Bacterial diversity in the marine sponge Halichondria panicea from Icelandic waters and host-specificity of its dominant symbiont Candidatus Halichondribacter symbioticus

Marine sponges can harbour diverse bacteria that contribute to host metabolism and defence. Identifying these stable members of sponge bacterial communities remains a necessary step in understanding their ecological roles and underlying co-evolutionary processes. In this study, we applied high-throughput sequencing of 16S rRNA gene amplicons, ribosomal nucleotide variant analysis and fluorescence in situ hybridisation to characterise the core members of the bacterial community in the marine sponge Halichondria panicea from Icelandic waters. We show that the core bacterial community across all samples consisted of a single, dominant bacterial taxon, for which we propose a candidate status 'Candidatus Halichondribacter symbioticus'. Comparison against public databases showed that 'Ca. H. symbioticus' is both a highly abundant specialist in H. panicea and a low abundant opportunist in other sponge species. Additionally, H. panicea with and without 'Ca. H. symbioticus' co-exist in similar locations in the North Atlantic. This dichotomy paired with the presence of geographically distinct ribosomal sequence variants of the symbiont make H. panicea an interesting sponge species for studying sponge-symbiont co-evolution and functional interactions.