Endozoicomonasprovides corals with steroid hormones during thermal stress

Abstract

Summary ParagraphRising temperatures are devastating coral populations throughout the globe1. The coral microbiome is believed to play a critical role in sustaining corals and enabling their adaptation to environmental changes, particularly thermal stress2–4. A ubiquitous group of coral-associated bacteria, known asEndozoicomonas, are hypothesized to provide their host with essential metabolites5. However, the nature of coral-Endozoicomonassymbiosis and the role these bacteria play in adaptation to thermal stress are largely unknown. Here, we show that symbioticEndozoicomonasadapt to the host environment by gaining the ability to degrade coral-derived steroids andSymbiodiniaceae-derived galactose while losing the ability to synthesize iron-binding siderophores, a common feature of free-living marine bacteria6. More importantly, under thermal stressEndozoicomonasutilizes 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 fungi7. 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.