[The influence of light upon the circadian rhythm in euglena gracilis in mixotrophic and autotrophic cultures].

The subject under investigation was the influence of light upon the circadian rhythm of mobility in Euglena gracilis. In mixotrophic cultures mitosis and reproduction are possible, whereas cells do not divide in a minimum medium without organic substances (autotrophic culture). The experimental results were: 1. In autotrophic and mixotrophic Euglena cultures "light-off" is a dominant "Zeitgeber", independent of the duration of the light stimulus. A minimum of mobility follows "light-off" after about 8h, a maximum after about 18h. Considering different period lengths, the response curves are nearly identical (for different light durations; for autotrophic and mixotrophic cultures). 2. Phase shifts after light interruption occur without transients. 3. The threshold for light intensity which still produces phase shifts under testing conditions is: 50 lux for a 2 h pulse, 10 lux for a 4 h pulse, and 1 lux for a 6 h pulse. 4. The range of entrainment is limited by a "Zeitgeber" period of 16 h (8:8 h) when the L:D ration is kept 1:1. The circadian component reappears with shorter "Zeitgeber" periods. The biological oscillation disappears for about 3 days after the cultures are returned from Zeitgeber conditions near the limits of entrainment to constant conditions. It reappears only slowly. We interpret this behavior to be the result of a perturbation of the timer "circadian order". However, this interpretation does not explain why the difference between Zeitgeber period and biological period is so great that there is no entrainment.