Cumulative risk of future bleaching for the world's coral reefs

Restoration cannot be scaled up globally to save reefs from loss and degradation

Coral restoration is gaining popularity as part of a continuum of approaches addressing the widespread, recurring mass mortality events of corals that-together with elevated and chronic mortality, slower growth and recruitment failure-threaten the persistence of coral reefs worldwide. However, the monetary costs associated with broad-scale coral restoration are massive, making widespread implementation challenging, especially with the lack of coordinated and ecologically informed planning. By combining a comprehensive dataset documenting the success of coral restoration with current and forecasted environmental, ecological and climate data, we highlight how such a coordinated and ecologically informed approach is not forthcoming, despite the extent of previous and ongoing efforts. We show that: (1) restoration sites tend to be disproportionally close to human settlements and therefore more vulnerable to local anthropogenic impacts; (2) the immediate outcomes of restoration do not appear to be influenced by relevant ecological and environmental predictors such as cumulative impact; and (3) most restored localities have a high and severe bleaching risk by the middle of this century, with more than half of recently restored sites already affected. Our findings highlight the need for the coral reef community to reinforce joint development of restoration guidelines that go beyond local objectives, with attention to ocean warming trends and their long-term impacts on coral resilience and restoration success.

Temperature-dependent responses of the hard corals Acropora sp. and Pocillopora verrucosa to molecular hydrogen

Coral reefs are increasingly threatened by mass bleaching events due to global ocean warming. Novel management strategies are urgently needed to support coral survival until global efforts can mitigate ocean warming. Given the strong antioxidant, anti-inflammatory and anti-apoptotic properties of molecular hydrogen, our study explores its potential to alleviate the negative effects of heat stress on corals. We investigated the ecophysiological responses of two common hard corals (Acropora sp. and Pocillopora verrucosa) from the Central Red Sea under ambient (26 °C) and elevated seawater temperatures (32 °C), with and without hydrogen addition ( ~ 150 µ M H2) over 48 h. Our results showed that at 32 °C without hydrogen addition, P. verrucosa exhibited high temperature tolerance, whereas Acropora sp. showed significant reductions in photosynthetic efficiency and maximum electron transport rate compared to the ambient condition (26 °C). The addition of hydrogen at 32 °C increased the maximum electron transport rate of Acropora sp. by 28%, maintaining it at levels compared to those at 26 °C. In contrast, the addition of hydrogen at 26 °C caused a significant decrease in the photophysiology of both Acropora sp. and P. verrucosa. This suggests that the short-term response of the coral holobiont to molecular hydrogen is temperature-dependent, potentially benefiting the coral holobiont under heat stress, while impairing the photophysiology under ambient temperatures. Our findings therefore provide the foundation for future long-term studies uncovering the mechanisms behind molecular hydrogen, potentially informing the development of new management strategies to enhance coral resilience to ocean warming.

The Influence of Symbiont Identity on the Proteomic and Metabolomic Responses of the Model Cnidarian Aiptasia to Thermal Stress

We examined the effects of symbiont identity and heat stress on the host metabolome and proteome in the cnidarian-dinoflagellate symbiosis. Exaiptasia diaphana ('Aiptasia') was inoculated with its homologous (i.e., native) symbiont Breviolum minutum or a heterologous (i.e., non-native) symbiont (Symbiodinium microadriaticum; Durusdinium trenchii) and thermally stressed. Integrated metabolome and proteome analyses characterised host thermal responses between symbioses, with clear evidence of enhanced nutritional deprivation and cellular stress in hosts harbouring heterologous symbionts following temperature stress. Host metabolomes were partially distinct at the control temperature; however, thermal stress caused metabolomes of anemones containing the two heterologous symbionts to become more alike and more distinct from those containing B. minutum. While these patterns could be partly explained by innate symbiont-specific differences, they may also reflect differences in symbiont density, as under control conditions D. trenchii attained 60% and S. microadriaticum 15% of the density attained by B. minutum, and at elevated temperature only D. trenchii-colonised anemones bleached (60% loss). Our findings add to a growing literature that highlights the physiological limits of partner switching as a means of adaptation to global warming. However, we also provide tentative evidence for improved metabolic functioning with a heterologous symbiont (D. trenchii) after sustained symbiosis.

Cryptic coral diversity is associated with symbioses, physiology, and response to thermal challenge

Coral persistence in the Anthropocene depends on interactions among holobiont partners (coral animals and microbial symbionts) and their environment. Cryptic coral lineages-genetically distinct yet morphologically similar groups-are critically important as they often exhibit functional diversity relevant to thermal tolerance. In addition, environmental parameters such as thermal variability may promote tolerance, but how variability interacts with holobiont partners to shape responses to thermal challenge remains unclear. Here, we identified three cryptic lineages of Siderastrea siderea in Bocas del Toro, Panamá that differ in distributions across inshore and offshore reefs, microbial associations, phenotypic traits of holobiont partners (i.e., phenomes), and skeleton morphologies. A thermal variability experiment failed to increase thermal tolerance, but subsequent thermal challenge and recovery revealed that one lineage maintained elevated energetic reserves, photochemical efficiency, and growth. Last, coral cores highlighted that this lineage also exhibited greater growth historically. Functional variation among cryptic lineages highlights their importance in predicting coral reef responses to climate change.

Shallow corals acclimate to mesophotic depths while maintaining their heat tolerance against ongoing climate change

Global warming poses a significant threat to coral reefs. It has been assumed that mesophotic coral ecosystems (MCEs, 30 to 150 m depths) may serve as refugia from ocean warming. This study examined the acclimation capacity and thermal tolerance of two shallow coral species, Porites cylindrica and Turbinaria reniformis, transplanted to mesophotic depths (40 m) for 12 months. Fragments from 5 and 40 m were exposed to control (28 °C), moderate (30 °C), and high (32 °C) temperatures over 14 days. MCE-acclimated fragments showed higher thermal thresholds and survival rates, delayed onset of bleaching, and less decline in photosynthesis efficiency (Fv/Fm) compared to shallow fragments. Both species maintained high thermal tolerance despite prolonged exposure to cooler temperatures of mesophotic depth. These findings suggest that low light intensity in MCEs can act as a modulator of bleaching, supporting the potential of these ecosystems as refugia for shallow corals in a rapidly changing world.

Climate refugia in the Great Barrier Reef may endure into the future

Although global warming is leading to more frequent mass coral bleaching events worldwide, parts of the Great Barrier Reef (GBR) have consistently escaped severe coral bleaching. Modeling and satellite observations show that climate refugia are created by the upwelling of cooler water to the surface through the interactions of tides and currents with dense reef structures. Here, we use a high-resolution nested regional ocean model to investigate the future status of two relatively large refugia. On the basis of model projections under a high-emission scenario, we find that the upwelling mechanisms will stay active in a warming climate, and these regions are likely to remain approximately more than 1°C cooler than surrounding waters until at least into the 2080s, providing thermal relief to corals. Identification and protection of these refugia may help facilitate reef survival and related biodiversity preservation by allowing their corals time to acclimatize and adapt and ultimately provide source populations to replenish the rest of the reef.

Coral responses to a catastrophic marine heatwave are decoupled from changes in total coral cover at a continental scale

The services provided by the world's coral reefs are threatened by increasingly frequent and severe marine heatwaves. Heatwave-induced degradation of reefs has often been inferred from the extent of the decline in total coral cover, which overlooks extreme variation among coral taxa in their susceptibility and responses to thermal stress. Here, we provide a continental-scale assessment of coral cover changes at 262 shallow tropical reef sites around Australia, using ecological survey data on 404 coral taxa before and after the 2016 mass bleaching event. A strong spatial structure in coral community composition along large-scale environmental gradients largely dictated how coral communities responded to heat stress. While heat stress variables were the best predictors of change in total coral cover, the pre-heatwave community composition best predicted the temporal beta-diversity index (an indicator of change in community composition over time). Indicator taxa in each coral community differed before and after the heatwave, highlighting potential winners and losers of climate-driven coral bleaching. Our results demonstrate how assessment of change in total cover alone may conceal very different responses in community structure, some of which showed strong regional consistency, and may provide a telling outlook of how coral reefs may reorganize in a warmer future.