Doctoral thesis (Dissertations and theses)
From photons to biomass in green microalgae: fluorimetric and oxymetric studies on the regulation of photosynthetic electron transport
de Marchin, Thomas
2015
 

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Keywords :
Photosynthesis; Microalgae; Fluorescence; CO2; Mehler; PSII
Abstract :
[en] Analyzing the photosynthetic performances of microalgae has become an important topic in view of the ecological importance of these microorganisms and of the growing field of their biotechnological applications. In this thesis, we developed methods which allow effective use of fluorescence and oxygen experimental signals for a more complete analysis of the functioning of the photosynthetic apparatus of green microalgae under different conditions. We applied these methods to analyze the adaptations of the photosynthetic apparatus under selected conditions, especially regarding CO2 supply. In the first part of this work, we applied a fluorescence-based method for determining the PSII antenna size based on the kinetics of the fast chlorophyll a fluorescence transient in presence of DCMU (or DCMU Fluorescence Rise). We then performed a detailed analysis of the different phases of this transient which are associated with different types of PSII formerly described as PSIIalpha and PSIIbeta (PSII heterogeneity). Our results on PSII heterogeneity during a transition from state 2 to state 1 in Chlamydomonas reinhardtii showed for the first time in vivo that this transition correlates with a conversion of PSIIbeta to PSIIalpha. We also discuss the possible relationships between PSIIalpha and PSIIbeta and the PSII mega-, super- and core- complexes described by biochemical studies. In photoautotophic atmospheric conditions, microalgae often have to deal with limited CO2 availability. 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. However, other functional adaptations of the photosynthetic apparatus to CO2 limitation have been much less studied. In the second part of this work, we used combined fluorescence-based electron transport and oxygen measurements to evaluate the responses to low and high CO2 in Chlamydomonas reinhardtii grown in photobioreactors at different light intensities. We developed a method to rationalize the relationship between the apparent quantum yields of oxygen evolution and of electron transport at PSII while taking into account the variations in the proportion of energy absorbed by PSII. We used this relationship as a tool to evidence a significant O2-dependent alternative electron transport in low CO2 acclimated cells. We showed that this alternative electron transport can represent up to 60% of the total electron transport in low CO2 cells even upon removal of the CO2 limitation by bicarbonate addition. In contrast, no significant alternative electron transport was detected in high CO2 cells. We suggest that the alternative electron transport to O2 observed in low CO2 cells represents an adaptation that could help to meet the higher ATP demand for the concentration of CO2 by the CCM. In contrast, in high CO2 conditions, the absence of the CCM would reduce the need for ATP and thus the need for electron transport to O2. Using mutants and inhibitors, we studied the involvement of known O2-dependent electron sinks such as the mitochondrial cytochrome and alternative oxidase pathways and also of chlororespiration in this light-dependent O2 uptake and we found no evidence for the involvement of any of these processes. The alternative electron transport was even higher in a mitochondrial mutant devoid of complexes I and III and we suggest that this alternative electron transport could compensate for the absence of mitochondrial ATP synthesis in this mutant. By elimination, our results suggest that the alternative electron transport to O2 evidenced here could be driven by a Mehler-type reaction although we did not find direct evidences for increased Mehler-type activity in low CO2 cells. Additionally from fluorescence measurements performed directly in the cultures, we concluded that low CO2 cells had a lowest PSII photosynthetic efficiency and developed strong qE NPQ. Our results indicate that Chlamydomonas reinhardtii is capable to induce an ample thermal dissipation of excitation energy and the extent of this process is influenced essentially by the CO2 availability. In contrast with results obtained in previous studies, low temperature fluorescence spectra showed that in high light, high CO2 cells were characterised by a higher proportion of light energy absorbed by PSI compared to low CO2 cells. This was accompanied by a decrease of PSII antenna size as shown by DCMU-FR measurements. These findings are discussed in relation with the conflicting theories concerning the role of state transitions in the regulation of the cyclic electron transport around PSI. Our results support the idea that CEF is regulated by the redox state of the chloroplast rather than by state transitions. Over the last decades, microalgae have been increasingly studied because of their potential applications in industry. Despite a growing interest in microalgae mass cultures, the majority of studies on microalgae have been carried out at a laboratory scale and only few studies have addressed the question of photosynthetic adaptations in mass cultures. It is well known that CO2 addition increases the growth rate of microalgae and for this reason, some mass microalgae cultures are CO2 supplemented. Among the studies on microalgae mass culture, as far as we know, none directly compared high CO2 (CO2 supplemented air) and low CO2 (air) conditions. In the last part of our work, two outdoor open thin-layer cascade systems operated as batch cultures with the alga Scenedesmus obliquus were used to compare the productivity and photosynthetic adaptations in control and CO2 supplemented cultures in relation with the outdoor light irradiance. We found that the culture productivity was limited by CO2 availability beyond a threshold of daily irradiance. In the CO2 supplemented culture, we obtained a productivity of up to 25 g dw.m-2.day-1 and found a photosynthetic efficiency of 2.6%. Fluorescence and oxygen evolution measurements showed that ETR and oxygen evolution light saturation curves, as well as alternative electron transport were similar in algal samples from both cultures when the CO2 limitation was removed. In contrast, we found that CO2 limitation conducted to a decreased PSII photochemical efficiency and an increased light-induced heat-dissipation in the control culture compared to the CO2 supplemented culture. These features may contribute to the lower productivity observed in absence of CO2 supplementation in outdoor mass cultures of Scenedesmus obliquus.
Disciplines :
Biochemistry, biophysics & molecular biology
Author, co-author :
de Marchin, Thomas ;  Université de Liège - ULiège > Labo de Bioénergétique
Language :
English
Title :
From photons to biomass in green microalgae: fluorimetric and oxymetric studies on the regulation of photosynthetic electron transport
Defense date :
13 March 2015
Number of pages :
172
Institution :
ULiège - Université de Liège
Degree :
Docteur in philosophy in sciences
Promotor :
Franck, Fabrice ;  Université de Liège - ULiège > Département des sciences de la vie > Génétique et physiologie des microalgues
President :
Dommes, Jacques ;  Université de Liège - ULiège > Département des sciences de la vie > Biologie végétale translationnelle
Secretary :
Ghysels, Bart ;  Université de Liège - ULiège > Integrative Biological Sciences (InBioS)
Jury member :
Valcke, Roland
Bassi, Roberto
Cardol, Pierre  ;  Université de Liège - ULiège > Integrative Biological Sciences (InBioS)
Bailleul, Benjamin ;  Université de Liège - ULiège > Département des sciences de la vie > Génétique et physiologie des microalgues
Funders :
FRIA - Fonds pour la Formation à la Recherche dans l'Industrie et dans l'Agriculture [BE]
Available on ORBi :
since 18 February 2015

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