Entrainment and Phase-Shifting of the Circadian Rhythm of Cell Division by Calcium in Synchronous Cultures of the Wild-Type Z Strain and of the ZC Achlorophyllous Mutant of Euglena gracilis

Cell division in exponentially increasing populations of the wild-type, photosynthetic Z strain of Euglena gracilis Klebs cultured autotrophically on an aerated, magnetically stirred, minimal mineral medium (pH 7.0) in constant light (LL) or in a light-dark 1 hour:1 hour cycle (LD:1,1) at 25 degrees C could be synchronized by a 10-hour:10-hour low (2 micromolar):normal (200 micromolar) cycle in the concentration of external calcium. Similar results were obtained with the photosynthesis-deficient, achlorophyllous ZC mutant cultured in darkness at 16 degrees C on mineral medium supplemented with 0.1% ethanol as a carbon source; even a single low-Ca(2+) (2 micromolar) pulse was effective in eliciting synchrony. In contrast, whereas the 20-hour entrained rhythm of cell division in ZC then free-ran with a circadian period (tau = 26 hours) for many cycles after the imposed calcium regimen was discontinued, division rhythmicity did not persist in the Z strain in LL. The rhythm in wild-type cultures (free-running in LD:1,1) could be phase-shifted by a single 2-hour increase (from 200 micromolar to 10 millimolar; HiCa) or decrease (from 200-2 micromolar; LoCa) in external Ca(2+) concentration (varied by the addition of CaCl(2) or EDTA, respectively, to the medium). Pulses were terminated by returning the cells to medium containing 200 micromolar Ca(2+) (the normal concentration), and the steady-state phase-shifts engendered (if any) after transients had subsided were calculated with reference to an unperturbed culture. For both HiCa and LoCa pulses given at different circadian times, strong (type 0) phase-response curves (PRCs) were obtained, but although the LoCa PRC was the same as that obtained for light signals, the HiCa PRC was the opposite (a mirror image). These results implicate calcium in clock function, although it is likely that only a small portion of the total intracellular Ca(2+) ion is playing a role since the period of the division rhythm in cultures grown in the continuous presence of excess Ca(2+) or under LoCa was not altered significantly.