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

Abstract

varying dark intervals were provided by chopping monochromatic light of 570 nm, which had been determined as the peak of the action spectrum of phototaxis in the material used (10), using a Jasco CRM-FL spectroirradiator with a rotating sector. The sector was driven by a speed-controllable induction motor (maximum output 25 w, 70-1,400 rpm; 4IK25RGK-A, Oriental Motor Co.Ltd., Tokyo) in combination with either speed-reducing gear heads 4GK-3 or 4GK-50 (Oriental Motor Co., Ltd.). The speed of revolution of the motor was monitored with an electric tachometer (PARM-2000A, Oriental Motor Co., Ltd.). The intensity of the stimulus light was adjusted with neutral density filters (Hoya Glass Co., Ltd., Tokyo) and was measured with a radiometer (YSI-Kettering, model 65).Phototactic responses of phytoflagellates have usually been analyzed using continuous light stimuli (2,5,6). It is important to study the effects of intermittent light stimuli on phototaxis, so that the over-all reaction can be separated into at least two subreactions, one requiring light and the other not requiring it.Diehn (1) RESULTSEffects of Frequency of Repeated Light Pulses. Actinic light of 0.5 w m-2 at 570 nm was chopped with a rotating sector at a frequency range between 0.125 and 32 Hz so that the duration of the light pulses was equal to that of the dark intervals. Responses of the cell population to the light pulses were compared with the response to continuous light of 0.25 w m-2, a total incident energy per unit time the same as that of the repeated stimuli (Fig. 1).The data clearly show a frequency dependence of the phototactic response: the responses at 16 and 32 Hz were as great as that to the continuous stimulus, while those at low frequencies (2 and I Hz) were reduced to approximately 20% ofthat ofthe continuous stimulus. In the range between 0.5 and 0.125 Hz, the lower the frequency the greater were the responses. As the incident light energy for a constant period of time was equal in all cases, the diverse responses (Fig. 1) Figure 2 show that the phototactic response is linearly dependent on the logarithm of the duration of the light pulses at this higher frequency.The effects of varying the intensity and duration of each light pulse were then tested at different total energies, and with various dark intervals, in fixed cycles with a period length of 32 msec (Table I). The phototactic responses correlated with the total dosages given for a unit period of time, so that the Bunsen-Roscoe reciprocity law holds in these data.Effects

Phototropic Responses of Vaucheria geminata to Intermittent Blue Light Stimuli

“…3, upper curve). Similar data were obtained in phototaxis of Cryptomonas (13). Interpretation of this data may be applied to our results, ie.…”

“…One of the possible approaches to this problem is to analyze the characteristics of dark reactions. An intermittent irradiation method has been used to detect the presence of dark reactions (1-3, 10,13), the longevity of intermediates produced in the dark period (2,10,12,13), or the number of light reactions involved in the reaction chain (4).…”

Photomovement of Motile Microorganisms

“…Repetitive light pulses are as effective as continuous light in causing positive phototaxis in Cryptornonus provided that the dark interval is shorter than 60 ms. When the dark interval exceeds 250 ms the responses drastically decrease even at higher fluence rates (Watanabe and Furuya, 1978). These results indicate that a periodic shading mechanism might be involved in the phototactic orientation.…”

Photomovement in Motile Microorganisms—ii

“…When the dark interval exceeds 250 ms the responses drastically decrease even at higher fluence rates (Watanabe and Furuya, 1978). These results indicate that a periodic shading mechanism might be involved in the phototactic orientation.…”