The ciliated protozoan Paramecium bursariu is photosensitive. Retinal, which is the chrome phore of the visual pigments of many multicellular animals, was extracted from P. bursaria and identified by HPLC. The procedure of identification was based on the change in polarity achieved by reducing retinal with sodium borohydrate to form retinol. This method is useful for the detection of small amounts of retinal in cells containing large amounts of other substances with similar polarity. The presence of retinal in P. bursaria suggests that this ciliate may contain a photoreceptor pigment with retinal as the chromophore.

Paramecium bursaria is photosensitive and accumulates in a lighted area. The cells can be deciliated by a brief suspension in dilute ethanol. Both intact and deciliated cells showed depolarization in response to light stimulation by a step-increase from dark to above 0.7 mW cm-2 (550 nm). On the other hand, after a step-increase to below 0.4 mW cm-1, intact cells showed hyperpolarization, while the deciliated cells showed no change in membrane potential. This difference in membrane potential response between ciliated and deciliated cells suggests that both somatic and ciliary structures are photosensitive. In our search for the photoreceptive molecules, a polyclonal antibody induced in rabbits against frog rhodopsin was found to cross-react with a 63x10(3) Mr protein of P. bursaria, by immunoelectrophoresis. Immunocytochemical studies showed that the antibody labeling was localized on both the ciliary and the somatic membranes. These results raise the possibility that P. bursaria may contain a rhodopsin-like protein as a photoreceptor molecule.