Mass mortality events are increasingly affecting benthic communities of Mediterranean coastal ecosystems, with the red gorgonian, Paramuricea clavata, being among the most visibly impacted. Over the past twenty years, studies focusing on the microbial communities of this species have revealed a link between its die-off and the dysbiosis of its bacterial community (i.e., changes of microbial community composition negatively impacting the host). Specifically, while the core microbiome of healthy colonies is stable and dominated by members of the Endozoicomonadaceae family, populations subjected to environmental stress and anthropogenic impacts are characterized by a higher microbial diversity with rare and opportunistic pathogens, such as members of the Vibrionaceae family. Despite these observations, current knowledge remains insufficient to identify common causative agents underlying disease onset and mortality events in this gorgonian.
In this study, we aimed to identify the origin and causative agent of a mass mortality event affecting P. clavata in Sicily, South Italy, following a severe thermal anomaly occurred during summer 2024. Specifically, we sought to determine whether the event resulted from a stress-induced physiological impairment of the host that facilitated colonization by opportunistic bacteria, or whether it was triggered by a pathogen that compromised the host's immune system, thereby increasing its vulnerability to environmental stress. To this aim, we compared the microbiomes of healthy and diseased P. clavata colonies from two different sites. We also characterized the microbial community of apparently healthy gorgonians within a diseased population that may have been delayed in
exhibiting the phenotypic signs of disease. We considered as diseased those colonies presenting more than 10% of tissue loss and as healthy the ones with no tissue loss. Our results shed light on the mechanisms underlying the development and spread of mass mortality events affecting one of the most vulnerable ecosystem-engineering species of the Mediterranean Sea.