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Most Earth organisms have evolved biological rhythms as an adaptive response to the daily light/dark cycle. Hypothetical life in the subsurface oceans of the icy moons Enceladus and Europa would lack sunlight as a stimulus for entrainment of biological rhythms. However, both moons orbit their parent planets in resonance with sister moons, resulting in strong tidal forces which are thought to cause periodic stressing of their icy crusts and rocky cores. As on Earth, tidal forces may create thermodynamic disequilibria essential for life by driving fluid movement in the crust below the seafloor. If simple life on Enceladus and Europa has evolved adaptive biological rhythms, it may entrain to rhythmic tidally-driven processes rather than to sunlight.
To explore the idea of biological rhythms of life in the subsurface oceans of icy moons, we present stimuli (other than light) to which life might entrain. Earth life is known to entrain to rhythmic fluctuations in temperature, a possible rhythmic stimuli on Enceladus and Europa. To survey the ability of microbial life on Earth to sense temperature, we identify temperature-sensing genes from the literature, catalog their function, identify microbes which may possess these genes, and summarize their diversity and distribution. We hope this work will encourage future exploration of non-light-entrained biological rhythms in Earth life, especially in model species for astrobiology and in organisms living in analog habitats for Enceladus and Europa.