We used procaryotic and eucaryotic phytoplankton (1.21 and 4.5 μm = Equivalent Spherical Diameter) and fluorescent carboxylated beads (0.2 and 0.75 μm ESD) to obtain direct measurements of grazing by Oikopleura dioica in the micron (0.914–6.025 μm
ESD) and submicron particle size range (0.2–0.75 μm ESD). Flow cytometry, used quantitatively, detected processes in the sub-micron range that could not be differentiated by other direct methods, such as low aperture techniques for electronic particle counting. We conclude
from our observations that (1) Based on the decrease of particle concentration in the food suspension, filtration by the food concentration filter is a non selective process down to 0.2 μm. (2) Collection by the: pharyngeal filter, and consequently ingestion, of the very fine 0.2
μm beads is less efficient than that of the larger 0.75 μm microspheres and apparently diminishes with their concentration in the food suspension. (3) The fraction of 0.2 μm beads resuspended after sieving by the pharyngeal filter is accumulated in the filter
house (possibly by adhering on the internal walls of the house due to physico-chemical interactions of their surfaces, though the properties of the beads are probably different from naturally occurring colloids), rather than being expelled as fecal pellets through the water sphincter exit.
These observations suggest that, as producers of biogenic particles, O. dioica influence the fate of the fine colloidal material (<0.2 μm) in a different way than that of the slightly larger material (e.g., 0.75 μm) the latter being exported preferentially with
fecal pellets while the finer particles would essentially contribute to the enrichment of the filter-house community microcosm.
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