Complexity of the Coral Microbiome Assembly

The coral holobiont, comprising corals and their associated microbiomes, is a complex entity critical to reef health and ecosystem function. The composition of these microbial assemblages varies across coral species, coral compartments (e.g., skeleton, tissue, and mucus), coral genotypes, and environments. At the same time, certain bacteria are repeatedly and predictively associated with specific coral species, suggesting that coral microbiomes consist of transient, resident, and core bacterial associates. The composition and assembly of these distinct communities are a consequence of coral host-related and environmental factors. A good example of this interplay is the correlation between different host genotypes and their associated microbial assemblages, and how the referred holobiont responds to environmental conditions and change. At large, the extent to which microbiomes can change with the environment varies among coral host species, a concept termed microbiome flexibility. Knowledge about the dependence of specific coral species on their microbiomes, the ability to change microbial association in different environments, and the extent of functional redundancy between host genotypes are essential for predicting coral responses to environmental change and developing effective microbiome-centered conservation strategies, such as probiotic therapy.