Inferring intraciliary dynamics from the gliding motility of Chlamydomonas reinhardtii
The unicellular microalga Chlamydomonas reinhardtii is widely recognized as a premier model living microswimmer for physicists and biophysicists. However, the interest around C. reinhardtii goes beyond its swimming capabilities. In fact, light can drastically alter its behavior: under blue illumination, the cell attaches to a nearby surface and intermittently glides on it. Such a gliding motility is powered by molecular-motor proteins operating on the cell's cilia, and the related machinery has established the cell as a prime reference for the study of intraciliary-transport mechanisms. This is what we focus on in the present work, by combining in-line holographic microscopy -which leads to unprecedented spatial and temporal resolutions on the gliding dynamics -and statistical inference. We show that, while gliding, the cells exhibit anomalous-diffusive features, including Lorentzian-like distributions of displacements, which are reminiscent of enhanced search strategies. The latter may be exploited by the cells to facilitate colony formation, or, more broadly, by organisms possessing an intraciliary-transport machinery for the transport of cargo molecules and signaling. Furthermore, gliding trajectories, by being intermittent, are valid candidates to infer forces at the molecular-motor scale that are necessary for the cells to move, or symetrically, to transport cargo molecules. We report a gliding threshold of about 20 pN, compatible with the activity of single molecular motors.