Towards a core microbiota in marine sponges

Marine sponges are well known for their associations with highly diverse, yet very specific and often highly similar microbial communities. Most previous molecular studies focused only on abundant community members, while the recent development of next-generation sequencing technologies now allows much deeper insights into the diversity of microbes including the variety of rare species. To test the hypothesis of a ‘sponge-specific microbial community’ and to define the core microbial community in sponges,16S rRNA gene amplicon pyrosequencing was applied to 32 sponge species from 8 locations around the world, thereby generating 2567 97% operational taxonomic units (OTUs) in total and up to 364 different OTUs per sponge species. The total sponge microbiota consists of 25 bacterial phyla with Proteobacteria, Chloroflexi and Poribacteria being most diverse. Eight candidate phyla, six of them found for the first time in sponges, belong to the rare biosphere. Bacterial symbionts are not distributed according to sponge phylogeny or sampling sites, suggesting that their biogeography is shaped by various factors which may act differently on abundant and rare symbionts. The most surprising finding was that only very few OTUs were shared among the 32 sponges, with the majority of OTUs being present in only a single species. High sequence similarity was found to previously published sponge sequence data and, on a higher taxonomic level, among sponges of this study which confirms sponge-specific microbial communities. Although each sponge contains a unique set of bacterial species, these bacteria are still more closely related to each other than to seawater microbes.