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1. Species‐specific responses to climate change and community composition determine future calcification rates of Florida Keys reefs

   https://doi.org/10.1111/gcb.13481  

   Okazaki, Remy R.; Towle, Erica K.; van Hooidonk, Ruben; Mor, Carolina; Winter, Rivah N.; Piggot, Alan M.; Cunning, Ross; Baker, Andrew C.; Klaus, James S.; Swart, Peter K.; et al (September 2016, Global Change Biology)

   Abstract Anthropogenic climate change compromises reef growth as a result of increasing temperatures and ocean acidification. Scleractinian corals vary in their sensitivity to these variables, suggesting species composition will influence how reef communities respond to future climate change. Because data are lacking for many species, most studies that model future reef growth rely on uniform scleractinian calcification sensitivities to temperature and ocean acidification. To address this knowledge gap, calcification of twelve common and understudied Caribbean coral species was measured for two months under crossed temperatures (27, 30.3 °C) andCO₂partial pressures (pCO₂) (400, 900, 1300 μatm). Mixed‐effects models of calcification for each species were then used to project community‐level scleractinian calcification using Florida Keys reef composition data andIPCC AR5 ensemble climate model data. Three of the four most abundant species,Orbicella faveolata, Montastraea cavernosa,andPorites astreoides, had negative calcification responses to both elevated temperature andpCO₂. In the business‐as‐usualCO₂emissions scenario, reefs with high abundances of these species had projected end‐of‐century declines in scleractinian calcification of >50% relative to present‐day rates.Siderastrea siderea, the other most common species, was insensitive to both temperature andpCO₂within the levels tested here. Reefs dominated by this species had the most stable end‐of‐century growth. Under more optimistic scenarios of reducedCO₂emissions, calcification rates throughout the Florida Keys declined <20% by 2100. Under the most extreme emissions scenario, projected declines were highly variable among reefs, ranging 10–100%. Without considering bleaching, reef growth will likely decline on most reefs, especially where resistant species likeS. sidereaare not already dominant. This study demonstrates how species composition influences reef community responses to climate change and how reducedCO₂emissions can limit future declines in reef calcification. 

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2. Divergent responses of the coral holobiont to deoxygenation and prior environmental stress

   https://doi.org/10.3389/fmars.2023.1301474  

   Swaminathan, Sara D.; Meyer, Julie L.; Johnson, Maggie D.; Paul, Valerie J.; Bartels, Erich; Altieri, Andrew H. (February 2024, Frontiers in Marine Science)

   Ocean deoxygenation is intensifying globally due to human activities – and is emerging as a grave threat to coral reef ecosystems where it can cause coral bleaching and mass mortality. However, deoxygenation is one of many threats to coral reefs, making it essential to understand how prior environmental stress may influence responses to deoxygenation. To address this question, we examined responses of the coral holobiont (i.e., the coral host, Symbiodiniaceae, and the microbiome) to deoxygenation in corals with different environmental stress backgrounds. We outplantedAcropora cervicornisfragments of known genotypes from anin situnursery to two sites in the Florida Keys spanning an inshore-offshore gradient. After four months, fragments from the outplanted corals were transferred to the laboratory, where we tested differences in survivorship, tissue loss, photosynthetic efficiency, Symbiodiniaceae cell density, and coral microbiome composition after persistent exposure to one of four oxygen treatments ranging from extreme deoxygenation (0.5 mg L^-1) to normoxia (6 mg L^-1). We found that, for the short duration of exposure tested in this study (four days), the entire coral holobiont was resistant to dissolved oxygen (DO) concentrations as low as 2.0 mg L^-1, but that the responses of members of the holobiont decoupled at 0.5 mg L^-1. In this most extreme treatment, the coral host showed decreased photosynthetic efficiency, tissue loss, and mortality, and lower Symbiodiniaceae densities in a bleaching response, but most microbial taxa remained stable. Although deoxygenation did not cause major community shifts in microbiome composition, the population abundance of some microbial taxa did respond. Site history influenced some responses of the coral host and endosymbiont, but not the coral microbiome, with corals from the more stressful inshore site showing greater susceptibility to subsequent deoxygenation. Our study reveals that coral holobiont members respond differently to deoxygenation, with greater sensitivity in the coral host and Symbiodiniaceae and greater resistance in the coral microbiome, and that prior stress exposure can decrease host tolerance to deoxygenation. 

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3. Parental effects provide an opportunity for coral resilience following major bleaching events

   https://doi.org/10.1371/journal.pone.0290479  

   Lenz, Elizabeth A; Donahue, Megan J; Gates, Ruth D; Putnam, Hollie M; van_der_Steeg, Eveline; Padilla-Gamiño, Jacqueline L (January 2025, PLOS ONE)

   Fujimura, Atsushi (Ed.)

   Identifying processes that promote coral reef recovery and resilience is crucial as ocean warming becomes more frequent and severe. Sexual reproduction is essential for the replenishment of coral populations and maintenance of genetic diversity; however, the ability for corals to reproduce may be impaired by marine heatwaves that cause coral bleaching. In 2014 and 2015, the Hawaiian Islands experienced coral bleaching with differential bleaching susceptibility in the speciesMontipora capitata, a dominant reef-building coral in the region. We tested the hypothesis that coral bleaching resistance enhances reproductive capacity and offspring performance by examining the reproductive biology of colonies that bleached and recovered (B) and colonies that did not bleach (NB) in 2015 in the subsequent spawning seasons. The proportion of colonies that spawned was higher in 2016 than in 2017. Regardless of parental bleaching history, we found eggs with higher abnormality and bundles with fewer eggs in 2016 than 2017. While reproductive output was similar between B and NB colonies in 2016, survivorship of offspring that year were significantly influenced by the parental bleaching history (egg donor × sperm donor: B × B, B × NB, NB × B, and NB × NB). Offspring produced by NB egg donors had the highest survivorship, while offspring from previously bleached colonies had the lowest survivorship, highlighting the negative effects of bleaching on parental investment and offspring performance. While sexual reproduction continues inM.capitatapost-bleaching, gametes are differentially impacted by recovery time following a bleaching event and by parental bleaching resistance. Our results demonstrate the importance of identifying bleaching resistant individuals during and after heating events. This study further highlights the significance of maternal effects through potential egg provisioning for offspring survivorship and provides a baseline for human-assisted intervention (i.e., selective breeding) to mitigate the effects of climate change on coral reefs. 

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4. Increased dominance of heat-tolerant symbionts creates resilient coral reefs in near-term ocean warming

   https://doi.org/10.1073/pnas.2202388120  

   Palacio-Castro, Ana M.; Smith, Tyler B.; Brandtneris, Viktor; Snyder, Grace A.; van Hooidonk, Ruben; Maté, Juan L.; Manzello, Derek; Glynn, Peter W.; Fong, Peggy; Baker, Andrew C. (February 2023, Proceedings of the National Academy of Sciences)

   Climate change is radically altering coral reef ecosystems, mainly through increasingly frequent and severe bleaching events. Yet, some reefs have exhibited higher thermal tolerance after bleaching severely the first time. To understand changes in thermal tolerance in the eastern tropical Pacific (ETP), we compiled four decades of temperature, coral cover, coral bleaching, and mortality data, including three mass bleaching events during the 1982 to 1983, 1997 to 1998 and 2015 to 2016 El Niño heatwaves. Higher heat resistance in later bleaching events was detected in the dominant framework-building genus, Pocillopora, while other coral taxa exhibited similar susceptibility across events. Genetic analyses of Pocillopora spp . colonies and their algal symbionts (2014 to 2016) revealed that one of two Pocillopora lineages present in the region ( Pocillopora “ type 1”) increased its association with thermotolerant algal symbionts ( Durusdinium glynnii ) during the 2015 to 2016 heat stress event. This lineage experienced lower bleaching and mortality compared with Pocillopora “type 3”, which did not acquire D. glynnii . Under projected thermal stress, ETP reefs may be able to preserve high coral cover through the 2060s or later, mainly composed of Pocillopora colonies that associate with D. glynnii . However, although the low-diversity, high-cover reefs of the ETP could illustrate a potential functional state for some future reefs, this state may only be temporary unless global greenhouse gas emissions and resultant global warming are curtailed. 

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5. Paleo reefs provide clues for contemporary climate-change refugia

   https://doi.org/10.1016/j.crsus.2024.100289  

   van_Woesik, Robert (February 2025, Cell Reports Sustainability)

   Plail, Melissa (Ed.)

   Marine heatwaves are increasing in intensity and frequency, causing worldwide coral bleaching, reductions in coral cover, and shifts in species composition. Recent studies have found, however, that inshore turbid reefs are more resistant to heat stress than offshore clear-water reefs. Inshore turbid reefs, therefore, may play a critical role as climate-change refuges for contemporary coral reefs subjected to marine heatwaves. This perspective explores the importance of inshore reefs in the past, present, and future. Paleo records show that inshore reefs were also crucial as refuges during historically warm periods. Yet, contemporary inshore reefs are especially vulnerable to pollution and land-use-change runoff, which were absent in paleo times. Therefore, inshore reefs need strategic management and protection to maintain their role as climate-change refugia as the oceans continue to warm. 

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