Thrust dynamics of hairy flagella with three-dimensional wave pattern

The pivotal role of unicellular flagellates in aquatic food webs is facilitated by their flagella that generate thrust for propulsion and feeding current generation. In important flagellates, the flagellum is equipped with rigid hairs that reverts thrust direction and increases its magnitude by a factor 5-10 relative to a naked flagellum. We understand how this functions in flagella with planar waves, but many flagellates have three-dimensional wave patterns. Here we show that a wave of twist and torsion of alternating sign propagating along the beating flagellum is required to yield thrust of the magnitude observed in living cells. The resulting dynamic rotation of the densely spaced hairs yields a flapping pattern that drives an efficient feeding current in a dominating group of oceanic flagellates.