Blooms of the diatom Haslea ostrearia are recorded worldwide in marine environments and occur frequently in oyster ponds in the Western French coast. This diatom produces a blue-green water soluble pigment named marennine that is known to turn oyster gills green, which provides a higher market value to the bivalves. Although H. ostrearia has been identified centuries ago, little is known about the ecological significance of the blue pigment. Marennine displays allelopathic, antioxidant or antibacterial activities in vitro and its use in aquaculture was considered as some prophylactic effects were demonstrated on farmed shellfish. The aims of this thesis were thus 1) to further identify the effects of this pigment on the growth of bacteria from the genus Vibrio that are frequently involved in bivalve mass mortality and 2) to ensure its safety on marine organisms (e.g. molluscs, crustaceans, echinoderms). Our results on bacterial growth demonstrated that the dose-response curves did not systematically present a linear pattern (e.g. “U shape”, hormetic responses) and that marennine could either totally inhibit the growth of Vibrio or stimulate it depending on the strain tested. Also, ecologically relevant doses could impair the survival and the development of the marine organisms tested. Such noxious effects seemed to only target early and fragile developmental stages as embryos and larvae, while adults appeared unaffected. Finally, our results question the exploitation of H. ostrearia in aquaculture and highlight the need to set exposure threshold doses to prevent any adverse effects but to benefit from the prophylactic strategy resulting from the use of marennine.

Haslea ostrearia est une diatomĂ©e cosmopolite pouvant prolifĂ©rer en milieu naturel et dont les efflorescences sont particulièrement frĂ©quentes dans les claires Ă  huĂ®tres de la cĂ´te Atlantique française. Cette diatomĂ©e produit la marennine, un pigment bleu-vert hydrosoluble connu pour induire le verdissement des branchies des huĂ®tres, augmentant ainsi leur valeur marchande. La marennine prĂ©sente des activitĂ©s allĂ©lopathiques, antioxydantes ou encore antibactĂ©riennes in vitro et son utilisation en aquaculture a Ă©tĂ© envisagĂ©e suite Ă  la mise en Ă©vidence d’effets prophylactiques chez des bivalves. Les objectifs de ce doctorat Ă©taient 1) d’identifier les effets de ce pigment sur la croissance des bactĂ©ries du genre Vibrio, frĂ©quemment impliquĂ©es dans la mortalitĂ© massive de bivalves et 2) de s’assurer de son innocuitĂ© sur des organismes marins (e.g. mollusques, Ă©chinodermes). Nos rĂ©sultats sur la croissance bactĂ©rienne ont montrĂ© que les relations dose-effet ne prĂ©sentaient pas systĂ©matiquement une tendance linĂ©aire et Ă©galement que la marennine pouvait totalement inhiber ou bien stimuler la croissance de Vibrio en fonction de la souche testĂ©e. De plus, des concentrations Ă©cologiquement pertinentes ont induit des effets nĂ©gatifs sur la survie et le dĂ©veloppement d’animaux marins. Ces effets indĂ©sirables n’ont cependant Ă©tĂ© observĂ©s que chez des stades fragiles de dĂ©veloppement tels que les embryons et les larves. Enfin, nos rĂ©sultats questionnent l’utilisation de H. ostrearia en aquaculture et soulignent la nĂ©cessitĂ© d’établir des doses seuils d’exposition afin de prĂ©venir tout effet indĂ©sirable, tout en bĂ©nĂ©ficiant des effets prophylactiques de la marennine.