Microbiome Dynamics in Early Life Stages of the Precious Mediterranean Red Coral Corallium rubrum.

Microbial colonisation in the early life stages of corals plays a critical role in their fitness, but the mechanisms by which microbial symbionts are acquired-whether from parental colonies, the environment, or both-remain poorly understood, particularly in octocorals. Although they are the second most abundant coral group on tropical reefs and contribute significantly to the structural complexity of Mediterranean coral reefs, their microbial dynamics are largely unexplored. In this study, we investigated the acquisition of the bacterial microbiota in the red coral Corallium rubrum, a precious coral. By analysing the composition of the bacterial community at different early life stages, including newly released larvae, 5- and 10-day-old larvae, 3-month-old settlers, 1-year-old recruits and 3-year-old juveniles, we are gaining new insights into the development of its microbiome. Using a direct PCR-based 16S rRNA metabarcoding approach, we performed high-resolution microbiome analyses at the level of individual larvae and settlers. Our results show that the bacterial microbiota of C. rubrum matures after the first year of life. Notably, dominant symbionts, such as Spirochaetaceae and BD72BR169 Gammaproteobacteria, were absent in larvae, settlers and recruits, suggesting that they were likely acquired horizontally from the environment. These findings improve our understanding of the microbial colonisation and development of C. rubrum and shed light on the potential role of its bacterial community in holobiont function.