Algae-specific immune modulation influences responses to heat and pathogen challenge in a symbiotic coral.

Abstract

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 <i>Durusdinium</i> 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 <i>Pocillopora acuta</i>. <i>Durusdinium</i>-hosting (<i>D</i>-hosting) <i>P. acuta</i> have distinct transcriptomic profiles, higher immune-related gene expression, and elevated baseline levels of immunity transcription factor nuclear factor κB as compared to corals hosting <i>Cladocopium</i> (<i>C</i>-hosting). Under heat challenge, <i>D</i>-hosting <i>P. acuta</i> exhibited tissue loss, oxidative stress, and immune and microbial dysregulation, whereas <i>C</i>-hosting <i>P. acuta</i> were more susceptible to bleaching, metabolic dysregulation, and decline in nitrogen-fixing and antioxidant-producing bacteria. Last, infection with the bacterium <i>Vibrio coralliilyticus</i> caused high tissue loss in <i>D</i>-hosting corals but not in <i>C</i>-hosting corals. Our results suggest a mechanism for how <i>Durusdinium</i> association enhances thermotolerance yet predisposes corals to tissue damage under stress, suggesting immune trade-offs that can compromise host survival under multiple stressors.