Algae-specific Immune Modulation Influences Responses to Heat and Pathogen Challenge in a Symbiotic Coral

The role of symbiotic algae in coral life history and host health is well documented, but the immune and physiological trade-offs of hosting these symbionts remain less explored. While association with the algal symbionts of the genus Durusdinium is known to confer thermotolerance, it has also been linked to coral tissue loss under stress. We investigated whether algal type influences host immunity and stress responses in the tropical coral Pocillopora acuta. Durusdinium-hosting (D-hosting) P. acuta have distinct transcriptomic profiles, higher immune-related gene expression, and elevated baseline levels of immunity transcription factor NF-{kappa}B as compared to corals hosting Cladocopium (C-hosting). Under heat challenge, D-hosting P. acuta exhibited tissue loss, oxidative stress, immune and microbial dysregulation, whereas C-hosting P. acuta were more susceptible to bleaching, metabolic dysregulation, and decline in nitrogen-fixing and antioxidant-producing bacteria. Finally, infection with the the bacterium Vibrio coralliilyticus caused high tissue loss in D-hosting corals, but not in C-hosting corals. Our results suggest a mechanism for how Durusdinium association enhances thermotolerance yet predisposes corals to tissue damage under stress, suggesting immune trade-offs that can compromise host survival under multiple stressors.