GRAZING IN THE MICRON AND THE SUBMICRON PARTICLE-SIZE RANGE - THE CASE OF OIKOPLEURA-DIOICA (APPENDICULARIA)

We used procaryotic and eucaryotic phytoplankton (1.21 and 4.5 mum = E quivalent Spherical Diameter) and fluorescent carboxylated beads (0.2 and 0.75 mum ESD) to obtain direct measurements of grazing by Oikopleu ra dioica in the micron (0.914-6.025 mum ESD) and submicron particle s ize range (0.2-0.75 mum ESD). Flow cytometry, used quantitatively, det ected processes in the sub-micron range that could not be differentiat ed by other direct methods, such as low aperture techniques for electr onic particle counting. We conclude from our observations that (1) Bas ed on the decrease of particle concentration in the food suspension, f iltration by the food concentration filter is a non selective process down to 0.2 mum. (2) Collection by the pharyngeal filter, and conseque ntly ingestion, of the very fine 0.2 mum beads is less efficient than that of the larger 0.75 mum microspheres and apparently diminishes wit h their concentration in the food suspension. (3) The fraction of 0.2 mum beads resuspended after sieving by the pharyngeal filter is accumu lated in the filter house (possibly by adhering on the internal walls of the house due to physico-chemical interactions of their surfaces, t hough the properties of the beads are probably different from naturall y occurring colloids), rather than being expelled as fecal pellets thr ough the water sphincter exit. These observations suggest that, as pro ducers of biogenic particles, O. dioica influence the fate of the fine colloidal material (<0.2 mum) in a different way than that of the sli ghtly larger material (e.g., 0.75 mum), the latter being exported pref erentially with fecal pellets while the finer particles would essentia lly contribute to the enrichment of the filter-house community microco sm.