Fig. 1

Cross-section of a coral skeleton, showing the microbiome associated with the tissue and mucus (left inset) and the skeleton (right inset). In the tissue, Symbiodiniaceae provide the coral with sugars through photosynthesis, and a rich prokaryotic microbiome is associated with mucus, epidermal and gastrodermal tissue layers, which often show bacterial aggregates. In the skeleton, a remarkably species-rich microbiome including the green alga Ostreobium, fungi and prokaryotes is found. The illustration depicts a relatively young massive coral species; older skeletons often have a more complex layered microbiome (see section "The skeleton as an environmental and biological record-keeper")