Microborings in Jurassic scleractinians: a glimpse into the ancient coral skeleton microbiome

Boring microorganisms (microendoliths) are ubiquitous in living corals, constituting the skeleton microbiome important for coral health and reef resilience. Numerous microborings were recognized in Upper Jurassic (ca. 160 million years ago) corals (Pomerania, Poland) providing a glimpse into the oldest scleractinian skeleton microbiome so far. Scanning electron microscope study of resin casts of microborings (ca. 4 μm) revealed that they represent mostly the ichnospecies Ichnoreticulina elegans, commonly considered as traces of Ostreobium quekettii, an alga adapted for a low-light environment. The distribution pattern of microborings (occurrence in the inner part of the skeletal elements, commonly upward orientation) implies that they were not done post-mortem, but by microendoliths inhabiting the coral skeleton during coral life. These findings imply that the most common boring microorganism inhabiting the skeleton of Jurassic corals was, like in modern corals, O. quekettii or a similar green alga. The microbiome of dead parts of modern living colonies revealed by analysis of microborings is an unexplored, but is a perspective topic for research by reef biologist