Rhythms, clocks and deterministic chaos in unicellular organisms

The cell generation or cell cycle time in a clonal population of identical unicellular organisms growing under steady-state conditions in a carbon (or energy)-limited continuous culture (chemostat) shows a broad distribution. This indicates that the rate of cell division cycle traverse shows considerable variability. The basis for this variable temporal organisation has been the subject of a great deal of speculation, and many models have been suggested. This process is highly temperature dependent, and like all chemical and biochemical reactions, its rate approximately doubles for every 10 °C rise in temperature over a certain range of growth temperatures (i.e. the Q10 ♎ 2). Clock-controlled biological processes on the other hand are temperature-compensated, so that Q10 ♎ 1; two well-established examples are the circadian (τ ♎ 24 h) and ultradian clocks (τ ♎ 40 min in Saccharomyces cerevisiae). Other biological processes proceeding in faster time domains often show reactive oscillatory dynamics. A well-studied example is glycolysis, although a function for glycolytic oscillations is not yet established. As all these examples depend on three or more variables and are often in coupled sets, departure from regular oscillatory behaviour into deterministic aperiodicity is to be expected. Chaos has been demonstrated in biochemical reactions (enzymecatalysed), as well as chemical reactions. It has also been shown in a metabolic pathway (glycolysis), and in the complex system of the cell division cycle. Chaos may also arise from coupled oscillators. One possible mechanism for spatio-temporal coherence (order) arises in unicells as an output from a tuneable multi-oscillator (controlled chaos). Self-similarity of oscillatory properties across multiple time domains indicates temporal coherence described by an inverse power law proportional to 1/fβ and long-term temporal correlations. Clocks, rhythms, oscillations, deterministic chaos and scale-free coherence are fundamental hallmarks of life on every time scale.