Day-night cycle as a key environmental factor affecting coral-Symbiodiniaceae symbiosis

Interpreting the coral-Symbiodiniaceae symbiosis in light of the day-night cycle may provide missing links in understanding the function of endosymbiosis. In this study, we found the photo-physiology, Symbiodinaiceae cell density and gene transcription of two coral holobionts (Acropora pruinosa-Cladocopium sp. and Pocillopora damicornis-Durusdinium sp.) have clear day-night oscillations. These two coral holobionts showed lower maximum quantum yield of photosystem II but higher Symbiodiniaceae cell density at day-time, as compared to at night-time. At day-time, the genes related to circadian rhythm and symbiosis in both hosts were up-regulated, while those related to immunity were down-regulated. Simultaneously, both symbionts had lower abundances of genes involved in the light reaction, Calvin cycle and glycolysis, but higher abundances of genes involved in the NH4+ assimilation. These results indicated the high density of Symbiodiniaceae at day-time might be attributed to up-regulating of genes involved in symbiosis and nitrogen metabolism but down-regulating of genes involved in immunity. Moreover, the A. pruinosa-Cladocopium sp. holobiont had larger day-night oscillations than P. damicornis-Durusdinium sp. holobiont, in terms of photo-physiology, Symbiodinaiceae cell density and gene transcription, revealing species-specific day-night oscillation. This study provides valuable insights into the cooperation strategies of coral holobionts to adapt to the day-night environmental changes.