INGESTION OF A BACTERIVOROUS CILIATE BY THE OYSTER CRASSOSTREA-GIGAS - PROTOZOA AS A TROPHIC LINK BETWEEN PICOPLANKTON AND BENTHIC SUSPENSION-FEEDERS

The linked concepts of 'microbial loop' and 'protozoan trophic link' h ave been very well documented in filter-feeding microzooplankton such as copepods, but have not been applied to energy transfer to benthic s uspension-feeding macrofauna, with the exception of the recent demonst ration of heterotrophic flagellate assimilation by mussels. The oyster Crassostrea gigas obtains energy resources by filtering microalgae (s imilar to 5 to 100 mu m). However, in turbid estuaries, light-limited phytoplanktonic production cannot entirely account for oyster energy r equirements. Conversely, picoplankters (<2 mu m), which are main effec ters of coastal energy flow and matter cycling, are not efficiently re tained by oyster filtration. Ciliate protozoal as both micro-sized cel ls (similar to 5 to 100 run) and bacteria grazers, may represent a maj or intermediary in trophic transfer between picoplankton and metazoa. The ciliate Uronema was intensely cultured and labelled, using the cya nobacteria Synechococcus as an auto-fluorescent biomarker. The labelle d ciliates were offered as potential prey to oysters. We report here t he first experimental evidence of a significant retention and ingestio n of ciliates by oysters, supporting the role of protozoa as a realist ic trophic link between picoplankters and filter-feeding bivalves and thus enhancing their potential importance in estuarine microbial food webs.