Circadian rhythms of the microscopic green alga Ostreococcus tauri: a robust and flexible clock in all weather and seasons

Circadian clocks rely on the alternation of light and dark to synchronize to the day/night cycle. However, a consequence of weather fluctuations and seasonal variations is that the driving signal received by the clock is highly variable not only from one day to the next but also throughout the year, which may compromise robust entrainment.

The microscopic green alga Ostreococcus tauri has recently emerged as a promising circadian model in the green lineage. Its clock is based on a central loop featuring orthologs of Arabidopsis TOC1 and CCA1 clock genes, yet seems to have a simpler architecture than Arabidopsis. The analysis of expression data from these two core clock genes and mathematical modeling have unveiled a simple yet effective strategy to protect the clock from fluctuations in daylight intensity, effectively decoupling the clock from the external cycle when it is on time. Being robust to these fluctuations appears to be sufficiently important that this strategy can be clearly evidenced for all photoperiods between 2 and 22 hours, despite the fact that the expression profiles significantly depend on day duration. This shows that a circadian clock can be both robust and flexible, using simple principles from nonlinear oscillator physics.