Circadian phase alignment promotes survival under predictive oxidative stress
Circadian clocks are endogenous time-keeping mechanisms that continue to oscillate with a period that approximates 24 h in the absence of cyclic environmental cues. This free-running period (FRP) varies from species to species and can vary within species across latitudinal clines. Because natural environments provide daily cues that reset circadian clocks to exactly 24 h, the importance of the FRP length is unclear. Using the cyanobacterium Synechococcus elongatus we show that the FRP length alters the alignment of a biological process with an environmental reference, known as phase angle of entrainment, across seasonal photoperiods, thereby repositioning stress response programs relative to the external environment. This phase relationship affects the overall fitness of the organism: strains that position stress-response programs to peak at the time at which the stressor peaks outcompete their early- or late-peaking counterparts. Our results suggest that the FRP length is selected for its ability to adjust the phase angle of entrainment across seasonal photoperiods: the wild-type FRP length was uniquely plastic in its ability to adjust stress-response programs to coincide with the stressor in all photoperiod conditions.