Stable and Enriched Cenarchaeum symbiosum and Uncultured Betaproteobacteria HF1 in the Microbiome of the Mediterranean Sponge Haliclona fulva (Demospongiae: Haplosclerida)

Spongesharbor characteristic microbiomesderived from symbiotic relationships shaping their lifestyle and survival. Haliclonafulva is encrusting marine spongespecies dwelling in coralligenous accretions or semidark caves of the Mediterranean Sea and the near Atlantic Ocean. In this work, we characterized the abundance and core microbial community composition found in specimens of H. fulva by means of electron microscopy and 16S amplicon Illumina sequencing. We provide evidence of its low microbial abundance (LMA) nature. We found that the H. fulva core microbiomeis dominated by sequences belonging to the orders Nitrosomonadalesand Cenarchaeales. Seventy percent of the reads assigned to these phylotypesgrouped in a very small number of high-frequency operational taxonomic units, representing niche-specific species Cenarchaeum symbiosumand uncultured BetaproteobacteriaHF1, a new eubacterial ribotypevariant found in H. fulva. The microbial composition of H. fulva is quite distinct from those reported in spongespecies of the same Haliclonagenus. We also detected evidence of an excretion/capturing loop between these abundant microorganisms and planktonic microbes by analyzing shifts in seawater planktonic microbial content exposed to healthy spongespecimens maintained in aquaria. Our results suggest that horizontal transmissionis very likely the main mechanism for symbionts’ acquisition by H. fulva. So far, this is the first shallow water spongespecies harboring such a specific and predominant assemblage composed of these eubacterial and archaeal ribotypes. Our data suggests that this symbiotic relationship is very stable over time, indicating that the identified core microbial symbionts may play key roles in the holobiontfunctioning.