Density-dependent collective behaviour in Chlamydomonas reinhardtii

Phototaxis is migration of motile cells in response to a light stimulus, a behaviour shown by many protists.  Besides survival and growth of single-cell, it plays a crucial role at the global scale of the aquatic ecosystem and has technological relevance in bioreactors, microbiopropellers and artificial microswimmers. Here we present a quantitative measurement of phototaxis of single-celled biflagellate microalgae Chlamydomonas reinhardtii using high spatiotemporal video microscopy. Our findings reveal that the phototactic efficiency as a function of cell concentration is non-monotonic and re-entrant in nature.   We demonstrate that collective enhancement in the phototactic efficiency originates from the density-dependent slowing down of the swim speed in the high-density regime. Finally, we show that the steady-state phototactic response is well captured by modelling it as active Brownian particles subject to density-dependent external torque.