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1. Local habitat heterogeneity rivals regional differences in coral thermal tolerance

   https://doi.org/10.1007/s00338-024-02484-x  

   Brown, Kristen T. ; Martynek, Marcelina P. ; Barott, Katie L. ( April 2024 , Coral Reefs)

   Variable temperature regimes that expose corals to sublethal heat stress have been recognized as a mechanism to increase coral thermal tolerance and lessen coral bleaching. However, there is a need to better understand which thermal regimes maximize coral stress hardening. Here, standardized thermal stress assays were used to determine the relative thermal tolerance of three divergent genera of corals (Acropora,Pocillopora,Porites) originating from six reef sites representing an increasing gradient of annual mean diel temperature fluctuations of 1–3 °C day⁻¹. Bleaching severity and dark-acclimated photochemical yield (i.e.,F_v/F_m) were quantified following exposure to five temperature treatments ranging from 23.0 to 36.3 °C. The greatest thermal tolerance (i.e.,F_v/F_meffective dose 50) was found at the site with intermediate mean diel temperature variability (2.2 °C day⁻¹), suggesting there is an optimal priming exposure that leads to maximal thermal tolerance. Interestingly,AcroporaandPocilloporaoriginating from the least thermally variable regimes (< 1.3 °C day⁻¹) had lower thermal tolerance than corals from the most variable sites (> 2.8 °C day⁻¹), whereas the opposite was true forPorites, suggesting divergent responses across taxa. Remarkably, comparisons across global studies revealed that the range in coral thermal tolerance uncovered in this study across a single reef (< 5 km) were as large as differences observed across vast latitudinal gradients (300–900 km). This finding indicates that local gene flow could improve thermal tolerance between habitats. However, as climate change continues, exposure to intensifying marine heatwaves is already compromising thermal priming as a mechanism to enhance coral thermal tolerance and bleaching resistance.

    

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2. Will coral reefs survive by adaptive bleaching?

   https://doi.org/10.1042/ETLS20210227  

   Cunning, Ross ( December 2021 , Emerging Topics in Life Sciences)

   Some reef-building corals form symbioses with multiple algal partners that differ in ecologically important traits like heat tolerance. Coral bleaching and recovery can drive symbiont community turnover toward more heat-tolerant partners, and this ‘adaptive bleaching’ response can increase future bleaching thresholds by 1–2°C, aiding survival in warming oceans. However, this mechanism of rapid acclimatization only occurs in corals that are compatible with multiple symbionts, and only when the disturbance regime and competitive dynamics among symbionts are sufficient to bring about community turnover. The full scope of coral taxa and ecological scenarios in which symbiont shuffling occurs remains poorly understood, though its prevalence is likely to increase as warming oceans boost the competitive advantage of heat-tolerant symbionts, increase the frequency of bleaching events, and strengthen metacommunity feedbacks. Still, the constraints, limitations, and potential tradeoffs of symbiont shuffling suggest it will not save coral reef ecosystems; however, it may significantly improve the survival trajectories of some, or perhaps many, coral species. Interventions to manipulate coral symbionts and symbiont communities may expand the scope of their adaptive potential, which may boost coral survival until climate change is addressed. 

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3. The Coral Bleaching Automated Stress System (CBASS): A low‐cost, portable system for standardized empirical assessments of coral thermal limits

   https://doi.org/10.1002/lom3.10555  

   Evensen, Nicolas R. ; Parker, Katherine E. ; Oliver, Thomas A. ; Palumbi, Stephen R. ; Logan, Cheryl A. ; Ryan, James S. ; Klepac, Courtney N. ; Perna, Gabriela ; Warner, Mark E. ; Voolstra, Christian R. ; et al ( May 2023 , Limnology and Oceanography: Methods)

   Ocean warming is increasingly affecting marine ecosystems across the globe. Reef‐building corals are particularly affected by warming, with mass bleaching events increasing in frequency and leading to widespread coral mortality. Yet, some corals can resist or recover from bleaching better than others. Such variability in thermal resilience could be critical to reef persistence; however, the scientific community lacks standardized diagnostic approaches to rapidly and comparatively assess coral thermal vulnerability prior to bleaching events. We present the Coral Bleaching Automated Stress System (CBASS) as a low‐cost, open‐source, field‐portable experimental system for rapid empirical assessment of coral thermal thresholds using standardized temperature stress profiles and diagnostics. The CBASS consists of four or eight flow‐through experimental aquaria with independent water masses, lighting, and individual automated temperature controls capable of delivering custom modulating thermal profiles. The CBASS is used to conduct daily thermal stress exposures that typically include 3‐h temperature ramps to multiple target temperatures, a 3‐h hold period at the target temperatures, and a 1‐h ramp back down to ambient temperature, followed by an overnight recovery period. This mimics shallow water temperature profiles observed in coral reefs and prompts a rapid acute heat stress response that can serve as a diagnostic tool to identify putative thermotolerant corals for in‐depth assessments of adaptation mechanisms, targeted conservation, and possible use in restoration efforts. The CBASS is deployable within hours and can assay up to 40 coral fragments/aquaria/day, enabling high‐throughput, rapid determination of thermal thresholds for individual genotypes, populations, species, and sites using a standardized experimental framework.

    

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4. The role of gene expression and symbiosis in reef-building coral acquired heat tolerance

   https://doi.org/10.1038/s41467-022-32217-z  

   Strader, Marie E. ; Quigley, Kate M. ( August 2022 , Nature Communications)

   Predicting how reef-building corals will respond to accelerating ocean warming caused by climate change requires knowledge of how acclimation and symbiosis modulate heat tolerance in coral early life-history stages. We assayed transcriptional responses to heat in larvae and juveniles of 11 reproductive crosses ofAcropora tenuiscolonies along the Great Barrier Reef. Larvae produced from the warmest reef had the highest heat tolerance, although gene expression responses to heat were largely conserved by cross identity. Juvenile transcriptional responses were driven strongly by symbiosis – when in symbiosis with heat-evolved Symbiodiniaceae, hosts displayed intermediate expression between its progenitorCladocopiumand the more stress tolerantDurusdinium, indicating the acquisition of tolerance is a conserved evolutionary process in symbionts. Heat-evolved Symbiodiniaceae facilitated juvenile survival under heat stress, although host transcriptional responses to heat were positively correlated among those hosting different genera of Symbiodiniaceae. These findings reveal the relative contribution of parental environmental history as well as symbiosis establishment in coral molecular responses to heat in early life-history stages.

    

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5. Palau’s warmest reefs harbor thermally tolerant corals that thrive across different habitats

   https://doi.org/10.1038/s42003-022-04315-7  

   Rivera, Hanny E. ; Cohen, Anne L. ; Thompson, Janelle R. ; Baums, Iliana B. ; Fox, Michael D. ; Meyer-Kaiser, Kirstin S. ( December 2022 , Communications Biology)

   Abstract Ocean warming is killing corals, but heat-tolerant populations exist; if protected, they could replenish affected reefs naturally or through restoration. Palau’s Rock Islands experience consistently higher temperatures and extreme heatwaves, yet their diverse coral communities bleach less than those on Palau’s cooler outer reefs. Here, we combined genetic analyses, bleaching histories and growth rates of Porites cf. lobata colonies to identify thermally tolerant genotypes, map their distribution, and investigate potential growth trade-offs. We identified four genetic lineages of P . cf. lobata . On Palau’s outer reefs, a thermally sensitive lineage dominates. The Rock Islands harbor two lineages with enhanced thermal tolerance; one of which shows no consistent growth trade-off and also occurs on several outer reefs. This suggests that the Rock Islands provide naturally tolerant larvae to neighboring areas. Finding and protecting such sources of thermally-tolerant corals is key to reef survival under 21 st century climate change. 

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