FIG 1.

Coral microbiome conceptualized into three distinct layers. We propose that the microbiome of each coral individual should be categorized into three distinct groups of bacteria with different levels of impact from the environment and host: (i) an environmentally responsive community with thousands of bacterial phylotypes, transient and highly variable across coral individuals; (ii) individual microbiome, ∼500 to 600 OTUs that vary among reefs at the level of OTUs but consistently belong to three major taxonomic classes; and (iii) core microbiome, few bacterial phylotypes, potentially symbiotic. Taxonomic (and potentially functional) redundancy is occurring in the resident community and the core microbiome. The reef picture was provided by Alexander J. Fordyce.