Shifts in marine invertebrate bacterial assemblages associated with tissue necrosis during a heatwave

Marine heatwaves (MHWs) are periods of extremely high seawater temperature that affect marine ecosystems in several ways. Anthozoans (corals and gorgonians) and Porifera (sponges) are usually among the taxa most affected by MHWs. Both are holobiont entities that form complex interactions with a wide range of microbes, which are an essential part of these organisms and play key roles in their health status. Here, we determine microbial community changes suffered in two corals ( Cladocora caespitosa and Oculina patagonica ), one gorgonian ( Leptogorgia sarmentosa ), and one sponge ( Sarcotragus fasciculatus ) during the 2015 MHW. The microbial communities were different among hosts and displayed shifts related to host health status, with a higher abundance in necrosed tissues of Ruegeria species or of potential pathogens like Vibrio. We also carry out a meta-analysis using 93 publicly accessible 16S rRNA gene libraries from O. patagonica , C. caespitosa and L. sarmentosa to establish a Mediterranean core microbiome in these species. We have identified one Ruegeria OTU that maintained a stable and consistent association with these species, which was also related with tissue necrosis in their hosts. Therefore, Ruegeria sp. could play an important and still underexplored role in the health status of its hosts.