Increased filtration efficiency of attached compared to free-swimming flagellates

Heterotrophic flagellates often attach to surfaces while feeding. A possible fluid dynamic reason is that attachment increases filtration efficiency. This was investigated experimentally and theoretically. The effect of attachment on flow field and clearance was studied by video microscopy in the nanoflagellates Paraphysomonas vestita and Pteridomonas danica. The flow field around feeding cells was visualised by latex bead suspensions. From a fluid dynamics point of view, it is advantageous to be sessile if the radius of the transectional area of the water flow that is swept for particles is smaller than 1.5 × cell radius. For flagellates with a smooth flagellum, this also requires that the flagellum beats with a large amplitude. For attached P. vestita, clearance is ca. 70% higher relative to free-swimming cells and for P. danica this value is ca. 34%. The difference between the 2 species accords with the fact that Pteridomonas sweeps a wider area of the water flow relative to its cell size than Paraphysomonas. Protozoa may attach for other reasons, such as to remain at sites with high food concentrations, but in the case of suspension feeders, it is also advantageous to attach in terms of fluid dynamics. It is likely that a large proportion of the heterotrophic nanoflagellates in plankton are associated with suspended particles that act as drift anchors, thus enhancing clearance.