Clocks in the Wild: Entrainment to Natural Light

Abstract

Entrainment denotes a process of coordinating the internal circadian clock to external rhythmic time-cues (Zeitgeber), mainly light. It is facilitated by stronger Zeitgeber signals and smaller period differences between the internal clock and the external Zeitgeber. The phase of entrainment ψ is a result of this process on the side of the circadian clock. On Earth, the period of the day-night cycle is fixed to 24 h, while the periods of circadian clocks distribute widely due to natural variation within and between species. The strength and duration of light depend locally on season and geographic latitude. Therefore, entrainment characteristics of a circadian clock vary under a local light environment and distribute along geoecological settings. Using conceptual models of circadian clocks, we investigate how local conditions of natural light shape global patterning of entrainment through seasons. This clock-side entrainment paradigm enables us to predict systematic changes in the global distribution of chronotypes.

Evidence for oscillating circadian clock genes in the copepodCalanus finmarchicusduring the summer solstice in the high Arctic

“…While it is possible that self-sustained clock gene cycling could exist without synchronization to environmental cycles, the concordance of synchronicity between large numbers of individuals strongly suggests that the populations sampled are synchronized by a common Zeitgeber. Our results therefore strongly suggest that even small fluctuations in light intensity, barely perceptible to the human eye, are sufficient to sustain the circadian clock [22]. This is potentially explained by high irradiance [25] and spectral light sensitivity [26] in these organisms.…”

Widely rhythmic transcriptome in Calanus finmarchicus during the high Arctic summer solstice period

“…However, it is still unclear how this temporal biological organization is facilitated in organisms inhabiting extreme photic environments. In high latitude marine environments without overt day/night cycles such as during the midnight sun period in the Arctic, the sun remains above the horizon for days or months ( Abhilash et al., 2017 ; Bertolini et al., 2019 ; Bloch et al., 2013 ; Schmal et al., 2020 ). Entrainment of the circadian clock by light and associated rhythmic gene oscillations is therefore considered unlikely ( Schmal et al., 2020 ).…”

Daily transcriptomes of the copepod Calanus finmarchicus during the summer solstice at high Arctic latitudes

“…We present here the first in situ daily transcriptomes from the high Arctic, where molecular investigations of biological rhythms are exceptionally limited 15,16 . The samplings have been realized during drastic Polar photic conditions, i.e.…”

“…The samplings have been realized during drastic Polar photic conditions, i.e. the summer solstice, when daily oscillations of the Sun are minimal, high in the sky and always above the horizon 15 . The proposed datasets are thus novel and of interest due to the unique geographical location and time of year, the ecological importance of C. finmarchicus, and the rigorous temporal sampling strategy.…”

“…Since the identification of circadian clock genes in C. finmarchicus 12 , studies have shown that this species possesses a functional clock that might be involved in the timing of both diel and seasonal events, such as the ecologically and biogeochemically important diel vertical migration (DVM) 13 or diapause 14 . However, the Arctic environment is characterized by dramatic seasonality resulting in permanent illumination during Midnight Sun and permanent darkness during Polar Night 15 . As the circadian clock is entrained and synchronized by daily light/dark cycles, the persistence of daily biological processes in Arctic organisms during the absence of those remains uncertain 16,17 , as well as the consequences for newcomer species due to global warming 8,9 .…”