Self-sustained rhythmic behavior ofSynechocystisPCC 6803 under continuous light conditions in the absence of light-dark entrainment

Circadian clocks regulate biological activities, providing organisms a fitness advantage under diurnal changing conditions by allowing them to anticipate and adapt to recurring external changes. In recent years attention was drawn to the entrainment by intracellular cycles. Photosynthetic Cyanobacteria coordinate their gene expression, metabolism, and other activities in a circadian fashion. Solely, three proteins, KaiA, KaiB, and KaiC, constitute the well-studied circadian clock of the cyanobacterial model,Synechococcus elongatusPCC 7942. It remained inconclusive for a long time whetherSynechocystissp. PCC 6803, an important organism for biotechnological applications, can also maintain circadian rhythms under continuous illumination. Using an approach, which does not require genetic modification, we investigated the growth behavior ofSynechocystisvia non-invasive online backscattering measurement and verified all three criteria for true circadian oscillators: temperature compensation, entrainment by external stimuli, and a self-sustained freerunning period of about 24 hours. Since manipulation of the circadian clock (SynechocystisΔkaiA1B1C1) led to a significant reduction in glycogen content, disruption of glycogen synthesis (SynechocystisΔglgC) entirely inhibited glycogen formation and both mutants lost oscillations, we hypothesize that the oscillations reflect glycogen metabolism.