Host rules: spatial stability of bacterial communities associated with marine sponges (Ircinia spp.) in the Western Mediterranean Sea

Dispersal limitation and environmental selection are the main processes shaping free-living microbial communities, but host-related factors may also play a major role in structuring symbiotic communities. Here, we aimed to determine the effects of isolation-by-distance and host species on the spatial structure of sponge-associated bacterial communities using as a model the abundant demosponge genus Ircinia. We targeted three co-occurring Ircinia species and used terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene sequences to explore the differentiation of their bacterial communities across a scale of hundreds of kilometres in the Western Mediterranean Sea. Multivariate analysis and nonmetric multidimensional scaling plots of T-RFLP profiles showed that bacterial communities in Ircinia sponges were structured by host species and remained stable across sampling locations, despite geographic distances (80-800 km) and diverse local conditions. While significant differences among some locations were observed in Ircinia variabilis-derived communities, no correlation between geographic distance and community similarity was consistently detected for symbiotic bacteria in any host sponge species. Our results indicate that bacterial communities are mostly shaped by host species-specific factors and suggest that evolutionary processes acting on long-term symbiotic relationships have favored spatial stability of sponge-associated bacterial communities.