MEPS 152:301-306 (1997)  -  doi:10.3354/meps152301

The linked concepts of 'microbial loop' and 'protozoan trophic link' have been very well documented in filter-feeding microzooplankton such as copepods, but have not been applied to energy transfer to benthic suspension-feeding macrofauna, with the exception of the recent demonstration of heterotrophic flagellate assimilation by mussels. The oyster Crassostrea gigas obtains energy resources by filtering microalgae (~5 to 100 µm). However, in turbid estuaries, light-limited phytoplanktonic production cannot entirely account for oyster energy requirements. Conversely, picoplankters (<2 µm), which are main effectors of coastal energy flow and matter cycling, are not efficiently retained by oyster filtration. Ciliate protozoa, as both micro-sized cells (~5 to 100 µm) and bacteria grazers, may represent a major intermediary in trophic transfer between picoplankton and metazoa. The ciliate Uronema was intensely cultured and labelled, using the cyanobacteria Synechococcus as an autofluorescent biomarker. The labelled ciliates were offered as potential prey to oysters. We report here the first experimental evidence of a significant retention and ingestion of ciliates by oysters, supporting the role of protozoa as a realistic trophic link between picoplankters and filter-feeding bivalves and thus enhancing their potential importance in estuarine microbial food webs.

Oyster · Food sources · Picoplankton · Protozoa · Trophic link