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1. 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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2. Sublethal changes to coral metabolism in response to deoxygenation

   https://doi.org/10.1242/jeb.249638  

   Mallon, J E; Altieri, A H; Cyronak, T; Melendez-Declet, C V; Paul, V J; Johnson, M D (February 2025, Journal of Experimental Biology)

   ABSTRACT Coastal deoxygenation poses a critical threat to tropical coral reefs. Dissolved oxygen (DO) depletion can cause hypoxia-induced stress and mortality in scleractinian corals. Coral hypoxic responses are species-specific and likely modulated by the duration and severity of low-DO conditions, although the physiological mechanisms driving hypoxia tolerance are not fully understood. In this study, the Caribbean corals Acropora cervicornis, Porites astreoides and Siderastrea siderea were exposed to either severe (1.5 mg l−1 DO) or moderate (3.5 mg l−1 DO) deoxygenation or a control treatment (6 mg l−1 DO). All corals survived 2 weeks of deoxygenation but exhibited sublethal changes to coral metabolism after 1- and 2-week exposures, compared with controls. Maximum quantum yield (Fv/Fm) was suppressed after 1 week in both deoxygenation treatments in A. cervicornis, and after 2 weeks in S. siderea and P. astreoides exposed to severe or moderate treatments, respectively. Respiration rates were lower than controls in A. cervicornis and S. siderea after 1 and 2 weeks of severe deoxygenation. The reduced respiration of P. astreoides after 1 week of moderate deoxygenation returned to control levels in week 2. Overall coral metabolic budgets, assessed by ratios of gross photosynthesis to respiration (Pg:R), were more autotrophic, or photosynthesis-dominant, after 1 week of severe deoxygenation in S. siderea and P. astreoides, whereas Pg:R was not significantly different in A. cervicornis between treatments. These results reveal that some corals shift their metabolism to tolerate low-oxygen conditions and avoid bleaching or mortality, indicating that metabolic plasticity is an important aspect of coral resistance to deoxygenation. 

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3. Divergent bleaching and recovery trajectories in reef-building corals following a decade of successive marine heatwaves

   https://doi.org/10.1073/pnas.2312104120  

   Brown, Kristen T.; Lenz, Elizabeth A.; Glass, Benjamin H.; Kruse, Elisa; McClintock, Rayna; Drury, Crawford; Nelson, Craig E.; Putnam, Hollie M.; Barott, Katie L. (December 2023, Proceedings of the National Academy of Sciences)

   Increasingly frequent marine heatwaves are devastating coral reefs. Corals that survive these extreme events must rapidly recover if they are to withstand subsequent events, and long-term survival in the face of rising ocean temperatures may hinge on recovery capacity and acclimatory gains in heat tolerance over an individual’s lifespan. To better understand coral recovery trajectories in the face of successive marine heatwaves, we monitored the responses of bleaching-susceptible and bleaching-resistant individuals of two dominant coral species in Hawai’i,Montipora capitataandPorites compressa, over a decade that included three marine heatwaves. Bleaching-susceptible colonies ofP. compressaexhibited beneficial acclimatization to heat stress (i.e., less bleaching) following repeat heatwaves, becoming indistinguishable from bleaching-resistant conspecifics during the third heatwave. In contrast, bleaching-susceptibleM. capitatarepeatedly bleached during all successive heatwaves and exhibited seasonal bleaching and substantial mortality for up to 3 y following the third heatwave. Encouragingly, bleaching-resistant individuals of both species remained pigmented across the entire time series; however, pigmentation did not necessarily indicate physiological resilience. Specifically,M. capitatadisplayed incremental yet only partial recovery of symbiont density and tissue biomass across both bleaching phenotypes up to 35 mo following the third heatwave as well as considerable partial mortality. Conversely,P. compressaappeared to recover across most physiological metrics within 2 y and experienced little to no mortality. Ultimately, these results indicate that even some visually robust, bleaching-resistant corals can carry the cost of recurring heatwaves over multiple years, leading to divergent recovery trajectories that may erode coral reef resilience in the Anthropocene. 

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4. 3D Photogrammetry Reveals Dynamics of Stony Coral Tissue Loss Disease (SCTLD) Lesion Progression Across a Thermal Stress Event

   https://doi.org/10.3389/fmars.2020.597643  

   Meiling, Sonora; Muller, Erinn M.; Smith, Tyler B.; Brandt, Marilyn E. (December 2020, Frontiers in Marine Science)

   null (Ed.)

   Stony coral tissue loss disease (SCTLD) was first observed in the United States Virgin Islands in January 2019 on a reef at Flat Cay off the island of St. Thomas. A year after its emergence, the disease had spread to several reefs around St. Thomas causing significant declines in overall coral cover. Rates of tissue loss are an important metric in the study of coral disease ecology, as they can inform many aspects of etiology such as disease susceptibility and resistance among species, and provide critical parameters for modeling the effects of disease among heterogenous reef communities. The present study quantified tissue loss rates attributed to SCTLD among six abundant reef building species ( Colpophyllia natans, Montastraea cavernosa, Diploria labyrinthiformis, Pseudodiploria strigosa, Orbicella annularis , and Porites astreoides ). Field-based 3D models of diseased corals, taken approximately weekly, indicated that the absolute rates of tissue loss from SCTLD slowed through time, corresponding with the accumulation of thermal stress that led to mass bleaching. Absolute tissue loss rates were comparable among species prior to the bleaching event but diverged during and remained different after the bleaching event. Proportional tissue loss rates did not vary among species or through time, but there was considerable variability among M. cavernosa colonies. SCTLD poses a significant threat to reefs across the Caribbean due to its persistence through time, wide range of susceptible coral species, and unprecedented tissue loss rates. Intervention and management efforts should be increased during and immediately following thermal stress events in order maximize resource distribution when disease prevalence is decreased. 

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5. Energetic and reproductive costs of coral recovery in divergent bleaching responses

   https://doi.org/10.1038/s41598-021-02807-w  

   Leinbach, Sarah E.; Speare, Kelly E.; Rossin, Ashley M.; Holstein, Daniel M.; Strader, Marie E. (December 2021, Scientific Reports)

   Abstract Mass thermal bleaching events are a primary threat to coral reefs, yet the sublethal impacts, particularly on energetics and reproduction, are poorly characterized. Given that the persistence of coral populations is contingent upon the reproduction of individuals that survive disturbances, there is an urgent need to understand the sublethal effects of bleaching on reproductive output to accurately predict coral recovery rates. In 2019, the French Polynesian island of Mo’orea experienced a severe mass bleaching event accompanied by widespread coral mortality. At the most heavily impacted sites, we observed Acropora hyacinthus individuals that were resistant to bleaching, alongside colonies that bleached but showed signs of symbiont recovery shortly after the bleaching event. We collected fragments from A. hyacinthus colonies five months post-bleaching and, using energetic assays and histological measurements, examined the physiological and reproductive consequences of these two distinct heat stress responses. Despite healthy appearances in both resistant and recovered corals, we found that recovered colonies had significantly reduced energy reserves compared to resistant colonies. In addition, we detected compound effects of stress on reproduction: recovered colonies displayed both a lower probability of containing gametes and lower fecundity per polyp. Our results indicate that bleaching inflicts an energetic constraint on the concurrent re-accumulation of energy reserves and development of reproductive material, with decreased reproductive potential of survivors possibly hampering overall reef resilience. These findings highlight the presence of intraspecific responses to bleaching and the importance of considering multiple trajectories for individual species when predicting population recovery following disturbance. 

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