Thèses sur le sujet « Non photochemical quencing »

When light energy input exceeds the capacity for photosynthesis the plant need to dissipate the excess energy and this is done through non-photo-chemical quenching (NPQ). Photochemical quenching (photosynthesis), NPQ and fluorescence are three alternative faiths of excited chlorophylls. PsbS associates to photosystem II and is involved in NPQ. The results presented in this thesis were generated on Arabidopsis plants and mainly based on wildtype Col-0 together with a mutant deficient in PsbS (npq4) and a transgene overexpressing PsbS (oePsbS). We connect light and herbivore stress and show that the level of PsbS influences the food preference of both a specialist (Plutella) and a generalist (Spodoptera) herbivore as well as oviposition of Plutella. Level of PsbS also affects both metabolomics and transcriptomics of the plant; up-regulation of genes in the jasmonic acid (JA) -pathway and amount of JA has been found in the npq4 plants after herbivory. Since many experiments were performed in field we have also characterized the field plant and how it differs from the commonly used lab plant. We have also studied the natural variation of NPQ in Arabidopsis plants both in the field and the lab. The results show surprisingly no correlation.
Överskottsenergi kan vara skadligt för en växts membran och fotosynteskomplex. Vid överskott av solenergi blir fotosystemen mättade och växten behöver därför ett sätt för att göra sig av med all överskottsenergi, detta kallas för ”icke-fotokemisk quenching” (NPQ). Fotokemisk quenching (fotosyntes), NPQ och fluoresens är tre alternativa vägar för exalterade klorofyller. PsbS är involverad i NPQ och associerar med fotosystem II. De resultat som presenteras i denna avhandling kommer från studier av modellväxten Arabidopsis thaliana (Backtrav), i huvudsak gjorda på vildtypen i jämförelse med en mutant som saknar PsbS (npq4) och en transgen som överuttrycker PsbS (oePsbS). Vi har försökt att undersöka kopplingen mellan ljus- och herbivoristress och visar här att mängden PsbS påverkar både en specialist (Plutella) och en generalist (Spodoptera) insekt vid val av föda, samt Plutella även vid äggläggning. Växternas nivå av PsbS visade sig även påverka metabolomet och transkriptomet, och vi fann en uppreglering av gener i biosyntesen för jasmonat samt mer av själva hormonet jasmonat i npq4 växter efter herbivori. Eftersom vi har gjort många av experimenten ute i fält har vi även karakteriserat en typisk Arabidopsis växt i fält samt hur denna skiljer sig från den vanligt använda lab-växten. Dessutom har vi även undersökt naturlig variation av NPQ av Arabidopsis både i fält och på lab och resultaten visar, till vår förvåning, att det inte går att finna någon korrelation mellan dessa.

Photoinhibition in three species of prymnesiophyte algae (Isochrysis galbana, Phaeocystis globosa and Emiliania huxleyi) was investigated using inhibitors to isolate various components. Results suggested that the three species show the same general responses, but timing and magnitude varied. I.galbana was the least susceptible to high light stress and showed a faster and greater non-photochemical quenching response than the other two species. This may be due to genetic adaptation, however characterisation of many more species is required before it becomes clear how photoinhibition responses vary in situ. A mathematical model was developed that simulates photon damage of photosystem II and subsequent effects on the initial slope of the photosynthesis/irradiance curve. Species specific differences in photoinhibition are proposed to be due to differences in pigment content and photoprotection. This was supported by the results presented, in that for each of the three species, the decline in the ratio of variable:maximum fluorescence normalised to Chlorophyll a for a given number of photons, is the same for a give species regardless of exposure irradiance. An additional model was constructed to simulate 3-step xanthophyll cycling (violaxanthin, antheraxanthin & zeaxanthin) in higher plants and chlorophyte algae. Both models were incorporated within existing photoacclimation models. The resultant model can simulate dynamic photoacclimation and photoinhibition under both nutrient replete and limiting conditions, and compares well with independent published experimental results. Experimental investigations into the 2-step xanthophyll cycle seen in prymnesiophyte algae (diadinoxanthin & diatoxanthin) presented here suggested that the relationship between non-photochemical quenching and the epoxidation state of the cycle was not as simple as the relationship seen in higher plants. Further research is required to clarify the relationship between non-photochemical quenching and this 2-step cycle, and the part of non-photochemical quenching that occurs in the presence of inhibitors of the de-epoxidation step of this cycle (DpH dependant quenching).

Les diatomées constituent le principal groupe du phytoplancton dans les océans, contribuant à près de 20% de la production primaire globale. Dans leur environnement très variable, les diatomées sont particulièrement efficaces dans leur capacité à ajuster leur activité photosynthétique en dissipant sous forme de chaleur l’énergie lumineuse absorbée en excès, par un processus appelé le « Non-Photochemical Quenching of chlorophyll fluorescence », (NPQ). Chez la diatomée modèle, Phaeodactylum tricornutum, il a été montré que LHCX1, une protéine proche des antennes photosynthétiques, est impliquée dans le NPQ. Par des approches intrégrées de génétique, biologie moléculaire, biochimie, imagerie des cinétiques de fluorescence et spectroscopie ultrarapide, j’ai étudié le rôle de la famille des LHCX chez P. tricornutum. J’ai tout d’abord pu corréler une expression différentielle des 4 gènes LHCX de P. tricornutum avec différentes dynamiques de NPQ et activités photosynthétiques, dans différentes conditions de lumiére et nutriments. En localisant les LHCX dans les differents complexes photosynthétiques et les différents sites de dissipation d’énergie, j’ai pu proposer un modèle de régulation dynamique du NPQ impliquant à court terme principalement LHCX1 au niveau des centres réactionnels, et une autre isoforme, possiblement LHCX3, au niveau des antennes lors d’un stress lumineux prolongé. Enfin, par le criblage d’une série de mutants potentiellement dérégulés dans leur contenu en LHCXs, j’ai pu identifier des lignées avec un NPQ altéré qui pourront constituer des nouveaux outils de recherche. Dans l’ensemble ce travail de thèse a permis de mettre en évidence la diversification fonctionnelle et l’importance de la famille des LHCX dans la fine modulation des capacités de collecte de lumière et de photoprotection, expliquant sans doute en partie le succès des diatomées dans leur environnement très fluctuant
Diatoms dominate phytoplanktonic communities in contemporary oceans, contributing to 20% of global primary productivity. In their extremely variable environment, diatoms are especially efficient in adjusting their photosynthetic activity by dissipating as heat the light energy absorbed in excess, through a process called “Non-Photochemical Quenching of chlorophyll fluorescence”, (NPQ). In the model diatom Phaeodactylum tricornutum, it has been shown that LHCX1, a photosynthetic antenna-related gene, is involved in the NPQ process. Through integrated approaches of genetics, molecular biology, biochemistry, study of the kinetics of chlorophyll fluorescence yields and ultrafast spectroscopy, I studied the role of the LHCX family in the photoprotection activity of P. tricornutum. I first correlated a differential regulation of the 4 P. tricornutum LHCX genes with different dynamics of NPQ and photosynthetic activity, in different light and nutrient conditions. By localizing the LHCXs in fractioned photosynthetic complexes and the different sites of energy dissipation, I was able to propose a model of dynamic regulation of NPQ capacity involving mainly the LHCX1 in the reaction centers, during short-term high light responses. During prolonged high light stress, the quenching occurs mainly in the antennas, potentially mediated by the LHCX3 isoform. Finally, using photosynthetic parameters, I screened a series of transgenic lines putatively deregulated in their LHCX amount, and I identified lines with altered NPQ, which could represent novel investigation tools. Altogether, this work highlighted the functional diversification and the importance of the LHCX protein family in the fine-tuning of light harvesting and photoprotection capacity, possibly contributing to explain diatoms success in their highly fluctuating environment

The increasing food demand as well as the need to predict the impact of warming climate on vegetation makes it critical to find the best tools to assess crop production and carbon dioxide (CO₂) exchange between the land and atmosphere. Photosynthesis is a good indicator of crop production and CO₂ exchange. Chlorophyll fluorescence (ChF) is directly related to photosynthesis. ChF can be measured at leaf-scale using active techniques and at field-scales using passive techniques. The measurement principles of both techniques are different. In this study, three overarching questions about ChF were addressed: Q1) How water, nutrient and ambient light conditions determine the relationships between photosynthesis and ChF? Which is the optimum irradiance level for detecting water and nutrient deficit conditions with ChF?; Q2) which are the limits within which active and passive techniques are comparable?; and Q3) What is the seasonal relationship between photosynthesis and ChF when nitrogen is the limiting factor? To address these questions, two main experiments were conducted: Exp1) Concurrent photosynthesis and ChF light-response curves were measured in camelina and wheat plants growing under (i) intermediate-light and (ii) high-light conditions respectively. Plant stress was induced by (i) withdrawing water, and (ii) applying different nitrogen levels; and Exp2) coincident active and passive ChF measurements were made in a wheat field under different nitrogen treatments. The results indicated ChF has a direct relationship with photosynthesis when water or nitrogen drives the relationship. This study demonstrates that the light level at which plants were grown was optimum for detecting water and nutrient deficit with ChF. Also, the results showed that for leaf-average-values, active measurements can be used to better understand the daily and seasonal behavior of passive ChF. Further, the seasonal relation between photosynthesis and ChF with nitrogen stress was not a simple linear function. Our study showed that at times in the season when nitrogen was sufficient and photosynthesis was highest, ChF decreased because these two processes compete for available energy. These results demonstrated that ChF is a reliable indicator of crop stress and has great potential for better understand the CO₂ exchange between the land and atmosphere.

The Small Cab-like Proteins (SCPs) in the cyanobacterium Synechocystis sp. PCC 6803 accumulate in cells grown under different stress conditions. Genes coding for SCPs have been found in all sequenced organisms performing oxygenic photosynthesis and even in the genomes of cyanophages. Deletion of multiple scp genes in Synechocystis resulted in mutants with severely impaired growth and altered pigment content. These findings indicate the importance of SCPs in photosynthesis; however, their specific function is not well understood. SCPs share a chlorophyll-binding motif with the plant light harvesting complex, suggesting that they bind chlorophyll. Here I describe my findings, which unambiguously show that SCPs are able to bind chlorophyll in vitro. Although they affect both the stoichiometric ratio of Photosystem I to II and chlorophyll stability, they do not seem to be directly involved in non-photochemical quenching. I was able to reveal the location of the SCPs within the cyanobacterial cell: in stressed cells they attach to Photosystem II in the thylakoid membrane. Furthermore, I revealed the presence of another light-harvesting like (Lil)/SCP protein in Synechocystis sp. PCC 6803. The gene, slr1544, codifying for this newly characterised LilA protein, co-transcribes together with scpD and also appears to bind to Photosystem II during stress.

Several species of Bryopsidales (Chlorophyta) are known for displaying functional absence of the xanthophyll cycle, a common photoprotection mechanism responsible for qE component of NPQ. To cope with the extreme variability of their natural environment, these algae must be able to avoid photodamage. Previous works reported significant accumulation of all-trans-neoxanthin and violaxanthin under high light acclimation in different Bryopsidales, and speculated that these xanthophylls might control the amount of energy that reaches the photosystems, causing photoprotection. In this work, we investigated photoacclimation and photoprotection strategies in two species of Bryopsidales (Codium tomentosum and Bryopsis plumosa). We first characterised the acclimation state of algae exposed for 7 days to low light or high light (respectively 20 and 1000 μmol photons m2 s−1) in terms of pigment content (HPLC) and chlorophyll a variable fluorescence (PAM). We confirmed that high light triggers significant alteration of pigment content with accumulation of trans-Neoxanthin and Violaxanthin, and for the first time we characterised thoroughly how the pigment pool is altered during acclimation. We also confirmed that no evidence of any xanthophyll cycle is present in high light acclimated cultures. On a second note we tried to answer another major question: are trans-neoxanthin and violaxanthin photoprotective? Using a novel chlorophyll a variable fluorescence approach (pNPQ assessment) and performing quantification of PSII repair capacity (via lincomycin treatment) we were not able to give a clear answer to this question. Nonetheless, we concluded that despite trans-Neoxanthin and Violaxanthin might contribute to photoprotection, this process in Bryopsidales algae is likely given by the coordination between different mechanisms that deserve to be further investigated, including chloroplast movement, PSII repair/modulation, state transitions, and PSI cyclic electron transport.

Cyanobacteria are a diverse group of microorganisms adapted to a range of environmental conditions that favour their ubiquity in waterbodies. Cyanobacteria harmful blooms (CyanoHABs) are events in which a cyanobacteria population grows rapidly, dominates the phytoplankton community and may release toxins or other noxious compounds in the waterbody. The frequency and magnitude of CyanoHABs are increasing as a result of climate change and increased pollution from urbanisation and agriculture expansion, representing a major risk to the public health and economy. Management of CyanoHABs is complicated by the temporal and spatial dynamic nature of these events, and by the large diversity of cyanobacteria species. Identification of the dominant cyanobacteria species is required to select appropriate mitigation and treatment alternatives. Therefore, water authorities have longed for reliable tools to support proactive and species-targeted CyanoHAB management. Emerging monitoring technologies and data-driven models represent a tangible opportunity to optimise CyanoHABs management by integrating rapid and taxa precise features into a single tool. Optical sensors, namely in-situ fluorescence sensors, allow rapid, remote estimation of the total phytoplankton and cyanobacteria concentration in a waterbody. This is done by measuring the fluorescence of the pigments chlorophyll a, common to all phytoplankton, and phycocyanin, exclusive to cyanobacteria. However, fluorescence estimates have limited taxa precision because they cannot discriminate between cyanobacteria species, and may have reduced accuracy, due to optical interferences. Data-driven models are increasingly being used to understand and predict complex ecological patterns, including cyanobacteria species succession, but the combination of high-frequency fluorescence data with data-driven models to optimise CyanoHAB management has seldom been investigated. The aim of this doctoral thesis is to develop an integrated model able to optimise CyanoHAB management by incorporating site-specific drivers of cyanobacteria succession and factors that affect fluorescence sensor estimates. This aim was achieved by addressing four objectives: (1) to systematically review the state-of-knowledge of forecasting and predictive CyanoHAB models and their application to freshwater lakes; (2) to test and quantify interferences, if any, on fluorescence probe measurements according to diel light variability and species composition; (3) to identify and quantify, through observational data analysis, dominance of cyanobacteria species according to site-specific environmental conditions; and (4) to establish a framework for implementation of integrated models considering fluorescence sensor calibration and prediction of cyanobacteria species succession. This research project’s combination of observational data analysis and analytical laboratory work underpins its novelty and relevance. Observational data analysis was performed for three Australian drinking-water reservoirs and correlations between environmental drivers and dominance of key cyanobacteria species were determined for Wivenhoe Lake (Queensland), Tingalpa Reservoir (Queensland) and Myponga Reservoir (South Australia). Two sets of controlled laboratory experiments were then performed. The first experiment analysed the fluorescence characteristics of four key cyanobacteria species (Aphanocapsa sp., Microcystis aeruginosa, Dolichospermum circinale and Raphidiopsis raciborskii) that are often dominant in the assessed drinking-water reservoirs. The experiment quantified the variability of the species’ fluorescence characteristics throughout their respective growth phases and also compared the differences among morphologically similar species. The second experiment analysed light-induced quenching in a cyanobacterium (Dolichospermum variabilis) and a green alga (Ankistrodesmus gracilis) by simulating diel light variability under controlled temperature and stratification conditions. Lastly, a framework combining the methodological procedures from the observational data analysis and the fluorescence calibration experiments was established with the aim of supporting the development of species-targeted models utilizing fluorescence sensors. An integrated model based on the framework was developed and tested in Myponga Reservoir, South Australia. Moreover, a continuous improvement process for CyanoHAB models and guidelines of best practices for fluorescence sensors deployment, calibration and operation were developed as a result of this research. The methods and findings are provided in four peer-reviewed journal papers included as chapters in this thesis (i.e., chapters 3, 5, 6 and 7) and a final discussion chapter (chapter 8). Objective 1 findings revealed that high-frequency data, such as the data from optical sensors, can improve performance of CyanoHAB models. For Objective 2, two key findings should be highlighted. First, fluorescence per cell was found to significantly vary among species, while fluorescence per unit of biomass (estimated from biovolume) was much more consistent among species. Second, diel light variability reduced fluorescence for both cyanobacteria and green algae up to 79% under the assessed conditions. Objective 3 findings indicated that environmental drivers for cyanobacteria succession and dominance are mostly site-specific. Species-specific traits, such as diazotrophy and gas vesicles, interact in complex ways with local environmental conditions leading to variable dominance succession among species. Finally, the key findings of Objective 4 showed that the required steps to develop a species-targeted CyanoHAB model using fluorescence sensors are feasible, given that constraints in data availability are met. Overall, the findings of this PhD research indicate that CyanoHAB management can be optimised through the combination of fluorescence sensors and forecasting models based on data-driven approaches, as long as rigorous calibration and data analysis procedures are undertaken. Importantly, the findings also highlight that even though generalisable patterns of species-specific drivers exist, site-specific analysis is required due to the complex interactions between the several factors involved in the occurrence of CyanoHABs.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Eng & Built Env
Science, Environment, Engineering and Technology
Full Text

La lumière est essentielle pour les organismes photosynthétiques qui convertissent l'énergie solaire en énergie chimique. Cependant, la lumière devient dangereuse lorsque l'énergie qui arrive aux centres réactionnels de l'appareil photosynthétique, est en excès par rapport à l’énergie consommée. Dans ce cas, la chaîne de transport d'électrons photosynthétiques se réduit et les espèces réactives de l'oxygène (ROS) sont accumulées, notamment au niveau des deux photosystèmes, PSI et PSII. Les cyanobactéries ont développé des mécanismes photoprotecteurs qui diminuent l'énergie transférée au PSII atténuant ainsi l'accumulation de ROS et les dommages cellulaires, comme l’extinction non-photochimique (NPQcya) induite par la lumière bleue-verte. La soluble Orange Caroténoïde Protéine (OCPo) est essentielle pour ce mécanisme de photoprotection. L'OCP agit comme un senseur de l’intensité lumineuse et un inducteur de la dissipation d'énergie des phycobilisomes (PBS), l'antenne extra-membranaire des cyanobactéries. L'OCP est la première protéine photo-active à caroténoïde connue comme senseur. Une forte lumière bleue-verte déclenche des changements structurels dans l'OCPo qui induisent une forme active, rouge (OCPr). Le domaine N-terminal de l’OCPr, en s’intercalant entre les trimères externes d’un des cylindres basaux du cœur du PBS, augmente la dissipation thermique de l'énergie au niveau de l'antenne. L'OCP possède aussi une autre fonction : l’extinction de l’oxygène singulet, qui protège les cellules du stress oxydatif. Pour récupérer pleinement la capacité de l’antenne en faible lumière, une deuxième protéine est nécessaire, la "Fluorescence Recovery Protein" (FRP), dont le rôle est de détacher l’OCPr des PBS et d’accélérer sa reconversion en OCPo inactive. Ce manuscrit est un état des lieux des connaissances et des dernières avancées sur le mécanisme de NPQ associé à l'OCP dans les cyanobactéries
Photosynthetic organisms use light energy from the sun in order to perform photosynthesis and to convert solar energy into chemical energy. Absorbance of excess light energy beyond what can be consumed in photosynthesis is dangerous for these organisms. Reactive oxygen species (ROS) are formed at the reaction centers and collecting light antennas inducing photooxidative damage which can lead to cell death. In cyanobacteria, one of these photoprotective mechanisms consists to reduce the amount of energy arriving to the reaction centers by thermal dissipation of the excess absorbed energy. Energy dissipation is accompanied by a decrease of Photosystem II-related fluorescence emission called non-photochemical quenching (NPQ). The soluble Orange Carotenoid Protein (OCPo) is essential for this photoprotective mechanism. The OCP is the first photo-active protein with a carotenoid known as light intensity sensor and acts as energy quencher of the phycobilisome (PB), the extra-membrane antenna of cyanobacteria. Structural changes occur when the OCPo absorbs a strong blue-green light leading to a red active form (OCPr). The N-terminal domain of OCPr burrows into the two external trimers of the core basal APC cylinders of the PB and increases thermal energy dissipation at the level of antenna. The OCP has an additional function in photoprotection as oxygen singlet quencher protecting cells from oxidative stress. Under low light conditions, to recover the full antenna capacity, a second protein is needed, the "Fluorescence Recovery Protein" (FRP), whose role is to detach the OCPr from the PB and accelerate its conversion into an inactive OCPo. In this manuscript, I will review the knowledge about the OCP, since the discovery of the mechanism and its characterization to the latest advances on the OCP-related-NPQ mechanism in cyanobacteria

Dans les écosystèmes littoraux, les communautés microphytobenthiques sont soumises à des conditions environnementales qui peuvent être extrêmes en particulier en ce qui concerne l'intensité lumineuse. Les mécanismes de protection mis en place dépendent étroitement du type d'habitat où se développent ces communautés et cette thèse a pour objectifs d'analyser les caractéristiques de l'activité photosynthétique et les mécanismes de protection développés par des assemblages microphytobenthiques dans deux écosystèmes littoraux très différents : les vasières intertidales atlantiques de la Baie de L'Aiguillon (France) et la lagune côtière non tidale de Puck Bay dans la Mer Baltique (Władysławowo, Pologne). Pour réaliser ces objectifs, trois études ont été réalisées : (1) la description des communautés microphytobenthiques, (2) la caractérisation de leur activité photosynthétique et (3) l'analyse des mécanismes de photoinhibition et de photoprotection.La structure taxonomique du microphytobenthos a été décrite en se basant sur des observations en microscopie optique et sur la mesure des caractéristiques des pigments photosynthétiques par chromatographie liquide à haute performance (HPLC). L'activité photosynthétique a été étudiée par des méthodes de microrespirométrie volumétrique et de spectrofluorométrie de la chlorophylle a. Les mécanismes de photoinhibition et de photoprotection ont été étudiés par fluorométrie en modulation d'amplitude pulsée (PAM).Les résultats obtenus nous ont permis de montrer que :1) Les communautés atlantiques sont fortement dominées par des diatomées épipéliques, alors que le microphytobenthos de la Mer Baltique est plus diversifié et comporte, outre des diatomées, une large part de cyanobactéries,2) Les microphytobenthos atlantique est bien acclimaté à des valeurs d'intensités lumineuses plutôt faibles, alors que les communautés de la Mer Baltique ont encore une bonne activité photosynthétique à de fortes irradiances,3) Les diatomées atlantiques présentent une plus forte photoinhibition que les microalgues de la Baltique,4) L'activité photosynthétique des communautés microphytobenthiques non perturbées montre un des rythmes circadien et tidal, qui semblent être contrôlés par des facteurs endogènes, qui mettent en jeu des adaptations comportementales comme la migration verticale pour les diatomées atlantiques,5) En ce qui concerne le microphytobenthos de la Mer Baltique, qui n'a pas de capacité migratoire, la photoprotection est assurée en premier lieu par la mise en jeu de processus physiologiques. Nous avons pu montrer la très grande flexibilité du photsystème PSII qui est capable de suivre très rapidement les changements à court terme de lumière ambiante
The scope of this thesis includes the characteristics and comparison of photosynthetic activity and photoprotection mechanisms of microphytobenthos assemblages inhabiting the Atlantic intertidal mudflats of Aiguillon Bay (Esnandes, France) and the littoral zone of the Baltic Sea in non-tidal Puck Bay (Władysławowo, Poland). In order to accomplish the main aims of the work the following tasks were carried out: (1) characterization of microphytobenthic assemblages; (2) characterization of their photosynthetic activity and (3) description of photoinhibition and photoprotective mechanisms. The structure of microphytobenthos was described based on observation of the material in light microscope (LM) and through the characteristics of photosynthetic pigments using high performance liquid chromatography (HPLC). Photosynthetic activity was described using various methods including classical (volumetric micro-respirometer) and modern (chlorophyll a fluorescence) ones. In addition, the measurements of variable fluorescence were also used to study photoinhibition and photoprotective mechanisms. Based on the obtained results it was stated that:1.) the Atlantic assemblages were strongly dominated by epipelic diatoms, while the Baltic microphytobenthos was more diverse and cyanobacteria, next to diatoms, were also very important component,2.) it was shown that the Atlantic microphytobenthos was well acclimated to rather low light intensities, while the Baltic assemblages showed good utilization of higher irradiance,3.) the Atlantic diatoms were more susceptible to photoinhibition than the Baltic microalgae,4.) the photosynthetic activity described for the undisturbed microphytobenthos communities revealed circadian and circatidal rhythms, which seemed to be controlled by endogenous factors, supporting diatoms’ behavioural adaptations i.e., vertical migration,5.) in case of the Baltic microphytobenthos, the lack of the ability to move caused their physiological processes the first line of defence against excess irradiances. The analysis revealed extreme flexibility of PSII which was able to follow rapidly the short-term changes in ambient light

碩士
國立交通大學
光電科技學程
106
In this thesis, I applied fluorescence lifetime technique to study the regulation of non-photochemistry quenching (NPQ) pathways in living Chlorella sp. cells. The goal is to find the best culture condition for Chlorella sp., so that the cells have balanced NPQ while having optimized photosynthesis efficiency. When exposed to light of different intensity, the starting lifetime of chlorophyll a during fluorescence transient changed. The stronger the intensity, the longer the starting fluorescence lifetime. This is because illumination intensity above certain level causes photosystem II reaction centers to close. It is a photoprotection mechanism that prevents excess energy to damage cells. The excess energy is then released as fluorescence. While the ratio of open/closed reaction centers decides photosynthesis efficiency, fluorescence lifetime measurement provides a way to find the optimal light intensity for desired ratio of open/closed reaction centers. Non-photochemistry quenching pathways include qE (Energy-dependent quenching), qT (State transition), and qI (Photo-inhibition). In this thesis, I discussed qE and qT mechanisms and their effects on photosynthesis efficiency. qT is activated under low light and lasts from seconds to minutes. qE is activated by stronger light, and responses differently as light wavelength varies; the reaction acts tens of minutes. I designed an illumination setup to stimulate either qE or qT response. I took continuous fluorescence lifetime measurement when either qE or qT was activated, and then calculated the corresponding photosynthesis efficiency. I demonstrated the advantages of fluorescence lifetime technique in studying non-photochemical quenching pathways and photosynthesis efficiency. The results in this thesis might contribute to research field related to food crisis and green energy.

abstract: Non-photochemical quenching (NPQ) is a photoprotective regulatory mechanism essential to the robustness of the photosynthetic apparatus of green plants. Energy flow within the low-light adapted reaction centers is dynamically optimized to match the continuously fluctuating light conditions found in nature. Activated by compartmentalized decreases in pH resulting from photosynthetic activity during periods of elevated photon flux, NPQ induces rapid thermal dissipation of excess excitation energy that would otherwise overwhelm the apparatus’s ability to consume it. Consequently, the frequency of charge separation decreases and the formation of potentially deleterious, high-energy intermediates slows, thereby reducing the threat of photodamage by disallowing their accumulation. Herein is described the synthesis and photophysical analysis of a molecular triad that mimics the effects of NPQ on charge separation within the photosynthetic reaction centers. Steady-state absorption and emission, time-resolved fluorescence, and transient absorption spectroscopies were used to demonstrate reversible quenching of the first singlet excited state affecting the quantum yield of charge separation by approximately one order of magnitude. As in the natural system, the populations of unquenched and quenched states and, therefore, the overall yields of charge separation were found to be dependent upon acid concentration.
Dissertation/Thesis
Doctoral Dissertation Chemistry 2015

Nonphotochemical quenching and state transitions are an important photoprotective mechanism against excessive irradiation. In this work I studied changes in the size of the effective crosssection of photosystem II antennae in regard to the level of nonphotochemical quenching (state transitions) under different levels of light induced stress.