Calcibacteria in sponge microbiomes
Endosymbiotic calcifying bacteria, named calcibacteria due to the calcareous spherules they produce, were first identified in the Altlanto-Mediterranean sponge Hemimycale columella (Uriz et al. 2012). Later, seven calcibacteria OTUs belonging to the Alphaproteobacteria were identified in sponges, corals and seawater (Garate et al. 2017). Moreover, similar calcareous spherules were observed in other sponges from different latitudes. The aim of this study was to analyze and compare the microbiota of the sponges with calcareous spherules to obtain more insight in the phylogeny of calcifying bacteria in sponges.The studied sponge species were: Hemimycale columella, H. mediterranea, and Cliona viridis from western Mediterranean Sea, H. mediterranea from Adriatic Sea, H. arabica and Crella cyathophora from Red Sea, C. cyathophora from Indo-Pacific Ocean, and Cinachyrella alloclada from Caribbean Sea. Tag-pyrosequencing analyses of 16S rRNA gene were performed and data were analyzed using QIIME 1.4.0 pipeline (Caporaso et al. 2010). The studied sponges had species-specific microbiotas with Proteobacteria as the most abundant phylum (> 70% relative abundance) in all the species reported as LMA sponges, except the one from the Caribbean Sea, which belong to the HMA sponges. All of the species harbored large numbers of calcareous spherules. The known calcibacteria OTUs were the most abundant sequences in the microbiota of the spherules-harboring species, except in C. alloclada and H. mediterranea. The microbiota of the latter species lacked the known Alphaproteobacteria but were dominated by a Gammaproteobacterium. Calcibacteria-sponge symbioses are widespread in temperate and warm seas, and calcibacteria are the most abundant bacteria in almost all the sponges with calcareous micro-spherules. Taxonomically distant bacteria seem to be involved in sponge calcification. Given the phylogenetic closeness of these OTUs to those present in corals, the hypothesis of the role of intracellular bacteria in calcification processes of Early Metazoa gains support.