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

Abstract

The circadian clock provides a mechanism for anticipating environmental cycles and is synchronized by temporal cues such as daily light/dark cycle or photoperiod. However, the Arctic environment is characterized by several months of Midnight Sun when the sun is continuously above the horizon and where sea ice further attenuates photoperiod. To test if the oscillations of circadian clock genes remain in synchrony with subtle environmental changes, we sampled the copepod Calanus finmarchicus, a key zooplankter in the north Atlantic, to determine in situ daily circadian clock gene expression near the summer solstice at a southern (74.5° N) sea ice-free and a northern (82.5° N) sea ice-covered station. Results revealed significant oscillation of genes at both stations, indicating the persistence of the clock at this time. While copepods from the southern station showed oscillations in the daily range, those from the northern station exhibited an increase in ultradian oscillations. We suggest that in C. finmarchicus , even small daily changes of solar altitude seem to be sufficient to entrain the circadian clock and propose that at very high latitudes, in under-ice ecosystems, tidal cues may be used as an additional entrainment cue.

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

“…Both stations revealed significant daily and ultradian cycling transcripts (adjusted-p-value < 0.001, Table 1 and Figure 1 C). This result corroborates with the bimodal oscillations of circadian clock gene transcripts recently described by Hüppe et al. (2020) and validated by the current transcriptomic analysis ( Figure S1 ), characterized by both daily and ultradian rhythms.…”

“…We sampled copepods at 74.5° N (south station, ice-free) and 82.5° N (north station, ice-covered) (hereafter referred to South and North respectively) on the 30° E longitude, within nine days of the summer solstice ( Figure 1 A). During the sampling periods, the sun remained above the horizon all day at both stations but still showed diel oscillations of altitude ( Figure 1 B) and photosynthetic active radiation ( Hüppe et al., 2020 ). North experienced lower diel oscillations of the sun's altitude due to the higher latitude and proximity to the summer solstice when compared to South .…”

“…As a consequence it will be exposed to greater annual photoperiodic ranges to which it has evolved ( Reygondeau and Beaugrand, 2011 ), with unknown impacts on its phenology and fitness ( Huffeldt, 2020 ; Saikkonen et al., 2012 ). A functional circadian clock has been described in this species under clear light/dark cycles ( Häfker et al., 2017 ), and a recent study revealed circadian clock gene transcript oscillation in C. finmarchicus in the high Arctic during summer solstice, showing that the core molecular clockwork remains synchronized at this time ( Hüppe et al., 2020 ). It is still unclear whether this clock synchronization leads to daily rhythms.…”

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

“…Solar altitude, tidal height and sea-ice cover during the sampling campaign at both latitudes are detailed in Supplementary Table 2. Temperature, pressure (depth), conductivity (salinity), oxygen saturation (SBE 43, Sea-Bird Electronics) and Chlorophyll a (Chl a) fluorescence (Aquatracka III fluorometer, Chelsea Technologies Group, UK) were measured from the surface to 200 m depth and are available in Hueppe et al 31 .…”

Adaptive value of circadian rhythms in High Arctic Svalbard ptarmigan

“…Nevertheless, several reports of sustained circadian behaviour and physiology in other arctic dwellers have emerged, e.g. Arctic ground squirrels [31,32], bumblebees [33], polar bears [34], copepods [35] and several migrating birds [36][37][38].…”