Etude de la structure des communautés bactériennes du sédiment et de l'écologie de Vibrio aestuarianus pathogène de l'huître creuse Crassostrea gigas dans deux sites ostréicoles

Since several years, the French oyster farming copes with high mortality affecting juveniles and some adults of the Pacific oyster Crassostrea gigas. Studies conducted in the framework of a multidisciplinary project (MOREST) showed that mortality was related to several factors like physiological and genetic status of oyster, pathogens including bacteria of the genus Vibrio, and revealed that the sediment, on which the oyster cages are installed, might play a particular role. This has been suggested because of the concomitance of the mortalities and increased toxic compounds like ammonia and sulphides in the sediment. Moreover, the presence of Vibrio bacteria in the sediment indicates a dual role of sediment in the mortality onset. The physico-chemical parameters, the structure of bacterial communities and the abundance of Vibrio aestuarianus, pathogen of C. gigas, have been determined in sediments below oyster tables and outside tables of the Aber Benoit (Nord-Finistère, French Atlantic coast) and Auray River (Gulf of Morbihan) in different seasons for 2 years. This work confirms the existence of a role of sediment risk and also the influence of oyster tables on sediment biogeochemistry. Indeed, high concentrations of ammonia and sulphides have been recorded in sediments under the tables of both sites. In the AberBenoît, the impact of oyster tables on the sediment biogeochemistry was more evident whereas the bacterial community structure showed certain stability. The microflora of sediments of the Auray River appeared more dynamic and was influenced by external inputs. These results demonstrate the importance of the characteristics related to each site in the response of bacterial communities following a disturbance. Furthermore, members of Vibrionaceae group were scarce in the sediments. Partial 16S rRNA gene sequences of two pathogenic species, namely V. ordalii and V aestuarianus, were detected, suggesting that they were part of the dominant community in the sediment. The study of the ecology of V. aestuarianus showed that it was present in the sediment during cold and warm seasons. However, presence of V. aestuarianus in sediment was not correlated to its abundance in oysters. Experiments conducted in microcosms with a GFP-transformed strain of V. aestuarianus showed that this bacterium could easily circulate between the sediment, the seawater and oysters and induce mortalities. Our results highlight the important of sediment as a reservoir of V. aestuarianus.