Telomere dysfunction is associated with dark-induced bleaching in the reef coral Stylophora pistillata

Telomere DNA length is a complex trait controlled by both multiple loci and environmental factors. A growing number of studies are focusing on the impact of stress and stress accumulation on telomere length and the link with survival and fitness in ecological contexts. Here, we investigated the telomere changes occurring in a symbiotic coral, Stylophora pistillata, that has experienced continuous darkness over 6 months. This stress condition led to the loss of its symbionts in a similar manner to that observed during large-scale bleaching events due to climate changes and anthropogenic activities, threatening reef ecosystems worldwide. We found that continuous darkness was associated with telomere length shortening. This result, together with a phylogenetic analysis of the telomere coral proteins and a transcriptome survey of the continuous darkness condition, paves the way for future studies on the role of telomeres in the coral stress response and the importance of environmentally induced telomere shortening in endangered coral species.

Deoxygenation lowers the thermal threshold of coral bleaching

Exposure to deoxygenation from climate warming and pollution is emerging as a contributing factor of coral bleaching and mortality. However, the combined effects of heating and deoxygenation on bleaching susceptibility remain unknown. Here, we employed short-term thermal stress assays to show that deoxygenated seawater can lower the thermal limit of an Acropora coral by as much as 1 °C or 0.4 °C based on bleaching index scores or dark-acclimated photosynthetic efficiencies, respectively. Using RNA-Seq, we show similar stress responses to heat with and without deoxygenated seawater, both activating putative key genes of the hypoxia-inducible factor response system indicative of cellular hypoxia. We also detect distinct deoxygenation responses, including a disruption of O2-dependent photo-reception/-protection, redox status, and activation of an immune response prior to the onset of bleaching. Thus, corals are even more vulnerable when faced with heat stress in deoxygenated waters. This highlights the need to integrate dissolved O2 measurements into global monitoring programs of coral reefs.

Ocean acidification elicits differential bleaching and gene expression patterns in larval reef coral Pocillopora damicornis under heat stress

The successful dispersal of coral larvae is vital to the population replenishment and reef recovery and resilience. Despite that this critical early stage is susceptible to ocean warming and acidification, little is known about the responses of coral larvae to warming and acidification across different biological scales. This study explored the influences of elevated temperature (29 °C versus 33 °C) and pCO₂ (500 μatm versus 1000 μatm) on brooded larvae of Pocillopora damicornis at the organismal, cellular and gene expression levels. Heat stress caused bleaching, depressed light-enhanced dark respiration, photosynthesis and autotrophy, whereas high pCO₂ stimulated photosynthesis. Although survival was unaffected, larvae at 33 °C were ten-times more likely to settle than those at 29 °C, suggesting reduced capacity to disperse and differentiate suitable substrate. Remarkably, heat stress induced greater symbiont loss at ambient pCO₂ than at high pCO₂, while cell-specific pigment concentrations of symbionts at 33 °C increased twofold under ambient pCO₂ relative to high pCO₂, suggesting pCO₂-dependent bleaching patterns. Considerable increases in activities of host antioxidants superoxide dismutase (SOD) and catalase (CAT) at 33 °C indicated oxidative stress, whereas lipid peroxidation and caspase activities were contained, thereby restraining larval mortality at 33 °C. Furthermore, the coral host mounted stronger transcriptional responses than symbionts. High pCO₂ stimulated host metabolic pathways, possibly because of the boosted algal productivity. In contrast, host metabolic processes and symbiont photosystem genes were downregulated at 33 °C. Interestingly, the upregulation of extracellular matrix genes and glycosaminoglycan degradation pathway at 33 °C was more evident under ambient pCO₂ than high pCO₂, suggesting compromised host tissue integrity that could have facilitated symbiont expulsion and bleaching. Our results provide insights into how coral larvae respond to warming and acidification at different levels of biological organization, and demonstrate that ocean acidification can mediate thermal bleaching and gene expression in coral larvae under heat stress.

The pulsating soft coral Xenia umbellata shows high resistance to warming when nitrate concentrations are low

The resistance of hard corals to warming can be negatively affected by nitrate eutrophication, but related knowledge for soft corals is scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to different levels of nitrate eutrophication (control = 0.6, medium = 6, high = 37 μM nitrate) in a laboratory experiment, with additional warming (27.7 to 32.8 °C) from days 17 to 37. High nitrate eutrophication enhanced cellular chlorophyll a content of Symbiodiniaceae by 168%, while it reduced gross photosynthesis by 56%. After additional warming, polyp pulsation rate was reduced by 100% in both nitrate eutrophication treatments, and additional polyp loss of 7% d⁻¹ and total fragment mortality of 26% was observed in the high nitrate eutrophication treatment. Warming alone did not affect any of the investigated response parameters. These results suggest that X. umbellata exhibits resistance to warming, which may facilitate ecological dominance over some hard corals as ocean temperatures warm, though a clear negative physiological response occurs when combined with nitrate eutrophication. This study thus confirms the importance of investigating combinations of global and local factors to understand and manage changing coral reefs.

Symbiont specificity differs among green hydra strains

The symbiotic hydra Hydra viridissima has a stable symbiotic relationship with the green alga Chlorella. This hydra appears to cospeciate with the symbiotic alga, and some strains are known to have strain-specific host/symbiont combinations. To investigate the mechanism of the specificity between host and symbiont, we explored the effect of the removal or exchange of symbionts in two distantly related H. viridissima strains (K10 and M9). In the K10 strain, severe morphological and behavioural changes were found in symbiont-removed and symbiont-exchanged polyps. Interestingly, both polyps showed a similar gene expression pattern. The gene ontology (GO) enrichment analysis revealed that the removal or exchange of symbionts caused the downregulation of genes involved in the electron transport chain and the upregulation of genes involved in translation in the K10 strain. On the other hand, symbiont-removed and symbiont-exchanged M9 polyps showed modest changes in their morphology and behaviour compared with the K10 strain. Furthermore, the patterns of the gene expression changes in the M9 strain were quite different between the symbiont-removed and symbiont-exchanged polyps. Our results suggested that the regulation of energy balance is one of the crucial mechanisms for maintaining symbiotic relationships in green hydra, and this mechanism differs between the strains.

Design Optimization of a Submersible Chemiluminescent Sensor (DISCO) for Improved Quantification of Reactive Oxygen Species (ROS) in Surface Waters

Reactive oxygen species (ROS) are key drivers of biogeochemical cycling while also exhibiting both positive and negative effects on marine ecosystem health. However, quantification of the ROS superoxide (O₂^-) within environmental systems is hindered by its short half-life. Recently, the development of the diver-operated submersible chemiluminescent sensor (DISCO), a submersible, handheld instrument, enabled in situ superoxide measurements in real time within shallow coral reef ecosystems. Here, we present a redesigned and improved instrument, DISCO II. Similar to the previous DISCO, DISCO II is a self-contained, submersible sensor, deployable to 30 m depth and capable of measuring reactive intermediate species in real time. DISCO II is smaller, lighter, lower cost, and more robust than its predecessor. Laboratory validation of DISCO II demonstrated an average limit of detection in natural seawater of 133.1 pM and a percent variance of 0.7%, with stable photo multiplier tube (PMT) counts, internal temperature, and flow rates. DISCO II can also be optimized for diverse environmental conditions by adjustment of the PMT supply voltage and integration time. Field tests showed no drift in the data with a percent variance of 3.0%. Wand tip adaptations allow for in situ calibrations and decay rates of superoxide using a chemical source of superoxide (SOTS-1). Overall, DISCO II is a versatile, user-friendly sensor that enables measurements in diverse environments, thereby improving our understanding of the cycling of reactive intermediates, such as ROS, across various marine ecosystems.

Comparative transcriptomics of two coral holobionts collected during the 2017 El Niño heat wave reveal differential stress response mechanisms

Although coral species exhibit differential susceptibility to stressors, little is known about the underlying molecular mechanisms. Here we compared scleractinian corals Montipora peltiformis and Platygyra carnosa collected during the 2017 El Niño heat wave. Zooxanthellae density and chlorophyll a content declined and increased substantially during and after heat stress event, respective. However, the magnitude of change was larger in M. peltiformis. Transcriptome analysis showed that heat-stressed corals corresponded to metabolic depression and catabolism of amino acids in both hosts which might promote their survival. However, only M. peltiformis has developed the bleached coral phenotype with corresponding strong stress- and immune-related responses in the host and symbiont, and strong suppression of photosynthesis-related genes in the symbiont. Overall, our study reveals differences among species in the homeostatic capacity to prevent the development of the bleached phenotype under environmental stressors, eventually determining their likelihood of survival in the warming ocean.

Coral taxonomy and local stressors drive bleaching prevalence across the Hawaiian Archipelago in 2019

The Hawaiian Archipelago experienced a moderate bleaching event in 2019—the third major bleaching event over a 6-year period to impact the islands. In response, the Hawai‘i Coral Bleaching Collaborative (HCBC) conducted 2,177 coral bleaching surveys across the Hawaiian Archipelago. The HCBC was established to coordinate bleaching monitoring efforts across the state between academic institutions, non-governmental organizations, and governmental agencies to facilitate data sharing and provide management recommendations. In 2019, the goals of this unique partnership were to: 1) assess the spatial and temporal patterns of thermal stress; 2) examine taxa-level patterns in bleaching susceptibility; 3) quantify spatial variation in bleaching extent; 4) compare 2019 patterns to those of prior bleaching events; 5) identify predictors of bleaching in 2019; and 6) explore site-specific management strategies to mitigate future bleaching events. Both acute thermal stress and bleaching in 2019 were less severe overall compared to the last major marine heatwave events in 2014 and 2015. Bleaching observed was highly site- and taxon-specific, driven by the susceptibility of remaining coral assemblages whose structure was likely shaped by previous bleaching and subsequent mortality. A suite of environmental and anthropogenic predictors was significantly correlated with observed bleaching in 2019. Acute environmental stressors, such as temperature and surface light, were equally important as previous conditions (e.g. historical thermal stress and historical bleaching) in accounting for variation in bleaching during the 2019 event. We found little evidence for acclimation by reefs to thermal stress in the main Hawaiian Islands. Moreover, our findings illustrate how detrimental effects of local anthropogenic stressors, such as tourism and urban run-off, may be exacerbated under high thermal stress. In light of the forecasted increase in severity and frequency of bleaching events, future mitigation of both local and global stressors is a high priority for the future of corals in Hawai‘i.

Viable protoplast formation of the coral endosymbiont alga Symbiodinium spp. in a microfluidics platform

Symbiodiniaceae is an important dinoflagellate family which lives in endosymbiosis with reef invertebrates, including coral polyps, making them central to the holobiont. With coral reefs currently under extreme threat from climate change, there is a pressing need to improve our understanding on the stress tolerance and stress avoidance mechanisms of Symbiodinium spp. Reactive oxygen species (ROS) such as singlet oxygen are central players in mediating various stress responses; however, the detection of ROS using specific dyes is still far from definitive in intact Symbiodinium cells due to the hindrance of uptake of certain fluorescent dyes because of the presence of the cell wall. Protoplast technology provides a promising platform for studying oxidative stress with the main advantage of removed cell wall, however the preparation of viable protoplasts remains a significant challenge. Previous studies have successfully applied cellulose-based protoplast preparation in Symbiodiniaceae; however, the protoplast formation and regeneration process was found to be suboptimal. Here, we present a microfluidics-based platform which allowed protoplast isolation from individually trapped Symbiodinium cells, by using a precisely adjusted flow of cell wall digestion enzymes (cellulase and macerozyme). Trapped single cells exhibited characteristic changes in their morphology, cessation of cell division and a slight decrease in photosynthetic activity during protoplast formation. Following digestion and transfer to regeneration medium, protoplasts remained photosynthetically active, regrew cell walls, regained motility, and entered exponential growth. Elevated flow rates in the microfluidic chambers resulted in somewhat faster protoplast formation; however, cell wall digestion at higher flow rates partially compromised photosynthetic activity. Physiologically competent protoplasts prepared from trapped cells in microfluidic chambers allowed for the first time the visualization of the intracellular localization of singlet oxygen (using Singlet Oxygen Sensor Green dye) in Symbiodiniaceae, potentially opening new avenues for studying oxidative stress.

Increased incompatibility of heterologous algal symbionts under thermal stress in the cnidarian-dinoflagellate model Aiptasia

Rising ocean temperatures are increasing the rate and intensity of coral mass bleaching events, leading to the collapse of coral reef ecosystems. To better understand the dynamics of coral-algae symbioses, it is critical to decipher the role each partner plays in the holobiont's thermotolerance. Here, we investigated the role of the symbiont by comparing transcriptional heat stress responses of anemones from two thermally distinct locations, Florida (CC7) and Hawaii (H2) as well as a heterologous host-symbiont combination composed of CC7 host anemones inoculated with the symbiont Breviolum minutum (SSB01) from H2 anemones (CC7-B01). We find that oxidative stress and apoptosis responses are strongly influenced by symbiont type, as further confirmed by caspase-3 activation assays, but that the overall response to heat stress is dictated by the compatibility of both partners. Expression of genes essential to symbiosis revealed a shift from a nitrogen- to a carbon-limited state only in the heterologous combination CC7-B01, suggesting a bioenergetic disruption of symbiosis during stress. Our results indicate that symbiosis is highly fine-tuned towards particular partner combinations and that heterologous host-symbiont combinations are metabolically less compatible under stress. These results are essential for future strategies aiming at increasing coral resilience using heterologous thermotolerant symbionts.

Low-level nutrient enrichment during thermal stress delays bleaching and ameliorates calcification in three Hawaiian reef coral species

Terrestrial-based nutrient pollution has emerged as one of the most detrimental factors to coral health in many reef habitats. Recent studies have shown that excessive dissolved inorganic nutrients can reduce coral thermal tolerance thresholds and even exacerbate bleaching during thermal stress, yet the effects of minor nutrient enrichment under heat stress have not been extensively studied. In this study, Lobactis scutaria, Montipora capitata, and Pocillopora acuta colonies under heated conditions (~30.5 °C) were exposed to low and balanced nitrogen and phosphorous concentrations over a 31-day heating period. Coral colonies were collected from Kāne'ohe Bay, O'ahu, which has a unique history of nutrient pollution, and held in mesocosms that allowed for environmental manipulation yet are also influenced by local field conditions. Principal findings included delays in the bleaching of nutrient-enriched heated colonies as compared to heated-only colonies, in addition to relatively greater calcification rates and lower proportions of early-stage paling. Species-specific outcomes were prevalent, with L. scutaria demonstrating no difference in calcification with enrichment under heat stress. By the end of the heating stage, however, many heated colonies were at least partially impacted by bleaching or mortality. Despite this, our findings suggest that low levels of balanced nutrient enrichment may serve as a mitigative force during thermal events. Further field-based studies will be required to assess these results in different reef habitats.

Coral-bleaching responses to climate change across biological scales

The global impacts of climate change are evident in every marine ecosystem. On coral reefs, mass coral bleaching and mortality have emerged as ubiquitous responses to ocean warming, yet one of the greatest challenges of this epiphenomenon is linking information across scientific disciplines and spatial and temporal scales. Here we review some of the seminal and recent coral-bleaching discoveries from an ecological, physiological, and molecular perspective. We also evaluate which data and processes can improve predictive models and provide a conceptual framework that integrates measurements across biological scales. Taking an integrative approach across biological and spatial scales, using for example hierarchical models to estimate major coral-reef processes, will not only rapidly advance coral-reef science but will also provide necessary information to guide decision-making and conservation efforts. To conserve reefs, we encourage implementing mesoscale sanctuaries (thousands of km² ) that transcend national boundaries. Such networks of protected reefs will provide reef connectivity, through larval dispersal that transverse thermal environments, and genotypic repositories that may become essential units of selection for environmentally diverse locations. Together, multinational networks may be the best chance corals have to persist through climate change, while humanity struggles to reduce emissions of greenhouse gases to net zero.

The seasonal investigation of Symbiodiniaceae in broadcast spawning, Acropora humilis and brooding, Pocillopora cf. damicornis corals

The density and diversity of Symbiodiniaceae associated with corals can be influenced by seasonal changes . This study provided the first annual investigation of Symbiodiniaceae density and diversity associated with Acropora humilis and Pocillopora cf. damicornis corals in the Gulf of Thailand using both zooxanthellae cell count and next-generation sequencing (ITS-1, ITS-2 regions) techniques, respectively. The results from this study indicated that zooxanthellae cell densities in both coral species differ significantly. The number of zooxanthellae was negatively correlated with the physical environment variable (light intensity). The diversity within A. humilis consisted of two genera, Cladocopium (Cspc_C3: 56.39%, C3w: 33.62%, C93type1: 4.42% and Cspf: 3.59%) and a small amount of Durusdinium (D1: 1.03%) whereas P. cf. damicornis was found to be 100% associated with Durusdinium (D1: 95.58%, D6: 1.01% and D10: 2.7%) suggesting that each coral species may select their appropriate genus/species of Symbiodiniaceae in response to local environmental stressors. The results of this study provided some information on the coral-Symbiodiniaceae relationship between seasons, which may be applied to predict the potential adaptation of corals in localized reef environments.

Nitric oxide production rather than oxidative stress and cell death is associated with the onset of coral bleaching in Pocillopora acuta

Elevated seawater temperatures associated with climate change lead to coral bleaching. While the ultimate causes of bleaching are well understood, the proximate physiological mechanisms underlying the bleaching response are not as well defined. Here we measured nitric oxide synthase activity, oxidative stress, and cell death in algal symbionts (Symbiodinaceae) freshly isolated from the reef-building coral Pocillopora acuta collected in the field under natural non-bleaching conditions and from corals experimentally exposed to elevated temperatures. Nitric oxide synthase activity in the algal symbionts was >3 orders of magnitude higher than that of the host and increased dramatically with increasing temperature and time of exposure (up to 72 h), consistent with the onset of bleaching for these corals. Oxidative stress and cell death among the algal symbionts were highest in coral holobionts exposed to intermediate as opposed to maximal temperatures, suggesting that these mechanisms are not proximal triggers for bleaching in this species. Our results point to nitric oxide production by the algal symbionts, rather than symbiont dysfunction, as a more important driver of coral bleaching under acute thermal stress in this coral.

Long-Term Heat Selection of the Coral Endosymbiont Cladocopium C1acro (Symbiodiniaceae) Stabilizes Associated Bacterial Communities

Heat-tolerant strains of the coral endosymbiont, Cladocopium C1^acro (Symbiodiniaceae), have previously been developed via experimental evolution. Here, we examine physiological responses and bacterial community composition (using 16S rRNA gene metabarcoding) in cultures of 10 heat-evolved (SS) and 9 wild-type (WT) strains, which had been exposed for 6 years to 31 °C and 27 °C, respectively. We also examine whether the associated bacterial communities were affected by a three-week reciprocal transplantation to both temperatures. The SS strains had bacterial communities with lower diversities that showed more stability and lower variability when exposed to elevated temperatures compared with the WT strains. Amplicon sequence variants (ASVs) of the bacterial genera Labrenzia, Algiphilus, Hyphobacterium and Roseitalea were significantly more associated with the SS strains compared with the WT strains. WT strains showed higher abundance of ASVs assigned to the genera Fabibacter and Tropicimonas. We hypothesize that these compositional differences in associated bacterial communities between SS and WT strains also contribute to the thermal tolerance of the microalgae. Future research should explore functional potential between bacterial communities using metagenomics to unravel specific genomic adaptations.

Thank you for biting: dispersal of beneficial microbiota through 'antagonistic' interactions

Multicellular organisms harbor populations of microbial symbionts; some of these symbionts can be dispersed through the feeding activities of consumers. Studies of consumer-mediated microbiota dispersal generally focus on pathogenic microorganisms; the dispersal of beneficial microorganisms has received less attention, especially in the context of 'antagonistic' trophic interactions (e.g., herbivory, parasitism, predation). Yet, this 'trophic transmission' of beneficial symbionts has significant implications for microbiota assembly and resource species (e.g., prey) health. For example, trophic transmission of microorganisms could assist with environmental acclimatization and help resource species to suppress other consumers or competitors. Here, we highlight model systems and approaches that have revealed these potential 'silver-linings' of antagonism as well as opportunities and challenges for future research.

Effects of Ocean Warming on the Underexplored Members of the Coral Microbiome

The climate crisis is one of the most significant threats to marine ecosystems. It is leading to severe increases in sea surface temperatures and in the frequency and magnitude of marine heatwaves. These changing conditions are directly impacting coral reef ecosystems, which are among the most biodiverse ecosystems on Earth. Coral-associated symbionts are particularly affected because summer heatwaves cause coral bleaching-the loss of endosymbiotic microalgae (Symbiodiniaceae) from coral tissues, leading to coral starvation and death. Coral-associated Symbiodiniaceae and bacteria have been extensively studied in the context of climate change, especially in terms of community diversity and dynamics. However, data on other microorganisms and their response to climate change are scarce. Here, we review current knowledge on how increasing temperatures affect understudied coral-associated microorganisms such as archaea, fungi, viruses, and protists other than Symbiodiniaceae, as well as microbe-microbe interactions. We show that the coral-microbe symbiosis equilibrium is at risk under current and predicted future climate change and argue that coral reef conservation initiatives should include microbe-focused approaches.

Implications of bleaching on cnidarian venom ecology

Cnidarian bleaching research often focuses on the effects on a cnidarian's physiological health and fitness, whilst little focus has been towards the impacts of these events on their venom ecology. Given the importance of a cnidarian's venom to their survival and the increasing threat of bleaching events, it is important to understand the effects that this threat may have on this important aspect of their ecology as it may have unforeseen impacts on their ability to catch prey and defend themselves. This review aims to explore evidence that suggests that bleaching may impact on each of the key aspects of a cnidarians' venom ecology: cnidae, venom composition, and venom toxicity. Additionally, the resulting energy deficit, compensatory heterotrophic feeding, and increased defensive measures have been highlighted as possible ecological factors driving these changes. Suggestions are also made to guide the success of research in this field into the future, specifically in regards to selecting a study organism, the importance of accurate symbiont and cnidae identification, use of appropriate bleaching methods, determination of bleaching, and animal handling. Ultimately, this review highlights a significant and important gap in our knowledge into how cnidarians are, and will, continue to be impacted by bleaching stress.

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Information on the effects of bleaching on cnidarian venom ecology is limited.

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There is evidence to suggest nematocysts, venom composition and venom toxicity may each be impacted by bleaching.

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Bleaching may result in depleted energy, increased heterotrophy and/or the need for stronger defensive strategies.

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To fully understand how cnidarians may be impacted by bleaching stress further research in this field is needed.

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Future studies should consider the model organism and methodologies, thereby minimising indirect confounding effects.

Elevated temperatures reduce the resilience of the Red Sea branching coral stylophora pistillata to copper pollution

Copper (Cu) is a common marine pollutant of coastal environments and can cause severe impacts on coral organisms. To date, only a few studies assessed the effects of Cu contamination in combination with elevated seawater temperatures on corals. Furthermore, experiments focusing on coral recovery during a depuration phase, and under thermal stress, are lacking. The present study investigated the physiological response of the common and thermally tolerant scleractinian coral Stylophora pistillata from the northern Red Sea to Cu contamination (2.5, 5 or 10 µg L ^- 1) in combination with thermal stress (5 °C above local ambient temperatures (26 °C)) for 23 days, and assessed the impact of elevated temperatures on its ability to recover from such pollution during a one-week depuration period. Variation in coral photo-physiological biomarkers including antioxidant defense capacity, were dose, time and temperature-dependent, and revealed additive effects of elevated temperatures. Successful recovery was achieved in ambient temperature only and was mediated by antioxidant defenses. Elevation of temperature altered the recovery dynamics during depuration, causing reduced Cu bioaccumulation and photosynthetic yield. The present study provides novel information on the effects of elevated temperature on the resilience (resistance and recovery processes) of a scleractinian coral exposed to a common marine pollutant. Our findings suggest that ocean warming may alter the resilience strategies of corals when exposed to local pollution, an impact that might have long-term consequences on the chances of survival of reefs in increasingly populated and warming coastal environments.

Bleaching physiology: who's the 'weakest link' - host vs. symbiont?

Environmental stress, such as an increase in the sea surface temperature, triggers coral bleaching, a profound dysfunction of the mutualist symbiosis between the host cnidarians and their photosynthetic dinoflagellates of the Family Symbiodiniaceae. Because of climate change, mass coral bleaching events will increase in frequency and severity in the future, threatening the persistence of this iconic marine ecosystem at global scale. Strategies adapted to coral reefs preservation and restoration may stem from the identification of the succession of events and of the different molecular and cellular contributors to the bleaching phenomenon. To date, studies aiming to decipher the cellular cascade leading to temperature-related bleaching, emphasized the involvement of reactive species originating from compromised bioenergetic pathways (e.g. cellular respiration and photosynthesis). These molecules are responsible for damage to various cellular components causing the dysregulation of cellular homeostasis and the breakdown of symbiosis. In this review, we synthesize the current knowledge available in the literature on the cellular mechanisms caused by thermal stress, which can initiate or participate in the cell cascade leading to the loss of symbionts, with a particular emphasis on the role of each partner in the initiating processes.

Exploring microbiome engineering as a strategy for improved thermal tolerance in Exaiptasia diaphana

Aims

Fourteen percent of all living coral, equivalent to more than all the coral on the Great Barrier Reef, has died in the past decade as a result of climate change‐driven bleaching. Inspired by the ‘oxidative stress theory of coral bleaching’, we investigated whether a bacterial consortium designed to scavenge free radicals could integrate into the host microbiome and improve thermal tolerance of the coral model, Exaiptasia diaphana.

Methods and Results

E. diaphana anemones were inoculated with a consortium of high free radical scavenging (FRS) bacteria, a consortium of congeneric low FRS bacteria, or sterile seawater as a control, then exposed to elevated temperature. Increases in the relative abundance of Labrenzia during the first 2 weeks following the last inoculation provided evidence for temporary inoculum integration into the E. diaphana microbiome. Initial uptake of other consortium members was inconsistent, and these bacteria did not persist either in E. diaphana’s microbiome over time. Given their non‐integration into the host microbiome, the ability of the FRS consortium to mitigate thermal stress could not be assessed. Importantly, there were no physiological impacts (negative or positive) of the bacterial inoculations on the holobiont.

Conclusions

The introduced bacteria were not maintained in the anemone microbiome over time, thus, their protective effect is unknown. Achieving long‐term integration of bacteria into cnidarian microbiomes remains a research priority.

Significance and Impact of the Study

Microbiome engineering strategies to mitigate coral bleaching may assist coral reefs in their persistence until climate change has been curbed. This study provides insights that will inform microbiome manipulation approaches in coral bleaching mitigation research.

Macro- and micro-scale adaptations allow distinct Stylophora pistillata-symbiodiniaceae holobionts to optimize performance across a broad light habitat

In sessile organisms, phenotypic plasticity represents an important strategy for dealing with environmental variability. Here we test if phenotypic plasticity enables the common coral Stylophora pistillata to occupy a broad niche. We find clear differences in the photo-physiology of four putative species of photosynthetic dinoflagellate symbionts associated with the coral S. pistillata, namely, Cladocopium 'C35a', 'C79', 'C78a' and 'C8a'. Coral phenotypic responses were also tightly linked to symbiont identity. Corals with Cladocopium 'C8a' have more "open" macro-morphology compared to colonies associating with depth-restricted Cladocopium 'C35a' or 'C78a' in the same shallow water habitat. Corals with Cladocopium 'C8a' had 40 to 60% lower symbiont cell densities compared to other holobionts but were more efficient at acclimating over a range of light levels, with clear mechanisms to dissipate excess light energy. This holobiont contains host-based green fluorescent pigments, increased concentrations of symbiont-based mycosporine amino acids, and xanthophyll cycling in high light habitats. Photosynthetic efficiency was also adjusted over the light habitat. In contrast, limited micro-scale responses were observed between three depth-restricted symbionts: Cladocopium 'C79', 'C35a', and 'C78a'. To optimize light levels reaching the photosynthetic unit, these colonies rely on a more closed macro-morphology under high light levels, which reduces incident light levels by up to 43%, and higher symbiont densities . Our results show that distinct macro- and micro-scale adaptations lead to functional differences between four distinct S. pistillata holobionts, allowing them to co-exist by filling specific niches on a small, but environmentally diverse, spatial scale. Key index words: Light, Symbiodiniaceae, coral, pigments, Stylophora pistillata, ITS2, phenotypic plasticity, niche diversification.

Host-symbiont transcriptomic changes during natural bleaching and recovery in the leaf coral Pavona decussata

Coral bleaching has become a major threat to coral reefs worldwide, but for most coral species little is known about their resilience to environmental changes. We aimed to understand the gene expressional regulation underlying natural bleaching and recovery in Pavona decussata, a dominant species of scleractinian coral in the northern South China Sea. Analyzing samples collected in 2017 from the field revealed distinct zooxanthellae density, chlorophyll a concentration and transcriptomic signatures corresponding to changes in health conditions of the coral holobiont. In the host, normal-looking tissues of partially bleached colonies were frontloaded with stress responsive genes, as indicated by upregulation of immune defense, response to endoplasmic reticulum, and oxidative stress genes. Bleaching was characterized by upregulation of apoptosis-related genes which could cause a reduction in algal symbionts, and downregulation of genes involved in stress responses and metabolic processes. The transcription factors stat5b and irf1 played key roles in bleaching by regulating immune and apoptosis pathways. Recovery from bleaching was characterized by enrichment of pathways involved in mitosis, DNA replication, and recombination for tissue repairing, as well as restoration of energy and metabolism. In the symbionts, bleaching corresponded to imbalance in photosystems I and II activities which enhanced oxidative stress and limited energy production and nutrient assimilation. Overall, our study revealed distinct gene expressional profiles and regulation in the different phases of the bleaching and recovery process, and provided new insight into the molecular mechanisms underlying the holobiont's resilience that may determine the species' fate in response to global and regional environmental changes.

Extra high superoxide dismutase in host tissue is associated with improving bleaching resistance in "thermal adapted" and Durusdinium trenchii-associating coral

Global warming threatens reef-building corals with large-scale bleaching events; therefore, it is important to discover potential adaptive capabilities for increasing their temperature resistance before it is too late. This study presents two coral species (Platygyra verweyi and Isopora palifera) surviving on a reef having regular hot water influxes via a nearby nuclear power plant that exhibited completely different bleaching susceptibilities to thermal stress, even though both species shared several so-called "winner" characteristics (e.g., containing Durusdinium trenchii, thick tissue, etc.). During acute heating treatment, algal density did not decline in P. verweyi corals within three days of being directly transferred from 25 to 31 °C; however, the same treatment caused I. palifera to lose < 70% of its algal symbionts within 24 h. The most distinctive feature between the two coral species was an overwhelmingly higher constitutive superoxide dismutase (ca. 10-fold) and catalase (ca. 3-fold) in P. verweyi over I. palifera. Moreover, P. verweyi also contained significantly higher saturated and lower mono-unsaturated fatty acids, especially a long-chain saturated fatty acid (C22:0), than I. palifera, and was consistently associated with the symbiotic bacteria Endozoicomonas, which was not found in I. palifera. However, antibiotic treatment and inoculation tests did not support Endozoicomonas having a direct contribution to thermal resistance. This study highlights that, besides its association with a thermally tolerable algal symbiont, a high level of constitutive antioxidant enzymes in the coral host is crucial for coral survivorship in the more fluctuating and higher temperature environments.

Elevated Temperature and Exposure to Copper Leads to Changes in the Antioxidant Defense System of the Reef-Building Coral Mussismilia harttii

The frequency and severity of coral bleaching events have increased in recent years. Global warming and contamination are primarily responsible for triggering these responses in corals. Thus, the objective of this study was to evaluate the isolated and combined effects of elevated temperature and exposure to copper (Cu) on responses of the antioxidant defense system of coral Mussismilia harttii. In a marine mesocosm, fragments of the coral were exposed to three temperatures (25.0, 26.6, and 27.3°C) and three concentrations of Cu (2.9, 5.4, and 8.6 μg/L) for up to 12 days. Levels of reduced glutathione (GSH) and the activity of enzymes, such as superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), and glutamate cysteine ligase (GCL), were evaluated on the corals and symbionts. The short exposure to isolated and combined stressors caused a reduction in GSH levels and inhibition of the activity of antioxidant enzymes. After prolonged exposure, the combination of stressors continued to reduce GSH levels and SOD, CAT, and GCL activity in symbionts and GST activity in host corals. GCL activity was the parameter most affected by stressors, remaining inhibited after 12-days exposure. Interesting that long-term exposure to stressors stimulated antioxidant defense proteins in M. harttii, demonstrating a counteracting response that may beneficiate the oxidative state. These results, combined with other studies already published suggest that the antioxidant system should be further studied in order to understand the mechanisms of tolerance of South Atlantic reefs.

Increased cellular detoxification, cytoskeletal activities and protein transport explain physiological stress in a lagoon sponge

Tropical lagoon-inhabiting organisms live in highly irradiated ecosystems and are particularly susceptible to thermal stress resulting from climate change. However, despite living close to their thermal maxima, stress response mechanisms found in these organisms are poorly understood. We used a novel physiological-proteomic approach for sponges to describe the stress response mechanisms of the lagoon-inhabiting sponge Amphimedon navalis, when exposed to elevated seawater temperatures of +2°C and +4°C relative to a 26°C ambient temperature for 4 weeks. After 4 weeks of thermal exposure, the buoyant weight of the sponge experienced a significant decline, while its pumping rates and oxygen consumption rates significantly increased. Proteome dynamics revealed 50 differentially abundant proteins in sponges exposed to elevated temperature, suggesting that shifts in the sponge proteome were potential drivers of physiological dysfunction. Thermal stress promoted an increase in detoxification proteins, such as catalase, suggesting that an excess of reactive oxygen species in sponge cells was responsible for the significant increase in oxygen consumption. Elevated temperature also disrupted cellular growth and cell proliferation, promoting the loss of sponge biomass, and the high abundance of multiple α-tubulin chain proteins also indicated an increase in cytoskeletal activities within sponge cells, which may have induced the increase in sponge pumping rate. Our results show that sustained thermal exposure in susceptible lagoonal sponges may induce significant disruption of cellular homeostasis, leading to physiological dysfunction, and that a combined physiological-proteomic approach may provide new insights into physiological functions and cellular processes occurring in sponges.

Establishment of Host-Algal Endosymbioses: Genetic Response to Symbiont Versus Prey in a Sponge Host

The freshwater sponge Ephydatia muelleri and its Chlorella-like algal partner is an emerging model for studying animal: algal endosymbiosis. The sponge host is a tractable laboratory organism, and the symbiotic algae are easily cultured. We took advantage of these traits to interrogate questions about mechanisms that govern the establishment of durable intracellular partnerships between hosts and symbionts in facultative symbioses. We modified a classical experimental approach to discern the phagocytotic mechanisms that might be co-opted to permit persistent infections, and identified genes differentially expressed in sponges early in the establishment of endosymbiosis. We exposed algal-free E. muelleri to live native algal symbionts and potential food items (bacteria and native heat-killed algae), and performed RNA-Seq to compare patterns of gene expression among treatments. We found a relatively small but interesting suite of genes that are differentially expressed in the host exposed to live algal symbionts, and a larger number of genes triggered by host exposure to heat-killed algae. The upregulated genes in sponges exposed to live algal symbionts were mostly involved in endocytosis, ion transport, metabolic processes, vesicle-mediated transport, and oxidation–reduction. One of the host genes, an ATP-Binding Cassette transporter that is downregulated in response to live algal symbionts, was further evaluated for its possible role in the establishment of the symbiosis. We discuss the gene expression profiles associated with host responses to living algal cells in the context of conditions necessary for long-term residency within host cells by phototrophic symbionts as well as the genetic responses to sponge phagocytosis and immune-driven pathways.

Mesophotic coral communities escape thermal coral bleaching in French Polynesia

Climate change and consequent coral bleaching are causing the disappearance of reef-building corals worldwide. While bleaching episodes significantly impact shallow waters, little is known about their impact on mesophotic coral communities. We studied the prevalence of coral bleaching two to three months after a heat stress event, along an extreme depth range from 6 to 90 m in French Polynesia. Bayesian modelling showed a decreasing probability of bleaching of all coral genera over depth, with little to no bleaching observed at lower mesophotic depths (greater than or equal to 60 m). We found that depth-generalist corals benefit more from increasing depth than depth-specialists (corals with a narrow depth range). Our data suggest that the reduced prevalence of bleaching with depth, especially from shallow to upper mesophotic depths (40 m), had a stronger relation with the light-irradiance attenuation than temperature. While acknowledging the geographical and temporal variability of the role of mesophotic reefs as spatial refuges during thermal stress, we ought to understand why coral bleaching reduces with depth. Future studies should consider repeated monitoring and detailed ecophysiological and environmental data. Our study demonstrated how increasing depth may offer a level of protection and that lower mesophotic communities could escape the impacts of a thermal bleaching event.

Symbiont shuffling induces differential DNA methylation responses to thermal stress in the coral Montastraea cavernosa

Algal symbiont shuffling in favour of more thermotolerant species has been shown to enhance coral resistance to heat-stress. Yet, the mechanistic underpinnings and long-term implications of these changes are poorly understood. This work studied the modifications in coral DNA methylation, an epigenetic mechanism involved in coral acclimatization, in response to symbiont manipulation and subsequent heat stress exposure. Symbiont composition was manipulated in the great star coral Montastraea cavernosa through controlled thermal bleaching and recovery, producing paired ramets of three genets dominated by either their native symbionts (genus Cladocopium) or the thermotolerant species (Durusdinium trenchi). Single-base genome-wide analyses showed significant modifications in DNA methylation concentrated in intergenic regions, introns and transposable elements. Remarkably, DNA methylation changes in response to heat stress were dependent on the dominant symbiont, with twice as many differentially methylated regions found in heat-stressed corals hosting different symbionts (Cladocopium vs. D. trenchii) compared to all other comparisons. Interestingly, while differential gene body methylation was not correlated with gene expression, an enrichment in differentially methylated regions was evident in repetitive genome regions. Overall, these results suggest that changes in algal symbionts favouring heat tolerant associations are accompanied by changes in DNA methylation in the coral host. The implications of these results for coral adaptation, along with future avenues of research based on current knowledge gaps, are discussed in the present work.

Intracellular Bacterial Symbionts in Corals: Challenges and Future Directions

Corals are the main primary producers of coral reefs and build the three-dimensional reef structure that provides habitat to more than 25% of all marine eukaryotes. They harbor a complex consortium of microorganisms, including bacteria, archaea, fungi, viruses, and protists, which they rely on for their survival. The symbiosis between corals and bacteria is poorly studied, and their symbiotic relationships with intracellular bacteria are only just beginning to be acknowledged. In this review, we emphasize the importance of characterizing intracellular bacteria associated with corals and explore how successful approaches used to study such microorganisms in other systems could be adapted for research on corals. We propose a framework for the description, identification, and functional characterization of coral-associated intracellular bacterial symbionts. Finally, we highlight the possible value of intracellular bacteria in microbiome manipulation and mitigating coral bleaching.

Eutrophication on Coral Reefs: What Is the Evidence for Phase Shifts, Nutrient Limitation and Coral Bleaching

Coral reefs continue to experience extreme environmental pressure from climate change stressors, but many coral reefs are also exposed to eutrophication. It has been proposed that changes in the stoichiometry of ambient nutrients increase the mortality of corals, whereas eutrophication may facilitate phase shifts to macroalgae-dominated coral reefs when herbivory is low or absent. But are corals ever nutrient limited, and can eutrophication destabilize the coral symbiosis making it more sensitive to environmental stress because of climate change? The effects of eutrophication are confounded not just by the effects of climate change but by the presence of chemical pollutants in industrial, urban, and agricultural wastes. Because of these confounding effects, the increases in nutrients or changes in their stoichiometry in coastal environments, although they are important at the organismal and community level, cannot currently be disentangled from each other or from the more significant effects of climate change stressors on coral reefs.

Physiological responses (Hsps 60 and 32, caspase 3, H2O2 scavenging, and photosynthetic activity) of the coral Pocillopora damicornis under thermal and high nitrate stresses

This study explored the physiological responses of the coral Pocillopora damicornis to high nitrate concentrations and thermal stresses. The expression of heat shock proteins Hsp60 and Hsp32, Symbiodiniaceae density, Chl a concentration, Fv/Fm, H₂O₂ scavenging, and caspase 3 activity varied during 60 h incubations at 28 °C or 32 °C, ambient or high nitrate (~10 μM) concentrations, and their combinations. In combined stresses, corals showed a rapid and high oxidation level negatively affecting the Symbiodiniaceae density and Chl a concentration at 12 h, followed by caspase 3 and Hsps upregulations that induced apoptosis, bleaching and tissue detachment. Corals under thermal stress showed the highest oxidation and upregulation of Hsps and caspase 3 resulting in coral discoloration. High nitrate treatment alone did not seriously affect the coral function. Results showed that combined stress treatment severely affected coral physiology and, judging from the condition of detached tissues, these corals might have lower chances to recover.

Different functional traits among closely related algal symbionts dictate stress endurance for vital Indo-Pacific reef-building corals

Reef-building corals in the genus Porites are one of the most important constituents of Indo-Pacific reefs. Many species within this genus tolerate abnormally warm water and exhibit high specificity for particular kinds of endosymbiotic dinoflagellates that cope with thermal stress better than those living in other corals. Still, during extreme ocean heating, some Porites exhibit differences in their stress tolerance. While corals have different physiological qualities, it remains unknown whether the stability and performance of these mutualisms is influenced by the physiology and genetic relatedness of their symbionts. We investigated two ubiquitous Pacific reef corals, Porites rus and Porites cylindrica, from warmer inshore and cooler offshore reef systems in Palau. While these corals harbored a similar kind of symbiont in the genus Cladocopium (within the ITS2 C15 subclade), rapidly evolving genetic markers revealed evolutionarily diverged lineages corresponding to each Porites species living in each reef habitat. Furthermore, these closely related Cladocopium lineages were differentiated by their densities in host tissues, cell volume, chlorophyll concentration, gross photosynthesis, and photoprotective pathways. When assessed using several physiological proxies, these previously undifferentiated symbionts contrasted in their tolerance to thermal stress. Symbionts within P. cylindrica were relatively unaffected by exposure to 32℃ for 14 days, whereas P. rus colonies lost substantial numbers of photochemically compromised symbionts. Heating reduced the ability of the offshore symbiont associated with P. rus to translocate carbon to the coral. By contrast, high temperatures enhanced symbiont carbon assimilation and delivery to the coral skeleton of inshore P. cylindrica. This study indicates that large physiological differences exist even among closely related symbionts, with significant implications for thermal susceptibility among reef-building Porites.

Elucidating gene expression adaptation of phylogenetically divergent coral holobionts under heat stress

As coral reefs struggle to survive under climate change, it is crucial to know whether they have the capacity to withstand changing conditions, particularly increasing seawater temperatures. Thermal tolerance requires the integrative response of the different components of the coral holobiont (coral host, algal photosymbiont, and associated microbiome). Here, using a controlled thermal stress experiment across three divergent Caribbean coral species, we attempt to dissect holobiont member metatranscriptome responses from coral taxa with different sensitivities to heat stress and use phylogenetic ANOVA to study the evolution of gene expression adaptation. We show that coral response to heat stress is a complex trait derived from multiple interactions among holobiont members. We identify host and photosymbiont genes that exhibit lineage-specific expression level adaptation and uncover potential roles for bacterial associates in supplementing the metabolic needs of the coral-photosymbiont duo during heat stress. Our results stress the importance of integrative and comparative approaches across a wide range of species to better understand coral survival under the predicted rise in sea surface temperatures.

Broad scale proteomic analysis of heat-destabilised symbiosis in the hard coral Acropora millepora

Coral reefs across the globe are threatened by warming oceans. The last few years have seen the worst mass coral bleaching events recorded, with more than one quarter of all reefs irreversibly impacted. Considering the widespread devastation, we need to increase our efforts to understanding the physiological and metabolic shifts underlying the breakdown of this important symbiotic ecosystem. Here, we investigated the proteome (PRIDE accession # PXD011668) of both host and symbionts of the reef-building coral Acropora millepora exposed to ambient (~ 28 °C) and elevated temperature (~ 32 °C for 2 days, following a five-day incremental increase) and explored associated biomolecular changes in the symbiont, with the aim of gaining new insights into the mechanisms underpinning the collapse of the coral symbiosis. We identified 1,230 unique proteins (774 host and 456 symbiont) in the control and thermally stressed corals, of which 107 significantly increased and 125 decreased in abundance under elevated temperature relative to the control. Proteins involved in oxidative stress and proteolysis constituted 29% of the host proteins that increased in abundance, with evidence of impairment to endoplasmic reticulum and cytoskeletal regulation proteins. In the symbiont, we detected a decrease in proteins responsible for photosynthesis and energy production (33% of proteins decreased in abundance), yet minimal signs of oxidative stress or proteolysis. Lipid stores increased > twofold despite reduction in photosynthesis, suggesting reduced translocation of carbon to the host. There were significant changes in proteins related to symbiotic state, including proteins linked to nitrogen metabolism in the host and the V-ATPase (-0.6 fold change) known to control symbiosome acidity. These results highlight key differences in host and symbiont proteomic adjustments under elevated temperature and identify two key proteins directly involved in bilateral nutrient exchange as potential indicators of symbiosis breakdown.

Reactive Oxygen Species and the Stress Response in Octocorals

AbstractReactive oxygen species (ROS) may damage cellular components but may also contribute to signaling that mitigates damage. In this context, the role of ROS in the stress response that leads to coral bleaching was investigated in three series of experiments with octocorals Sarcothelia sp. and Sympodium sp. Using video and fluorescent microscopy, the first experiments examined ROS and symbiont migration. Colonies mildly stressed with increased temperature and light showed increases in both ROS and numbers of migrating symbionts compared with stress-free controls. Symbionts migrating in the gastrovascular lumen may escape programmed cell death and provide a reservoir of healthy symbionts once conditions return to normal. In the second series of experiments, colonies were mildly stressed with elevated temperature and light. During stress, treated colonies were incubated in seawater enriched with two concentrations of bicarbonate (1 and 3 mmol/L), while controls were incubated in normal seawater. Bicarbonate enrichment provides additional carbon for photosynthesis and at some concentrations diminished the ROS emissions of stressed colonies of Sympodium sp. and Sarcothelia sp. In all experiments, the latter species tended to exhibit more ROS. Sympodium sp. contains Cladocopium sp. symbionts, which are less tolerant of stress, while Sarcothelia sp. contains the more resistant Durusdinium sp. Indeed, in direct comparisons, Sarcothelia sp. experienced higher levels of ROS under stress-free conditions and thus is conditioned to endure the stress associated with bleaching. Generally, ROS levels provide important insight into the cnidarian stress response and should be measured more often in studies of this response.

Rapid compensatory evolution can rescue low fitness symbioses following partner switching

Partner switching plays an important role in the evolution of symbiosis, enabling local adaptation and recovery from the breakdown of symbiosis. Because of intergenomic epistasis, partner-switched symbioses may possess novel combinations of phenotypes but may also exhibit low fitness due to their lack of recent coevolutionary history. Here, we examine the structure and mechanisms of intergenomic epistasis in the Paramecium-Chlorella symbiosis and test whether compensatory evolution can rescue initially low fitness partner-switched symbioses. Using partner-switch experiments coupled with metabolomics, we show evidence for intergenomic epistasis wherein low fitness is associated with elevated symbiont stress responses either in dark or high irradiance environments, potentially owing to mismatched light management traits between the host and symbiont genotypes. Experimental evolution under high light conditions revealed that an initially low fitness partner-switched non-native host-symbiont pairing rapidly adapted, gaining fitness equivalent to the native host-symbiont pairing in less than 50 host generations. Compensatory evolution took two alternative routes: either hosts evolved higher symbiont loads to mitigate for their new algal symbiont's poor performance, or the algal symbionts themselves evolved higher investment in photosynthesis and photoprotective traits to better mitigate light stress. These findings suggest that partner switching combined with rapid compensatory evolution can enable the recovery and local adaptation of symbioses in response to changing environments.

Symbiosis and the Anthropocene

Recent human activity has profoundly transformed Earth biomes on a scale and at rates that are unprecedented. Given the central role of symbioses in ecosystem processes, functions, and services throughout the Earth biosphere, the impacts of human-driven change on symbioses are critical to understand. Symbioses are not merely collections of organisms, but co-evolved partners that arise from the synergistic combination and action of different genetic programs. They function with varying degrees of permanence and selection as emergent units with substantial potential for combinatorial and evolutionary innovation in both structure and function. Following an articulation of operational definitions of symbiosis and related concepts and characteristics of the Anthropocene, we outline a basic typology of anthropogenic change (AC) and a conceptual framework for how AC might mechanistically impact symbioses with select case examples to highlight our perspective. We discuss surprising connections between symbiosis and the Anthropocene, suggesting ways in which new symbioses could arise due to AC, how symbioses could be agents of ecosystem change, and how symbioses, broadly defined, of humans and “farmed” organisms may have launched the Anthropocene. We conclude with reflections on the robustness of symbioses to AC and our perspective on the importance of symbioses as ecosystem keystones and the need to tackle anthropogenic challenges as wise and humble stewards embedded within the system.

Effect of Cryopreservation on Proteins from the Ubiquitous Marine Dinoflagellate Breviolum sp. (Family Symbiodiniaceae)

Coral reefs around the world are exposed to thermal stress from climate change, disrupting the delicate symbiosis between the coral host and its symbionts. Cryopreservation is an indispensable tool for the preservation of species, as well as the establishment of a gene bank. However, the development of cryopreservation techniques for application to symbiotic algae is limited, in addition to the scarceness of related studies on the molecular level impacts post-thawing. Hence, it is essential to set up a suitable freezing protocol for coral symbionts, as well as to analyze its cryo-injury at the molecular level. The objective of this study was to develop a suitable protocol for the coral symbiont Breviolum subjected to two-step freezing. The thawed Breviolum were then cultured for 3, 7, 14, and 28 days before they were analyzed by Western blot for protein expression, light-harvesting protein (LHP), and red fluorescent protein (RFP) and tested by adenosine triphosphate bioassay for cell viability. The results showed the highest cell viability for thawed Breviolum that was treated with 2 M propylene glycol (PG) and 2 M methanol (MeOH) and equilibrated with both cryoprotectants for 30 min and 20 min. Both treatment groups demonstrated a significant increase in cell population after 28 days of culture post-thawing, especially for the MeOH treatment group, whose growth rate was twice of the PG treatment group. Regarding protein expression, the total amounts of each type of protein were significantly affected by cryopreservation. After 28 days of culture, the protein expression for the MeOH treatment group showed no significant difference to that of the control group, whereas the protein expression for the PG treatment group showed a significant difference. Breviolum that were frozen with MeOH recovered faster upon thawing than those frozen with PG. LHP was positively and RFP was negatively correlated with Symbiodiniaceae viability and so could serve as health-informing biomarkers. This work represents the first time to document it in Symbiodiniaceae, and this study established a suitable protocol for the cryopreservation of Breviolum and further refined the current understanding of the impact of low temperature on its protein expression. By gaining further understanding of the use of cryopreservation as a way to conserve Symbiodiniaceae, we hope to make an effort in the remediation and conservation of the coral reef ecosystem and provide additional methods to rescue coral reefs.

Title: Characterizing environmental stress responses of aposymbiotic Astrangia poculata to divergent thermal challenges

Anthropogenic climate change threatens corals globally and both high and low temperatures are known to induce coral bleaching. However, coral stress responses across wide thermal breadths remain understudied. Disentangling the role of symbiosis on the stress response in obligately symbiotic corals is challenging because this response is inherently coupled with nutritional stress. Here, we leverage aposymbiotic colonies of the facultatively symbiotic coral, Astrangia poculata, which lives naturally with and without its algal symbionts, to examine how broad thermal challenges influence coral hosts in the absence of symbiosis. A. poculata were collected from their northern range limit and thermally challenged in two independent 16-day common garden experiments (heat and cold challenge) and behavioral responses to food stimuli and genome-wide gene expression profiling (TagSeq) were performed. Both thermal challenges elicited significant reductions in polyp extension. However, there were five times as many differentially expressed genes (DEGs) under cold challenge compared to heat challenge. Despite an overall stronger response to cold challenge, there was significant overlap in DEGs between thermal challenges. We contrasted these responses to a previously identified module of genes associated with the environmental stress response (ESR) in tropical reef-building corals. Cold challenged corals exhibited a pattern consistent with more severe stressors while the heat challenge response was consistent with lower intensity stressors. Given that these responses were observed in aposymbiotic colonies, many genes previously implicated in ESRs in tropical symbiotic species may represent the coral host's stress response in or out of symbiosis.

Cloudiness reduces the bleaching response of coral reefs exposed to heat stress

Climate change and warming ocean temperatures are a threat to coral reef ecosystems. Since the 1980s, there has been an increase in mass coral bleaching and associated coral mortality due to more frequent and severe thermal stress. Although most research has focused on the role of temperature, coral bleaching is a product of the interacting effects of temperature and other environmental variables such as solar radiation. High light exacerbates the effects of thermal stress on corals, whereas reductions in light can reduce sensitivity to thermal stress. Here, we use an updated global dataset of coral bleaching observations (n = 35,769) from 1985 to 2017 and satellite-derived datasets of SST and clouds to examine for the first time at a global scale the influence of cloudiness on the likelihood of bleaching from thermal stress. We find that among coral reefs exposed to severe bleaching-level heat stress (Degree Heating Weeks >8°Cˑweek), bleaching severity is inversely correlated with the interaction of heat stress and cloud fraction anomalies (p < 0.05), such that higher cloudiness implies reduced bleaching response. A Random Forest model analysis employing different set of environmental variables shows that a model employing Degree Heating Weeks and the 30-day cloud fraction anomaly most accurately predicts bleaching severity (Accuracy = 0.834; Cohen's Kappa = 0.769). Based on these results and global warm-season cloudiness patterns, we develop a 'cloudy refugia' index which identifies the central equatorial Pacific and French Polynesia as regions where cloudiness is most likely to protect corals from bleaching. Our findings suggest that incorporating cloudiness into prediction models can help delineate bleaching responses and identify reefs which may be more resilient to climate change.

Coral microbiome manipulation elicits metabolic and genetic restructuring to mitigate heat stress and evade mortality

Beneficial microorganisms for corals (BMCs) ameliorate environmental stress, but whether they can prevent mortality and the underlying host response mechanisms remains elusive. Here, we conducted omics analyses on the coral Mussismilia hispida exposed to bleaching conditions in a long-term mesocosm experiment and inoculated with a selected BMC consortium or a saline solution placebo. All corals were affected by heat stress, but the observed "post-heat stress disorder" was mitigated by BMCs, signified by patterns of dimethylsulfoniopropionate degradation, lipid maintenance, and coral host transcriptional reprogramming of cellular restructuration, repair, stress protection, and immune genes, concomitant with a 40% survival rate increase and stable photosynthetic performance by the endosymbiotic algae. This study provides insights into the responses that underlie probiotic host manipulation. We demonstrate that BMCs trigger a dynamic microbiome restructuring process that instigates genetic and metabolic alterations in the coral host that eventually mitigate coral bleaching and mortality.

What is the role of zooxanthellae during coral bleaching? Review of zooxanthellae and their response to environmental stress

Coral reefs are diverse and productive but sensitive ecosystems. Due to the impact of climate change, these organisms are in danger of dying out, mainly through the process of coral bleaching, which is the process by which zooxanthellae (algal endosymbionts) are expelled from their respective coral hosts, causing the coral to lose colour and become white. Coral bleaching has been linked to increases in sea surface temperatures as well as an increase in light intensity. We reviewed the different zooxanthellae taxa and their ecological traits, as well as the information available on the protective mechanisms present in zooxanthellae cells when they experience environmental stress conditions, such as temperature fluctuations, specifically concentrating on heat shock proteins and their response to antioxidant stress. The eight clades (A–H) previously recognised were reorganised into seven existing genera. Different zooxanthellae taxa exhibit different ecological traits such as their photosynthetic stress responses to light and temperature. Zooxanthellae have the ability to regulate the number and type of heat shock proteins (Hsps) they produce during a heat response. They can also regulate the host’s respective Hsps. Antioxidant responses that can prevent coral hosts from expelling the zooxanthellae, can be found both within exposed coral tissue and the zooxanthellae cells. Despite the lower likelihood of bleaching in South African coral reefs, genetic engineering presents a useful tool to understand and adapt traits within zooxanthellae genotypes to help mitigate coral bleaching in the future.

Photosynthesis and respiration of the soft coral Xenia umbellata respond to warming but not to organic carbon eutrophication

Eutrophication with dissolved organic carbon (DOC) as a far under-investigated stressor, and ocean warming, can strongly affect coral reefs and hard corals as major reefs ecosystem engineers. However, no previous studies have investigated the metabolic responses of soft corals to DOC eutrophication, or its interaction with ocean warming. Thus, we investigated respiration and photosynthesis response of Xenia umbellata, a common mixotrophic soft coral from the Indo-pacific, to (1) three levels of DOC eutrophication simulated by glucose addition over the first 21 days of experiment and (2) ocean warming scenarios where the temperature was gradually increased from 26 °C (control condition) to 32 °C over another 24 days in an aquarium experiment. We found no significant difference in response to DOC treatments and all corals survived regardless of the DOC concentrations, whilst subsequent exposure to simulated ocean warming significantly decreased gross photosynthesis by approximately 50% at 30 °C, and 65% at 32 °C, net photosynthesis by 75% at 30 °C and 79% at 32 °C, and respiration by a maximum of 75% at 30 °C; with a slight increase at 32 °C of 25%. The ratio between gross photosynthesis and respiration decreased by the end of the warming period but remained similar between controls and colonies previously exposed to DOC. Our findings suggest that soft corals may be more resistant than hard corals to DOC eutrophication and in consequence, may potentially experiment in less magnitude the negative effects of increased temperature or subsequently both stressors. The results of this study may contribute to explain the successful role of soft corals in phase shifts as reported from many coral reefs. Where predicted declines in reef ecosystems health due to increased eutrophication levels can be exacerbated by future warming.

Functional Characterization of Hexacorallia Phagocytic Cells

Phagocytosis is the cellular defense mechanism used to eliminate antigens derived from dysregulated or damaged cells, and microbial pathogens. Phagocytosis is therefore a pillar of innate immunity, whereby foreign particles are engulfed and degraded in lysolitic vesicles. In hexacorallians, phagocytic mechanisms are poorly understood, though putative anthozoan phagocytic cells (amoebocytes) have been identified histologically. We identify and characterize phagocytes from the coral Pocillopora damicornis and the sea anemone Nematostella vectensis. Using fluorescence-activated cell sorting and microscopy, we show that distinct populations of phagocytic cells engulf bacteria, fungal antigens, and beads. In addition to pathogenic antigens, we show that phagocytic cells engulf self, damaged cells. We show that target antigens localize to low pH phagolysosomes, and that degradation is occurring within them. Inhibiting actin filament rearrangement interferes with efficient particle phagocytosis but does not affect small molecule pinocytosis. We also demonstrate that cellular markers for lysolitic vesicles and reactive oxygen species (ROS) correlate with hexacorallian phagocytes. These results establish a foundation for improving our understanding of hexacorallian immune cell biology.

Coral environmental memory: causes, mechanisms, and consequences for future reefs

The apparent ability of corals to acquire and maintain enhanced stress tolerance through a dose-dependent environmental memory, which may persist for multiple years, has critical implications for coral reef conservation research. Such responses are variable across coral species and environmental stressors, with primed corals exhibiting a modified response to secondary stress exposures. While the mechanisms underlying coral memory responses are poorly understood, they likely involve both the coral host and microbiome. With advances in molecular technologies, it is now possible to investigate potential memory mechanisms in non-model organisms, including transcriptional regulation through epigenetic modifications. We integrate evidence of coral environmental memory and suggest future research directions to evaluate the potential for this process to enhance coral resilience under climate change.

Development of a free radical scavenging bacterial consortium to mitigate oxidative stress in cnidarians

Corals are colonized by symbiotic microorganisms that profoundly influence the animal’s health. One noted symbiont is a single‐celled alga (in the dinoflagellate family Symbiodiniaceae), which provides the coral with most of its fixed carbon. Thermal stress increases the production of reactive oxygen species (ROS) by Symbiodiniaceae during photosynthesis. ROS can both damage the algal symbiont’s photosynthetic machinery and inhibit its repair, causing a positive feedback loop for the toxic accumulation of ROS. If not scavenged by the antioxidant network, excess ROS may trigger a signaling cascade ending with the coral host and algal symbiont disassociating in a process known as bleaching. We use Exaiptasia diaphana as a model for corals and constructed a consortium comprised of E. diaphana–associated bacteria capable of neutralizing ROS. We identified six strains with high free radical scavenging (FRS) ability belonging to the families Alteromonadaceae, Rhodobacteraceae, Flavobacteriaceae and Micrococcaceae. In parallel, we established a consortium of low FRS isolates consisting of genetically related strains. Bacterial whole genome sequences were used to identify key pathways that are known to influence ROS.

The history, biological relevance, and potential applications for polyp bailout in corals

Corals have evolved a variety of stress responses to changing conditions, many of which have been the subject of scientific research. However, polyp bailout has not received widespread scientific attention, despite being described more than 80 years ago. Polyp bailout is a drastic response to acute stress in which coral colonies break down, with individual and patches of polyps detaching from the colony and the calcareous skeleton Polyps retain their symbiotic partners, have dispersal ability, and may undergo secondary settlement and calcification. Polyp bailout has been described worldwide in a variety of anthozoan species, especially in Scleractinia. It can be induced by multiple natural stressors, but also artificially. Little is known about the evolutionary and ecological potential and consequences of breaking down modularity, the dispersal ability, and reattachment of polyps resulting from polyp bailout. It has been shown that polyp bailout can be used as a model system, with promise for implementation in various research topics. To date, there has been no compilation of knowledge on polyp bailout, which prompted us to review this interesting stress response and provide a basis to discuss research topics and priorities for the future.