Robert Carpentier

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Alternative Photosystem I-Driven Electron Transport Routes: Mechanisms and Functions

In addition to the linear electron transport, several alternative Photosystem I-driven (PS I) electron pathways recycle the electrons to the intersystem electron carriers mediated by either ferredoxin:NADPH reductase, NAD(P)H dehydrogenase, or putative ferredoxin:plastoquinone reductase. The following functions have been proposed for these pathways: adjustment of ATP/NADPH ratio required for CO2 fixation, generation of the proton gradient for the down-regulation of Photosystem II (PS II), and ATP supply the active transport of inorganic carbon in algal cells. Unlike ferredoxin-dependent cyclic electron transport, the pathways supported by NAD(P)H can function in the dark and are likely involved in chlororespiratory-dependent energization of the thylakoid membrane. This energization may support carotenoid biosynthesis and/or maintain thylakoid ATPase in active state. Active operation of ferredoxin-dependent cyclic electron transport requires moderate reduction of both the intersystem electron carriers and the acceptor side of PS I, whereas the rate of NAD(P)H-dependent pathways under light depends largely on NAD(P)H accumulation in the stroma. Environmental stresses such as photoinhibition, high temperatures, drought, or high salinity stimulated the activity of alternative PS I-driven electron transport pathways. Thus, the energetic and regulatory functions of PS I-driven pathways must be an integral part of photosynthetic organisms and provides additional flexibility to environmental stress.

Photosystem II thermostability in situ: environmentally induced acclimation and genotype-specific reactions in Triticum aestivum L.

Photosystem II (PSII) thermostability and acclimation effects on PSII photochemical efficiency were analyzed in thirty field grown winter wheat (Triticum aestivum L.) genotypes using prompt chlorophyll a fluorescence kinetics before and after dark heat treatment. A gradual increase in temperature caused the appearance of K-bands at 300 μs on the chlorophyll fluorescence induction curve, indicating the impairment of the PSII donor side (even by heat treatment at 38 °C). An increase in basal fluorescence, commonly used as a criterion of PSII thermostability, was observed beyond a temperature threshold of 44 °C. Moreover, an acclimation shift (increase of critical temperature) was observed at the 3.5 °C identified for K-band appearance, but only by 1.1 °C for a steep increase in F(0). The single temperature approach with regular weekly observations completed within two months using dark heat treatment at 40 °C demonstrated that the acclimation effect is not gradual, but occurs immediately and is associated with an increase of daily temperature maxima over 30 °C. The acclimated heat treated samples had less effect on the donor side of PSII, the higher fraction of active Q(A)(-) reducing reaction centers and causing a much lower decrease of connectivity among PSII units compared to non-acclimated samples. In the non-treated plants the reduction of antennae size, increase of PSII connectivity and changes in the acceptor side occurred as a result of heat acclimation. The enhancement of PSII thermostability persisted over several weeks regardless of weather conditions. The genotype comparison identified three groups that differed either in initial PSII thermostability or in acclimation capacity; these groupings were clearly associated with the origin of the genotypes.

Glycinebetaine protects the D1/D2/Cytb559 complex of photosystem II against photo-induced and heat-induced inactivation

The presence of 1.0 mol/L glycinebetaine during isolation of D1/D2/Cytb559 reaction centre (RC) complexes from photosystem II (PSII) membrane fragments preserved the photochemical activity, monitored as the light-induced reduction of pheophytin and electron transport from diphenylcarbazide to 2.6-dichlorophenol-indophenol.-Glycinebetaine also protected the D1/D2/Cytb559 complexes against strong light-induced damage to the photochemical reactions and the irreversible bleaching of beta-carotene and chlorophyll. The presence of glycinebetaine also enhanced thermotolerance of the D1/D2/Cytb559 complexes isolated in the presence of 1.0 mol/L betaine with an increase in the temperature for 50% inactivation from 29 degrees C to 35 degrees C. The results indicate an increased supramolecular structural stability in the presence of glycinebetaine.

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

Photosynthetic activities of Anacystis nidulans can be detected by photoacoustic spectroscopy. Algae treated by a photosynthetic inhibitor are used to provide the signal from the photochemically inactive sample. The results of these measurements correspond well with the activities which can be monitored by conventional biochemical assays. Acoustic data from A. nidulans are used to obtain the action spectrum for photochemical energy storage. It is concluded that phycocyanin harvests light for both photoreactions but that chlorophyll alpha molecules convey most of their excitation energy to photoreaction I. As judged from the relationship between the modulation frequency and the acoustic signal intensity, at least 60% of the photons absorbed at 630 nm perform photochemical work and about half of the useful energy is stored at stable products. Although it cannot be separated from the purely thermal effect, the contribution of modulated oxygen evolution to the acoustic signal of algae is estimated to be relatively small. Due to structural peculiarities, the opposite situation predominates in low frequency measurements performed with leaves from Impatiens petersiana.

A quantitative secondary structure analysis of the 33 kDa extrinsic polypeptide of photosystem II by FTIR spectroscopy

In chloroplast photosystem II, the extrinsic polypeptide of 33 kDa is involved in the stabilization the Mn cluster in charge of water splitting and in the fulfillment of the Ca2+‐cofactor requirement for oxygen evolution. The conformational analysis of the purified 33 kDa extrinsic polypeptide was carried out using FTIR spectroscopy with its self‐deconvolution and second derivative resolution enhancement as well as curve‐fitting procedures. The FTIR spectroscopic results showed that the isolated polypeptide is characterized by a major proportion β‐sheet conformation (36%) with 27% α‐helix, 24% turn, and 13% β‐antiparallel structures.

Stabilization of oxygen evolution and primary electron transport reactions in photosystem II against heat stress with glycinebetaine and sucrose

The protective action of co-solutes, such as sucrose and glycinebetaine, against the thermal inactivation of photosystem II function was studied in untreated and Mn-depleted photosystem II preparations. It was shown that, in addition to the reactions that depend on the oxygen evolving activity of the photosystem, those that implicate more intimately the reaction center itself are protected by high concentrations of osmolytes. However, the temperature required to inhibit oxygen evolution totally in the presence of osmolytes is lower than that required to eliminate reactions, such as P680 (primary electron donor in photosystem II) photo-oxidation and pheophytin photo reduetion, which only involve charge separation and primary electron transport processes. The energy storage measured from the thermal dissipation yield during photoacoustic experiments and the yield of variable fluorescence are also protected to a significant degree (up to 30%) at temperatures at which oxygen evolution is totally inhibited. It is suggested that a cyclic electron transport reaction around photosystem II may be preserved under these conditions and may be responsible for the energy storage measured at relatively high temperatures. This interpretation is also supported by thermoluminescence data involving the recombination between reduced electron acceptors and oxidized electron donors at - 30 and - 55 °C. The data also imply that a high concentration of osmolyte allows the stabilization of the photosystem core complex together with the oxygen-evolving complex. The stabilization effect is understood in terms of the minimization of protein-water interactions as proposed by the theory of Arakawa and Timasheff (Biophys. J., 47 (1985) 411--414).

Fluorescence parameters as early indicators of light stress in barley

Photosynthetic efficiency of two Syrian barley landraces Arabi Aswad and Arabi Abiad grown under different light intensities were studied by the application of qualitative and quantitative analysis of chlorophyll a fluorescence. Different values of fluorescence parameters, quantum efficiencies, specific and phenomenological energy fluxes were obtained for each cultivar. Both low and high light stresses decreased photosystem II (PSII) activity in barley seedlings depending on the stress type and its duration. Cultivar Arabi Aswad was more tolerant to high light while Arabi Abiad was more tolerant to low light stress. The results allowed us to select chlorophyll a fluorescence parameters related to energy flux within PSII which were specifically affected under low or high light stress. We found that the performance index parameter is a sensitive indicator to explore the effect of light changes on PSII activity immediately after stress application, while maximal quantum yield of PSII and phenomenological parameters were only modified after a long period of stress application indicating PSII damage. Thus, we recommend the former parameter for early detection of light stress.

Hg2+, Cu2+, and Pb2+ -induced changes in Photosystem II photochemical yield and energy storage in isolated thylakoid membranes: A study using simultaneous fluorescence and photoacoustic measurements

Simultaneous fluorescence and photoacoustic measurements have been used to study the effects of metal ions (copper, lead, and mercury) during dark incubation of thylakoid membranes. The values of the chlorophyll fluorescence parameters Fo (initial fluorescence yield with the reaction centers in the open state), Fm (maximal fluorescence yield), Ft (steady state fluorescence yield) and the calculated parameters, Φo (maximal quantum yield of Photosystem II photochemistry) and Φt (actual quantum yield of Photosystem II photochemistry), strongly decreased in the presence of the metal ions coinciding with an increase in the non-photochemical deexcitation rate constant k(N). It was observed that photosynthetic energy storage measured by photoacoustic spectroscopy also decreased but a large portion of energy storage remained unaffected even at the highest metal ion concentrations used. A maximal inhibition of photosyntheti c energy storage of 80% and 50% was obtained with Hg2+ and Cu2+-treated thylakoids, respectively, while energy storage was insensitive to Pb2+. The results are consistent with the known predominant inhibition of the donor side of Photosystem II by the metal ions. The insensitive portion of energy storage is attributed to the possible recurrence of cyclic electron transport around Photosystem II that would depend on the extent of inhibition produced on the acceptor side by the metal ion used.

Photoacoustic spectroscopy of Anacystis nidulans. I. Effect of sample thickness on the photoacoustic signal.

A photoacoustic spectroscopy study of the cyanobacteria Anacystis nidulans has been undertaken. It is demonstrated, by using a filter deposition technique, that the photoacoustic signal intensity becomes progressively saturated as the thickness of the algal layer is increased. This saturation effect originates mostly from the limited optical penetration of the sample and distorts the photoacoustic spectrum from its true shape. A theoretical model is proposed to explain these results, and practical means to obviate the limitations of this spectroscopic technique are suggested.