Coral probiotics induce tissue-specific and putative beneficial microbiome restructuring in a coral-dwelling fish

The ongoing fourth mass global coral bleaching event reinforces the need for active solutions to support corals through this major crisis. The use of beneficial microorganisms for corals (BMCs) offers a promising nature-based solution to rehabilitate coral's dysbiotic microbiomes. While the benefits to corals are increasingly recognized, the impacts on associated reef organisms, such as fish, remain unexplored. This study investigated the effects of BMCs on the tissue-associated microbiomes of <i>Dascyllus abudafur</i> (<i>Pomacentridae</i>), a damselfish that lives closely associated with coral colonies. Over three months, we applied BMCs three times per week to healthy <i>Pocillopora verrucosa</i> colonies in the central Red Sea and analyzed the resultant changes in the inhabiting fish's microbiomes. Our findings reveal significant, tissue-specific shifts in bacterial communities, particularly skin and gut, with moderate changes observed in gills. Notably, putative fish beneficial bacteria such as <i>Mitsuokella</i> spp. were enriched in the skin, while various Firmicutes taxa increased in the gut. There was also a marked decrease in potential fish pathogens. This study highlights the potential extended benefits of BMCs on coral reef fish and sets a foundation for understanding the broader ecological interactions between BMCs and reef-associated organisms.