Channelrhodopsins (ChRs) are the light-gated ion channels that have opened the research field of optogenetics. They were originally identified in the green alga Chlamydomonas reinhardtii, a biciliated unicellular alga that inhabits in freshwater, swims with the cilia, and undergoes photosynthesis. It has various advantages as an experimental organism and is used in a wide range of research fields including photosynthesis, cilia, and sexual reproduction. ChRs function as the primary photoreceptor for the cell's photo-behavioral responses, seen as changes in the manner of swimming after photoreception. In this chapter, we will introduce C. reinhardtii as an experimental organism and explain our current understanding of how the cell senses light and shows photo-behavioral responses.
Keywords: Channelrhodopsin; Chlamydomonas; Cilia; Eyespot; Photoresponse; Phototaxis.