How Does the Coral Microbiome Cause, Respond to, or Modulate the Bleaching Process?

Abstract

Corals holobionts are formed by a dynamic multipartite symbiosis with intracellular photoautotrophs in the genus Symbiodinium and a consortium of other microbes that include bacteria, archaea, viruses, fungi, and protists. Coral bleaching, or the loss of Symbiodinium and their pigments, has caused significant global declines in reef-building corals, particularly in the past several decades as global sea surface temperatures continue to rise. The majority of the bleaching literature has primarily focused on Symbiodinium populations, with increasing interest in the role of the coral host in the bleaching process. However, much less attention has been paid to the coral-associated prokaryotes and viruses and their role in coral bleaching and the bleaching response. Microbes (e.g., Symbiodinium, prokaryotes, and viruses) exhibit extensive diversity and abilities to respond to and withstand environmental pressures and, as with other multipartite mutualisms, have the ability to recruit colonizers with distinct environmental tolerances that may provide the ability to adapt or acclimatize to environmental change. Here, we provide some context for the importance of the coral microbiome to holobiont function from the perspective of the bleaching response. We survey what is currently known about the relationship among Symbiodinium, temperature stress, and the associated prokaryotes and viruses. We also examine the impact of the coral bleaching response and environmental stressors conducive to bleaching (e.g., temperature, irradiance) on coral-associated microorganisms with particular emphasis on diazotrophic (nitrogen-fixing) bacteria. In conclusion, we summarize how a shifting and potentially dysbiotic microbiome may impact the coral host in the context of bleaching.