Phototaxis in the ciliated protozoan Ophryoglena flava: dose-effect curvesand action spectrum determination

The sensitivity of positive phototactic orientation of cells of the ciliate d protozoan Ophryoglena flava has been measured for white Light, broad-band blue and red light, and narrow-band monochromatic light, using a laborator y-developed computer aided system. The white-light fluence rate-response cu rve shows that there is no negative phototaxis in the fluence rate range in vestigated (0-15 W/m(2)) and no adaptation phenomena; it is very well fitte d by a hyperbolic function; the fluence rate curves under broad band blue a nd red Light (full width at half maximum, FWHM=100 nm) can be fitted by the same model. The saturation level is, within experimental errors, the same for the three curves, indicating that there are no chromaticity effects and that if there is more than one photoreceptor pigment, they act independent ly of each other. The fluence rate-response curves determined under narrow band monochromatic light (FWHM= 10 nm) can also be fitted by the same model and show, within experimental errors, the same saturation level. An action spectrum for positive phototaxis at 10-nm intervals has been calculated fr om fluence rate-response curves: it shows three maxima, at 420, 540 and 590 nm. This action spectrum is significantly different from the ones for phot omotile responses in Blepharisma japonicum, Stentou coeruleus and Chlamydod on mnemosyne, whereas it resembles the ones of Paramecium bursaria and Fabr ea salina. (C) 2000 Elsevier Science S.A. All rights reserved.