Holobiont Variability and the ecosystem impact of coral bleaching: lessons learned from a molecular time series

Andrew Thurber, Oregon State University, Corvallis, OR, United States, Sarah Seabrook, Oregon State University, CEOAS, Corvallis, OR, United States, Kaitlin McConnell, Oregon State University, Microbiology, OR, United States, Emily Schmeltzer, Oregon State University, Microbiology, Corvallis, CA, United States, Adrienne Correa, Rice University, BioSciences, Houston, United States and Rebecca Vega Thurber, Oregon State University, Department of Microbiology, Corvallis, OR, United States
Abstract:
Reef-building corals are integral parts of many tropical marine habitats, yet are increasingly challenged by a diversity of stressors. Corals succeed in these oligotrophic waters largely through mutualistic symbioses, especially with Symbiodiniaceae, however, they also host a diversity of taxa whose interactions with the coral and each other range from mutualistic to parasitic. The nature of these symbioses can be altered by external stressors resulting in coral bleaching or a disease outbreak that can in turn alter the role of the coral in reef ecosystems. Here, we (1) explore islandwide spatio-temporal variability in holobiont (coral and associated microbes) community structure over multiple years, (2) quantify how that structure is impacted by stress, and (3) test how this stress impacts the flux of C, N, and microbes from corals during both empirically induced and a naturally occurring bleaching event on the Island of Mo’orea. Using a three-year time-series where we repeatedly sampled ~300 focal coral colonies, we produced a dataset of >1200 samples thus far and found clear differences in holobiont community beta-diversity among coral species, island region (West, North, East), and reef type (Back, Fore, Fringing). Using mesocosm pulse-chase experiments to quantify how the coral microbiome shifts during stress, we also discovered that while temperature and nutrients both impacted the holobiont, DOM and POM exudate from bleaching corals caused a rapid decline of otherwise healthy corals, as observed by bleaching, significant loss of photosystem functioning (Fv/Fm), and finally coral death within 48 hours.This bleaching and death also resulted in the release of DOM and POM and a rapid reduction in oxygen in the mesocosms. Using the 2019 island-wide bleaching event in Mo’orea, we further explore reef-holobiont cascades, which we define as shifts in the overall reef health (altered C, N, and O dynamics and increased pathogenicity and heterotrophy within the reef-wide microbiome) precipitated by coral bleaching. Viewed together, this work provides a mechanism and realized example of how coral stress can induce a positive feedback loop that expedites the decline of corals and coral reefs in tropical marine systems.
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