Oceanic differences in coral-bleaching responses to marine heatwaves

Gene expression plasticity governing symbiosis during natural coral bleaching

The increasing global frequency and severity of coral bleaching events, driven by the loss of endosymbiotic algae, pose a significant threat to these vital ecosystems. However, gene expression plasticity offers a potential mechanism for rapid and effective acclimatization to environmental changes. We employed dual transcriptomics to examine the gene expression profile of Seriatopora hystrix, an ecologically important scleractinian coral, across healthy, mildly bleached, and severely bleached colonies collected from the waters of Likupang, North Sulawesi, Indonesia. Our analysis revealed that coral bleaching is associated with gene plasticity in calcium signaling and focal adhesion within coral hosts, as well as with endoplasmic reticulum stress in symbionts. Notably, we identified specific genes associated with innate immunity that were predominantly overexpressed in mildly bleached coral hosts. This overexpression implies that high expression plasticity of these key genes might contribute to bleaching resistance and the preservation of the host-symbiont relationship. Our findings offer a detailed insight into the dynamics of bleaching resistance in S. hystrix, shedding light on the variability of bleaching risks in Indonesian reefs and underscoring the coral's ability to utilize gene expression plasticity for immediate survival and potential long-term adaptation to climate changes.

Climate adaptive loci revealed by seascape genomics correlate with phenotypic variation in heat tolerance of the coral Acropora millepora

One of the main challenges in coral reef conservation and restoration is the identification of coral populations resilient under global warming. Seascape genomics is a powerful tool to uncover genetic markers potentially involved in heat tolerance among large populations without prior information on phenotypes. Here, we aimed to provide first insights on the role of candidate heat associated loci identified using seascape genomics in driving the phenotypic response of Acropora millepora from New Caledonia to thermal stress. We subjected 7 colonies to a long-term ex-situ heat stress assay (4 °C above the maximum monthly mean) and investigated their physiological response along with their Symbiodiniaceae communities and genotypes. Despite sharing similar thermal histories and associated symbionts, these conspecific individuals differed greatly in their tolerance to heat stress. More importantly, the clustering of individuals based on their genotype at heat-associated loci matched the phenotypic variation in heat tolerance. Colonies that sustained on average lower mortality, higher Symbiodiniaceae/chlorophyll concentrations and photosynthetic efficiency under prolonged heat stress were also the closest based on their genotypes, although the low sample size prevented testing loci predictive accuracy. Together these preliminary results support the relevance of coupling seascape genomics and long-term heat stress experiments in the future, to evaluate the effect size of candidate heat associated loci and pave the way for genomic predictive models of corals heat tolerance.

Corals that survive repeated thermal stress show signs of selection and acclimatization

Climate change is transforming coral reefs by increasing the frequency and intensity of marine heatwaves, often leading to coral bleaching and mortality. Coral communities have demonstrated modest increases in thermal tolerance following repeated exposure to moderate heat stress, but it is unclear whether these shifts represent acclimatization of individual colonies or mortality of thermally susceptible individuals. For corals that survive repeated bleaching events, it is important to understand how past bleaching responses impact future growth potential. Here, we track the bleaching responses of 1,832 corals in leeward Maui through multiple marine heatwaves and document patterns of coral growth and survivorship over a seven-year period. While we find limited evidence of acclimatization at population scales, we document reduced bleaching over time in specific individuals that is indicative of acclimatization, primarily in the stress-tolerant taxa Porites lobata. For corals that survived both bleaching events, we find no relationship between bleaching response and coral growth in three of four taxa studied. This decoupling suggests that coral survivorship is a better indicator of future growth than is a coral's bleaching history. Based on these results, we recommend restoration practitioners in Hawai'i focus on colonies of Porites and Montipora with a proven track-record of growth and survivorship, rather than devote resources toward identifying and cultivating bleaching-resistant phenotypes in the lab. Survivorship followed a latitudinal thermal stress gradient, but because this gradient was small, it is likely that local environmental factors also drove differences in coral performance between sites. Efforts to reduce human impacts at low performing sites would likely improve coral survivorship in the future.

A global database on coral recovery following marine heatwaves

Coral reefs support the world's most diverse marine ecosystem and provide invaluable goods and services for millions of people worldwide. They are however experiencing frequent and intensive marine heatwaves that are causing coral bleaching and mortality. Coarse-grained climate models predict that few coral reefs will survive the 3 °C sea-surface temperature rise in the coming century. Yet, field studies show localized pockets of coral survival and recovery even under high-temperature conditions. Quantifying recovery from marine heatwaves is central to making accurate predictions of coral-reef trajectories into the near future. Here we introduce the world's most comprehensive database on coral recovery following marine heatwaves and other disturbances, called Heatwaves and Coral-Recovery Database (HeatCRD) encompassing 29,205 data records spanning 44 years from 12,266 sites, 83 countries, and 160 data sources. These data provide essential information to coral-reef scientists and managers to best guide coral-reef conservation efforts at both local and regional scales.

Systematic review of the uncertainty of coral reef futures under climate change

Climate change impact syntheses, such as those by the Intergovernmental Panel on Climate Change, consistently assert that limiting global warming to 1.5 °C is unlikely to safeguard most of the world's coral reefs. This prognosis is primarily based on a small subset of available models that apply similar 'excess heat' threshold methodologies. Our systematic review of 79 articles projecting coral reef responses to climate change revealed five main methods. 'Excess heat' models constituted one third (32%) of all studies but attracted a disproportionate share (68%) of citations in the field. Most methods relied on deterministic cause-and-effect rules rather than probabilistic relationships, impeding the field's ability to estimate uncertainty. To synthesize the available projections, we aimed to identify models with comparable outputs. However, divergent choices in model outputs and scenarios limited the analysis to a fraction of available studies. We found substantial discrepancies in the projected impacts, indicating that the subset of articles serving as a basis for climate change syntheses may project more severe consequences than other studies and methodologies. Drawing on insights from other fields, we propose methods to incorporate uncertainty into deterministic modeling approaches and propose a multi-model ensemble approach to generating probabilistic projections for coral reef futures.

Environmental Concentrations of Herbicide Prometryn Render Stress-Tolerant Corals Susceptible to Ocean Warming

Global warming has caused the degradation of coral reefs around the world. While stress-tolerant corals have demonstrated the ability to acclimatize to ocean warming, it remains unclear whether they can sustain their thermal resilience when superimposed with other coastal environmental stressors. We report the combined impacts of a photosystem II (PSII) herbicide, prometryn, and ocean warming on the stress-tolerant coral Galaxea fascicularis through physiological and omics analyses. The results demonstrate that the heat-stress-induced inhibition of photosynthetic efficiency in G. fascicularis is exacerbated in the presence of prometryn. Transcriptomics and metabolomics analyses indicate that the prometryn exposure may overwhelm the photosystem repair mechanism in stress-tolerant corals, thereby compromising their capacity for thermal acclimation. Moreover, prometryn might amplify the adverse effects of heat stress on key energy and nutrient metabolism pathways and induce a stronger response to oxidative stress in stress-tolerant corals. The findings indicate that the presence of prometryn at environmentally relevant concentrations would render corals more susceptible to heat stress and exacerbate the breakdown of coral Symbiodiniaceae symbiosis. The present study provides valuable insights into the necessity of prioritizing PSII herbicide pollution reduction in coral reef protection efforts while mitigating the effects of climate change.

Diversification of refugia types needed to secure the future of coral reefs subject to climate change

Identifying locations of refugia from the thermal stresses of climate change for coral reefs and better managing them is one of the key recommendations for climate change adaptation. We review and summarize approximately 30 years of applied research focused on identifying climate refugia to prioritize the conservation actions for coral reefs under rapid climate change. We found that currently proposed climate refugia and the locations predicted to avoid future coral losses are highly reliant on  excess heat metrics, such as degree heating weeks. However, many existing alternative environmental, ecological, and life-history variables could be used to identify other types of refugia that lead to the desired diversified portfolio for coral reef conservation. To improve conservation priorities for coral reefs, there is a need to evaluate and validate the predictions of climate refugia with long-term field data on coral abundance, diversity, and functioning. There is also the need to identify and safeguard locations displaying resistance toprolonged exposure to heat waves and the ability to recover quickly after thermal exposure. We recommend using more metrics to identify a portfolio of potential refugia sites for coral reefs that can avoid, resist, and recover from exposure to high ocean temperatures and the consequences of climate change, thereby shifting past efforts focused on avoidance to a diversified risk-spreading portfolio that can be used to improve strategic coral reef conservation in a rapidly warming climate.

Exposure to pentachlorophenol destructs the symbiotic relationship between zooxanthellae and host and induces pathema in coral Porites lutea

Stress from chemical pollutants is among the key issues that have adverse impacts on coral reefs. As a persistent organic pollutant, pentachlorophenol (PCP) has been detected in the seawater of Weizhou Island and was proved to have significant adverse effects on aquatic animals. However, little is known about its effects on scleractinian coral. Therefore, we investigated the response of the coral Porites lutea to PCP stress. Coral bleaching, photosynthesis parameters and antioxidant enzyme activities of P. lutea under PCP exposure were documented. After 96 h of exposure, significant tissue loss and bleaching occurred when the PCP concentration exceeded 100 μg/L. The density of symbiotic zooxanthellae decreased from 2.06 × 106 cells/cm2 to 0.93 × 106 cells/cm2 when the PCP concentration increased from 1 μg/L- 1000 μg/L. Long-term exposure of 120 days to PCP at 0.1 μg/L also led to coral bleaching, the maximum photochemical quantum yield of PSII in P. lutea nubbins significantly decreased to 0.482. The analysis of microbial community distribution indicated that the increase of the pathogenic bacterium Citrobacter may be one of the inducers of coral bleaching. Conjoint analysis of transcriptomics and proteomics showed that the metabolism of amino acids and carbohydrates in zooxanthellae was abnormal, leading to the destruction of its symbiotic relationship with the host. The immune system of the host was disrupted, which could be linked to the prevalence of coral pathema. The toxic responses of PCP on both zooxanthellae and its host were further confirmed by the upregulation of the differential metabolites including 1-naphthylamine and phosphatidylcholine, etc.

Past disturbances and local conditions influence the recovery rates of coral reefs

Corals are being increasingly subjected to marine heatwaves. Theory suggests that increasing the intensity of disturbances reduces recovery rates, which inspired us to examine the recovery rates of coral cover following marine heatwaves, cyclones, and other disturbances at 1921 study sites, in 58 countries and three oceans, from 1977 to 2020. In the Atlantic Ocean, coral cover has decreased fourfold since the 1970s, and recovery rates following disturbances have been relatively slow, except in the Antilles. By contrast, reefs in the Pacific and Indian Oceans have maintained coral cover and recovery rates over time. There were positive relationships between rates of coral recovery and prior cyclone and heatwave frequency, and negative relationships between rates of coral recovery and macroalgae cover and distance to shore. A recent increase in the variance in recovery rates in some ecoregions of the Pacific and Indian Oceans suggests that some reefs in those ecoregions may be approaching a phase shift. While marine heatwaves are increasing in intensity and frequency, our results suggest that regional and local conditions influence coral recovery rates, and therefore, effective local management efforts can help reefs recover from disturbances.

Standardization of in situ coral bleaching measurements highlights the variability in responses across genera, morphologies, and regions

Marine heatwaves and regional coral bleaching events have become more frequent and severe across the world's oceans over the last several decades due to global climate change. Observational studies have documented spatiotemporal variation in the responses of reef-building corals to thermal stress within and among taxa across geographic scales. Although many tools exist for predicting, detecting, and quantifying coral bleaching, it remains difficult to compare bleaching severity (e.g., percent cover of bleached surface areas) among studies and across species or regions. For this review, we compiled over 2,100 in situ coral bleaching observations representing 87 reef-building coral genera and 250 species of common morphological groups from a total of 74 peer-reviewed scientific articles, encompassing three broad geographic regions (Atlantic, Indian, and Pacific Oceans). While bleaching severity was found to vary by region, genus, and morphology, we found that both genera and morphologies responded differently to thermal stress across regions. These patterns were complicated by (i) inconsistent methods and response metrics across studies; (ii) differing ecological scales of observations (i.e., individual colony-level vs. population or community-level); and (iii) temporal variability in surveys with respect to the onset of thermal stress and the chronology of bleaching episodes. To improve cross-study comparisons, we recommend that future surveys prioritize measuring bleaching in the same individual coral colonies over time and incorporate the severity and timing of warming into their analyses. By reevaluating and standardizing the ways in which coral bleaching is quantified, researchers will be able to track responses to marine heatwaves with increased rigor, precision, and accuracy.

The 2022 summer marine heatwaves and coral bleaching in China's Greater Bay Area

From July to August 2022, scleractinian coral communities in China's Greater Bay Area (GBA) in the northern South China Sea (nSCS) experienced an unprecedented bleaching event, despite the fact that coral communities in this area are often considered coral thermal refugia due to their high latitude distribution. Field surveys of six sites covering three main coral distribution areas of the GBA revealed that coral bleaching occurred at all sites. Bleaching was more severe in shallow water (1-3 m) than in deep water (4-6 m), as indicated by both percent bleached cover (51.80 ± 10.04% vs. 7.09 ± 7.37%) and bleached colonies (45.86 ± 11.22% vs. 6.58 ± 6.53%). Coral genera Acropora, Favites, Montipora, Platygyra, Pocillopora, and Porites showed high susceptibility to bleaching, and Acropora and Pocillopora suffered high post-bleaching mortality. In the three areas surveyed, analysis of oceanographic data detected marine heatwaves (MHWs) during the summer, with mean intensities between 1.62 and 1.97 °C and durations between 5 and 22 days. These MHWs were primarily driven by increased shortwave radiation due to strong western Pacific Subtropical High (WPSH), combined with reduced mixing between the surface and deep upwelling waters due to reduced wind speed. Comparing with histological oceanographic data showed that the 2022 MHWs were unprecedented, and there was a significant increase in the frequency, intensity, and total days of MHWs during 1982-2022. Furthermore, the heterogeneous distribution of summer MHW characteristics indicates that the coastal upwelling may modulate the spatial distribution of summer MHWs in nSCS through its cooling effect. Overall, our study indicates that MHWs may have affected the structure of the subtropical coral communities in the nSCS, and impaired their potential as thermal refugia.