  Phototaxis is one of the most fundamental stimulus-response behaviors in
biology wherein motile micro-organisms sense light gradients to swim towards
the light source. Apart from single cell survival and growth, it plays a major
role at the global scale of aquatic ecosystem and bio-reactors. We study
photoaxis of single celled algae Chalmydomonas reinhardtii as a function of
cell number density and light stimulus using high spatio-temporal video
microscopy. Surprisingly, the phototactic efficiency has a minimum at a
well-defined number density, for a given light gradient, above which the
phototaxis behaviour of collection of cells can even exceed the performance
obtainable from single isolated cells. We show that the origin of enhancement
of performance above the critical concentration lies in the slowing down of the
cells which enables them to sense light more effectively. We also show that
this steady state phenomenology is well captured by a modelling the phototactic
response as a density dependent torque acting on an active Brownian particle.
