Flagellar swimmers oscillate between pusher- and puller-type swimming.

Self-propulsion of cellular microswimmers generates flow signatures, commonly classified as pusher and puller type, which characterize hydrodynamic interactions with other cells or boundaries. Using experimentally measured beat patterns, we compute that the flagellated green alga Chlamydomonas oscillates between pusher and puller, rendering it an approximately neutral swimmer, when averaging over its full beat cycle. Beyond a typical distance of 100μm from the cell, inertia attenuates oscillatory microflows. We show that hydrodynamic interactions between cells oscillate in time and are of similar magnitude as stochastic swimming fluctuations. From our analysis, we also find that the rate of hydrodynamic dissipation varies in time, which implies that flagellar beat patterns are not optimized with respect to this measure.