Heavy water slows the Gonyaulax clock: a test of the hypothesis that D2O affects circadian oscillations by diminishing the apparent temperature.

In order to test the hypothesis that deuterium oxide acts in circadian systems by simulating a lower environmental temperature, we have examined the effects of D(2)O on Gonyaulax polyedra, where the period of the circadian rhythm of bioluminescent glow in constant environmental conditions is shorter at 16 degrees than at 22 degrees , and the phase of the glow peak relative to a light-dark cycle is advanced at 16 degrees relative to 22 degrees . Addition of low concentrations of D(2)O to Gonyaulax does not produce the same effects on period and phase as lowering the temperature; the free running period is lengthened and the phase is delayed. These results show that in the Gonyaulax rhythm, the effect of added D(2)O is not equivalent to lowering the temperature.