Ingestion of a bacterivorous ciliate by the oyster Crassostrea gigas:protozoa as a trophic link between picoplankton and benthic suspension-feeders

Abstract

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 suspensionfeedlng macrofauna, with the exception of the recent demonstration of heterotrophlc flagellate assimilation by mussels. The oyster Crassostrea gigas o b t a~n s energy resources by filtering microalgae (-5 to 100 pm). However, in turbld estuaries, lighl-limited phvtoplanktonic production cannot entirely account for oyster energy requirements. Conversely, picoplankters ( 1 2 pm), which are main effectors of c o a s t~~l energy flow and matter cycling, are not efficiently retained by oyster filtration C~liate protozoa, as both micro-sized cells (-5 to 100 pm) and bacteria grazers, may represent a malor intermediary in trophic transfer between picoplankton and met~lzoa. The ciliate Uronema was intensely cultured and labelled, using the cyanobacteria Synechococcus a s a n 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 clllates by oysters, supporting the role of protozoa as a real~stic trophic link between picoplankters and filter-feeding bivalves and thus enhancing their potential importance in estuarine microbial food webs.

Numerical analysis of the food web of an intertidal mudflat ecosystem on the Atlantic coast of France

“…Pelagic secondary producers (protozoa and zooplankton): Heterotrophic protozoa are a trophic intermediary between bacteria and oysters but have not been studied quantitatively in MOB (Le Gall et al 1997, Dupuy et al 1999, 2000. Thus, concentration data measured in the English Channel (Newell & Linley 1984) are used instead.…”

Phytoplankton composition and selective feeding of the pearl oyster Pinctada margaritifera in the Takapoto lagoon (Tuamotu Archipelago, French Polynesia):in situ study using optical microscopy and HPLC pigment analysis

“…This low retention is attributed to the absence of eulatero-frontal cirri on the gill filaments. However the indirect ingestion of the picoplanktonic biomass can be mediated by heterotrophic protists which then represent a trophic link between cyanobacteria and suspension-feeders (Le Gall et al 1997, Dupuy et al 1999.…”

Retention of ciliates and flagellates by the oyster Crassostrea gigas in French Atlantic coastal ponds:protists as a trophic link between bacterioplankton and benthic suspension-feeders

“…Recently, Bardouil et al (1996) showed that Crassostrea gigas easily consumes a nontoxic dinoflagellate and Kreeger & Newel1 (1996) clearly demonstrated in mussels the ingestion and assimilation of bacterial carbon via heterotrophic flagellates. From experimental work, Le Gall et al (1997) reported significant retention and ingestion of cultured bacterivorous ciliates, Uronema sp., by the oyster C. gigas. Heterotrophic protists, which are abundant in coastal ecosystems, may thus constitute an alternative or complementary food resource for benthic filter feeders, allowing the indirect recuperation of DOM and picoplanktonic production otherwise not accessible to them.…”