Temporal Changes in Swimming Direction during the Phototactic Orientation of Individual Cells in Cryptomonas sp.

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The response of individual Cryptomonas cells to continuous light was recorded using infrared video-micrography. Swimming directions and temporal shifts in swimming direction of each cell were measured. White light of 0.1-1 W m^<-2> elicited a positive phototactic orientation, but did not induce any photophobic response. Light of 100 W m^<-2> induced a photophobic response at the onset of actinic irradiation, but did not induce positive phototactic orientation. No correlation between positive phototactic orientation and photophobic response was found in this species. The direction toward the light source was defined as 0°, and the direction away from the source as 180°. Within 2s after the onset of lateral monochromatic light of 570 nm at 0.1 W m^<-2>, cells which were swimming in a direction of less than 120°predominantly shifted their course toward the light source. Cells swimming in directions of larger than 120°shifted their course as randomly as those in the dark. Thus, for phototactic orientation, the cells must perceive the light from their anterior side.
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