Defining Coral Bleaching as a Microbial Dysbiosis within the Coral Holobiont

Abstract

Coral microbiomes are critical to holobiont health and functioning, but the stability of host–microbial interactions is fragile, easily shifting from eubiosis to dysbiosis. The heat-induced breakdown of the symbiosis between the host and its dinoflagellate algae (that is, “bleaching”), is one of the most devastating outcomes for reef ecosystems. Yet, bleaching tolerance has been observed in some coral species. This review provides an overview of the holobiont’s diversity, explores coral thermal tolerance in relation to their associated microorganisms, discusses the hypothesis of adaptive dysbiosis as a mechanism of environmental adaptation, mentions potential solutions to mitigate bleaching, and suggests new research avenues. More specifically, we define coral bleaching as the succession of three holobiont stages, where the microbiota can (i) maintain essential functions for holobiont homeostasis during stress and/or (ii) act as a buffer to mitigate bleaching by favoring the recruitment of thermally tolerant Symbiodiniaceae species (adaptive dysbiosis), and where (iii) environmental stressors exceed the buffering capacity of both microbial and dinoflagellate partners leading to coral death.

Contrasting heat stress response patterns of coral holobionts across the Red Sea suggest distinct mechanisms of thermal tolerance

“…The overall lower correlation may be attributable to site-speci c bacterial community composition differences. Corals from the GoA, however, exhibited a low correlation between replicate colonies at 30° and 33 °C that increased considerably at 36 °C, due to the substantial relative increase of Vibrionaceae and Rhodobacteraceae, driving the similarity between samples and indicating dysbiosis/loss of microbiome structure, as alluded earlier (Sweet and Bulling 2017;Boilard et al 2020;Lima et al 2020). Thus, the decreasing correlation coe cient(s) with increasing temperature observed in GoA corals is that of a population approaching a tipping point with subsequent systemic collapse: GoA corals ramp up their response to the increasing stressor until they reach a tipping point upon which the system collapses.…”

Neither Coral- nor Symbiont- Genetic Diversity may Explain the Resistance of the Coral <i>Echinopora lamellosa</i> to Bleaching

“…Coral E. lamellosa has been reported as moderately susceptible to bleaching, in the Great Barrier Reefs (Marshall & Baird 2000). Recent studies are accumulating to see the importance of the coral holobiont to the physiology of the coral, including its thermal stress tolerance (Boilard et al 2020). Holobiont is a consortium of coral, algal symbiont, and microbiota.…”

“…The microbiota is composed of prokaryotes, eukaryotes, and viruses. Coral susceptibility to bleaching is dependent on the robustness of the complex mutual relationship within the coral holobiont (Bourne et al 2016;Boilard et al 2020). Coral trophic strategy may also explain the resistance of E. lamellosa.…”

Mangrove forests mitigate coral bleaching under thermal stress from climate change

“…Coral reefs are continuing to decline globally, primarily driven by anthropogenic heating (1-3). Increased sea water temperatures and marine heatwaves are inciting coral bleaching, a process whereby photosynthetic endosymbionts are expelled by the cnidarian host (4)(5)(6)(7)(8).…”