ABSTRACT
The mammalian phosphagen creatine has been shown to shorten the circadian period (t) of the marine alga Gonyaulax polyedra. We have investigated the effect of creatine on t under constant light of different spectral qualities as well as its effects on the phase shifting capacity of light pulses. In constant red light, creatine has modest t-shortening effect, but only at intermediate concentrations (3-10 µM), whereas in blue and white light (short wavelength), creatine is highly effective, shortening t up to 20%. Over the tested range (10 to 40 µE m-2 x sec-1), this effect appears to be independent of blue light intensity .
When cells are kept in constant red light, creatine greatly enhances phase shifting by light pulses containing short wavelengths (white and blue) without changing the maximum phase shift. The enhancing effect on phase shifting by light pulses depends on the creatine concentration. The phase response curve (PRC) to dim light pulses can be changed from a weak (type 1) PRC to a strong (type 0) PRC by simply adding creatine to the culture medium two days prior to the pulse. In G. polyedra , the shape of the incremental light PRC is asymetrical; while advance phase shifts can exceed 12 hours, light pulses, even at intensities of 150 µE m-2 x sec-1, delay the clock no more than one hour. However, when creatine is added to the culture medium, light pulses result in a typical type 0 PRC, where advances and delays are continual at 12 hours.
The effect of creatine is not restricted to the circadian system alone: The sensitivity to light is also increased in phototactic responses (monitored with the help of daytime cell aggregations). A second strain of Gonyaulax, GP81, shows greater sensitivity to creatine than our usual strain, GP70, and will thus prove useful in elucidating the mechanism of creatine action on the clock.