Collective response in phototaxis of <i>Chlamydomonas reinhardtii</i>

Many photosynthetic microorganisms exhibit phototaxis behavior wherein the organism moves towards or away from the light gradient to inhabit in an optimal light condition for adequate health and growth. Phototaxis also plays a crucial role in the aquatic ecosystem by affecting phytoplankton mass transfer through diel vertical migration and algal bloom and has significant application in bioreactor, microbiopropellers and artificial microswimmers technologies. Here we present a quantitative measurement of phototaxis in Chlamydomas reinhardtii cells, a unicellular biflagellate microalgae, using high spatio-temporal video microscopy. We find that at a given light intensity, phototactic efficiency as a function of cell concentration is non-monotonic and has a re-entrant nature. We demonstrate that in the high-density collective regime, enhancement in phototactic efficiency with the cell concentration is due to the decrease in the linear speed of the cell. Furthermore, we also show that the re-entrant nature of phototactic efficiency is well captured by density-dependent aligning torque on active Brownian particle.