All-trans-retinal is the chromophore bound to the photoreceptor of the alga Chlamydomonas reinhardtii.

Rhodopsin is the general name for a family of visual pigments that receive light and transmit this signal to the rest of an organism. Chlamydomonas reinhardtii is a unicellular eukaryote whose light-tracking system consists of a single eye. Through spectral studies of Chlamydomonas' reaction to light of different wavelengths (action spectroscopy), it has been shown in vivo that the photoreceptor of Chlamydomonas is functionally similar to vertebrate rhodopsin. We seek to characterize the photoreceptor further by identifying the molecule that is incorporated into the rhodopsin of Chlamydomonas forming the chromophore. High performance liquid chromatography analysis of organic extracts of retinaloximes from membrane fractions enriched in eye-spots and in cells virtually free of interfering carotenoids identified syn-all-trans as the existing retinaloxime isomer. We conclude that all-trans-retinal is the native molecule that is available to be incorporated into the rhodopsin of Chlamydomonas and therefore forms the functioning chromophore on binding.