Temporal transcriptional rhythms govern coral-symbiont function and microbiome dynamics

Diel rhythms align physiological processes with light/dark cycles, driving predictable oscillations in gene expression and protein activity through tightly controlled transcriptional-translational feedback loops. This study presents in situ transcriptomic analyses of the stony coral Pseudodiploria strigosa and its photosymbionts, Breviolum sp., at key daily time points. P. strigosa shows precise transcriptional control: dawn triggers a molecular reset marked by RNA metabolism and protein turnover; midday emphasizes anabolic and phosphate-regulated pathways; dusk reflects transitional lipid and amino acid metabolism; and midnight reveals stress responses, mRNA catabolism, and mitochondrial organization. Photosymbionts display subtler diel patterns, with photoprotection at dawn, metabolite transport and nitrogen cycling through midday/dusk, and cell cycle and ion homeostasis at night. Microbial communities show time-dependent restructuring of co-occurrence networks, driving diel-related functional consequences like changes in microbial metabolism. These findings present a system-level molecular framework of diel regulation across the coral-photosymbiont-microbe holobiont, revealing time-specific transcriptional control of coordinated function and homeostasis.