Phototactic Responses of Cell Population to Repeated Pulses of Yellow Light in a Phytoflagellate Cryptomonas sp.

Abstract

Positive phototaxis in cell populations of a phytoflagellate Cryptomonas sp. was recorded photoelectrically when the duration and intensity of repeated pulses of monochromatic yellow light (570 nm) interspersed with darkness were varied. Irrespective of the duration of the light pulses, phototactic responses to repeated pulses were as great as those to continuous irradiation and were linearly dependent on the logarithm of total incident light energy when the dark interval was shorter than 60 ms. Reciprocity between duration and intensity held well. In contrast, when the dark interval exceeded 250 ms, the responses were remarkably reduced regardless of light duration and were not affected by increasing the intensity of actinic light pulses. Apparently continuous stimulation with actinic light is not essential for the maximum effect, but the length of dark interval is crucial in phototactic response.