Steroid hormone production by coral-associated Endozoicomonas during thermal stress

The coral microbiome is believed to play a critical role in sustaining corals and enabling adaptation to thermal stress. A ubiquitous group of coral-associated bacteria, known as Endozoicomonas, are hypothesized to provide essential metabolites to corals, though the roles these bacteria play during thermal stress are largely unknown. We demonstrate that Endozoicomonas synthesizes the siderophore aerobactin in vitro-a trait potentially suppressed in hospite. Integrated metagenomics, metabolomics, and lab validations reveal that Endozoicomonas is the only microbe capable of metabolising coral-derived testosterone and converting ubiquitous cholesterol into testosterone and progesterone. These hormones accumulate within the coral holobiont during thermal stress, suggesting active microbial involvement in immune priming and stress mitigation. These findings highlight a previously unrecognized mechanism of microbial contribution to coral resilience. Finally, the capacity of bacteria to synthesize eukaryotic steroids implies deep evolutionary connections between microbial metabolism and the origin of steroid-mediated processes in early eukaryogenesis.