Self-sustained rhythmic behavior of <i>Synechocystis</i> sp. PCC 6803 under continuous light conditions in the absence of light–dark entrainment

Circadian clocks regulate biological activities, providing organisms with a fitness advantage under diurnal conditions by enabling anticipation and adaptation to recurring external changes. Three proteins, KaiA, KaiB, and KaiC, constitute the circadian clock in the cyanobacterial model <i>Synechococcus elongatus</i> PCC 7942. Several techniques established to measure circadian output in <i>Synechococcus</i> yielded comparably weak signals in <i>Synechocystis</i> sp. PCC 6803, a strain important for biotechnological applications. We applied an approach that does not require genetic modifications to monitor the circadian rhythms in <i>Synechococcus</i> and <i>Synechocystis</i>. We placed batch cultures in shake flasks on a sensor detecting backscattered light via noninvasive online measurements. Backscattering oscillated with a period of ∼24 h around the average growth. Wavelet and Fourier transformations are applied to determine the period's significance and length. In <i>Synechocystis</i>, oscillations fulfilled the circadian criteria of temperature compensation and entrainment by external stimuli. Remarkably, dilution alone synchronized oscillations. Western blotting revealed that the backscatter was ∼6.5 h phase-delayed in comparison to KaiC3 phosphorylation.