Mechanisms and fluid dynamics of foraging in heterotrophic nanoflagellates

Heterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean. In their microscale world, viscosity impedes predator–prey contact, and the mechanisms that allow flagellates to daily clear a volume of water for prey corresponding to 10⁶ times their own volume is unclear. It is also unclear what limits observed maximum ingestion rates of about 10⁴ bacterial preys per day. We used high-speed video microscopy to describe feeding flows, flagellum kinematics, and prey searching, capture, and handling in four species with different foraging strategies. In three species, prey handling times limit ingestion rates and account well for their reported maximum values. Similarly, observed feeding flows match reported clearance rates. Simple point force models allowed us to estimate the forces required to generate the feeding flows, between 4 and 13 pN, and consistent with the force produced by the hairy (hispid) flagellum, as estimated using resistive force theory. Hispid flagella can produce a force that is much higher than the force produced by a naked (smooth) flagellum with similar kinematics, and the hairy flagellum is therefore key to foraging in most nanoflagellates. Our findings provide a mechanistic underpinning of observed functional responses of prey ingestion rates in nanoflagellates.