bComplex microbiomes reside in marine sponges and consist of diverse microbial taxa, including functional guilds that may contribute to host metabolism and coastal marine nutrient cycles. Our understanding of these symbiotic systems is based primarily on static accounts of sponge microbiota, while their temporal dynamics across seasonal cycles remain largely unknown. Here, we investigated temporal variation in bacterial symbionts of three sympatric sponges (Ircinia spp.) over 1.5 years in the northwestern (NW) Mediterranean Sea, using replicated terminal restriction fragment length polymorphism (T-RFLP) and clone library analyses of bacterial 16S rRNA gene sequences. Bacterial symbionts in Ircinia spp. exhibited host species-specific structure and remarkable stability throughout the monitoring period, despite large fluctuations in temperature and irradiance. In contrast, seawater bacteria exhibited clear seasonal shifts in community structure, indicating that different ecological constraints act on free-living and on symbiotic marine bacteria. Symbiont profiles were dominated by persistent, sponge-specific bacterial taxa, notably affiliated with phylogenetic lineages capable of photosynthesis, nitrite oxidation, and sulfate reduction. Variability in the sponge microbiota was restricted to rare symbionts and occurred most prominently in warmer seasons, coincident with elevated thermal regimes. Seasonal stability of the sponge microbiota supports the hypothesis of host-specific, stable associations between bacteria and sponges. Further, the core symbiont profiles revealed in this study provide an empirical baseline for diagnosing abnormal shifts in symbiont communities. Considering that these sponges have suffered recent, episodic mass mortalities related to thermal stresses, this study contributes to the development of model sponge-microbe symbioses for assessing the link between symbiont fluctuations and host health.
Sponges are sessile invertebrates that form a species-rich phylum at the base of the metazoan tree of life (Ͼ8,500 valid species [65]). Renowned for their efficient filter-feeding capabilities and bioactive secondary metabolite production, sponges have important ecological and biotechnological relevance as major players in marine nutrient cycles (11,12,26) and the most prolific producers of marine natural products (Ͼ6,600 secondary metabolites [16]). The discovery and characterization of diverse microbial symbionts inhabiting the sponge body have prompted the adoption of the holobiont concept, thereby incorporating microbial symbionts in the study of sponge ecology and evolution (55). The resulting field of sponge microbiology has grown rapidly in the past 2 decades (59) and revealed a tight ecological link between host health and symbiont composition. Indeed, sponge-associated microbes have been implicated in host metabolism and growth (20,22,75), chemical defense production (21), and susceptibility to biotic (e.g., disease) and abiotic (e.g., temperature stress) stressors (33,66,72).The remarkable...