Putative promiscuous symbionts in deep-sea corals and crinoids may contribute to nitrogen cycling

Crinoids (feather stars) are frequently found in association with corals, yet the physiological and microbial interactions between these organisms remain poorly understood. Both corals and crinoids host symbiotic microorganisms, but the functional roles of these symbionts, particularly in deep-sea environments, are largely unexplored. This study characterizes the microbiomes of the deep-sea corals Desmophyllum pertusum and Solenosmilia variabilis and their associated crinoid Koehlermetra sp. (Thalassometridae) from the Campos Basin, Brazil, to investigate potential cross-host microbial interactions and their ecological implications. We used multiple approaches for this investigation, including amplicon sequencing surveys, genome-resolved metagenomics, and fluorescence in situ hybridization.

We found that the same endosymbiotic members of the families Endozoicomonadaceae and Nitrosopumilaceae inhabit both corals and the crinoids, suggesting promiscuity in host-symbiont relationships. Metagenomic analysis revealed a novel and dominant Endozoicomonas species (E. promiscua sp. nov.), whose genome encodes pathways for dissimilatory nitrate reduction to ammonia (DNRA). This metabolic capability could provide a substrate for ammonia-oxidizing archaea (Nitrosopumilaceae), indicating a potential cross-host nitrogen-cycling network. Shared microbial taxa between corals and crinoids further support the hypothesis of symbiont promiscuity, where metabolic redundancy may facilitate colonization across species.

Our findings suggest that nitrogen cycling plays a key role in structuring microbial symbioses in deep-sea coral-crinoid holobionts. The promiscuous distribution of symbionts across hosts implies that metabolic interactions, such as DNRA-driven ammonia provisioning, could underpin resilience in nutrient-limited environments. This study highlights the importance of microbial versatility in deep-sea ecosystems and provides new insights into how cross-host symbiosis may contribute to biogeochemical cycling in the ocean. Video Abstract.