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1. Increasing comparability among coral bleaching experiments

   https://doi.org/10.1002/eap.2262  

   Grottoli, A. G.; Toonen, R. J.; van Woesik, R.; Vega Thurber, R.; Warner, M. E.; McLachlan, R. H.; Price, J. T.; Bahr, K. D.; Baums, I. B.; Castillo, K. D.; et al (May 2021, Ecological Applications)

   Abstract Coral bleaching is the single largest global threat to coral reefs worldwide. Integrating the diverse body of work on coral bleaching is critical to understanding and combating this global problem. Yet investigating the drivers, patterns, and processes of coral bleaching poses a major challenge. A recent review of published experiments revealed a wide range of experimental variables used across studies. Such a wide range of approaches enhances discovery, but without full transparency in the experimental and analytical methods used, can also make comparisons among studies challenging. To increase comparability but not stifle innovation, we propose a common framework for coral bleaching experiments that includes consideration of coral provenance, experimental conditions, and husbandry. For example, reporting the number of genets used, collection site conditions, the experimental temperature offset(s) from the maximum monthly mean (MMM) of the collection site, experimental light conditions, flow, and the feeding regime will greatly facilitate comparability across studies. Similarly, quantifying common response variables of endosymbiont (Symbiodiniaceae) and holobiont phenotypes (i.e., color, chlorophyll, endosymbiont cell density, mortality, and skeletal growth) could further facilitate cross‐study comparisons. While no single bleaching experiment can provide the data necessary to determine global coral responses of all corals to current and future ocean warming, linking studies through a common framework as outlined here, would help increase comparability among experiments, facilitate synthetic insights into the causes and underlying mechanisms of coral bleaching, and reveal unique bleaching responses among genets, species, and regions. Such a collaborative framework that fosters transparency in methods used would strengthen comparisons among studies that can help inform coral reef management and facilitate conservation strategies to mitigate coral bleaching worldwide. 

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2. Thirty years of coral heat-stress experiments: a review of methods

   https://doi.org/10.1007/s00338-020-01931-9  

   McLachlan, Rowan H.; Price, James T.; Solomon, Sarah L.; Grottoli, Andréa G. (August 2020, Coral Reefs)

   null (Ed.)

   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. 

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3. Role of host genetics and heat‐tolerant algal symbionts in sustaining populations of the endangered coral Orbicella faveolata in the Florida Keys with ocean warming

   https://doi.org/10.1111/gcb.14545  

   Manzello, Derek P.; Matz, Mikhail V.; Enochs, Ian C.; Valentino, Lauren; Carlton, Renee D.; Kolodziej, Graham; Serrano, Xaymara; Towle, Erica K.; Jankulak, Mike (January 2019, Global Change Biology)

   Abstract Identifying which factors lead to coral bleaching resistance is a priority given the global decline of coral reefs with ocean warming. During the second year of back‐to‐back bleaching events in the Florida Keys in 2014 and 2015, we characterized key environmental and biological factors associated with bleaching resilience in the threatened reef‐building coralOrbicella faveolata. Ten reefs (five inshore, five offshore, 179 corals total) were sampled during bleaching (September 2015) and recovery (May 2016). Corals were genotyped with 2bRADand profiled for algal symbiont abundance and type.O. faveolataat the inshore sites, despite higher temperatures, demonstrated significantly higher bleaching resistance and better recovery compared to offshore. The thermotolerantDurusdinium trenchii(formerlySymbiondinium trenchii) was the dominant endosymbiont type region‐wide during initial (78.0% of corals sampled) and final (77.2%) sampling; >90% of the nonbleached corals were dominated byD. trenchii. 2bRADhost genotyping found no genetic structure among reefs, but inshore sites showed a high level of clonality. While none of the measured environmental parameters were correlated with bleaching, 71% of variation in bleaching resistance and 73% of variation in the proportion ofD. trenchiiwas attributable to differences between genets, highlighting the leading role of genetics in shaping natural bleaching patterns. Notably,D. trenchiiwas rarely dominant inO. faveolatafrom the Florida Keys in previous studies, even during bleaching. The region‐wide high abundance ofD. trenchiiwas likely driven by repeated bleaching associated with the two warmest years on record for the Florida Keys (2014 and 2015). On inshore reefs in the Upper Florida Keys,O. faveolatawas most abundant, had the highest bleaching resistance, and contained the most corals dominated byD. trenchii, illustrating a causal link between heat tolerance and ecosystem resilience with global change. 

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4. A review of coral bleaching specimen collection, preservation, and laboratory processing methods

   https://doi.org/10.7717/peerj.11763  

   McLachlan, Rowan H.; Dobson, Kerri L.; Schmeltzer, Emily R.; Vega Thurber, Rebecca; Grottoli, Andréa G. (January 2021, PeerJ)

   Under current climate warming predictions, the future of coral reefs is dire. With projected coral reef decline, it is likely that coral specimens for bleaching research will increasingly become a more limited resource in the future. By adopting a holistic approach through increased collaborations, coral bleaching scientists can maximize a specimen’s investigative yield, thus reducing the need to remove more coral material from the reef. Yet to expand a specimen’s utility for additional analytic methods, information on how corals are collected is essential as many methods are variably sensitive to upstream handling and processing. In an effort to identify common practices for coral collection, sacrifice, preservation, and processing in coral bleaching research, we surveyed the literature from the last 6.5 years and created and analyzed the resulting dataset of 171 publications. Since January 2014, at least 21,890 coral specimens were collected for bleaching surveys or bleaching experiments. These specimens spanned 122 species of scleractinian corals where the most frequently sampled were Acropora millepora , Pocillopora damicornis , and Stylophora pistillata . Almost 90% of studies removed fragments from the reef, 6% collected skeletal cores, and 3% collected mucus specimens. The most common methods for sacrificing specimens were snap freezing with liquid nitrogen, chemical preservation (e.g., with ethanol or nucleic acid stabilizing buffer), or airbrushing live fragments. We also characterized 37 distinct methodological pathways from collection to processing of specimens in preparation for a variety of physiological, -omic, microscopy, and imaging analyses. Interestingly, almost half of all studies used only one of six different pathways. These similarities in collection, preservation, and processing methods illustrate that archived coral specimens could be readily shared among researchers for additional analyses. In addition, our review provides a reference for future researchers who are considering which methodological pathway to select to maximize the utility of coral bleaching specimens that they collect. 

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5. Natural Variability in Caribbean Coral Physiology and Implications for Coral Bleaching Resilience

   https://doi.org/10.3389/fmars.2021.811055  

   Chapron, Leila; Schoepf, Verena; Levas, Stephen J.; Aschaffenburg, Matthew D.; Warner, Mark E.; Grottoli, Andréa G. (January 2022, Frontiers in Marine Science)

   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. 

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