Impact of the oyster Crassostrea gigas on a microbial community in Atlantic coastal ponds near La Rochelle

To assess the in situ impact of oysters Crassostrea gigas on planktonic protist and bac- teria communities and the potential contribution of protozoa to their food resource intake, the abun- dance and the diversity of protists and bacteria were followed in 2 Atlantic coastal ponds, with and without oysters. The protist biomass in such ponds was high, with a maximum in spring of 982 µg C l -1 and a minimum in winter of 179 µg C l -1 . Whatever the season, the presence of oysters (20 m -2 cor- responding to an average of 23 mg dry weight m -2 ) induced a significant decrease in > 5 µm protist abundance. On the contrary, planktonic organisms 5 µm particles. In spring, oyster grazing triggered dramatic changes in the protist community by lowering the taxo- nomic diversity. In autumn and winter, the presence of oysters deeply influenced the taxonomic structure of the protist communities: > 5 µm protists could only develop in the control pond, whereas they were removed by filtration in the oyster pond; on the contrary, 5 µm were the main protist resource for C. gigas; ciliates represented the second resource, with a substantial contribution in autumn; diatoms and dinoflagellates, though efficiently removed, represented a weak carbon resource. Our study supports the hypothesis that oysters may access the strong bacterioplanktonic production through hetero/mixotrophic protists, which would thus allow the transfer of carbon from the microbial loop towards C. gigas.

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