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Three periodicities in the circadian range are observed when measuring circadian parameters of unicellular organisms in long running experiments (more than 15 days). This is demonstrated for different organisms (Chlamydomonas, Euglena, Chlorella) and different parameters (auto­ kinesis, extracellular pH, CO 2 - and O 2 -partial pressure). Having excluded analytical and experimental artefacts (i.e. filter leakage and subpopulation effects), the multiple periodicities have to be interpreted in a physiological model. The three periodicities always exhibit two common features: The locations of the side peaks are symmetrical to the middle peak and their energy contribution is always the same. We therefore favour the model of multiplicative coupling between the circadian oscillator and a low frequency oscillator modulating the amplitude of the circadian rhythm. Since the low frequency rhythm is not correlated to any exogenously running periodicity of the experimental surroundings, it is considered as generated by an endogenous oscillator. This shows the existence of different biological long time oscillators in one single cell and contradicts the so-called master-clock hypothesis stating that one cell has only one clock related oscillator.

Multiple Periodicities in the Circadian System of Unicellular Algae