Thèse de Pierre Ramond

Directeurs de thèse : C. de Vargas SBR Roscoff et Raffaele Siano Ifremer Dyneco Pelagos

Résumé

Protists are the eukaryotic share of microbial communities, in the ocean they represent the first link between the harsh aquatic environment and its biocenosis. The distinct roles and adaptations of marine protists to their environment constitutes their functional diversity. A number of marine protist have been discovered by DNA-based taxonomy, however due to their recent discovery the functional diversity of these organisms is still unknown. In this project, the functional diversity of marine protist is studied by coupling a genetic survey (V4 marker of 18S rDNA) of 1145 distinct samples from various coastal ecosystems and a trait approach constituted of 13 traits describing the ecological strategies of marine protists. As a first step, in terms of functional redundancy, changes in the community of marine protists were tightly coupled with changes in the functional role it expressed. These results contrasts with observations about prokaryotes and the distinct evolutionary process at stake are commented. The smallest size-fractions also displayed a higher functional diversity probably influenced by less stringent requirement and the higher pelagic resource availability for this compartment. In a second application associated to a tidal front, the influence of the environment on marine protists is studied. The phototrophic protists presented a maximum of taxonomic and functional diversity at the front. The diversity maximum was influenced by dispersal (at an ecotone) but also by physical cycles of nutrient inputs and stratification, which allowed to decrease competitive exclusion and to alternate the dominant ecological strategy. Reversely, the diversity of heterotrophic protists was less structured over this environment. It is postulated that heterotrophic protists could be influenced by similar processes as dispersal and resource availability, however because their nutrition is related to biological interactions, their distribution is less influenced by the environment. In a last section, parasitism of a single dinoflagellate species was showed to be carried out by few specialized protistan parasites. These results underline that the predation role of protistan communities might be dictated by the extent of specialized interactions involving heterotrophic protists and their prey.

Mots clés

Marine protists, ecology, functional diversity, metabarcoding, coastal ecosystems, environmental microbiolog