Endozoicomonas provides corals with steroid hormones during thermal stress

Rising temperatures are devastating coral populations throughout the globe ¹ . The coral microbiome is believed to play a critical role in sustaining corals and enabling their adaptation to environmental changes, particularly thermal stress ^2–4 . A ubiquitous group of coral-associated bacteria, known as Endozoicomonas , are hypothesized to provide their host with essential metabolites ⁵ . However, the nature of coral- Endozoicomonas symbiosis and the role these bacteria play in adaptation to thermal stress are largely unknown. Here, we show that symbiotic Endozoicomonas adapt to the host environment by gaining the ability to degrade coral-derived steroids and Symbiodiniaceae- derived galactose while losing the ability to synthesize iron-binding siderophores, a common feature of free-living marine bacteria ⁶ . More importantly, under thermal stress Endozoicomonas utilizes coral-derived cholesterol partially as a carbon source while simultaneously converting it to the hormones testosterone and progesterone. Both steroids prime the innate immune system and inhibit pathogenic bacteria and fungi ⁷ . These findings highlight an unknown interaction between corals and their microbiome that may be critical to coral health as oceans warm up. The ability of bacteria to synthesize eukaryotic steroid hormones underscores the importance of these molecules in inter-kingdom interactions and suggests that their origin may have evolved as a result of eukaryogenesis.