Microbial landscapes on the outer tissue surfaces of the reef-building coral Porites compressa

Microbial-coral interactions are increasingly recognized as important for coral health and disease. Visualizing these interactions is important for understanding where, when, and how the coral animal and microbes interact. Porites compressa, preserved using Parducz fixative and examined by scanning electron microscopy, revealed a changing microbial landscape. The external cell layers of this coral were invariably clean of directly adhering microbes, unlike coral-associated mucus. In colonies with expanded polyps, secreted mucus rapidly dissipated, although blobs of new mucus were common; the coral epidermal cells expressed cilia, which are presumably used to clean the surface, and coral-associated microbes were present as flocs, possibly enmeshed in mucus. In colonies with permanently contracted polyps, the coral epidermis had lost cilia and a stable, multi-lamellar mucous sheet covered the surface of the animal. This sheet became heavily colonized by both prokaryotic and eukaryotic microbes, however these microbes did not penetrate the mucous sheet and the animal's epidermal cell surfaces remained sterile. These observations show that relationships between this coral animal and associated microbes are highly dynamic.