C hloroflexi bacteria are more diverse, abundant, and similar in high than in low microbial abundance sponges

Some marine sponges harbor dense and phylogenetically complex microbial communities [high microbial abundance ( HMA ) sponges] whereas others contain only few and less diverse microorganisms [low microbial abundance ( LMA ) sponges]. We focused on the phylum C hloroflexi that frequently occurs in sponges to investigate the different associations with three HMA and three LMA sponges from N ew Z ealand. By applying a range of microscopical and molecular techniques a clear dichotomy between HMA and LMA sponges was observed: C hloroflexi bacteria were more abundant and diverse in HMA than in LMA sponges. Moreover, different HMA sponges contain similar C hloroflexi communities whereas LMA sponges harbor different and more variable communities which partly resemble C hloroflexi seawater communities. A comprehensive phylogenetic analysis of our own and publicly available sponge‐derived C hloroflexi 16 S rRNA gene sequences (> 780 sequences) revealed the enormous diversity of this phylum within sponges including 29 sponge‐specific and sponge‐coral clusters ( SSC / SCC ) as well as a ‘supercluster’ consisting of > 250 sponge‐derived and a single nonsponge‐derived 16 S rRNA gene sequence. Interestingly, the majority of sequences obtained from HMA sponges, but only a few from LMA sponges, fell into SSC / SCC clusters. This indicates a much more specific association of C hloroflexi bacteria with HMA sponges and suggests an ecologically important role for these prominent bacteria.