A New “Business as Usual” Climate Scenario and the Stress Response of the Caribbean Coral Montastraea cavernosa

The climate change related decline of shallow (<30 m) coral reef ecosystems has been driven by the mortality of scleractinian corals caused primarily by the phenomenon known as “coral bleaching.” But despite pervasive phase shifts and macroalgal dominance on many coral reefs, some coral species have persisted. One of those species is Montastraea cavernosa which has been categorized as resilient to a range of biotic and abiotic stressors. In order to understand the mechanism(s) of resistance in this coral, we present the results of a thermal stress and ocean acidification (OA) experiment on M. cavernosa, both its brown and orange color morphs, representing conditions predicted by the Representative Concentration Pathway (RCP) 6.0 scenario in the year 2100. We assessed the community response of the prokaryotic microbiome, the photophysiological response of the endosymbiotic Symbiodiniaceae and the molecular responses of critical pathways in the host by quantifying transcript abundances of genes encoding fluorescent proteins, heat shock proteins, antioxidant enzymes and regulators of apoptosis. After a 12 d acclimatization experiment, no visible bleaching was observed in any treatment, and the excitation pressure on photosystem II of the symbiotic Symbiodiniaceae showed no effects of the independent or interactive effects of thermal stress and OA, while only minor, but significant, changes in the prokaryotic microbiome were observed when exposed to RCP 6.0 predicted OA conditions. At the end of the experiment, the host heat shock protein 90 showed an increase in transcript abundance under the combined effects of thermal stress and OA compared to high temperatures alone, but these treatment groups were not significantly different from treatments under normal temperatures. While Bax, an activator of apoptosis, was significantly higher under thermal stress alone compared to control samples. Taken together, M. cavernosa, exhibits ecological stability over time and this may be based on its physiological persistence, resistance and resilience when experimentally exposed to the ecologically realistic RCP 6.0 climate model predictions.