Time and function in the living world

We begin by discussing some historical ideas about the natural dynamics of living organisms and their complex states, from the very transient to the very persistent. We classify ultradian rhythms as being more important than most oscillatory behaviour in their distinctive properties. Then we summarise rhythmicity in three yeasts and five representative protist species. Recent discoveries about the single-celled photosynthetic alga Chlamydomonas reinhardii are then discussed as revealed by computer-controlled semi-continuous monitoring: automatic periodic measurement of the rate of phototaxis and chlorophyll a content over extended times for up to 12 days. Methodological approaches for analysing time series are discussed, including the recently developed 'GaMoSEC' procedure, providing published references to the detection and understanding of the complex behaviour of natural systems. Finally, we conclude by summarising the general significance of ultradian rhythms to the vital aspects of the biological timekeeping of coherent functionality of living systems from single-cell organisms to humans from their healthy to declining states.