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Abstract Climate change poses a major threat to coral reefs. We conducted an outdoor 22-month experiment to investigate if coral could not just survive, but also physiologically cope, with chronic ocean warming and acidification conditions expected later this century under the Paris Climate Agreement. We recorded survivorship and measured eleven phenotypic traits to evaluate the holobiont responses of Hawaiian coral: color, Symbiodiniaceae density, calcification, photosynthesis, respiration, total organic carbon flux, carbon budget, biomass, lipids, protein, and maximumArtemiacapture rate. Survivorship was lowest inMontipora capitataand only some survivors were able to meet metabolic demand and physiologically cope with future ocean conditions. MostM. capitatasurvivors bleached through loss of chlorophyll pigments and simultaneously experienced increased respiration rates and negative carbon budgets due to a 236% increase in total organic carbon losses under combined future ocean conditions.Porites compressaandPorites lobatahad the highest survivorship and coped well under future ocean conditions with positive calcification and increased biomass, maintenance of lipids, and the capacity to exceed their metabolic demand through photosynthesis and heterotrophy. Thus, our findings show that significant biological diversity within resilient corals likePorites, and some genotypes of sensitive species, will persist this century provided atmospheric carbon dioxide levels are controlled. SincePoritescorals are ubiquitous throughout the world’s oceans and often major reef builders, the persistence of this resilient genus provides hope for future reef ecosystem function globally. 

Abstract Corals obtain nutrition from the photosynthetic products of their algal endosymbionts and the ingestion of organic material and zooplankton from the water column. Here, we use stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to assess the proportionate contribution of photoautotrophic and heterotrophic sources to seven Hawaiian coral species collected from six locations around the island of O‘ahu, Hawaiʻi. We analyzed the δ¹³C and δ¹⁵N of coral tissues and their algal endosymbionts, as well as that of dissolved inorganic matter, particulate organic matter, and zooplankton from each site. Estimates of heterotrophic contribution varied among coral species and sites. Bayesian mixing models revealed that heterotrophic sources (particulate organic material and zooplankton) contributed the most toPocillopora acutaandMontipora patulacoral tissues at 49.3% and 48.0%, respectively, and the least toPorites lobataat 28.7%, on average. Estimates of heterotrophic contribution based on the difference between δ¹³C of the host and algal endosymbiont (δ¹³C_h–e) and isotopic niche overlap often differed, while estimates based on δ¹⁵N_h–ewere slightly more aligned with the estimates produced using Bayesian mixing models. These findings suggest that the utility of each approach may vary with coral health status, regions, and coral species. Overall, we find that the mean heterotrophic contribution to Hawaiian coral tissues ranges from 20% to 50%, suggesting a variety of trophic strategies. However, these findings did not always match past direct measurements of heterotrophic feeding, indicating that heterotrophically acquired nutrition does not necessarily get incorporated into tissues but can be respired or exuded in mucus. 

The plasticity of some coral-associated microbial communities under stressors like warming and ocean acidification suggests the microbiome has a role in the acclimatization of corals to future ocean conditions. Here, we evaluated the acclimatization potential of coral-associated microbial communities of four Hawaiian coral species (Porites compressa,Porites lobata,Montipora capitata, andPocillopora acuta) over 22-month mesocosm experiment. The corals were exposed to one of four treatments: control, ocean acidification, ocean warming, or combined future ocean conditions. Over the 22-month study, 33–67% of corals died or experienced a loss of most live tissue coverage in the ocean warming and future ocean treatments while only 0–10% died in the ocean acidification and control. Among the survivors, coral-associated microbial communities responded to the chronic future ocean treatment in one of two ways: (1) microbial communities differed between the control and future ocean treatment, suggesting the potential capacity for acclimatization, or (2) microbial communities did not significantly differ between the control and future ocean treatment. The first strategy was observed in bothPoritesspecies and was associated with higher survivorship compared toM.capitataandP.acutawhich exhibited the second strategy. Interestingly, the microbial community responses to chronic stressors were independent of coral physiology. These findings indicate acclimatization of microbial communities may confer resilience in some species of corals to chronic warming associated with climate change. However,M.capitatagenets that survived the future ocean treatment hosted significantly different microbial communities from those that died, suggesting the microbial communities of the survivors conferred some resilience. Thus, even among coral species with inflexible microbial communities, some individuals may already be tolerant to future ocean conditions. These findings suggest that coral-associated microbial communities could play an important role in the persistence of some corals and underlie climate change-driven shifts in coral community composition. 

Coral reefs are among the most diverse and complex ecosystems in the world that provide important ecological and economical services. Increases in sea surface temperature linked to global climate change threatens these ecosystems by inducing coral bleaching. However, it is not fully known if natural intra- or inter-annual physiological variability is linked to bleaching resilience or recovery capacity of corals. Here, we monitored the coral physiology of three common Caribbean species ( Porites divaricata, Porites astreoides, Orbicella faveolata ) at six time points over 2 years by measuring the following traits: calcification, biomass, lipids, proteins, carbohydrates, chlorophyll a , algal endosymbiont density, stable carbon isotopes of the host and endosymbiotic algae, and the stable carbon and oxygen isotopes of the skeleton. The overall physiological profile of all three species varied over time and that of P. divaricata was consistently different from the two other coral species. Porites divaricata had higher energy reserves coupled with higher contributions of heterotrophically derived carbon to host tissues than both P. astreoides and O. faveolata . Consistently higher overall energy reserves and heterotrophic contributions to tissues appear to buffer against environmental stress, including bleaching events. Thus, natural physiological variability among coral species appears to be a stronger predictor of coral bleaching resilience than intra- or inter-annual physiological variability within a coral species. 

Evidence has shown that individually feeding or reduced light can mitigate the negative effects of elevated temperature on coral physiology. We aimed to evaluate if simultaneous low light and feeding would mitigate, minimize, or exacerbate negative effects of elevated temperature on coral physiology and carbon budgets. Pocillopora damicornis, Stylophora pistillata, and Turbinaria reniformis were grown for 28 days under a fully factorial experiment including two seawater temperatures (ambient temperature of 25 °C, elevated temperature of 30 °C), two light levels (high light of 300 μmol photons m−2 s−1, low light of 150 μmol photons m−2 s−1), and either fed (Artemia nauplii) or unfed. Coral physiology was significantly affected by temperature in all species, but the way in which low light and feeding altered their physiological responses was species-specific. All three species photo-acclimated to low light by increasing chlorophyll a. Pocillopora damicornis required feeding to meet metabolic demand irrespective of temperature but was unable to maintain calcification under low light when fed. In T. reniformis, low light mitigated the negative effect of elevated temperature on total lipids, while feeding mitigated the negative effects of elevated temperature on metabolic demand. In S. pistillata, low light compounded the negative effects of elevated temperature on metabolic demand, while feeding minimized this negative effect but was not sufficient to provide 100% metabolic demand. Overall, low light and feeding did not act synergistically, nor additively, to mitigate the negative effects of elevated temperature on P. damicornis, S. pistillata, or T. reniformis. However, feeding alone was critical to the maintenance of metabolic demand at elevated temperature, suggesting that sufficient supply of heterotrophic food sources is likely essential for corals during thermal stress (bleaching) events.