Heterotrophic protists as a trophic link between picocyanobacteria and thepearl oyster Pinctada margaritifera in the Takapoto lagoon (Tuamotu Archipelago, French Polynesia)

Pearl oysters are farmed in oligotrophic tropical atoll lagoons where plank tonic communities are dominated by production from cyanobacteria smaller th an 2 pm. Paradoxically, the pearl oyster Pinctada margaritifera only retain s particles larger than 2 Pm. In this study, we assess the relative contrib ution of hetero/mixotrophic microbiota to the available planktonic resource . In Takapoto Atoll, picocyanobacteria are the dominant biomass (20 mug C l (-1)). The carbon biomass of ciliates and dinoflagellates ranges from 1 to 24 and 0.5 to 5 mug C l(-1) respectively, with a mean of 6 mug C l(-1) for ciliates and 2 mug C l(-1) for dinoflagellates. The possible retention by P . margaritifera on a natural protist suspension was investigated. Due to it s high clearance rates (ca 20 l h(-1) g(-1)) the pearl oyster retained 85 m ug C h(-1) g(-1) from ciliates and 55 mug C h(-1) g(-1) from dinoflagellate s. Conversely, cyanobacteria were not efficiently retained by the bivalve a nd did not efficiently contribute to its diet. From our experiments, we con cluded that hetero/mixotrophic protists rapidly and efficiently process the picoplanktonic resource towards filter-feeders, particularly pearl oysters .