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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 For over three decades, scientists have conducted heat-stress experiments to predict how coral will respond to ocean warming due to global climate change. However, there are often conflicting results in the literature that are difficult to resolve, which we hypothesize are a result of unintended biases, variation in experimental design, and underreporting of critical methodological information. Here, we reviewed 255 coral heat-stress experiments to (1) document where and when they were conducted and on which species, (2) assess variability in experimental design, and (3) quantify the diversity of response variables measured. First, we found that two-thirds of studies were conducted in only three countries, three coral species were more heavily studied than others, and only 4% of studies focused on earlier life stages. Second, slightly more than half of all heat-stress exposures were less than 8 d in duration, only 17% of experiments fed corals, and experimental conditions varied widely, including the level and rate of temperature increase, light intensity, number of genets used, and the length of acclimation period. In addition, 95%, 55%, and > 35% of studies did not report tank flow conditions, light–dark cycle used, or the date of the experiment, respectively. Finally, we found that 21% of experiments did not measure any bleaching phenotype traits, 77% did not identify the Symbiodiniaceae endosymbiont, and the contribution of the coral host in the physiological response to heat-stress was often not investigated. This review highlights geographic, taxonomic, and heat-stress duration biases in our understanding of coral bleaching, and large variability in the reporting and design of heat-stress experiments that could account for some of the discrepancies in the literature. Development of some best practice recommendations for coral bleaching experiments could improve cross-studies comparisons and increase the efficiency of coral bleaching research at a time when it is needed most. 

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.