Les réseaux génétiques, constitués de gènes qui interagissent entre eux par l'intermédiaire de protéines régulatrices modulant leurs activités, sont des systèmes non linéaires qui présentent une variété de comportements dynamiques tels que la multistabilité ou les oscillations. Le développement des approches systémiques en biologie a permis l'identification de modules génétiques dont le comportement est quantitativement modélisable de sorte que leur fonction et leur structure puissent être étudiées et comprises. Notre expérience des systèmes non linéaires, ainsi que de la modélisation de systèmes expérimentaux, nous a conduit à l'étude de réseaux minimaux ayant la capacité d'osciller.Tout d'abord, nous avons revisité la dynamique d'un gène réprimé par sa propre protéine dans le cas où le taux de transcription ne s'adapte pas instantanément à la concentration en protéine mais est une variable dynamique. En effet, de nouvelles techniques de détection in vivo de l'ARN ont mis en évidence les salves de transcription d'un gène dans une cellule vivante. Nous avons obtenu un critère analytique pour l'apparition des oscillations entretenues et avons trouvé qu'elles nécessitent des mécanismes de dégradation moins non linéaires que pour une régulation infiniment rapide. Les prédictions obtenues par une approche déterministe ont été confirmées par des simulations stochastiques.Nous avons ensuite étudié un modèle mathématique minimal d'oscillateur circadien qui ajuste de façon surprenante les profils d'expression de deux gènes centraux de l'horloge de l'algue verte microscopique Ostreococcus tauri, TOC1 et CCA1. Outre cet accord entre théorie et expérience, nous avons constaté que le meilleur ajustement des données d'expression enregistrées en alternance jour/nuit est obtenu lorsqu'aucun paramètre du modèle ne dépend de l'intensité lumineuse, comme si l'oscillateur n'était pas sensible au cycle jour/nuit. Nous avons montré que ce phénomène contre-intuitif est en fait compatible avec un couplage à la lumière restreint à une fenêtre temporelle courte et judicieusement placée dans la journée. Cela confère à cette horloge circadienne une grande robustesse, de telle sorte que l'oscillateur est à la fois sensible à un éventuel déphasage nécessitant une remise à l'heure, et insensible aux fluctuations de l'intensité de la lumière du jour. / Networks of genes interacting via regulatory proteins modulating their activities are highly nonlinear systems wich display a variety of dynamical behaviour, such multistability or oscillations. The development of systemic approaches in biology has put emphasis on identifying genetic modules whose behavior can be modeled quantitatively so that their function and structure can be studied and understood. Our experience in nonlinear systems and modeling of experimental systems has led us to study minimal oscillating networks. First, we have revisited the dynamics of a gene repressed by its own protein in the case where the transcription rate does not adapt instantaneously to protein concentration but is a dynamical variable. Indeed, burst-like gene transcription has been monitored with new in vivo technique for tracking single-RNA molecule. We have derived analytical criteria for the appearance of sustained oscillations and found that they require degradation mechanisms much less nonlinear than for infinitely fast regulation. Deterministic predictions are confirmed by stochastic simulations of this minimal genetic oscillator. Secondly, we have studied a minimal mathematical model of a circadian oscillator, wich is in surprisingly good agreement with expression profiles of two central clock genes TOC1 and CCA1 of the microscopic green alga Ostreococcus tauri. We not only found that this two-gene transcriptional loop model can reproduce almost perfectly transcript and protein profiles but observed that excellent adjustment of data recorded under light/dark alternation is obtained when no model parameter depends on light intensity. Furthermore, we have shown that this paradoxical behaviour is in fact compatible with a coupling to light that is confined to short temporal windows and judiciously scheduled during the day. This circadian clock is robust in that the oscillator is both sensitive to phase shifts when resetting is required and insensitive to daylight fluctuations.

Relógios endógenos controlam grande parte de processos biológicos através de osciladores bioquímicos que coordenam a sinalização de pistas ambientais até vias metabólicas, permitindo a percepção do tempo e adaptação a mudanças rítmicas. Comportamentos cíclicos diários foram primordialmente descritos em plantas e, mais recentemente, têm fornecido informações valiosas sobre os ciclos de retroalimentação da transcrição e tradução de genes que controlam estes osciladores. O florescimento é um exemplo bem conhecido da importância da percepção do comprimento do dia através do relógio, processo intimamente regulado por fotorreceptores e pelos genes centrais e periféricos do relógio biológico. Em organismos multicelulares há uma combinação específica de genes mais expressa em cada tecido, podendo ter funções, fases e períodos diferentes, o que aumenta a complexidade desse mecanismo. Devido a isso, tem-se buscado modelos alternativos mais simples dentro dos eucariotos fotossintetizantes relacionados às plantas terrestres. Modelos simplificados facilitam, por exemplo, a avaliação da combinação de fatores que induzem o estresse e como o relógio biológico se altera, permitindo a antecipação de mudanças ambientais e sincronização da fisiologia com o meio ambiente. Neste trabalho, verificou-se como o relógio circadiano se ajusta ao estresse em 3 diferentes modelos: Gracilaria tenuistipitata (Rhodophyta), Ostreococcus tauri (Chlorophyta) e Saccharum sp (Embryophyta). Para isso, estabeleceu-se em G. tenuistipitata métodos para avaliação de crescimento e da fluorescência da clorofila de modo automático, comprovando da existência de ritmos circadianos. Além disso, após padronização de genes de referência para normalização das RT-qPCRs, o gene TRX ficou superexpresso durante a primeira hora após o déficit hídrico. Já em O. tauri, onde os genes centrais do relógio são conhecidos, mudanças na expressão de LOV-HK e TOC1 estão relacionadas com maior crescimento em baixa e alta temperatura, respectivamente. Uma combinação específica de luz, temperatura e salinidade pode ser um importante indutor de eflorescências que reflete mudanças transcricionais no oscilador central, o que pode ser comparado às florescências de plantas terrestres. Já em Saccharum sp tolerante à seca, ritmos de fotossíntese e de expressão de CCA1 sofrem mudanças de fase em suas oscilações e transcritos de HVA-22 e DRP são significativamente mais expressos sob dessecação. Em suma, o estresse em Saccharum sp reseta o relógio, aumentando o período de oscilação da fotossíntese. Em O. tauri induz maior crescimento, mantendo as características do relógio. Não foi possível avaliar o efeito do estresse no relógio de G. tenuistipitata, mas ferramentas foram desenvolvidas visando este objetivo. Estudos de respostas do relógio podem fornecer informações valiosas para o entendimento da reprodução e crescimento de organismos com elevado potencial de aplicações biotecnológicas. / Endogenous clocks control a large range of biological processes through biochemical oscillators that coordinate the signaling of environmental cues to metabolic pathways, allowing the perception of time and adjust to rhythmic changes. Cyclical daily behaviors were first noticed in plants and, more recently, revealed information about the transcriptional-translational feedback loops of genes that control these oscillators. Flowering is a well-known process where the perception of day length by the clock is intimately regulated by photoreceptors and by the central and peripheric genes of the biological clock. Multicellular organisms have a tissue-specific combination of expressed clock genes that may have different phase and period, increasing the complexity of this mechanism. Due to this reason, alternative models have been proposed for land plants-related photosynthetic eukaryotes. New models can simplify, for example, which combination of factors induce stress and how the biological clock is altered, allowing the anticipation of environmental changes and synchronization of physiology and environmental factors. This work aimed to verify how the biological clock adjusts to different kinds of stresses in 3 species: Gracilaria tenuistipitata (Rhodophyta), Ostreococcus tauri (Chlorophyta) and Saccharum sp (Embryophyta). Automated measurement techniques for growth rate and photosynthesis were stablished for the red alga. This alga also showed, after establishment of reference genes for RT-qPCRs normalization, an overexpression of TRX during the first hour under water deficit. In O. tauri, where the central clock genes are known, changes in LOV-HK and TOC1 gene expression are related to a higher growth rate under low and high temperatures, respectively. Besides, a specific combination of light, temperature and salinity can be an important trigger of seasonal blooms that causes important transcriptional changes at the central oscillator, what is similar to land plants. In Saccharum sp tolerant to drought, photosynthesis rhythms and CCA1 expression change their phase under simulated water deficit and drought responsive transcripts like HVA-22 and DRP are significantly up-regulated. In short, stress resets the clock in Saccharum sp, increasing the period of photosynthesis oscillation. In O.tauri, it induces a higher growth, keeping clock features. It was not possible to verify clock responses to stress in G.tenuistipitata, but methods to do so were stablished. The biological clock responses to stress can provide invaluable information for the better understanding about the growth and reproduction of organisms with a high biotechnological potential

Les microalgues du phytoplancton sont exposées à des variations fréquentes et rapides de la qualité et de l'intensité spectrale en milieu marin. On peut donc supposer qu'il existe des mécanismes de photoperception spécifiques aux microalgues, différents de ceux identifiés chez les organismes terrestres. L'importance de l'horloge circadienne dans la transmission de l'information lumineuse et notamment la photopériode a largement été caractérisée chez plusieurs organismes modèles terrestres. Le principal objectif de ma thèse était d'étudier les régulations des gènes de l'horloge en réponse à la lumière, chez la microalgue Ostreococcus tauri. Le développement récent des techniques de génomique fonctionnelle chez cette microalgue eucaryote l'a promue comme un nouvel organisme modèle pour l'étude de mécanismes complexes tels que horloge circadienne. Mon étude s'est focalisée sur la caractérisation d'une voie de signalisation de type système à deux composants susceptible de transmettre le signal lumineux vers l'oscillateur central de l'horloge. J'ai étudié les régulations des principaux acteurs de l'horloge d'Ostreococcus par la lumière, et en particulier celles du gène TOC1. J'ai aussi caractérisé la protéine LOV-HK, un nouveau type de photorécepteur à la lumière bleue chez les eucaryotes, dont l'activité est requise pour le bon fonctionnement de l'horloge d'Ostreococcus. L'importance des régulations transcriptionnelles de TOC1 et de LOV-HK, ainsi que leurs fonctions dans l'oscillateur central ont été abordées par l'utilisation d'un promoteur inductible. Enfin, j'ai montré que LOV-HK et plus globalement l'horloge régulent la croissance cellulaire et la biomasse, démontrant leur rôle central dans le contrôle de la physiologie d'Ostreococcus tauri. / Light quality and intensity change frequently in the water column. Therefore marine microalgae are exposed to large changes in light spectrum. Photoperception mechanisms in microalgae are expected to differ from those of land plants since the marine environment has unique properties of light transmission. The focus of my PhD project concerns two mains topics, circadian clock regulation and photoperception in the microalga Ostreococcus tauri. In recent years, O. tauri has emerged as a promising model organism using functional genomics approaches to study complex processes such as the circadian clock regulations. My study was focused on the involvement of a two components system in light transmission to the circadian clock of Ostreococcus. I have studied the molecular mechanisms underlying the regulation of the core clock component TOC1. I have also characterized a novel eukaryotic blue light photoreceptor called LOV-HK, which regulates circadian clock function in Ostreococcus. Using an inducible promoter system to modulate the levels of TOC1 and LOV-HK, I have analyzed the importance of their transcriptional regulations in the clock. Finally, I have shown that LOV-HK and more generally the circadian clock, regulates cell growth and biomass in Ostreococcus tauri.

Relógios endógenos controlam grande parte de processos biológicos através de osciladores bioquímicos que coordenam a sinalização de pistas ambientais até vias metabólicas, permitindo a percepção do tempo e adaptação a mudanças rítmicas. Comportamentos cíclicos diários foram primordialmente descritos em plantas e, mais recentemente, têm fornecido informações valiosas sobre os ciclos de retroalimentação da transcrição e tradução de genes que controlam estes osciladores. O florescimento é um exemplo bem conhecido da importância da percepção do comprimento do dia através do relógio, processo intimamente regulado por fotorreceptores e pelos genes centrais e periféricos do relógio biológico. Em organismos multicelulares há uma combinação específica de genes mais expressa em cada tecido, podendo ter funções, fases e períodos diferentes, o que aumenta a complexidade desse mecanismo. Devido a isso, tem-se buscado modelos alternativos mais simples dentro dos eucariotos fotossintetizantes relacionados às plantas terrestres. Modelos simplificados facilitam, por exemplo, a avaliação da combinação de fatores que induzem o estresse e como o relógio biológico se altera, permitindo a antecipação de mudanças ambientais e sincronização da fisiologia com o meio ambiente. Neste trabalho, verificou-se como o relógio circadiano se ajusta ao estresse em 3 diferentes modelos: Gracilaria tenuistipitata (Rhodophyta), Ostreococcus tauri (Chlorophyta) e Saccharum sp (Embryophyta). Para isso, estabeleceu-se em G. tenuistipitata métodos para avaliação de crescimento e da fluorescência da clorofila de modo automático, comprovando da existência de ritmos circadianos. Além disso, após padronização de genes de referência para normalização das RT-qPCRs, o gene TRX ficou superexpresso durante a primeira hora após o déficit hídrico. Já em O. tauri, onde os genes centrais do relógio são conhecidos, mudanças na expressão de LOV-HK e TOC1 estão relacionadas com maior crescimento em baixa e alta temperatura, respectivamente. Uma combinação específica de luz, temperatura e salinidade pode ser um importante indutor de eflorescências que reflete mudanças transcricionais no oscilador central, o que pode ser comparado às florescências de plantas terrestres. Já em Saccharum sp tolerante à seca, ritmos de fotossíntese e de expressão de CCA1 sofrem mudanças de fase em suas oscilações e transcritos de HVA-22 e DRP são significativamente mais expressos sob dessecação. Em suma, o estresse em Saccharum sp reseta o relógio, aumentando o período de oscilação da fotossíntese. Em O. tauri induz maior crescimento, mantendo as características do relógio. Não foi possível avaliar o efeito do estresse no relógio de G. tenuistipitata, mas ferramentas foram desenvolvidas visando este objetivo. Estudos de respostas do relógio podem fornecer informações valiosas para o entendimento da reprodução e crescimento de organismos com elevado potencial de aplicações biotecnológicas. / Endogenous clocks control a large range of biological processes through biochemical oscillators that coordinate the signaling of environmental cues to metabolic pathways, allowing the perception of time and adjust to rhythmic changes. Cyclical daily behaviors were first noticed in plants and, more recently, revealed information about the transcriptional-translational feedback loops of genes that control these oscillators. Flowering is a well-known process where the perception of day length by the clock is intimately regulated by photoreceptors and by the central and peripheric genes of the biological clock. Multicellular organisms have a tissue-specific combination of expressed clock genes that may have different phase and period, increasing the complexity of this mechanism. Due to this reason, alternative models have been proposed for land plants-related photosynthetic eukaryotes. New models can simplify, for example, which combination of factors induce stress and how the biological clock is altered, allowing the anticipation of environmental changes and synchronization of physiology and environmental factors. This work aimed to verify how the biological clock adjusts to different kinds of stresses in 3 species: Gracilaria tenuistipitata (Rhodophyta), Ostreococcus tauri (Chlorophyta) and Saccharum sp (Embryophyta). Automated measurement techniques for growth rate and photosynthesis were stablished for the red alga. This alga also showed, after establishment of reference genes for RT-qPCRs normalization, an overexpression of TRX during the first hour under water deficit. In O. tauri, where the central clock genes are known, changes in LOV-HK and TOC1 gene expression are related to a higher growth rate under low and high temperatures, respectively. Besides, a specific combination of light, temperature and salinity can be an important trigger of seasonal blooms that causes important transcriptional changes at the central oscillator, what is similar to land plants. In Saccharum sp tolerant to drought, photosynthesis rhythms and CCA1 expression change their phase under simulated water deficit and drought responsive transcripts like HVA-22 and DRP are significantly up-regulated. In short, stress resets the clock in Saccharum sp, increasing the period of photosynthesis oscillation. In O.tauri, it induces a higher growth, keeping clock features. It was not possible to verify clock responses to stress in G.tenuistipitata, but methods to do so were stablished. The biological clock responses to stress can provide invaluable information for the better understanding about the growth and reproduction of organisms with a high biotechnological potential

Small but mighty phytoplankton can be used as excellent model organisms to answer questions that are of importance to marine biologists and researchers in experimental evolution alike. For example, marine biologists are interested in finding out, how, in a changing ocean, the phytoplankton foundation of the ocean ecosystem is going to change - can we use short-term data to extrapolate to longer timescales? What are the physiological consequences of selection in stable and fluctuating high-pCO2 environments? From a more evolutionary perspective, is elevated pCO2 alone enough to drive evolution in marine algae? Can we select organisms to maintain plasticity in fluctuating environments, and how does selection in a fluctuating environment affect their ability to evolve? Can we detect a cost of plasticity? I have used theoretical and practical approaches from both disciplines to answer these questions, as they are ultimately similar questions that are important to address, and the lack of communication between disciplines has lead to conflicting predictions on the fate of populations in changing environments. Using evolutionary theory and applying it to an organism with a known function in the marine environment allows us to make ecologically relevant predictions while also enabling us to disentangle the underlying evolutionary mechanisms. While there have been some studies focusing on evolution of marine algae in climate change scenarios since I started my PhD, my study is the first to test the link between phenotypic plasticity and adaptation empirically, and it is also the first to use 16 rather than single or few genotypes of an algae, thereby creating the statistical power necessary to make any predictions. In a short-term CO2 enrichment study, and a selection experiment, those 16 physiologically and genetically distinct lineages of Ostreococcus, the smallest free living eukaryote, were selected for 400 generations in fluctuating and stable high pCO2 environments. I have shown that short-term plastic responses in phenotype can predict the magnitude of long-term evolutionary ones. Ostreococcus lineages in fluctuating environments evolve to be more plastic with no associated costs, and the adaptive response to selection in a high pCO2 environment is to grow more slowly in monoculture, but to be more successful competitors in mixed culture. High-pCO2 evolved lineages are genetically and physiologically different from their ancestors. Importantly, their quality as a food source for zooplankton will change, with possible repercussions for the ocean ecosystem at a whole. Furthermore, the lineages’ ability to perceive pCO2 levels in the surrounding medium is altered after evolution in fluctuating and high pCO2 environment, allowing them to broaden the window in which they can respond to changes in their environment without suffering metabolic stress.

La picoalgue verte marine Ostreococcus tauri est un eucaryote minimal développé en système modèle et qui a servi de ressource de gènes en biologie des lipides. Des informations détaillées sur ces caractéristiques lipidiques étaient cependant manquantes. Lors de ma thèse j’ai caractérisé le glycérolipidome d’O. tauri et ai cherché à déterminer quelles sont les cibles enzymatiques responsables de la dynamique des acides gras (FA) et de la régulation du métabolisme lipidique en réponse à des modifications de l’environnement (carences nutritives et refroidissement). O. tauri présente des caractéristiques uniques de composition en lipides et en FA mixtes entre les algues vertes et les Chromalveolates, et a été validé comme espèce modèle pour la classe des Mamiellophyceae. L’acide docosahexaénoïque (DHA) est confiné dans les lipides présumés extraplastidiaux : le phosphatidyldimethylpropanethiol (PDPT) et le bétaïne-lipide diacylglyceryl-hydroxymethyl-trimethyl-β-alanine (DGTA), tous deux marqueurs lipidiques des Chromalveolates. Les lipides plastidiaux de type procaryotique sont caractérisés par une prépondérance de FA polyinsaturés (PUFA) en C18 n-3, le 18:5 n-3 étant restreint aux galactolipides. Le 16:4 n-3, PUFA typique des galactolipides des microalgues vertes, est également un composant majoritaire des lipides extraplastidiaux chez O. tauri. Les triacylglycérols (TAG) présentent tout le panel d’acides gras d’O. tauri et leurs combinaisons moléculaires indiquent une origine plastidiale majoritaire. La carence azote provoque une forte accumulation de TAG, notamment des espèces présentant des combinaisons sn-1/sn-2 en 18:X/16:X et s'accompagne d'un transfert de carbone du phosphatidylglycérol (PG) et du monogalactosyldiacylglycérol (MGDG) aux TAG ce qui indique une contribution croissante de la voie plastidiale à la synthèse des TAG. Des expériences préliminaires de RT-qPCR sur des gènes du métabolisme des TAG révèlent une forte activation transcriptionnelle de certaines diacylglycérol acyltransférases (DAGT). Les carences nutritives répriment sévèrement l’activité Δ6 désaturase, générant une inversion du ratio 18:3/18:4 dans les lipides plastidiaux qui se répercute dans les TAG. La régulation fine et dynamique de ce ratio suggère un rôle important du 18:3 et du 18:4 dans les membranes plastidiales. Le refroidissement engendre une augmentation spécifique du 18:5 des galactolipides. La recherche active de la désaturase responsable de ce phénotype par une approche d'expression de gènes candidats en systèmes homologue et hétérologues (S. cerevisiae, N. Benthamiana) a conduit à l’indentification de deux Δ6 désaturases plastidiales jamais caractérisées dans d'autres systèmes. Celles-ci possèdent des spécificités non redondantes et originales entre elles et par rapport à l'acyl-CoA-Δ6 d'O. tauri. / The marine green picoalga Ostreococcus tauri is a minimal eukaryote implemented as model system that has been used as gene resource for lipid biology. Detailed information about its lipidic features was however missing. During my PhD, I characterized O. tauri glycerolipidome and associated dynamics under environmental stresses such as nutrient starvations and chilling and investigated transcriptional variations of putative target enzymes responsible for these changes. O. tauri which could be validated as model for related species of the class Mamiellophyceae, was found to display unique lipidic features related to both green and Chromalveolates microalgae. Docosahexaenoic acid (DHA) is confined to presumed extraplastidial lipids i.e. phosphatidyldimethylpropanethiol (PDPT) and the betaine lipid diacylglyceryl-hydroxymethyl-trimethyl-β-alanine (DGTA); all of these compounds are hallmarks of Chromalveolates. Plastidial lipids found to be of prokaryotic type are characterized by the overwhelming presence of C18 n-3 polyunsaturated FA (PUFA), 18:5 n-3 being restricted to galactolipids. C16:4 n-3, an FA typical of green microalgae galactolipids, also was a major component of O. tauri extraplastidial lipids. Triacylglycerols (TAGs) display the complete panel of FAs, and their molecular combinations designate a major plastidial origin of DAG precursors. Nitrate starvation greatly increases TAG content, in particular 18:X/16:X (sn-1/sn-2) combinations, and was associated with the transfer of carbon from phosphatidylglycerol (PG) and monogalactosyldiacylglycerol (MGDG) to TAG indicating an increased contribution of the plastidial pathway to the TAG synthesis. Preliminary RT-qPCR experiments on TAG metabolism genes revealed an important transcriptional activation of some diacylglycerol acyltransferases (DGAT). Nutrient starvations severely repress Δ6 desaturase activity and result in the inversion of the 18:3/18:4 ratio in plastidial lipids that was feedback into TAG. The fine-tuning and dynamic regulation of the 18:3/18:4 ratio suggests an important physiological role of these FAs in photosynthetic membranes. Chilling generates an increase of 18:5 in galactolipids. The active quest for the desaturases responsible for this phenotype was achieved by expressing candidate genes in homologuous and heterologous (S. cerevisiae, N. Benthamiana) systems and led to the identification of two yet uncharacterized plastidial Δ6 desaturases. These desaturases display original and non-redundant specificity between each other and with the previously characterized in O. tauri Δ6 acyl-CoA desaturase.

The smallest free living eukaryote known as Ostreococcus tauri became along with some related species great experimental models for iron uptake research in marine picoplankton. The ecological context of such topic is very interesting considering the nature of adaptation to iron limitation and its connection to the copper metabolism. Our experiments has simulated iron and copper limiting conditions of the ocean, as a control we used iron and copper repleted cultivation media. The maximum cell counts were two- to threefold higher in iron-repleted medium compared to iron- depleted one. There was the only exception showing no difference in growth - O. lucimarinus coming from open ocean with high irradiance levels and very low iron concentrations, which all together made it the best adapted species. The reinoculation of cell cultures after a week cultivation into to the same iron/copper containing media led to unmasking the copper effect on growth, which was much weaker than encountered with iron (again except of O. lucimarinus). Iron sufficiency positively affects heme b and chlorophyll a and b content with no significant copper dependency. The circadian timing of heme b and chlorophyll a and b content shows the increasing trend during the day followed by decrease at night. This might be caused by...

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In marine environments, eukaryotic marine microalgae coexist with the viruses that infect them. Marine microalgae are the main primary producers in the oceans and are at the base of the marine food web. Viruses play important roles in top-down control of algae populations, cycling of organic matter, and as evolutionary drivers of their hosts. Algae must adapt in response to the strong selection pressure that viruses impose for resistance to infection. In addition to biotic selection pressures such as viral infections, algae must also adapt to their abiotic environment. Global climate change is affecting temperature, salinity, pH, light and nutrient concentrations in the oceans, particularly in surface waters, where microalgae live. Currently, little is known about how consistent the effects of viruses on their hosts are, whether the cost of host resistance varies across environments, and whether there is a trade-off between maintaining resistance to viruses and adapting to other environmental changes. The marine picoeukaryote Ostreococcus tauri is abundant in Mediterranean lagoons, where it experiences large fluctuations in environmental conditions and co-occurs with lytic viruses (Ostreococcus tauri viruses – OtVs). Viral infection causes lysis of susceptible (S) cells, however a small proportion of cells are resistant (R) and avoid lysis. Some resistant O. tauri populations can coexist with infectious viruses, and it has been proposed that these viruses are produced by a minority of susceptible cells within a mainly resistant population. These populations are referred to as resistant producers (RP). Virus production in RP populations is unstable and eventually they shift to R populations. I used O. tauri and one of its viruses, OtV5, as a model system to investigate whether cells that are susceptible or resistant to virus infection adapt to environmental change differently and whether there is a cost of being resistant. For the first time, I evolved susceptible and resistant hosts of a marine alga separately under a range of environments and directly compared their plastic and evolved responses. I showed that resistant populations of O. tauri maintained their resistance for more than 200 generations in the absence of viruses across all environments, indicating that the resistance mechanism is difficult to reverse. Furthermore, I did not detect a cost of being resistant, as measured by population growth rate and competitive ability. Virus production in RP populations stopped in all environments and all populations became R. In addition, I found that virus production in RP O. tauri populations can fluctuate before completely ceasing, and that phosphate affected the length of time it took for virus production to stop. These results, combined with mathematical modelling of O. tauri infection dynamics, provide support for the prediction that RP populations consist of a mixed population of susceptible and resistant cells. By examining multiple environments and resistance types, we can better understand first, how microalgae populations adapt to environmental change and second, the ecological and evolutionary consequences of maintaining resistance to viruses in common marine picoeukaryotes.

Le fer est un élément présent en abondance dans la croûte terrestre, indispensable à la quasi-totalité des êtres vivants. Cependant, en milieu marin la biodisponibilité du fer est souvent faible et sporadique. Les micro-algues du phytoplancton ont développé des stratégies pour faire face à cette limitation en fer et s’adapter à des niches écologiques variables. Les micro-algues vertes du genre Ostreococcus (Prasinophyceae) présentent une large distribution géographique dans l’océan mondial, et de nombreux écotypes venant de milieux contrastés ont été isolés. L’objectif principal de ma thèse était d’étudier les différentes stratégies mises en place par le genre Ostreococcus, et notamment l’influence de la lumière et de l’horloge circadienne, dans la gestion de la carence en fer. Mon travail s’est focalisé sur l’étude d’Ostreococcus tauri, écotype lagunaire (Clade C), que de récentes techniques de transformation par insertion et recombinaison homologue ont promu comme un organisme modèle pour des approches de génétique fonctionnelle. J’ai étudié la ferritine, une protéine impliquée dans la gestion de la réserve en fer chez de nombreux organismes, et mis en évidence sa régulation par l’alternance jour/nuit et l’horloge circadienne. J’ai montré son rôle dans l’assimilation du fer, la régulation de l’homéostasie du fer et le recyclage du fer intracellulaire lors d’une carence. Enfin, j’ai caractérisé les stratégies d’acclimatation et d’adaptation à la carence en fer chez plusieurs écotypes d’Ostreococcus, dont O. tauri, RCC 802 (Clade A), RCC 809 (Clade B) et un mutant de taille/biomasse. Une stratégie d’acclimatation par réduction de la biomasse cellulaire a été mise en évidence. / Iron is an abundant element in the earth crust and is essential for almost organisms. In the marine environment, however, its bioavailability is often low and the iron supplies sporadic. Phytoplanktonic species have developed various strategies to face iron limitation and adapt to different ecological niches. Green picoalgae from the genus Ostreococcus (Prasinophyceae) are widespread in the global ocean and numerous ecotypes have been isolated from contrasted environments. The main objective of my thesis was to identify the strategie(s) used by the genus Ostreococcus in response to iron starvation and in particular the influence of the day/night cycle and the circadian clock in the regulation of iron homeostasis. I focused my work on the lagoon ecotype, Ostreococcus tauri (Clade C), which has emerged as a model organism for functional genomics approaches thanks to the development of genetic transformation by random insertion and homologous recombination. I have studied ferritin, a protein involved in iron storage which is present throughout the tree of life. I showed that ferritin is regulated by the light/dark cycle and the circadian clock and that it is a key player in the regulation of iron uptake and the recycling. Finally, I characterized the acclimation and adaptation strategies to iron limitations of several Ostreococcus ecotypes including O.tauri, RCC802 (Clade A), RCC809 (Clade B) and a cell biomass mutant of O.tauri. The reduction of cell biomass appears to be a main mechanism of acclimation in response to iron limitation.