side of PSI. These inhibitory actions were also accompanied by an impairment of the energy transfer from light harvesting complex (LHCI) to RC of PSI, following the disconnection of LHCI antenna as illustrated by an enhancement of chlorophyll fluorescence emission spectra at low temperature (77K). The above results coincided with FTIR measurements that indicated a conformational change of the protein secondary structures in PSI complex where 25% of α-helix was converted into β-sheet, β-antiparallel and turn structures. These structural changes in PSI complex proteins are closely related with the alteration photochemical activity of PSI including the inhibition of the electron transport through both acceptor and donor sides of PSI.

Protective action of spermine and spermidine against photoinhibition of photosystem I in isolated thylakoid membranes

The photo-stability of photosystem I (PSI) is of high importance for the photosynthetic processes. For this reason, we studied the protective action of two biogenic polyamines (PAs) spermine (Spm) and spermidine (Spd) on PSI activity in isolated thylakoid membranes subjected to photoinhibition. Our results show that pre-loading thylakoid membranes with Spm and Spd reduced considerably the inhibition of O2 uptake rates, P700 photooxidation and the accumulation of superoxide anions (O2(-)) induced by light stress. Spm seems to be more effective than Spd in preserving PSI photo-stability. The correlation of the extent of PSI protection, photosystem II (PSII) inhibition and O2(-) generation with increasing Spm doses revealed that PSI photo-protection is assumed by two mechanisms depending on the PAs concentration. Given their antioxidant character, PAs scavenge directly the O2(-) generated in thylakoid membranes at physiological concentration (1 mM). However, for non-physiological concentration, the ability of PAs to protect PSI is due to their inhibitory effect on PSII electron transfer.

Effect of biogenic polyamine spermine on the structure and function of photosystem I

We located the binding sites of spermine (Spm) to PSI sub-membrane proteins and the impact of this interaction on the photoprotection of PSI activity, using spectroscopic methods and molecular modeling. Our results showed that at high Spm content the polyamine binds PSI polypeptides through H-bonding and induces major protein conformational changes with the reduction of α-helix from 52% to 42% and an increase of the β-sheet from 26% to 29%. However, polyamine does not affect significantly the photooxidizable P700 in control sample and considerably protects it against strong illumination. On the contrary, protein conformational changes coincide with an important inhibition of O2 uptake rates by polyamine, which revealed that the protein of the PSI donor side plastocyanin is a main target for Spm inhibition. The photoprotection of PSI photochemical activity may be due to the stabilization of the PSI stromal polypeptides by Spm as shown by the docking results. Spm binds to different amino acids with hydrophilic and hydrophobic characters, while the presence of several H-bondings stabilizes Spm-PSI complexation.

Monitoring electron transfer by photoacoustic spectroscopy in native and immobilized thylakoid membranes

Photoacoustic spectroscopy was used to monitor photo synthetic electron transfer in native and immobilized thylakoid membranes. The photoacoustic parameter phi(r)' (the percentage of absorbed energy that is stored in photo chemical intermediates) and i(50) (the half-saturation modulated light intensity) were directly correlated to electron transfer rates. As previously shown, thylakoids immobilized in an albumin-glutaraldehyde matrix were more resistant to aging. The inhibitory effects of the immobilization procedure and of aging at 4 degrees C were detected as a decrease in i(50) values. In analogy with enzyme kinetic analysis, the effect could be characterized as a competitive type of inhibition. Photoacoustic measurements are performed in conditions similar to a working bioreactor cell with regards to the sample preparation.

Characterization of photosystem I from spinach: effect of solution pH

Our previous work has demonstrated the isolation of photosystem I (PSI) from spinach using ultrafiltration with a final purity of 84%. In order to get a higher purity of PSI and more importantly to develop a practical bioseparation process, key physiochemical properties of PSI and their dependence on operational parameters must be assessed. In this study, the effect of solution pH, one of the most important operating parameters for membrane process, on the property of PSI was examined. Following the isolation of crude PSI from spinach using n-dodecyl-beta-D: -maltoside as detergent, the isoelectric point, aggregation size, zeta potential, low-temperature fluorescence, atomic force microscopy imaging, secondary structure, and thermal stability were determined. Solution pH was found to have a significant effect on the activity, aggregation size and thermal stability of PSI. The results also suggested that the activity of PSI was related to its aggregation size.

Regulation of cyclic electron flow in C₃ plants: differential effects of limiting photosynthesis at ribulose-1,5-bisphosphate carboxylase/oxygenase and glyceraldehyde-3-phosphate dehydrogenase

Cyclic electron flow around photosystem I (CEF1) is thought to augment chloroplast ATP production to meet metabolic needs. Very little is known about the induction and regulation of CEF1. We investigated the effects on CEF1 of antisense suppression of the Calvin-Benson enzymes glyceraldehyde-3-phosphate dehydrogenase (gapR), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit (SSU), in tobacco (Nicotiana tabacum cv. Wisconsin 38). The gapR, but not ssuR, mutants showed substantial increases in CEF1, demonstrating that specific intermediates, rather than slowing of assimilation, induce CEF1. Both types of mutant showed increases in steady-state transthylakoid proton motive force (pmf) and subsequent activation of the photoprotective q(E) response. With gapR, the increased pmf was caused both by up-regulation of CEF1 and down-regulation of the ATP synthase. In ssuR, the increased pmf was attributed entirely to a decrease in ATP synthase activity, as previously seen in wild-type plants when CO₂ levels were decreased. Comparison of major stromal metabolites in gapR, ssuR and hcef1, a mutant with decreased fructose 1,6-bisphosphatase activity, showed that neither the ATP/ADP ratio, nor major Calvin-Benson cycle intermediates can directly account for the activation of CEF1, suggesting that chloroplast redox status or reactive oxygen species regulate CEF1.

Rapid purification of photosystem I chlorophyll-binding proteins by differential centrifugation and vertical rotor

Photosystem I (PSI), which consists of a core complex and light-harvesting complex I (LHCI), is an important multisubunit pigment-protein complex located in the photosynthetic membranes of cyanobacteria, algae and plants. In the present study, we described a rapid method for isolation and purification of PSI and its subfractions. For purification of PSI, crude PSI was first prepared by differential centrifugation, which was applicable on a large scale at low cost. Then PSI was purified by sucrose gradient ultracentrifugation in a vertical rotor to reduce the centrifugation time from more than 20 h when using a swinging bucket rotor to only 3 h. Similarly, for subfractionation of PSI into the core complex and light-harvesting complex I, sucrose gradient ultracentrifugation in a vertical rotor was also used and it took only 4 h to obtain the PSI core, LHCI-680, and LHCI-730 at the same time. The resulting preparations were characterized by sodium dodecyl-sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), absorption spectroscopy, and 77 K fluorescence spectroscopy. In addition, their pigment composition was analyzed by high-performance liquid chromatography and the results showed that each Lhca could bind 1.5-1.6 luteins, 1.0 Violaxanthins, and 0.8-1.1 beta-carotenes on average, demonstrating that fewer carotenoids were released than with the slower traditional centrifugation. These results showed that the rapid isolation procedure, based on differential centrifugation and sucrose gradient ultracentrifugation in a vertical rotor, was efficient, and it should significantly facilitate preparation and studies of plant PSI. Moreover, the vertical rotor, rather than the swinging bucket rotor, may be a good choice for isolation of some other proteins.

Highly efficient photoactivation of Mn-depleted photosystem II by imidazole-liganded manganese complexes

The oxygen-evolving complex (OEC) of Mn-depleted photosystem II (PSII) can be reconstituted in the presence of exogenous Mn or a Mn complex under weak illumination, a process called photoactivation. Synthetic Mn complexes could provide a powerful system to analyze the assembly of the OEC. In this work, four mononuclear Mn complexes, [(terpy)2Mn(II)(OOCH3)] x 2 H2O (where terpy is 2,2':6',2''-terpyridine), Mn(II)(bzimpy)2, Mn(II)(bp)2(CH3CH2OH)2 [where bzimpy is 2,6-bis(2-benzimidazol-2-yl)pyridine] and [Mn(III)(HL)(L)(py)(CH3OH)]CH3OH (where py is pyridine) were used in photoactivation experiments. Measurements of the photoreduction of 2,6-dichorophenolindophenol and oxygen evolution demonstrate that photoactivation is more efficient when Mn complexes are used instead of MnCl2 in reconstructed PSII preparations. The most efficient recoveries of oxygen evolution and electron transport activities are obtained from a complex, [Mn(III)(HL)(L)(py)(CH3OH)]CH3OH, that contains both imidazole and phenol groups. Its recovery of the rate of oxygen evolution is as high as 79% even in the absence of the 33-kDa peptide. The imidazole ligands of the Mn complex probably accelerate P680*+ reduction and consequently facilitate the process of photoactivation. Also, the strong intermolecular hydrogen bond probably facilitates interaction with the Mn-depleted PSII via reorganization of the hydrogen-bonding network, and therefore promotes the recovery of oxygen evolution and electron transport activities.

The function of chloroplastic NAD(P)H dehydrogenase in tobacco during chilling stress under low irradiance

The function of chloroplastic NAD(P)H dehydrogenase (NDH) was examined by comparing a tobacco transformant (DeltandhB) in which the ndhB gene had been disrupted with its wild type, upon exposure to chilling temperature (4 degrees C) under low irradiance (100 micro mol m(-2) s(-1) PFD). During the chilling stress, the maximum photochemical efficiency of PSII (F(v)/F(m)) decreased markedly in both the wild type and DeltandhB. However, both F(v)/F(m) and P700(+), as well as the PSII-driven electron transport rate (ETR), in DeltandhB were lower than that in the wild type, implying that NDH-dependent cyclic electron flow around PSI functioned to protect the photosynthetic apparatus from chilling stress under low irradiance. Under the stress, non-photochemical quenching (NPQ), particularly the fast relaxing NPQ component (qf) and the de-epoxidized ratio of the xanthophyll cycle pigments, (A+Z)/(V+A+Z), were distinguishable in DeltandhB from those in the wild type. The lower NPQ in DeltandhB might be related to an inefficient proton gradient across thylakoid membranes (DeltapH) because of lacking an NDH-dependent cyclic electron flow around PSI at chilling temperature under low irradiance.

Energy storage in the photosynthetic electron-transport chain: An analogy with Michaelis-Menten kinetics

Simultaneous measurements of fluorescence and thermal emission have been performed by applying combined fluorescence and photoacoustic techniques on isolated thylakoids pretreated by prolonged illumination with saturating light. The traces were used to create Lineweaver-Burk type plots, proving clearly at least a formal analogy between the kinetics of the mechanisms governing fluorescence and thermal emission from isolated thylakoids and Michaelis-Menten kinetics of enzymatic reactions. Two characteristic parameters were calculated from them (energy storage and half-saturation light intensity) in order to obtain a basic, initial response of the photosynthetic apparatus functioning under photoinhibition stress.

Cyclic electron flow around photosystem I in C(3) plants. In vivo control by the redox state of chloroplasts and involvement of the NADH-dehydrogenase complex

Cyclic electron flow around photosystem (PS) I has been widely described in vitro in chloroplasts or thylakoids isolated from C(3) plant leaves, but its occurrence in vivo is still a matter of debate. Photoacoustic spectroscopy and kinetic spectrophotometry were used to analyze cyclic PS I activity in tobacco (Nicotiana tabacum cv Petit Havana) leaf discs illuminated with far-red light. Only a very weak activity was measured in air with both techniques. When leaf discs were placed in anaerobiosis, a high and rapid cyclic PS I activity was measured. The maximal energy storage in far-red light increased to 30% to 50%, and the half-time of the P(700) re-reduction in the dark decreased to around 400 ms; these values are comparable with those measured in cyanobacteria and C(4) plant leaves in aerobiosis. The stimulatory effect of anaerobiosis was mimicked by infiltrating leaves with inhibitors of mitochondrial respiration or of the chlororespiratory oxidase, therefore, showing that changes in the redox state of intersystem electron carriers tightly control the rate of PS I-driven cyclic electron flow in vivo. Measurements of energy storage at different modulation frequencies of far-red light showed that anaerobiosis-induced cyclic PS I activity in leaves of a tobacco mutant deficient in the plastid Ndh complex was kinetically different from that of the wild type, the cycle being slower in the former leaves. We conclude that the Ndh complex is required for rapid electron cycling around PS I.

Degradation of the photosystem I complex during photoinhibition

Exposure of isolated photosystem I (PSI) complexes to illumination (2300 microE m(-2) s(-1)) for various periods of time resulted in striking changes in their absorption spectra. A 6 nm blueshift of the absorption maximum in the red was detected after 100 min illumination. The fourth derivative of the absorption spectra verifies that the main change of the red peak was attributed to the 682 nm absorption band. Further, it was also shown that a shoulder in the absorption spectra located around 470 nm decreased after the first 5 min of illumination and almost disappeared after 40 min illumination, suggesting that chlorophyll b bound to light-harvesting complex I (LHCI) is also sensitive to excess light. A maximum inhibitory effect on the oxygen uptake rates and a strong stimulation were observed when the PSI complexes were exposed to illumination for about 20 and 40 min, respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that LHCI-680 started to degrade during the first 5 min of illumination and almost completely disappeared after 40 min of illumination. These observations demonstrated that LHCI was more sensitive to illumination than the PsaA/B subunits which also presented some degradation signs after 40 min illumination. In addition, insoluble-cohesive-denatured proteins also appeared between the stacking and resolving gel after prolonged illumination (100 min). A photoprotective function of LHCI for the PSI reaction center is proposed.

The Effect of Cholesterol on the Solution Structure of Proteins of Photosystem II. Protein Secondary Structure and Photosynthetic Oxygen Evolution

Cholesterol induces large perturbations in the physical properties of membranes, especially in the structural organization of the phospholipid bilayers and the aggregation and solubility of proteins at physiological temperatures. This study was designed to examine the interaction of cholesterol with lipid and proteins of chloroplasts photosystem II (PSII) submembrane fractions in air dried film at pH 6-7 with cholesterol concentrations of 0.01 to 20 mM. Fourier transform infrared difference spectroscopy with its self-deconvolution and second derivative methods as well as curve-fitting procedures are used, in order to determine the cholesterol binding mode, the protein conformational changes, and the structural properties of cholesterol-protein complexes. Correlations between the effect of cholesterol on the protein secondary structure and the rate of oxygen evolution in PSII are also established. Spectroscopic evidence showed that at low cholesterol concentration (0.01 and 0.1 mM), minor chol-protein and chol-lipid interactions (through hydrogen bonding) occur with no major perturbations of the protein secondary structure. As cholesterol concentration increases (5 and 10 and 20 mM), major alterations of the protein secondary structure are observed from that of the alpha-helix 47% (uncomplexed protein) to 43-39% (complexes) and the beta-sheet structure 18% (uncomplexed protein) to 22-26% (complexes). Those changes coincide with a partial decrease in the rate of the oxygen evolution (8-33%) is observed in the presence of cholesterol at high concentration. Copyright 1999 Academic Press.

Bill Arnold and calorimetric measurements of the quantum requirement of photosynthesis-once again ahead of his time

The approach of photocalorimetry to decide on the true quantum requirement of photosynthesis - one of the main issues of the research in the first half of the century and a source of a bitter debate - is described. Bill Arnold's original approach to get into the true answer is reflected from the point of view of present day calorimetric techniques.

Relationship between quenching of variable fluorescence and thermal dissipation in isolated thylakoid membranes: similar terminology and mathematical treatments may be used

Simultaneous measurements of chlorophyll fluorescence and thermal emission using photoacoustic spectroscopy have been done in isolated thylakoid membranes to study the relationship between the photochemical quenching of fluorescence (qPF) and energy storage measured in photoacoustic experiments. It is shown that energy storage can be interpreted as the photochemical quenching of a variable component of thermal dissipation termed qPH. The parameters qPF were similarly sensitive to light intensity as demonstrated by their half-saturation light intensity. However, the nonvariable part of thermal dissipation (Ho) represented a greater proportion of the maximal thermal dissipation yield in comparison with the corresponding non-variable component of fluorescence (Fo) as a result of the thermal energy losses occurring during electron transport. A residual qPH found when qPF was removed indicated the participation of cyclic photosystem I or photosystem II in the measured qPH. The participation of cyclic photosystem I was also suggested by a low constant K, representing the quasi equilibria between (re)oxidized and reduced photosystem II quinone acceptors as determined from the logarithmic plots of the hyperbolic relationship obtained between qPH and light intensity. It is finally concluded that the terminology and mathematical treatments used for fluorescence measurements can be applied to thermal dissipation.

Changes in the cyanobacterial photosynthetic apparatus during acclimation to macronutrient deprivation

When the cyanobacterium Synechococcus sp. Strain PCC 7942 is deprived of an essential macronutrient such as nitrogen, sulfur or phosphorus, cellular phycobiliprotein and chlorophyll contents decline. The level of β-carotene declines proportionately to chlorophyll, but the level of zeaxanthin increases relative to chlorophyll. In nitrogen- or sulfur-deprived cells there is a net degradation of phycobiliproteins. Otherwise, the declines in cellular pigmentation are due largely to the diluting effect of continued cell division after new pigment synthesis ceases and not to net pigment degradation. There was also a rapid decrease in O2 evolution when Synechococcus sp. Strain PCC 7942 was deprived of macronutrients. The rate of O2 evolution declined by more than 90% in nitrogen- or sulfur-deprived cells, and by approximately 40% in phosphorus-deprived cells. In addition, in all three cases the fluorescence emissions from Photosystem II and its antennae were reduced relative to that of Photosystem I and the remaining phycobilisomes. Furthermore, state transitions were not observed in cells deprived of sulfur or nitrogen and were greatly reduced in cells deprived of phosphorus. Photoacoustic measurements of the energy storage capacity of photosynthesis also showed that Photosystem II activity declined in nutrient-deprived cells. In contrast, energy storage by Photosystem I was unaffected, suggesting that Photosystem I-driven cyclic electron flow persisted in nutrient-deprived cells. These results indicate that in the modified photosynthetic apparatus of nutrient-deprived cells, a much larger fraction of the photosynthetic activity is driven by Photosystem I than in nutrient-replete cells.

Variable thermal dissipation in a Photosystem I submembrane fraction

Photoacoustic spectroscopy was used to study the thermal deactivation processes in a Photosystem I submembrane fraction isolated from spinach. A large part of the thermal dissipation was variable. The yield of this variable thermal emission depended on the redox state of the Photosystem. It increased with the measuring modulated light intensity coinciding with the gradual closure of the reaction centers. Thermal deactivation was maximal when the reaction centers were closed by a saturating illumination. Extrapolation of the data at zero light intensity indicated that the yield of non-variable thermal emission represented about 37% of the maximal emission. The presence of methylviologen as artificial electron acceptor decreased the yield of variable thermal emission whereas inhibition following heat stress treatments increased it. The significance of the variable and non-variable components of thermal dissipation is discussed and the measured energy storage is suggested to originate from the reduction of the plastoquinone pool during cyclic electron transport around Photosystem I.

Variable thermal emission and chlorophyll fluorescence in photosystem II particles

In photosynthetic systems, the absorbed light energy is used to generate electron transport or it is lost in the form of fluorescence and thermal emission. While fluorescence can be readily measured, the detection of thermal deactivation processes can be achieved by the photoacoustic technique. In that case, the pressure wave generated by the thermal deactivations in a sample irradiated with modulated light is detected by a sensitive microphone. The relationships between the yield of fluorescence and thermal emissions measured simultaneously were analyzed by using a spinach photosystem II (PSII)-enriched preparation. It is shown that the quenching of fluorescence due to the photochemical activity of the preparations (photochemical quenching) increases in proportion to the fraction of thermal deactivations that is not immediately released as heat but is stored in photochemical intermediates (energy-storage yield) as the intensity of the photoacoustic modulated measuring beam (35 Hz) is decreased. Maximal levels of fluorescence and thermal emissions were both decreased in similar proportions upon photoreduction of pheophytin (Pheo), the primary acceptor of PSII. The variable components of fluorescence and thermal emissions were strongly decreased upon depletion of Mn from the Mn complex that catalyzes water oxidation and were recovered proportionally during reconstitution with Mn2+ at various Mn2+/reaction center ratios. Finally, depletion of Mn from the Mn complex together with the Fe of the QA-Fe-QB complex that is composed of the secondary quinone acceptors of PSII resulted in an increased initial level of fluorescence Fo and in the loss of the variable components of fluorescence and thermal emissions. The initial Fo and the variable components could be partially recovered by reconstitution of both donor and acceptor sides with Mn2+, Co2+, HCO3- and plastoquinone. It is concluded that the photochemical fluorescence quenching is correlated with a simultaneous "quenching" of a variable component of thermal emission. It is proposed that the measured component of variable thermal emission is related to the decay of the pair [P680+ Pheo-]. The suggestion is also made that a bicarbonate-induced protonation of reduced QA or QB or conformational change in the PSII complex, or both, adds an additional entropic factor to the variable thermal emission component.

Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O2 in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b 6-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b 6-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and 'poises' the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO2 can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic photophosphorylation will depend on experiments made on living cells and measurements of cyclic photophosphorylation in vivo.

Heat-stress stimulation of oxygen uptake by Photosystem I involves the reduction of superoxide radicals by specific electron donors

A Photosystem I submembrane fraction isolated from spinach was used to study the mechanism of heat-stress stimulation of oxygen uptake by the photosystem. Various artificial electron donors were shown to generate electron transport reactions with various degrees of thermally induced stimulation. A strong stimulation was observed with durohydroquinone as electron donor with a maximal effect at 50 °C. The degree of stimulation obtained was independent from the redox potential of the electron donors and from their oxidation site because the enzyme superoxide dismutase fully inhibited the stimulation. Instead, it is proposed that thermal stress causes the release of membrane bound superoxide dismutase from the thylakoids thus allowing the reduced form of electron donors with specific properties to reduce O2 (-) radicals to H2O2 besides the usual disproportionation of O2 (-) into O2 and H2O2.

Interaction of Zn(2+) with the donor side of Photosystem II

The inhibitory effect of Zn(2+) on photosynthetic electron transport was investigated in native and CaCl2-treated (depleted in extrinsic polypeptides) Photosystem II (PS II) submembrane preparations. Inhibition of 2,6-dichlorophenolindophenol photoreduction by Zn(2+) was much stronger in protein-depleted preparations in comparison to the native form. It was found that Ca(2+) significantly reduced the inhibition in the native PS II preparations, as did Mn(2+) in a combination with H2O2 in the protein-depleted counterparts. No other tested monovalent or divalent cations could replace Ca(2+) or Mn(2+) in the respective experiments. Diphenylcarbazide could partially relieve (40-45%) the inhibition in both types of preparations. The above indicates the presence of an active Zn(2+) inhibitory site on the donor side of PS II. However, neither Ca(2+) nor Mn(2+) could completely prevent inhibition by high concentrations of Zn(2+) (>1 mM). We propose that elevated levels of Zn(2+) strongly perturb the conformation of the PS II core complex and might also affect the acceptor side of the photosystem.

Photosynthetic energy storage of Photosystems I and II in the spectral range of photosynthetically active radiation in intact sugar maple leaves

The relative activity of Photosystems (PS) I and II in the spectral range between 400 and 720 nm was studied by measuring photosynthetic energy storage (ES) of an intact sugar maple leaf using photoacoustic spectroscopy. ES, determined with a modulated (80 Hz) monochromatic light beam in the presence of saturating intensity of background non-modulated white light, indicated the total energy stored by both photosystems (EST). Using background far-red light, ES of PS I (ESPS I) was quantified. ESPS II was derived from EST-ESPS I. EST dependence on intensity and wavelength of modulated light was studied at 470, 560, 640 and 680 nm. EST was maximum in red light and minimum in blue light. It decreased with an increase in modulated light intensity. The ratio ESPS II/ESPS I, measured at 640 nm, remained nearly constant with an increase in modulated light intensity. The relative quantum yield of EST spectrum showed two peaks around 610 and 660 nm, and declined sharply after 680 nm, revealing a clear red drop. ESPS I spectrum presented peaks around 610 and 670 nm, and a minimum between 440 and 470 nm. ESPS I was observed beyond 700 nm up to 720 nm, indicating the energy stored by cyclic electron transport. ESPS II spectrum showed broad peaks, around 460, 490, 600 and 660 nm, and a shoulder between 530 and 560 nm. ESPS II was always higher than ESPS I between 400 and 690 nm and reached zero around 700 nm.

Detection of photosynthetic energy storage in a photosystem I reaction center preparation by photoacoustic spectroscopy

Thermal emission and photochemical energy storage were examined in photosystem I reaction center/core antenna complexes (about 40 Chl a/P700) using photoacoustic spectroscopy. Satisfactory signals could only be obtained from samples bound to hydroxyapatite and all samples had a low signal-to-noise ratio compared to either PS I or PS II in thylakoid membranes. The energy storage signal was saturated at low intensity (half saturation at 1.5 W m(-2)) and predicted a photochemical quantum yield of >90%. Exogenous donors and acceptors had no effect on the signal amplitudes indicating that energy storage is the result of charge separation between endogenous components. Fe(CN)6 (-3) oxidation of P700 and dithionite-induced reduction of acceptors FA-FB inhibited energy storage. These data are compatible with the hypothesis that energy storage in PS I arises from charge separation between P700 and Fe-S centers FA-FB that is stable on the time scale of the photoacoustic modulation. High intensity background light (160 W m(-2)) caused an irreversible loss of energy storage and correlated with a decrease in oxidizable P700; both are probably the result of high light-induced photoinhibition. By analogy to the low fluorescence yield of PS I, the low signal-to-noise ratio in these preparations is attributed to the short lifetime of Chl singlet excited states in PS I-40 and its indirect effect on the yield of thermal emission.

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.

Effect of sulfur dioxide and sulfite on photochemical energy storage of isolated chloroplasts--a photoacoustic study

Photoacoustic spectroscopy was used to study the effect of sulfite and SO(2) on isolated corn mesophyll chloroplasts by monitoring the photochemical energy storage. Sulfite incubation of isolated chloroplasts, either in light or in darkness, caused a decrease in photochemical energy storage. The more pronounced decrease in light indicates a light-dependent sulfite inhibitory site(s) in chloroplasts. Also diphenylcarbazide caused a partial recovery of energy storage in sulfite treated chloroplasts indicating a possible site of damage at the water oxidizing system. Although the chloroplast membranes were found to be insensitive to high concentrations of SO(2) for relatively short exposure periods (10 min) in light, exposure of chloroplasts to 28.5 ng cm(-3) SO(2) for 10 min caused a decrease in energy storage. An attempt was made to explain the mechanism of action of sulfite and SO(2) in chloroplasts.