HIGH CO2 LEVELS OVERCOME THE NIGHTTIME LETHALITY OF CIRCADIAN MUTANTS IN CYANOBACTERIA
Cyanobacteria have adapted to daily fluctuations in light intensity through a circadian clock that aligns their physiology and metabolism to the external daytime. In the model cyanobacterium Synechococcus elongatus PCC 7942, the rhythm generated by a molecular pacemaker is converted into a global transcriptional oscillation by the central regulator RpaA. Mutants in RpaA and other components have been instrumental in elucidating the clocks molecular architecture and physiological role. Extensive evidence suggests that one of the central functions of circadian regulation is to prevent the accumulation of reactive oxygen species (ROS) during nighttime. However, circadian mutants are unable to grow under natural day/night cycles and require constant illumination, hindering our ability to study the clock function during the night, a phase where circadian regulation is critical for redox homeostasis. Here, we show that the darkness lethality phenotype of circadian mutants can be overcome by high CO2 levels. When grown under a 3% CO2 atmosphere, RpaA-null mutants exhibited growth rates similar to the wt. An analysis of the ROS levels under different CO2 and light intensity conditions revealed carbon scarcity to be the most significant contributor to redox stress. Nighttime ROS accumulation can be modulated by CO2 abundance, an observation that will allow the characterization of heretofore lethal mutants in circadian regulation. The dispensability of the circadian clock in a high CO2 environment suggests that the clock may have evolved as an adaptation to the decrease in atmospheric CO2 levels that occurred after the Great Oxygenation Events, in the Paleoproterozoic era. IMPORTANCECyanobacterial circadian clocks are considered essential for survival under natural day/night conditions, and many circadian mutants are lethal under diel cycles. Results presented here challenge this assumption, showing that these mutants grow normally when CO2 is abundant. This suggests that the major adaptive role of the clock is to manage the redox stress caused by carbon limitation. By separating the effects of carbon availability from circadian control, we identify a key environmental factor that shaped the evolution of biological timekeeping. Using high CO2 to bypass clock dependency will allow the study of circadian function in otherwise lethal mutants. These findings reframe the clock as a specific adaptation to atmospheric carbon, stressing its key role in the regulation of the redox balance.