Bacterial Microbiota Signatures Suggest Acclimation of the Gorgonian Leptogorgia sarmentosa to Highly Impacted Environments in the Mediterranean Sea

Anthropogenic disturbances increasingly compromise marine environments, with severe consequences for vulnerable coral ecosystems. While global stressor impacts on stony corals are well-documented, the mechanisms by which local perturbations influence gorgonians remain poorly characterized. This limits our ability to predict the tolerance of temperate octocorals in human-dominated coastal landscapes. The Mediterranean gorgonian Leptogorgia sarmentosa is remarkably resilient, thriving in both marine protected areas (MPAs) and highly impacted urban habitats. To investigate the microbiota's role in this adaptability, we characterized the bacterial communities of L. sarmentosa from a protected site (Western Mediterranean) and an impacted seaport (Barcelona). For broader regional context, results were compared in parallel with a re-analysis of datasets from similarly contrasting Mediterranean sites (Cassis and La Spezia). Our results reveal distinct site-specific microbial signatures, but no differences in alpha diversity or dispersion between seaport and the MPA. However, significant compositional shifts occurred in the impacted Barcelona seaport, characterized by a reconfiguration of the dominant symbiont genus Endozoicomonas and an enrichment of Spongiibacteraceae_clade BD1-7. Similarly, Endozoicomonas strain abundances differed between Cassis and the high-runoff environment of La Spezia, which exhibited increased Mycoplasma abundance. Collectively, these findings suggest a high degree of microbiome flexibility. This microbiota plasticity, alongside predicted functional pathways, suggests a contribution to preserving and acquiring key holobiont functions, highlighting the capacity of L. sarmentosa to persist in disturbed habitats through strategic microbial reconfiguration. This study provides crucial insights into the mechanisms underlying coral acclimation; essential for developing conservation strategies and predicting the long-term viability of Mediterranean marine biodiversity.