Fig. 2. Phototaxis experiments.

(a) Gonium colonies swim in a sealed chamber made of two glass slides, with nonphototactic red illumination from above, on the stage of an inverted microscope connected to a high-speed video camera. Two blue LEDs on the right- and left-hand sides of the chamber can independently shine light with controllable intensities. (b) Micropipette experiments. A micropipette of inner diameter ~20 µm holds a colony (green disk) in a chamber made of two glass slides, spaced to allow room for optical fiber connected to a blue LED to enter the chamber.