Heavy Water Slows the Gonyaulax Clock: A Test of the Hypothesis That D 2 O 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 D2 O onGonyaulax polyedra , where the period of the circadian rhythm of bioluminescent glow in constant environmental conditions is shorter at 16° than at 22°, and the phase of the glow peak relative to a light-dark cycle is advanced at 16° relative to 22°. Addition of low concentrations of D2 O toGonyaulax 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 theGonyaulax rhythm, the effect of added D2 O is not equivalent to lowering the temperature.