A universal mechanism for chiral swimming at low Reynolds number

Eukaryotes evolved radically new ways to sense and respond to a constantly changing environment. Even single-celled microorganisms display near-determinstic navigation towards sensory cues. Helical alignment towards gradients is a ubiquitous mechanism by which ciliated and flagellated organisms achieve reorientation in three-dimensional space. This navigational strategy usually requires the presence of a specialised organelle which detects stimulus direction and an about-axis rotation of the organism. Here, we consider the biflagellate green alga Chlamydomonas reinhardtii which has an eyespot photosensor and rotates steadily at 2Hz in the absence of stimuli. We combine theory and experiment to reveal how Chlamydomonas actively modulates asymmetries in flagellar beating to produce axial rotation and steering. In previous models, helical swimming is usually assumed rather than derived. In contrast, in our model chiral axial rotation emerges naturally from the 3D flagellar dynamics. We perform high-speed imaging to measure key simulation parameters, which successfully reproduce the experimentally observed dynamics. Finally, we demonstrate how this coupling between signal detection and motor output constitutes a universal strategy for responding to an arbitrary, vectorial stimulus.