Green algae have evolved several photo-protective responses to cope with high-light stress. The present study examines the metabolic changes during photoacclimation to high-light in Chlamydomonas reinhardtii using nuclear magnetic resonance and mass spectrometry. Using principal component analysis, a clear metabolic response to highlight intensity was observed on global metabolite pools in Chlamydomonas, with major changes in the levels of amino acids and related nitrogen metabolites. Amino acid biosynthesis was induced during short-term photoacclimation presumably to alleviate excess excitation pressure in the plastid. An increase in mitochondrial metabolism through downstream photorespiratory and glyoxylate metabolism, pathways thought to act in a photo-protective capacity, was also observed. Long-term light stress resulted in a significant increase in antioxidant metabolites, ascorbate and dehydroascorbate. These results suggest that metabolism plays a direct role in coping with the imbalance in the excess excitation pressure generated during high-light stress; however, this metabolomics survey has generated additional questions about the roles of nitrogen assimilation associated metabolites in photoacclimatory responses to high-light in Chlamydomonas.

Photoacclimation is a process by which photosynthetic capacity is regulated in response to environmental adjustments in terms of light regime. Photoacclimation is essential in determining the photosynthetic capacity to optimize light use and to avoid potentially damaging effects. Previous work in our laboratory has identified a gene, gpt2 (At1g61800) that is essential for plants to acclimate to an increase in growth irradiance. Furthermore, we observed that the accession Columbia-0 (Col-0) is unable to respond to increases in light. Therefore, a Quantitative Trait Locus (QTL) mapping analysis was performed in Landsberg erecta (Ler)/Columbia (Col) recombinant inbred line population to identify novel genes responsible for this variation to acclimation. In order to investigate the photoacclimation in Arabidopsis thaliana, photosynthetic capacity was measured in plants of the accession Wassileskija (WS) and in plants lacking expression of the gene At1g61800 (WS-gpt2) during acclimation from high to low light. Plants were grown for 6 weeks under high light (400 μmol.m-2.s-1) and half of them were transferred to low light (100 μmol.m-2.s-1) after six weeks. Gas exchange measurements were performed in order to measure the maximum capacity for photosynthesis. Acclimation to a decrease in light resulted in a decrease in the photosynthetic capacity in WS and WS-gpt2 plants. This shows that under lower or limiting light, photosynthesis was slowed down. Chlorophyll fluorescence analysis was carried out to measure changes in the quantum efficiency of PSII (ΦPSII) and non-photochemical quenching (NPQ) during acclimation. ΦPSII decreased in both WS and WS-gpt2 plants showing that under low light, PSII is more saturated. However, it was found that there was no significant changes in NPQ level for both WS and WS-gpt2. To estimate the total chlorophyll and chl a/b ratio, a chlorophyll composition analysis was performed. There was no significant changes in the total chlorophyll for both WS and WS-gpt2. However, the chlorophyll a/b ratio was seen to be decreased in low light plants representing an increase in light harvesting complexes relative to reaction centre core. Plants of WS and WS-gpt2 were also grown under natural variable light in an unheated greenhouse in Manchester, UK. This experiment was carried out to study the photosynthetic acclimation of plants under fluctuating light condition. A preliminary work on gene expression of gpt2 was conducted by doing reverse transcriptase PCR (RT-PCR). It shows that the gene expression of gpt2 decreased following transfer to low light plants in WS. Microarray analysis was also performed to investigate the role of GPT2 (if any) and to identify any potential gene that is important in high to low light acclimation.

No description available.

Study of the Arabidopsis thaliana knockout mutant lacking Lhcb3 (koLhcb3) have revealed a close similarity to the wild type plants. Growth rate, NPQ, qP, Φ(PSII), circular dichroism spectra, pigment composition and content of LCHII trimers have been found to be unaffected by this mutation. The proteomic analysis shows only some minor increases in the amount of Lhcb1 and Lhcb2. PAM fluorometry revealed a significant increase in the rate of the state 1 to state 2 state transition in the koLhcb3. None the less, the extent of state transition is identical to wild type. Alterations in the PSII-LHCII supercomplex structure have been demonstrated as well. The M-trimer was found to be rotated ~21° CCW. This altered binding of the LHCII M-trimer is likely the cause of the altered affinity resulting in the increased rate of state transition. Proteomic analysis of the phosphorylation of LHCII revealed a significant increase in state 1 and 2 LHCII phosphorylation relative to wild type. Investigation whether phosphorylation or the altered LHCII binding is the cause of the accelerated rate of state transition have not been conclusive so far. A Lhcb6 depleted mutant (koLhcb6) showed a significant alteration of the PSII-LHCII supercomplex structure and photosynthetic acclimation processes. The LHCII M-trimer is depleted in the PSII-LHCII supercomplexes causing the state transition process to be “stuck” in state 2 and the mutants ability to preform NPQ is inhibited as well. The Lhcb6 protein was concluded to be essential for the binding of the LHCII M-trimer to the PSII core as well as energy transfer. The depletion of LHCII M-trimer was linked to the reduced ability to photoacclimate using NPQ as well.

Picophytoplanktonic cells (0.2-2 μm) are the dominant phytoplankters in the largest marine biomes on Earth: the subtropical gyres. The overaching aim of this thesis is to develop algorithms that use remote-sensing observables to map the distribution of the smallest and most abundant member of picophytoplankton, Prochlorococcus, and assess its contribution to the marine carbon cycle. To understand how the photoacclimatory status and growth of Prochlorococcus and its sister genera Synechococcus are influenced by light and nutrients, experiments were conducted in the South Atlantic Gyre (SAG). Results from the manipulation experiments show that, in the central region of the SAG, nutrient addition can induce marked changes in the optical properties of Prochlorococcus cells when subjected to saturating light levels, leading to a decrease in cell abundance, whereas at the gyre periphery no substantive changes in cell growth or optical characteristics were observed. Since light plays a central role in shaping the distribution of cyanobacteria, an empirical algorithm based on relationships between Prochlorococcus abundance and remotely-sensed observables was developed. The outputs were then used in a modified primary production model to predict the vertical distribution of carbon fixation by Prochlorococcus. The models estimate that ∼ 3.4 x 10²⁷ Prochlorococcus cells in the global ocean fix 4.7 Gt C year^-1. Most of the cell biomass and primary productivity is concentrated in the subtropical gyres and areas near the Equatorial Convergence, and 61% of the carbon fixation occurs in the upper water column (0-45 metres), where only 43% of the cells reside. However, in the gyres, carbon fixation is highest (62%) in deeper layers (45-200m), and both cell abundance and carbon fixation show marked seasonal patterns. The models developed in this study provide an unprecedented view of the vertical distribution of Prochlorococcus cells and their corresponding rates of carbon fixation in the global ocean.

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

No description available.