Differential light effects on the dark motility rhythm in Euglena gracilis by series of short light pulses: Induction of long-term fluctuations and holding of the circadian oscillator

Dark motility was measured by means of an infrared beam in stationary, 8- to 10-months-old autotrophic cultures of Euglena gracilis, strain Z. Controls in constant darkness exhibited circadian rhythms for several weeks. Cells were subjected to series of short light pulses of 800 lx covering, in most of the experiments, not more than 1/240 of the intermittent dark periods. When the overall amount of light per day was kept constant, the results of the light treatment strongly depended on the frequency of the light pulses. In light/dark cycles of more than 1 h, the circadian rhythmicity persisted, whereas the oscillator was affected by cycles of 40 min or less. Such treatment by a high-frequency series of light pulses led to long-term fluctuations lasting for several days and a subsequent holding of the oscillator. The temporal position of extreme values of the fluctuations correlated with the frequency of the light pulses. The potency of light to suppress rhythmicity, as known from fading-out phenomena in constant illumination, is not restricted to the continuous presence of light, but is also immanent in the Zeitgeber signals of dark/light transitions. Hence, the results reflect differential effects in the action of light.