Cell size control driven by the circadian clock and environment in cyanobacteria

Area of Science:

• Cell biology
• Microbiology
• Systems biology

Background:

• Cell size control is crucial for cellular function and is well-studied under constant conditions.
• Real-world cellular environments are dynamic, featuring fluctuating light and temperature cycles.
• The 24-hour circadian clock plays a significant role in regulating cellular processes in response to environmental cues.

Purpose of the Study:

• To investigate how the circadian clock and environmental cycles modulate cell size control and division timing in *Synechococcus elongatus*.
• To understand the interplay between cell size control, circadian rhythms, and environmental conditions.
• To develop a predictive model for cell division timing based on these interactions.

Main Methods:

• Single-cell time-lapse microscopy of *Synechococcus elongatus* under controlled and natural light-dark cycles.
• Development and application of a stochastic model to analyze cell division behavior.
• Iterative cycles of computational modeling and experimental validation.

Main Results:

• Under constant light, *Synechococcus elongatus* exhibits an apparent sizer-like cell division, but the circadian clock creates distinct subpopulations with different division rules.
• The circadian clock continuously modulates cell division probability, rather than acting as a simple on-off switch.
• Cell division timing is effectively coupled to the time of day, influenced by both the environment and the clock, narrowing the division window and shifting it away from low-growth periods.

Conclusions:

• The circadian clock and environmental cycles are critical regulators of cell size control and division timing in *Synechococcus elongatus*.
• A dynamic interplay exists between the circadian clock, environmental cycles, and cell size control mechanisms.
• This study provides a framework for disentangling and predicting the complex interactions governing cell division in fluctuating environments.