3) Interactions between circadian and seasonal rhythms
Seasonal shifts in gut microbiome composition and function have been
well described 78–83, but emerging evidence suggests
that changes to function may be mediated via increasing or decreasing
the amplitude of host circadian rhythms 76. In giant
pandas, seasonal switching of diet from bamboo leaves to shoots causes
an increase in the bacterial metabolite butyrate in the gut microbiota,
and when transferred to mice, this causes the upregulation of clock genePer2, which increases lipid production and fat deposition in
spring 76. This study does not measure gut microbial
oscillations directly however, and it is unclear whether microbial
rhythms also increase in amplitude during spring. Yet, the findings
suggest that seasonal cycling of the gut microbiota functions via
interacting with host circadian rhythms.
In addition to seasonal diet switches, seasonal changes to life history
stages that involve metabolic restructuring such as migration,
hibernation, and even reproduction may also be paired with changes to
the amplitude of their gut microbial rhythms (Fig. 3). Shifts in the gut
microbiota during hibernation adaptively lower metabolism and recycle
nitrogen 84–86, yet it remains unknown how these
functional changes interact with or are mediated by diurnal rhythms.
Seasonal switches in strategies may take more unpredictable and
fascinating forms. For instance, the circadian rhythms of some
arctic-breeding shorebirds become uncoupled from environmental cues
during breeding due to pressures of incubation and predators, with
social cues becoming the dominant form of entrainment87. How might such changes be reflected in the gut
microbiome?