Unravelling the microbiome of fossil corals: a message from microborings

The coral microbiome comprises skeleton-associated microendoliths (boring algae, bacteria and fungi) which are important for coral health and reef conditions. The analysis of microborings is a key tool in deciphering of the skeletal microbiome of fossil corals. Even micrometre-size traces have a high preservation potential and indicate the trace-maker organisms that inhabited skeletons of live corals. SEM studies of resin replicas and naturally occurring internal moulds of microborings were performed on skeletons of Jurassic, Cretaceous, Palaeogene, and Miocene colonial corals. This study and a literature review show that micro-borings (mostly 2–10 μm in diameter) which were formed in coral skeletons during their lifetimes are common in some fossil corals. They are ubiquitous throughout the whole colony, concentrated in the inner part of skeletal elements, and located commonly roughly along their longitudinal axis, in the direction of the coral growth. Post-mortem microborings, occurring at the margin of the skeletal elements, were rare in the examined samples. The coral-associated microendoliths were like in modern reef corals, mainly represented by Ostreobium quekettii or similar green algae, and less commonly by red algae, cyanobacteria, and fungi. Analyses of microborings can be also applied in studies of the skeleton microbiome of modern living corals.