Observation of Photobehavior in <em>Chlamydomonas reinhardtii</em>

For the survival of the motile phototrophic microorganisms, being under proper light conditions is crucial. Consequently, they show photo-induced behaviors (or photobehavior) and alter their direction of movement in response to light. Typical photobehaviors include photoshock (or photophobic) response and phototaxis. Photoshock is a response to a sudden change in light intensity (e.g., flash illumination), wherein organisms transiently stop moving or move backward. During phototaxis, organisms move toward the light source or in the opposite direction (called positive or negative phototaxis, respectively). The unicellular green alga Chlamydomonas reinhardtii is an excellent organism to study photobehavior because it rapidly changes its swimming pattern by modulating the beating of cilia (a.k.a., flagella) after photoreception. Here, various simple methods are shown to observe photobehaviors in C. reinhardtii. Research on C. reinhardtii's photobehaviors has led to the discovery of common regulatory mechanisms between eukaryotic cilia and channelrhodopsins, which may contribute to a better understanding of ciliopathies and the development of new optogenetics methods.