FILTRATION OF THE ENTERIC BACTERIA ESCHERICHIA-COLI BY 2 FILTER-FEEDING BIVALVES, VENUS-VERRUCOSA AND MYTILUS-GALLOPROVINCIALIS .1. EXPERIMENTAL-STUDY

We investigated the filtration and utilization of the enteric bacteria Escherichia coli by two suspension-feeding bivalves, Venus verrucosa (Linne), collected, during April 1990, in Port-Vendres harbour (France), and Mytilus galloprovincialis (Lamarck) collected from rearing units in the bay of Banyuls-sur-mer during June 1990. Because of a significant decrease in the concentration of culturable E. coli in filtered seawater, we used C-14 glutamic acid to label the bacteria. Labelling efficiency was low (20%) compared to the 30% reported for the marine bacteria Lactobacillus sp. by Amouroux (1982). However, the labelling by this radioisotope was very stable, enabling its use to monitor filtration. Concentrations of culturable E. coli decreased more rapidly in the presence of M. galloprovincialis than in the presence of V. verrucosa. In both bivalve species, changes of radioactivity within the bivalve, particulate organic matter (POM), dissolved organic matter (DOM) and CO2 compartments were similar and resulted from the interaction of several processes: filtration, biodeposition, and recycling. This interaction complicates the determination of the actual ingestion and assimilation rates, and necessitates the use of mathematical modelling.