Ca. Steroidedax gorgoniicola, a heterotrophic coral associate with horizontally acquired genes from Endozoicomonadaceae

The BD1-7 clade has been found in coral microbial metabarcoding libraries, but its evolutionary position and metabolic role were unclear, with speculation of phototrophy. Researchers assembled and annotated the genome of an octocoral associate from Swiftia exserta, showing it belongs to a distinct and divergent clade of coral associates, proposed as Ca. Steroidedax gorgoniicola. Its genome lacks light-driven ATP generation pathways, indicating heterotrophy, and instead encodes enzymes for degrading steroids, chitin, and collagen, along with potential toxins, antimicrobial compounds, and detoxification systems for reactive oxygen and nitrogen species. Notably, several genes, including transposases and genes related to virulence and cell adhesion, appear to have been horizontally transferred from Endozoicomonadaceae, shedding light on the role of horizontal gene transfer in the evolution of coral symbiosis and underscoring the value of genome-resolved studies for understanding coral-associated microbial lineages.