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See detailLa Spiruline : a star is born (a long time ago)
de Marchin, Thomas ULg

Scientific conference (2014, May 06)

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See detailAnalysis of PSII antenna size heterogeneity of Chlamydomonas reinhardtii during state transitions
de Marchin, Thomas ULg; Ghysels, Bart ULg; Nicolay, Samuel ULg et al

in Biochimica et Biophysica Acta-Bioenergetics (2014), 1837(1), 121-130

PSII antenna size heterogeneity has been intensively studied in the past. Based on DCMU fluorescence rise kinetics, multiple types of photosystems with different properties were described. However, due to ... [more ▼]

PSII antenna size heterogeneity has been intensively studied in the past. Based on DCMU fluorescence rise kinetics, multiple types of photosystems with different properties were described. However, due to the complexity of fluorescence signal analysis, multiple questions remain unanswered. The number of different types of PSII is still debated as well as their degree of connectivity. In Chlamydomonas reinhardtii we found that PSIIα possesses a high degree of connectivity and an antenna 2-3 times larger than PSIIβ, as described previously. We also found some connectivity for PSIIβ in contrast with the majority of previous studies. This is in agreement with biochemical studies which describe PSII mega-, super- and core- complexes in Chlamydomonas. In these studies, the smallest unit of PSII in vivo would be a dimer of two core complexes hence allowing connectivity. We discuss the possible relationships between PSIIα and PSIIβ and the PSII mega-, super- and core- complexes. We also showed that strain and medium dependent variations in the half-time of the fluorescence rise can be explained by variations in the proportions of PSIIα and PSIIβ. When analyzing the state transition process in vivo, we found that this process induces an inter-conversion of PSIIα and PSIIβ. During a transition from state 2 to state 1, DCMU fluorescence rise kinetics are satisfactorily fitted by considering two PSII populations with constant kinetic parameters. We discuss our findings about PSII heterogeneity during state transitions in relation with recent results on the remodeling of the pigment-protein PSII architecture during this process. [less ▲]

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See detailStudy of photosynthesis of Chlamydomonas reinhardtii under High and Low CO2 conditions.
de Marchin, Thomas ULg; Ghysels, Bart ULg; Franck, Fabrice ULg

Poster (2013, August 12)

In photoautotophically air-grown microalgae, CO2 availability is usually limited. The unicellular green algae Chlamydomonas reinhardtii can adapt to low CO2 concentration with the inorganic carbon ... [more ▼]

In photoautotophically air-grown microalgae, CO2 availability is usually limited. The unicellular green algae Chlamydomonas reinhardtii can adapt to low CO2 concentration with the inorganic carbon concentration mechanism (CCM). This has been extensively studied in the past but functional adaptation of the photosynthetic apparatus has been less studied. Photosynthetic organisms can cope with CO2 limitation by dissipating excess absorbed energy with the help of different energy dissipating mechanisms like energy-dependent non-photochemical quenching (NPQ). In Chlamydomonas reinhardtii, this process only seems to develop to high levels in extreme conditions combining high light and strong CO2 limitation. Under moderate CO2 limitation, absence of important energy-dependent NPQ suggests the development of another energy dissipating mechanism. We compared the growth and functional adaptations of the photosynthetic apparatus of the wild-type strain 1690 grown in photobioreactor under low and high CO2 bubbling, 0.039% and 10%, respectively. Under low CO2, where growth was 2 to 4 times slower than under high CO2, the non-linear relationship between electron transport rate (derived from PAM fluorescence measurements) and gross oxygen evolution rate suggested that a significant portion of the electron flux is directed to oxygen at light intensities approaching photosynthetic saturation (either at PSI or at PTOX). The use of the mutant strain PTOX2 indicated that O2 reduction occurs mainly at PSI and not at PTOX. Low temperature fluorescence emission spectra indicated no significant difference in excitation energy distribution between PSI and PSII. Western blot analysis showed no major differences in abundance of Rubisco or of photosystem subunits between the two conditions. In contrast, cytochrome f abundance was lower in high CO2 condition. Although energy-dependent NPQ remained weak, low CO2 cells were characterized by a higher xanthophyll deepoxydation index which usually indicates more dissipation as heat, as also suggested by increased Lhcsr3 expression. Despite a higher ATP requirement of the CCM mechanism in low CO2 condition, only minor difference in cyclic electron transport could be found if compared to high CO2 condition (as determined by P700 spectroscopic measurements). In Chlamydomonas, conflicting views were expressed in earlier studies on the amplitude and role of Mehler-type O2-uptake at steady state. Our analysis of oxygen evolution, electron transport and NPQ after growth under different combinations of light intensities and CO2 supply rates allows us to define Mehler-type alternative electron transport as an important and flexible response to photosynthetic electron transport saturation in Chlamydomonas. [less ▲]

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See detailAnalysis of PSII antenna size heterogeneity of Chlamydomonas reinhardtii during state transitions - Colloque annuel de la Société Française de Photosynthèse
de Marchin, Thomas ULg; Ghysels, Bart ULg; Nicolay, Samuel ULg et al

Conference (2013, June 18)

PSII antenna size heterogeneity has been extensively studied in the past. Based on in vivo DCMU fluorescence rise kinetics, at least two types of photosystems were described. They differ by their apparent ... [more ▼]

PSII antenna size heterogeneity has been extensively studied in the past. Based on in vivo DCMU fluorescence rise kinetics, at least two types of photosystems were described. They differ by their apparent antenna size and connectivity (this last term refers to the transfer of absorbed energy from a closed PSII unit to an open neighboring unit). In this study, we analysed PSII heterogeneity in Chlamydomonas reinhardtii using non-linear linear regression fitting on in vivo DCMU fluorescence rise kinetics, with a focus on changes in PSII heterogeneity associated with state transitions. We found that PSIIα possesses a high degree of connectivity and an antenna about 3 times larger than PSIIβ, as described previously. In contrast with most earlier studies, we found some connectivity for PSIIβ (although it was highly variable). This is in agreement with recent models based on biochemical and structural analysis of PSII after gel filtration separation which describe PSII mega-, super- and core- complexes in Chlamydomonas. According to these studies, the smallest unit of PSII in vivo would be a dimer of two core complexes hence still allowing connectivity. We also showed that strain and medium dependent variations in the half-time of the fluorescence rise, generally taken as an indicator of the average cross-section of PSII, can be explained by variations in the proportions of PSIIα and PSIIβ. When analyzing the state transition process, we showed for the first time in vivo that it induces an inter-conversion of PSIIα and PSIIβ. These findings are discussed with respect to the latest insights on the remodeling of the pigment-protein PSII architecture during this process. [less ▲]

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See detailFinding the bottleneck: a research strategy for improved biomass production
Bassi, Roberto; Cardol, Pierre ULg; Choquet, Yves et al

in Posten, Clemens; Walter, Christian (Eds.) Microalgal Biotechnology: integration and economy (2012)

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See detailLa réponse photosynthétique d'une algue verte à la carence en soufre
de Marchin, Thomas ULg; Ghysels, Bart ULg; Franck, Fabrice ULg

Book published by Editions universitaires europeennes (2010)

Chlamydomonas reinhardtii possède la capacité de produire de l'hydrogène à la lumière en absence d'oxygène. Cette condition peut être obtenue en cultivant les algues dans un milieu carencé en soufre. La ... [more ▼]

Chlamydomonas reinhardtii possède la capacité de produire de l'hydrogène à la lumière en absence d'oxygène. Cette condition peut être obtenue en cultivant les algues dans un milieu carencé en soufre. La carence en soufre entraîne une forte diminution de l'activité du photosystème II tout en maintenant une respiration élevée, ce qui provoque un passage de cultures fermées en anoxie et induit la production d'hydrogène. Dans cette étude, nous avons caractérisé la réponse photosynthétique à la carence en soufre chez la souche sauvage et la souche déficiente en oxydase alternative mictochondriale (AOX) dans des milieux contenant de l'ammonium ou du nitrate comme source d'azote. L'AOX, inductible par le nitrate, fait partie de la chaîne de transport d'électrons mitochondriale et catalyse l'oxydation de l'ubiquinol en transférant directement ses électrons à l'oxygène. Ainsi l'AOX entre en compétition avec le complexe III et est impliquée dans une voie de dissipation du pouvoir réducteur en excès. [less ▲]

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See detailCaractérisation de la réponse photosynthétique de Chlamydomonas reinhardtii à la carence en S : effet de la source d'azote et influence de l'oxydase alternative mitochondriale
de Marchin, Thomas ULg

Master's dissertation (2010)

Chlamydomonas reinhardtii possède la capacité de produire de l'hydrogène à la lumière en absence d'oxygène. Cette condition peut être obtenue en cultivant les algues dans un milieu carencé en soufre. La ... [more ▼]

Chlamydomonas reinhardtii possède la capacité de produire de l'hydrogène à la lumière en absence d'oxygène. Cette condition peut être obtenue en cultivant les algues dans un milieu carencé en soufre. La carence en soufre entraîne une forte diminution de l'activité du photosystème II tout en maintenant une respiration élevée, ce qui provoque un passage de cultures fermées en anoxie et induit la production d'hydrogène. Dans des observations préliminaires, le laboratoire a mis en évidence des différences de vitesse de réponse à la carence en soufre entre la souche sauvage (contrôle) et une souche portant une construction RNA-i l'empêchant d'exprimer l'oxydase alternative mitochondriale (AOX). Cette enzyme, inductible par le nitrate, fait partie de la chaîne de transport d'électrons mitochondriale et catalyse l’oxydation de l’ubiquinol en transférant directement ses électrons à l’oxygène. Ainsi l'AOX entre en compétition avec le complexe III et est impliquée dans une voie de dissipation du pouvoir réducteur en excès. Dans cette étude, nous avons caractérisé la réponse photosynthétique à la carence en soufre chez la souche sauvage et la souche déficiente en AOX dans des milieux contenant de l'ammonium ou du nitrate comme source d'azote. Nos observations ont montré que la carence en soufre affecte drastiquement les capacités photosynthétiques de la souche sauvage en ammonium. Nous avons relevé une chute de l'abondance de RbcL et de D1, du rendement photochimique du PSII et une dé-époxydation des xanthophylles ainsi que l'inactivation du cycle de Calvin. Cependant, nous avons observé que le déclin de plusieurs paramètres photosynthétiques en réponse à une carence en soufre est ralenti par la présence de nitrate et dans la souche déficiente en AOX. Nous avons pu constater que les différences d'importance et de vitesse du déclin de la photosynthèse entraînent aussi des différences dans la durée d'accumulation d'amidon au cours de la carence. Nous avons également montré que l'expression de l'AOX et la capacité de la voie alternative sont stimulées par la carence en soufre. Ces observations suggèrent un rôle important à la voie AOX dans la réponse de C.reinhardtii à la carence en soufre. [less ▲]

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