Asymmetries in the three-dimensional beat of Chlamydomonas reinhardtii flagella revealed by holographic microscopy.

We present the first three-dimensional time-resolved imaging of the Chlamydomonas reinhardtii flagellar waveform. This freshwater alga is a model system for eukaryotic flagella that allow cells to move and pump fluid. During the power stroke, the flagella show rotational symmetry about the centre line of the cell, but during the recovery stroke they display mirror symmetry about the same axis. Furthermore, and in contrast to the usual assumptions about beat planarity, we show a subtle rotational motion of the flagella at the initiation of the power stroke, which is mechanically rectified into a quasi-planar mode. We apply resistive force theory to infer the swimming speed and rotational speed of the cells, when a force-free configuration is approximated using a cell on a micropipette, showing good agreement with experimental results on freely swimming cells.