Despite being one of the simplest metazoans, corals harbor some of the most highly diverse and abundant microbial communities. Differentiating core, symbiotic bacteria from this diverse host-associated consortium is essential for characterizing the functional contributions of bacteria but has not been possible yet. Here we characterize the coral core microbiome and demonstrate clear phylogenetic and functional divisions between the micro-scale, niche habitats within the coral host. In doing so, we discover seven distinct bacterial phylotypes that are universal to the core microbiome of coral species, separated by thousands of kilometres of oceans. The two most abundant phylotypes are co-localized specifically with the corals’ endosymbiotic algae and symbiont-containing host cells. These bacterial symbioses likely facilitate the success of the dinoflagellate endosymbiosis with corals in diverse environmental regimes.

Bleaching of the Great Barrier Reef: The Australian Great Barrier Reef (GBR) is one of Earth's most extraordinary natural wonders, but it is vulnerable to climate change. Ainsworth et al. have tracked the effects of three decades of increasing heat stress on coral organisms. In the past, pulses of elevated temperatures that presaged hot seasons stimulated the acclimation of coral organisms and resilience to thermal stress. More recently, temperature hikes have been severe and precluded acclimation. The result has been increasing bleaching and death; notably extreme during 2016 in the wake of El Niño. Science , this issue p. 338