Phototactic behavior of individual cells of Cryptomonas sp. in response to continuous and intermittent light stimuli.

The phototactic response of cells of Cryptomonas sp. to stimulation with continuous or intermittent lateral light was determined by an individual cell method using photomicrography and videomicrography. The cells showed positive phototaxis under the conditions studied. The phototactic orientation of individual cells was induced most effectively by irradiation with light of 570 nm; blue light was less effective, and no orientation was found in red light. An intermittent stimulus regime with a long dark interval (250 ms) elicited a weaker phototactic orientation than did a regime with a short dark interval (63 ms) irrespective of the duration of light pulses (16, 250 and 1000 ms). The swimming rate was ca. 240 ums -1 and the rotation period ca. 450 ms in the dark, neither of which was greatly affected by stimulation with continuous or intermittent light. Neither step-up nor step-down photophobic responses were observed at the time of onset or removal of the light stimulus under the experimental conditions. The swimming direction of individual cells became gradually oriented toward the light source. Phototactic response was detectable within 4 s after the onset of light stimulation, reaching a saturation level after more than 30 s