Tackling the specificity of the marine sponge microbiome: a biogeographical approach. Biogeography of the Sponge Microbiome

The ability of a given sponge host to maintain a core microbiota across biogeographical gradients has been poorly approached, in spite of being integral to our understanding of the marine sponge holobiont, its resilience and degree of specificity. Here, we used next generation sequencing technologies to determine the extent to which the shape of the spongeassociated microbiome is driven by the host organism and its biogeographical background, and to define the complementary role of seawater and marine sediments as “seed banks” of microorganisms, which make up its composition. To this end, specimens of the genera Ircinia, Sarcotragus (Irciniidae) and Spongia (Spongiidae) were sampled at the Algarve coast, the Madeira Island, and the Azores archipelago and subjected to 454 pyrosequencing profiling of bacterial 16S rRNA genes. After quality filtering, 232.251 sequences were obtained, representing 5.601 different OTUs. Sponge microbiomes presented sharply contrasting composition of bacterial phyla in comparison with those present in seawater and sediments. The predominant taxonomic groups in Sarcotragus and Spongia were Acidobacteria and Actinobacteria, followed by Proteobacteria, Poribacteria, PAUC34f, Bacteroidetes and Chloroflexi. In Ircinia spp., the Chloroflexi displayed dominance at the Madeira site but was found to be a minority taxon at the Algarve and Azores sites, where Proteobacteria, Acidobacteria, and Actinobacteria were dominant groups at varying degrees. Regarding the biogeographical patterns exhibited by the studied communities at the approximate species (i.e., OTU) level, only the seawater samples were clearly dependent of the sampling location. Sediments and sponges clustered otherwise together independently of the sampling site they were collected. Our results suggest that the composition of the sponge associated microbiome is shaped by an intricate and cooperative interaction between the host organism and its environmental background. Further, compelling evidence for the recognition of marine sediments as the pivotal environmental source of highly specialized and abundant spongeassociated Acidobacteria was found.