New trends in photobiology: Photomotile responses in ciliated protozoa

Abstract

Various colourless and coloured ciliated protozoa alter their swimming behaviour when exposed to light stimulation. Typically, the cells stop their forward motion, swim backwards and then resume their normal motion. They can react to a light increase (step-up photophobic response) and to a light decrease (step-down photophobic response). They usually also show a dependence of swimming speed on the intensity of steady illumination (photokinesis). In the case of Paramecium bursaria the sensitivity to light is altered by the presence of symbiotic algae, whereas Loxodes require oxygen to express their photomotile response. Action spectra of the different photoresponses have been determined; they show maxima at various wavelengths, and the nature of the pigments involved is still a matter of debate. Motile responses to light in Paramecium bursaria are accompanied by a membrane depolarization which is probably brought about by a light-dependent conductance change of calcium channels in the plasmalemma.