Dynamics of odd microalgae

Chirality is a defining feature of life, visible at all scales from molecules to tissues and organisms. It is also an intrinsically out-of-equilibrium feature that has to be actively maintained by the cells and is regulated by specific pathways, though its function often remain elusive. The propagation of chirality across scales is thus a central question in biology. Recently, the self-organization of chiral units and the new properties emerging at the collective level have also attracted great interest among physicists. Here, we introduce an experimental system made of spinning microalgae, and investigate their dynamics. Importantly, all the algae spin in the same direction, breaking chiral symmetry. By comparing our experiments with theory and simulations, we show that hydrodynamic interactions between the cells are responsible for their self-organization. We then explore the phenomena that emerge at the collective level in this chiral active fluid.