**3.8 Circadian clock**

Cyanobacteria are responsive to diurnal photoperiods by adjusting their photosynthesis and respiration. In *S. elongatus* PCC 7942, the circadian clock controls the genes using promoters in light and dark conditions. Control of promoters is time-dependent, which sequentially maintains energy metabolism, cell division, and chromosome structure. Some CBCR domain (KaiABC), CikA (circadian input kinase A) and PsR in the *S. elongatus* oscillator become natural sensors that identify the change from light to dark by detecting the redox condition of the quinone pool [45].
