Steroid hormones produced by coral-associated Endozoicomonas influence coral immune response during thermal stress.

The coral microbiome is believed to play a crucial role in sustaining corals and enabling adaptation to perturbations. A ubiquitous group of coral-associated bacteria, known as Endozoicomonas, are hypothesized to provide essential metabolites to corals, although detailed dynamics of the interactions between host and microbe are poorly understood. Here multi-omics and laboratory validation experiments reveal that Endozoicomonas encodes genes for the metabolism of coral-derived testosterone and converts ubiquitous cholesterol into testosterone and progesterone, both important steroids that accumulate within the coral holobiont during thermal stress. Field sampling along a natural temperature gradient showed general suppression of coral steroid hormones under thermal stress and during white syndrome manifestation. Provision of testosterone or progesterone to corals during laboratory thermal stress induced transcriptional reprogramming of the coral innate immune response, suggesting microbial involvement in host immune modulation. These findings highlight a previously unrecognized potential mechanism of microbial contribution to coral resilience.