Phototaxis in Chlamydodon mnemosyne: Determination of the illuminance-response curve and the action spectrum

The cyrtophorid ciliate Chlamydodon mnemosyne shows accurate orientation with respect to the direction of light. Positive phototaxis occurs during mild starvation, while the cells are negatively phototactic in the well-fed state. Beside the determination of the illuminance-response curve for white light, we have measured the action spectrum of phototaxis in this ciliate by means of an automatic cell-tracking system. Phototactic reactions occur at illuminances from 100 lx to 100 klx, with an optimum at 10 klx. The action spectrum has one main peak centred at 470 nm, and two secondary ones at 450 and 390 nm. Responses also occur at other wavelengths in the blue and near-UV range of the spectrum, while no significant phototactic reactions are measured between 500 and 710 nm. Results confirm the view that bluelight photoreceptor molecules, e.g., flavins or flavin-like pigments, are responsible for light perception in C. mnemosyne and trigger phototactic orientation.