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"Coral Bleaching"
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Coral whisperers : scientists on the brink
\"In recent years, a catastrophic global bleaching event devastated many of the world's precious coral reefs. Working on the front lines of ruin, today's coral scientists are struggling to save these important coral-reef ecosystems from the imminent threats of rapidly warming, acidifying, and polluted oceans. Coral Whisperers captures a critical moment in the history of coral-reef science. Based on over one hundred interviews with leading scientists and conservation managers, Irus Braverman documents a community caught in an existential crisis and alternating between despair and hope. In this important new book, corals emerge as signs and measures, but also as a way out of the projected collapse of life on earth\"--Provided by publisher.
Book
Thirty years of coral heat-stress experiments: a review of methods
For over three decades, scientists have conducted heat-stress experiments to predict how coral will respond to ocean warming due to global climate change. However, there are often conflicting results in the literature that are difficult to resolve, which we hypothesize are a result of unintended biases, variation in experimental design, and underreporting of critical methodological information. Here, we reviewed 255 coral heat-stress experiments to (1) document where and when they were conducted and on which species, (2) assess variability in experimental design, and (3) quantify the diversity of response variables measured. First, we found that two-thirds of studies were conducted in only three countries, three coral species were more heavily studied than others, and only 4% of studies focused on earlier life stages. Second, slightly more than half of all heat-stress exposures were less than 8 d in duration, only 17% of experiments fed corals, and experimental conditions varied widely, including the level and rate of temperature increase, light intensity, number of genets used, and the length of acclimation period. In addition, 95%, 55%, and > 35% of studies did not report tank flow conditions, light–dark cycle used, or the date of the experiment, respectively. Finally, we found that 21% of experiments did not measure any bleaching phenotype traits, 77% did not identify the Symbiodiniaceae endosymbiont, and the contribution of the coral host in the physiological response to heat-stress was often not investigated. This review highlights geographic, taxonomic, and heat-stress duration biases in our understanding of coral bleaching, and large variability in the reporting and design of heat-stress experiments that could account for some of the discrepancies in the literature. Development of some best practice recommendations for coral bleaching experiments could improve cross-studies comparisons and increase the efficiency of coral bleaching research at a time when it is needed most.
Journal Article
Large-scale bleaching of corals on the Great Barrier Reef
In 2015–2016, record temperatures triggered a pan-tropical episode of coral bleaching. In the southern hemisphere summer of March–April 2016, we used aerial surveys to measure the level of bleaching on 1,156 individual reefs throughout the 2,300 km length of the Great Barrier Reef, the world’s largest coral reef system. The accuracy of the aerial scores was ground-truthed with detailed underwater surveys of bleaching at 260 sites (104 reefs), allowing us to compare aerial and underwater bleaching data with satellite-derived temperatures and with associated model predictions of bleaching. The severity of bleaching on individual reefs in 2016 was tightly correlated with the level of local heat exposure: the southernmost region of the Great Barrier Reef escaped with only minor bleaching because summer temperatures there were close to average. Gradients in nutrients and turbidity from inshore to offshore across the Great Barrier Reef had minimal effect on the severity of bleaching. Similarly, bleaching was equally severe on reefs that are open or closed to fishing, once the level of satellite-derived heat exposure was accounted for. The level of post-bleaching mortality, measured underwater after 7–8 months, was tightly correlated with the aerial scores measured at the peak of bleaching. Similarly, reefs with a high aerial bleaching score also experienced major shifts in species composition due to extensive mortality of heat-sensitive species. Reefs with low bleaching scores did not change in composition, and some showed minor increases in coral cover. Two earlier mass bleaching events occurred on the Great Barrier Reef in 1998 and 2002, that were less severe than 2016. In 2016, <9% of scored reefs had no bleaching, compared to 42% in 2002 and 44% in 1998. Conversely, the proportion of reefs that were severely bleached (>60% of corals affected) was four times higher in 2016. The geographic footprint of each of the three events is distinctive, and matches satellite-derived sea surface temperature patterns. Our aerial surveys indicate that past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. This data set of aerial bleaching scores provides a historical baseline for comparison with future bleaching events. No copyright restrictions apply to the use of this data set other than citing this publication.
Journal Article
Empirically derived thermal thresholds of four coral species along the Red Sea using a portable and standardized experimental approach
Global warming is causing an unprecedented loss of species and habitats worldwide. This is particularly apparent for tropical coral reefs, with an increasing number of reefs experiencing mass bleaching and mortality on an annual basis. As such, there is a growing need for a standardized experimental approach to rapidly assess the thermal limits of corals and predict the survival of coral species across reefs and regions. Using a portable experimental system, the Coral Bleaching Automated Stress System (CBASS), we conducted standardized 18 h acute thermal stress assays to quantitively determine the upper thermal limits of four coral species across the length of the Red Sea coastline, from the Gulf of Aqaba (GoA) to Djibouti (~ 2100 km). We measured dark-acclimated photosynthetic efficiency (Fv/Fm), algal symbiont density, chlorophyll a, and visual bleaching intensity following heat stress. Fv/Fm was the most precise response variable assessed, advancing the Fv/Fm effective dose 50 (ED50, i.e., the temperature at which 50% of the initial Fv/Fm is measured) as an empirically derived proxy for thermal tolerance. ED50 thermal thresholds from the central/southern Red Sea and Djibouti populations were consistently higher for Acropora hemprichii, Pocillopora verrucosa, and Stylophora pistillata (0.1–1.8 °C above GoA corals, respectively), in line with prevailing warmer maximum monthly means (MMMs), though were lower than GoA corals relative to site MMMs (1.5–3.0 °C). P. verrucosa had the lowest thresholds overall. Despite coming from the hottest site, thresholds were lowest for Porites lobata in the southern Red Sea, suggesting long-term physiological damage or ongoing recovery from a severe, prior bleaching event. Altogether, the CBASS resolved historical, taxonomic, and possibly recent environmental drivers of variation in coral thermal thresholds, highlighting the potential for a standardized, short-term thermal assay as a universal approach for assessing ecological and evolutionary variation in the upper thermal limits of corals.
Journal Article
First insights into coral recruit and juvenile abundances at remote Aldabra Atoll, Seychelles
Coral recruitment and successive growth are essential for post-disturbance reef recovery. As coral recruit and juvenile abundances vary across locations and under different environmental regimes, their assessment at remote, undisturbed reefs improves our understanding of early life stage dynamics of corals. Here, we first explored changes in coral juvenile abundance across three locations (lagoon, seaward west and east) at remote Aldabra Atoll (Seychelles) between 2015 and 2019, which spanned the 2015/16 global coral bleaching event. Secondly, we measured variation in coral recruit abundance on settlement tiles from two sites (lagoon, seaward reef) during August 2018–August 2019. Juvenile abundance decreased from 14.1 ± 1.2 to 7.4 ± 0.5 colonies m-2 (mean ± SE) during 2015–2016 and increased to 22.4 ± 1.2 colonies m-2 during 2016–2019. Whilst juvenile abundance increased two- to three-fold at the lagoonal and seaward western sites during 2016–2018 (from 7.7–8.3 to 17.3–24.7 colonies m-2), increases at the seaward eastern sites occurred later (2018–2019; from 5.8–6.9 to 16.6–24.1 colonies m-2). The composition of coral recruits on settlement tiles was dominated by Pocilloporidae (64–92% of all recruits), and recruit abundance was 7- to 47-fold higher inside than outside the lagoon. Recruit abundance was highest in October–December 2018 (2164 ± 453 recruits m-2) and lowest in June–August 2019 (240 ± 98 recruits m-2). As Acroporid recruit abundance corresponded to this trend, the results suggest that broadcast spawning occurred during October–December, when water temperature increased from 26 to 29°C. This study provides the first published record on coral recruit abundance in the Seychelles Outer Islands, indicates a rapid (2–3 years) increase of juvenile corals following a bleaching event, and provides crucial baseline data for future research on reef resilience and connectivity within the region.
Journal Article
Remote Sensing of Coral Bleaching Using Temperature and Light: Progress towards an Operational Algorithm
The National Oceanic and Atmospheric Administration’s Coral Reef Watch program developed and operates several global satellite products to monitor bleaching-level heat stress. While these products have a proven ability to predict the onset of most mass coral bleaching events, they occasionally miss events; inaccurately predict the severity of some mass coral bleaching events; or report false alarms. These products are based solely on temperature and yet coral bleaching is known to result from both temperature and light stress. This study presents a novel methodology (still under development), which combines temperature and light into a single measure of stress to predict the onset and severity of mass coral bleaching. We describe here the biological basis of the Light Stress Damage (LSD) algorithm under development. Then by using empirical relationships derived in separate experiments conducted in mesocosm facilities in the Mexican Caribbean we parameterize the LSD algorithm and demonstrate that it is able to describe three past bleaching events from the Great Barrier Reef (GBR). For this limited example, the LSD algorithm was able to better predict differences in the severity of the three past GBR bleaching events, quantifying the contribution of light to reduce or exacerbate the impact of heat stress. The new Light Stress Damage algorithm we present here is potentially a significant step forward in the evolution of satellite-based bleaching products.
Journal Article
Fast and pervasive transcriptomic resilience and acclimation of extremely heat-tolerant coral holobionts from the northern Red Sea
Corals from the northern Red Sea and Gulf of Aqaba exhibit extreme thermal tolerance. To examine the underlying gene expression dynamics, we exposed Stylophora pistillata from the Gulf of Aqaba to short-term (hours) and long-term (weeks) heat stress with peak seawater temperatures ranging from their maximum monthly mean of 27 °C (baseline) to 29.5 °C, 32 °C, and 34.5 °C. Corals were sampled at the end of the heat stress as well as after a recovery period at baseline temperature. Changes in coral host and symbiotic algal gene expression were determined via RNA-sequencing (RNA-Seq). Shifts in coral microbiome composition were detected by complementary DNA (cDNA)-based 16S ribosomal RNA (rRNA) gene sequencing. In all experiments up to 32 °C, RNA-Seq revealed fast and pervasive changes in gene expression, primarily in the coral host, followed by a return to baseline gene expression for the majority of coral (>94%) and algal (>71%) genes during recovery. At 34.5 °C, large differences in gene expression were observed with minimal recovery, high coral mortality, and a microbiome dominated by opportunistic bacteria (including Vibrio species), indicating that a lethal temperature threshold had been crossed. Our results show that the S. pistillata holobiont can mount a rapid and pervasive gene expression response contingent on the amplitude and duration of the thermal stress. We propose that the transcriptomic resilience and transcriptomic acclimation observed are key to the extraordinary thermal tolerance of this holobiont and, by inference, of other northern Red Sea coral holobionts, up to seawater temperatures of at least 32 °C, that is, 5 °C above their current maximum monthly mean
Journal Article
Short-term heat stress assays resolve effects of host strain, repeat stress, and bacterial inoculation on Aiptasia thermal tolerance phenotypes
The ongoing loss of corals and their reef ecosystems hastens the need to develop approaches that mitigate the impacts of climate change. Given the strong reliance of corals on their associated prokaryotic and microalgal symbionts, microbiome-targeted interventions in the form of probiotics or microbiome transplants are emerging as potential solutions. Although inoculation with beneficial microorganisms was shown to improve coral bleaching recovery, the mechanistic underpinnings and extent to which microbiomes can be manipulated are largely unknown. Research progress in this regard is often hindered by coral holobiont complexity and a lack of standardized diagnostics to assess physiological and phenotypic changes following microbial manipulation. Here we address these shortcomings by establishing short-term acute thermal stress assays using the Coral Bleaching Automated Stress System (CBASS) as a standardized and reproducible experimental platform to assess stress tolerance phenotypes of the coral model Aiptasia. We show that thermal tolerance phenotypes following acute heat stress assays are highly reproducible, host species-specific, and can exert legacy effects with consequences for long-term thermal resilience. We further demonstrate the ability to resolve phenotypic differences in thermal tolerance following incubation with the coral bleaching pathogen Vibrio coralliilyticus, providing an avenue for screening bacteria for their ability to affect holobiont thermal performance. By employing acute heat stress assays in conjunction with a tractable model organism, we posit CBASS assays as a standardized experimental platform that allows functional screening for microbes that affect thermal stress tolerance. Such effort may accelerate the discovery of microbes and microbial mechanisms mediating thermal tolerance and our ability to harness them to increase stress resilience.
Journal Article
Higher spatial resolution is not always better: evaluating satellite-sensed sea surface temperature products for a west Pacific coral reef system
As marine heatwaves and mass coral bleaching events rise in frequency and severity, there is an increasing need for high-resolution satellite products that accurately predict reef thermal environments over large spatio-temporal scales. Deciding which global sea surface temperature (SST) dataset to use for research or management depends in part on the desired spatial resolution. Here, we evaluate two SST datasets – the lower-resolution CoralTemp v3.1 (0.05° ~ 5 km grid) and the Multiscale Ultra-high Resolution MUR v4.1 (0.01° ~ 1 km grid) – in their ability to predict in situ reef thermal environments (nightly mean and daily maximum) and the severity of past bleaching in Palau, western Pacific Ocean. We expected higher-resolution data to improve prediction accuracy, yet CoralTemp data explained 10% additional variability of in situ temperatures and 70% additional variance in past bleaching. This likely relates to differential data protocols; despite MUR using a higher spatial resolution grid, CoralTemp achieves higher raw satellite observation density in coastal areas by utilising geostationary satellites. MUR SSTs were also consistently more variable. These results reinforce calls to develop more accurate high-resolution SST products for coral reefs. Our paper demonstrates that higher spatial resolution SST data is not necessarily better at predicting in situ thermal environments of coral reefs and local marine heatwave impacts.
Journal Article
All-inclusive coral reef restoration: How the tourism sector can boost restoration efforts in the caribbean
Following a strong decline in the health of Caribbean coral reefs in the 1970s, disease outbreaks, overfishing, and warming events have continued to push these reefs towards a point of no return. As such, researchers and stakeholders have turned their attention to restoration practices to overcome coral recovery bottlenecks on Caribbean reefs. However, successful restoration faces many challenges, including economical and logistical feasibility, long-term stability, and biological and ecological factors yet to fully understand. The tourism sector has the potential to enhance and scale restoration efforts in the Caribbean, beyond simple financial contributions. Its strengths include long-term presence in several locations, logistical and human resources, and a business case focused on preserving the ecosystem services on which it depends. Here, we present the restoration program of Iberostar Hotels and Resorts which includes a scientific team that incorporates science-based solutions into resort operations to promote reef resilience in the face of climate change. We exemplify the potential of our program to scale up science-based reef restoration in collaboration with academia, local community, and government by presenting the first utilization of the Coral Bleaching Automated Stress System (CBASS) in Latin America and the Latin American Caribbean, with the aim of applying findings on coral thermotolerance directly to Iberostar’s reef restoration program across the Caribbean. This program presents a new model for tourism involvement in coral restoration and illustrates its capacity to scale up existing restoration practices by utilizing the strengths of the sector while maintaining science-based decision making.
Journal Article