  Chlamydomonas shows both positive and negative phototaxis. It has a single
eyespot near its equator and as the cell rotates during forward motion the
light signal received by the eyespot varies. We use a simple mechanical model
of Chlamydomonas that couples the flagellar beat pattern to the light intensity
at the eyespot to demonstrate a mechanism for phototactic steering that is
consistent with observations. The direction of phototaxis is controlled by a
parameter in our model and the steering mechanism is robust to noise. Our model
shows switching between directed phototaxis when the light is on and
run-and-tumble behaviour in the dark.
