Phototaxis and photophobic responses in Stentor coeruleus. Action spectrum and role of Ca2+ fluxes.

Negative phototactic orientation, step-up photophobic responses and light-induced action potentials have been studied in the ciliate Stentor coeruleus. A resolved action spectrum, based on fluence rate-response curves, is consistent with stentorin as the photoreceptor. Calcium flux blockers prolong the response time for ciliary stop and reversal and inhibit step-up photophobic responses. Drugs believed to affect the membrane-bound calcium pump likewise inhibit phobic responses. On the other hand, alpha-phosphatidic acid promotes Ca2+-influx and enhances the photophobic sensitivity of the organism, thus providing an unambiguous evidence for the role of Ca2+ influx. A change in the response time decreases the degree of phototactic orientation, indicating that negative phototaxis in this organism is brought about by subsequent phobic responses of individual rows of cilia as the associated photoreceptor granules experience an increase in light intensity when the organism rotates during forward locomotion in lateral light.