The cold-water coral
(Scleractinia, Caryophylliidae) is a key species in the formation of cold-water reefs, which are among the most diverse deep-sea ecosystems. It occurs in two color varieties: white and red. Bacterial communities associated with
have been investigated in recent years, but the role of the associated bacteria remains largely obscure. This study uses catalyzed reporter deposition fluorescence in situ hybridization to detect the in situ location of specific bacterial groups on coral specimens from the Trondheimsfjord (Norway). Two tissue-associated groups were identified: (i) bacteria on the host's tentacle ectoderm, “
Mycoplasma corallicola,” are flasklike, pointed cells and (ii) endoderm-associated bona fide TM7 bacteria form long filaments in the gastral cavity. These tissue-bound bacteria were found in all coral specimens from the Trondheimsfjord, indicating a closer relationship with the coral compared to bacterial assemblages present in coral mucus and gastric fluid.