Transcriptional responses of P. astreoides to reciprocal transplantation across depths
Coral reefs are increasingly threatened by local stressors and climate change. Mesophotic reefs have been proposed as climate refugia for endangered shallow populations, yet mechanisms of depth-related acclimation remain largely unknown. Here, we cross-transplanted Porites astreoides between shallow and mesophotic reefs in the Cayman Islands to assess seasonal and depth-related variation and evaluate acclimatory capacity after 7.5 months. Using an integrated framework linking skeletal morphology and growth, host and symbiont physiology, symbiont composition and quantitative transcriptomics, this study identifies mechanisms underlying coral plasticity pre- and post-transplantation. Shallow corals showed higher productivity and calcification and enrichment of metabolic and stress-related genes, whereas mesophotic corals had reduced productivity and skeletal extension and abundant skeletal organic matrix proteins. Shallow-to-deep transplants exhibited coordinated physiological and transcriptional acclimation, while deep-to-shallow colonies showed higher mortality and largely uncoordinated responses. This asymmetry supports the role of shallow reefs as reservoirs of tolerant individuals and suggests that mesophotic reefs may have limited refugial capacity under environmental change.