Allocation du carbone et métabolisme azoté chez l'haptophyte Tisochrysis lutea

The aim of this thesis is to improve knowledge on mechanisms involved in the response to nitrogen limitation and in lipid accumulation in the microalgae haptophyte Tisochrysis lutea. The wild type strain and a lipid accumulating mutant strain were grown under different nitrogen limitation and starvation and analyzed by functional genomics. Four genes of high-affinity nitrate/nitrite transporter (Nrt2) were identified and characterized to reveal the mechanisms involved in mineral absorption in this species. Transcriptomes of both strains were sequenced and proteins affected by nitrogen starvation and differentially expressed between the two strains were identified. We so identified the functions regulated by nitrogen deficiency and potentially involved in the accumulation of storage lipids. The responses of both strains to thin variations of nitrogen limitation were studied. The results of high-throughput proteomic analyzes suggest that the lipid-accumulation in the mutant strain is the result of carbon metabolism impacted overall, this spurred on signaling mechanisms. Two proteins have been studied since probably involved in carbon and nitrogen remobilization from amino acids catabolism during nitrogen limitation. This work increases knowledge on haptophytes, and brings assumptions on metabolic key involved in nitrogen limitation and carbon allocation in microalgae.

Ecophysiologie de la maturation sexuelle et de la ponte de l'huître creuse Crassostrea gigas : réponses métaboliques (respiration) et alimentaires (filtration, absorption) en fonction des différents stades de maturation

The study of maturation and spawning of the oyster is part of a research program to investigate the summer mortalities of the oysters, Crassostrea gigas in Marennes-Oléron Bay. Four maturity stages were simultaneously obtained by diet and thermal conditioning (immature, low maturation, mature and post-spawning stages). Measurements of clearance, filtration, absorption and respiration rates allowed a calculation of the scope for growth and hence an estimation of the oyster's energetic budget at various maturity stages. Male and female oysters had similar physiological responses. The filtration rate ranged from 2.4 to 2.6 1.h(-1) at the early stages of maturation and decreased to 1.8 1.h.' during the maturity stage. Growth rate resulting from gonad development did not induce filtration rate changes. Mature 2.5 and 1.5-year-old oysters showed a negative energy budget reaching -15 and -90 J.h(-1) respectively. By contrast, non-ripe oysters had scope for growth in the range 110 to 170 J.h(-1). A negative energy budget during the high maturation stage resulted from a reduced absorption efficiency. A new allometric relationship for the respiration model of C. gigas was defined during vitellogenesis with a 0.574 coefficient value. Based on Our results, the oyster's physiological weakness during vitellogenesis should be considered as a part of explanation for spring and summer mortalities of cultured oysters in Marennes-Oléron Bay.

Ecophysiologie de la maturation sexuelle et de la ponte de l'huître creuse Crassostrea gigas : réponses métaboliques (respiration) et alimentaires (filtration, absorption) en fonction des différents stades de maturation

The study of sexual maturation and spawning in the Pacific oyster (Crassostrea gigas) is part of a vast research programme that endeavours to understand the causes of mortality that occur sporadically during the spring and summer seasons in the Marennes-Oléron Bay. Thermal and diet conditioning were used to obtain oysters at each stage of maturity simultaneously. Using the measured rates of clearance, consumption, absorption and respiration provided estimates of growth potential and gave the energetic budget of oysters at different stages of sexual maturity. Physiological responses were similar for males and females. Filtration decreased from 2.4 to 2.6 l.h (-1) to 1.8 l.h (-1) with increasing maturity. Weight gain was associated with gonad development and did not appear to have an effect on the clearance rate. Oysters 2.5 years old showed a negative energy budget (-15 J h (-1)) at later maturity stages. This deficit was confirmed (90 J.h (-1)) in oysters 1.5 years old at the same stage of maturity. On the contrary, immature oysters, in the early stages of maturity or post-spawning, had a growth potential of 110 to 170 J.h (-1). The energy deficit observed at later stages of maturity was primarily due to absorption, which decreased sharply during peak gametogenesis. Using measured respiration rates, an allometric relationship specific to gonad growth was determined with a coefficient of 0.574. Low physiological performance of oysters, observed at later stages of sexual maturity, must be taken into account in research on the factors responsible for spring and summer mortalities affecting oyster farms in Marennes-Oléron.

Bases génétiques de la différenciation adaptative en milieu anthropisé chez Macoma balthica, un bivalve marin à fort flux génique

Dans un contexte environnemental anthropisé, fragmenté et soumis à un changement climatique rapide, l’appréhension des processus d'adaptation locale des organismes marins par l'étude de zones de contact entre taxa proches constitue une approche privilégiée. Dans ces zones, des génotypes hybrides persistent malgré un état de maladaptation liée à des incompatibilités génétiques endogènes et/ou des barrières exogènes. L'histoire biogéographique complexe de la telline baltique Macoma balthica fait émerger quatre zones hybrides européennes, dont l'une, localisée autour de la Pointe Finistère (France), est le résultat d’un contact entre deux stocks génétiques ayant divergé en allopatrie. Ces divergences sont susceptibles de rompre la coadaptation entre génomes nucléaire et mitochondrial en raison de l'émergence d'incompatibilités mitonucléaires (MNIs). Ainsi, les sous-unités protéiques des cinq complexes de la chaine OXPHO sont codées à la fois par des gènes nucléaires et mitochondriaux, et une coévolution intergénomique étroite est requise pour maintenir la production énergétique cellulaire. De précédentes données de transcriptomique dévoilent de probables MNIs chez M. balthica au niveau des complexes respiratoires I et V, Afin d’apporter des éléments de compréhension aux mécanismes de maintien des zones hybrides dans un contexte de pression anthropique, le présent travail se propose de tester l'hypothèse de putatives MNIs dans cette zone de contact. Pour cela, (i) six mitogénomes correspondant à cinq lignées haplotypiques divergentes en Europe ont été séquencés et l'architecture génomique a été étudiée conjointement à une cartographie des mutations des 13 gènes mitochondriaux, (ii) le niveau de transcription de 5 gènes nucléaires et 8 gènes mitochondriaux (complexe I à V) des individus hybrides a été comparé à celui des lignées parentales après détermination du statut d'hybridation de chaque individu (six populations françaises). A défaut d'apporter des éléments de réponses concrets quant à l'existence de MNIs chez M. balthica, et ses répercussions évolutives en terme de dépression d'hybridation, ce travail constitue un tremplin vers une étude approfondie de la zone hybride française en développant de nouveaux outils moléculaires, et de solides techniques expérimentales pour la conduite de futurs croisements artificiels.

Analyse du compartiment mésozooplanctonique et écologie alimentaire printanière de la sardine, Sardina pilchardus (Walbaum, 1782), et de l’anchois, Engraulis encrasicolus (Linné, 1758) adultes dans le Golfe de Gascogne

Dietary studies of marine species constitute an important key to improve the understanding of its biology and of its role in the ecosystem. Thus, prey-predator relationships structure and determine population dynamics and the trophic network at the ecosystem scale. Among the major study sites, the marine ecosystem is submitted to natural and anthropogenic constraints. In the North-Eastern part of the Atlantic Ocean, the Bay of Biscay is a large open area surrounded South by Spain and East by France. This bay is an historic place of intense fishery activities for which the main small pelagic species targeted are the pilchard, Sardina pilchardus and the anchovy, Engraulis encrasicolus. The aim of this work is to analyze the trophic ecology of these two small pelagic fish in spring in the Bay of Biscay. To do this, a first section is devoted to their prey composed by the mesozooplanktonic compartment, through a two-fold approach: the characterization of their spatio-temporal dynamics during the decade 2003-2013 and the measurement of their energetic content in spring. For this season, it appears that all prey types are not worth energetically and that the Bay of Biscay represents a mosaic of dietary habitat. Moreover, the spring mesozooplankton community presents a strong spatial structuration, a temporal evolution marked by a major change in abundance and a control by the microphytoplankton biomass. The second section of this work is relative to a methodological approach of the trophic ecology of S. pilchardus and E. encrasicolus. Three different trophic tracers have been used: isotopic ratios of carbon and nitrogen, parasitological fauna and mercury contamination levels. To improve the use of the first of these trophic tracers, an experimental approach has been conducted with S. pilchardus to determine a trophic discrimination factor. Finally, it appears that the use of these three trophic tracers has always been permitted to highlight a temporal variability of the relative trophic ecology of these fish. However, no spatial dynamics could be identified through these three trophic tracers.