Higher stress and immunity responses are associated with higher mortality in reef-building coral exposed to a bacterial challenge
Understanding the drivers of intraspecific variation in susceptibility is essential to manage increasingly frequent coral disease outbreaks. We challenged replicate fragments of eight Acropora millepora genotypes with Vibrio spp. to quantify variation in lesion development and to identify host and coral-associated microbial community properties associated with resistance. While Vibrio spp. remained relatively rare in the microbiome of challenged corals, other stress-associated microbial taxa significantly increased in abundance. Contrary to expectations, higher constitutive immunity and more active immune responses did not confer higher resistance to bacterial challenge. Furthermore, more pronounced gene expression responses to bacterial challenge were associated with higher rather than lower mortality. A newly developed gene expression assay based on two genes related to inflammation and immune responses, deleted in malignant brain tumors 1 and a matrix metalloproteinase, predicted mortality under Vibrio treatment both in the initial experiment and in a validation experiment involving another 20 A. millepora genotypes. Instead of mounting more robust responses, resistant corals were largely unaffected by the bacterial challenge and maintained gene expression signatures of healthier condition, including elevated fluorescent proteins and ribosomal biosynthesis along with diminished ubiquitination. Overall, our results support the view that coral disease and mortality is commonly due to opportunistic pathogens exploiting physiologically compromised hosts rather than specific infections, and show, contrary to the prevailing wisdom, that greater immune responses do not necessarily translate into greater disease resistance.