The circadian clocks in unicellular phototrophic organisms are known to display remarkable reliability. In contrast, little is known about circadian clocks in a multicellular setting. Are the clocks in multicellular cyanobacteria coupled and synchronized with each other through cell–cell communication or do they only react to external cues? What is the spatial extent of synchronization? To tackle these and other questions, we studied the dynamics of a circadian clock-controlled gene in undifferentiated and differentiated Anabaena sp. PCC 7120 filaments, a multicellular cyanobacterium in which cells are arranged in a line and coupled by protein channels. We followed its expression at the level of individual cells in real time as the filaments grew, and the synchronization and spatial coherence along filaments was quantitatively measured. Our study sheds light on the importance of circadian clocks in the regulation of a variety of essential processes in cyanobacteria.
