Recent progress in understanding the temporal behavior of unicellular organisms.

This survey summarizes the findings concerning endogenous oscillations of three unicellular organisms: the dinophyte Gonyaulax polyedra, the ciliate Tetrahymena thermophila and the euglenophyte Euglena gracilis. All of them behave rhythmically and show the common features of zeitgeber action, differential sensitivity and temperature compensation; however, they exhibit some species-specific peculiarities that make each of them suitable for addressing particular chronobiological questions. Although ultradian rhythms have been described for Tetrahymena thermophila and Euglena gracilis, they appear under different conditions: in the first case, a modulation of the period in relation to the concentration of nutrients is observed, whereas Euglena oscillates in an ultradian and circadian fashion simultaneously. Transitions between periodic and aperiodic states can be induced in Euglena gracilis and Gonyaulax polyedra: Euglena gracilis can enter an aperiodic state after repeated exposure to short light pulses (up to 10 sec) given at intervals of 40 min or less, whereas in Gonyaulax polyedra the circadian oscillator is arrested at temperatures below 12 degrees C. In the arrhythmic state, the oscillator might be driven into singularity within the phase space of a limit cycle attractor; re-initiation from the holding point occurs by transition to a relatively precisely defined new phase. Photoperiodism as another important chronobiological phenomenon can be studied in Gonyaulax polyedra: cells enter the dormant stage of an asexual cyst under short days and a temperature below 16 degrees C. This response can be mimicked by 5-methoxylated indoleamines such as melatonin and 5-methoxytryptamine, which are synthesized by this organism. Melatonin concentration exhibit an endogenous circadian rhythm characterized by a rapid increase shortly after the onset of darkness. Encystment, as induced by indoleamines, is associated with stimulations of bioluminescence. The coupling of the two processes involves, as a common element, the release of protons from an acidic vacuole.