High variability and limited host specificity characterise the microbiome of Desmophyllum pertusum across a cold-water coral reef
Cold-water corals (CWC), like their tropical counterparts, are ecosystem engineers that create complex habitats supporting high biodiversity. Despite their broad distribution and increasing exposure to climate change and anthropogenic stressors, CWC remain comparatively understudied, particularly with respect to their associated microbiomes. Unlike tropical corals, CWC lack photosynthetic algal symbionts, suggesting that microbial associates may play a particularly important role in nutrient acquisition, biogeochemical cycling, and host resilience. Here, we investigated microbial community composition across the reef ecosystem formed by the cosmopolitan CWC Desmophyllum pertusum at Tisler Reef (NE Skagerrak, Kosterfjord, Norway). Using Nanopore sequencing of the full-length 16S rRNA gene, we characterised microbial communities associated with coral, seawater, substrate, and coral rubble to (i) assess reef-scale patterns, (ii) characterise variation in microbial assemblages between coral colonies and between branch and mucus compartments, as well as according to branch position, and (iii) identify putative core microbiome members and indicator taxa associated with D. pertusum relative to its surrounding environment. At the reef scale, microbial community structure differed among habitat compartments. Within D. pertusum, both colony identity and compartment (branch vs. mucus) contributed to structuring microbial assemblages, while branch position also explained an additional but limited proportion of the variation. The coral-associated microbiome contained a set of recurrent taxa, but many of these were also widespread in the surrounding environment, and few were consistently enriched in coral samples. Together, these findings indicate that the D. pertusum microbiome at Tisler Reef comprises recurrent microbial associates embedded within a highly variable assemblage, rather than a large and abundant core microbiome exclusive to the coral habitat. These results contribute to establishing a baseline understanding of D. pertusum microbiomes and provide insights into microbial community structure across cold-water coral reef ecosystems.