Microbial diversity in the marine sponge Aplysina cavernicola (formerly Verongia cavernicola) analyzed by fluorescence in situ hybridization (FISH)

We have employed electronmicroscopical methods (SEM, TEM) to document the m icrobial community associated with the marine sponge Aplysina cavernicola ( formerly Verongia cavernicola, class Demospongiae). Five dominant bacterial types were identified, three of which resemble the morphotypes originally described by Vacelet (1975). One bacterial type possesses morphological pro perties that are characteristic of the genus Planctomyces. In addition, mor phologically uniform bacteria which reside inside the nuclei of host cells were observed. Using in situ hybridization with fluorescently labelled rRNA probes directed against known bacterial groups, the phylogenetic affiliati on of the mesohyl bacteria was assessed. It could be shown that the vast ma jority of mesohyl bacteria belongs to the domain Bacteria with a low GC con tent. Among the Bacteria, the delta-Proteobacteria were most abundant, foll owed by the gamma-Proteobacteria and representatives of the Bacteroides clu ster. Clusters of Gram-positive bacteria with a high GC content were also f ound consistently in low amounts. No hybridization signal was obtained with probes specific to the domain Archaea, to the alpha- and beta-Proteobacter ia and to the Cytophaga/Flavobacterium cluster. This study describes for th e first time the application of the "top-to-bottom approach" using 16S rRNA probes and in situ hybridization to assess the microbial diversity in Aply sina sponges.