Abstract
How cells maintain their size has been extensively studied under constant conditions. In the wild, however, cells rarely experience constant environments. Here, we examine how the 24-hour circadian clock and environmental cycles modulate cell size control in the cyanobacterium Synechococcus elongatus using single cell time-lapse microscopy. Under constant light, WT cells follow an apparent sizer-like principle. Closer inspection reveals that the clock generates two subpopulations, with cells born in the subjective day or night following different division rules. A stochastic model explains how this behaviour emerges from the interaction of size control with the clock. Combining modelling and experiments, we go onto show how under physiological light-dark cycles, the clock shifts cell division away from dusk and dawn, causing division to occur in more energetically favourable conditions.