Early cellular changes are indicators of pre-bleaching thermal stress in the coral host

Thermal stress causes the coral-dinoflagellate symbiosis to disassociate and the coral tissues to whiten. The onset and occurrence of this coral bleaching is primarily defined via the dinoflagellate responses. Here we demonstrate that thermal stress responses occur in the coral host tissues in the days before the onset of coral bleaching. The observed sequence of thermal responses includes reductions in thickness of coral tissue layers and apoptosis of the cells prior to reductions in symbiont density. In the days before the onset of coral bleaching the outer coral tissue layer (epithelium) thickness reduces and apoptosis occurs within the gastrodermis. Two days following this, coinciding with an initial reduction of symbiont density (by approximately 25%), gastrodermal thickness decreased and apoptosis of host cells was identified in the epithelium. This was eventually followed by large reduction in symbiont density (by approximately 50%) consistent with coral bleaching. Both pro-apoptotic and anti-apoptotic genes are identified in the reef building coral Acropora aspera, demonstrating the necessary pathways are present for fine control of host apoptosis. Our study shows that defining periods of host stress based on the responses defined by dinoflagellate symbiont underestimates the importance of early cellular events and the cellular complexity of coral host.     

Thermal Stress Promotes Host Mitochondrial Degradation in Symbiotic Cnidarians: Are the Batteries of the Reef Going to Run Out?

The symbiotic relationship between cnidarians and their dinoflagellate symbionts, Symbiodinium spp, which underpins the formation of tropical coral reefs, can be destabilized by rapid changes to environmental conditions. Although some studies have concluded that a breakdown in the symbiosis begins with increased reactive oxygen species (ROS) generation within the symbiont due to a decoupling of photosynthesis, others have reported the release of viable symbionts via a variety of host cell derived mechanisms. We explored an alternative model focused upon changes in host cnidarian mitochondrial integrity in response to thermal stress. Mitochondria are often likened to being batteries of the cell, providing energy in the form of ATP, and controlling cellular pathway activation and ROS generation. The overall morphology of host mitochondria was compared to that of associated symbionts under an experimental thermal stress using confocal and electron microscopy. The results demonstrate that hyperthermic stress induces the degradation of cnidarian host mitochondria that is independent of symbiont cellular deterioration. The potential sites of host mitochondrial disruption were also assessed by measuring changes in the expression of genes associated with electron transport and ATP synthesis using quantitative RT-PCR. The primary site of degradation appeared to be downstream of complex III of the electron transport chain with a significant reduction in host cytochrome c and ATP synthase expression. The consequences of reduced expression could limit the capacity of the host to mitigate ROS generation and maintain both organelle integrity and cellular energy supplies. The disruption of host mitochondria, cellular homeostasis, and subsequent cell death irrespective of symbiont integrity highlights the importance of the host response to thermal stress and in symbiosis dysfunction that has substantial implications for understanding how coral reefs will survive in the face of climate change.     

Investigating the causes and consequences of symbiont shuffling in a multi-partner reef coral symbiosis under environmental change

Dynamic symbioses may critically mediate impacts of climate change on diverse organisms, with repercussions for ecosystem persistence in some cases. On coral reefs, increases in heat-tolerant symbionts after thermal bleaching can reduce coral susceptibility to future stress. However, the relevance of this adaptive response is equivocal owing to conflicting reports of symbiont stability and change. We help reconcile this conflict by showing that change in symbiont community composition (symbiont shuffling) in Orbicella faveolata depends on the disturbance severity and recovery environment. The proportion of heat-tolerant symbionts dramatically increased following severe experimental bleaching, especially in a warmer recovery environment, but tended to decrease if bleaching was less severe. These patterns can be explained by variation in symbiont performance in the changing microenvironments created by differentially bleached host tissues. Furthermore, higher proportions of heat-tolerant symbionts linearly increased bleaching resistance but reduced photochemical efficiency, suggesting that any change in community structure oppositely impacts performance and stress tolerance. Therefore, even minor symbiont shuffling can adaptively benefit corals, although fitness effects of resulting trade-offs are difficult to predict. This work helps elucidate causes and consequences of dynamism in symbiosis, which is critical to predicting responses of multi-partner symbioses such as O. faveolata to environmental change.     

Metabolite profiling of symbiont and host during thermal stress and bleaching in the coral <Emphasis Type="Italic">Acropora aspera</Emphasis>

Rising seawater temperatures pose a significant threat to the persistence of coral reefs. Despite the importance of these systems, major gaps remain in our understanding of how thermal stress and bleaching affect the metabolic networks that underpin holobiont function. We applied gas chromatography–mass spectrometry (GC–MS) metabolomics to detect changes in the intracellular free metabolite pools (polar and semi-polar compounds) of in hospite dinoflagellate symbionts and their coral hosts (and any associated microorganisms) during early- and late-stage thermal bleaching (a reduction of approximately 50 and 70% in symbiont density, respectively). We detected characteristic changes to the metabolite profiles of each symbiotic partner associated with individual cellular responses to thermal, oxidative and osmotic stress, which progressed with the severity of bleaching. Alterations were also indicative of changes to energy-generating and biosynthesis pathways in both partners, with a shift to the increased catabolism of lipid stores. Specifically, in symbiont intracellular metabolite pools, we observed accumulations of multiple free fatty acids, plus the chloroplast-associated antioxidant alpha-tocopherol. In the host, we detected a decline in the abundance of pools of multiple carbohydrates, amino acids and intermediates, in addition to the antioxidant ascorbate. These findings further our understanding of the metabolic changes that occur to symbiont and host (and its associated microorganisms) during thermal bleaching. These findings also provide further insight into the largely undescribed roles of free metabolite pools in cellular homeostasis, signalling and acclimation to thermal stress in the cnidarian–dinoflagellate symbiosis.     

Estimating the Potential for Adaptation of Corals to Climate Warming

The persistence of tropical coral reefs is threatened by rapidly increasing climate warming, causing a functional breakdown of the obligate symbiosis between corals and their algal photosymbionts (Symbiodinium) through a process known as coral bleaching. Yet the potential of the coral-algal symbiosis to genetically adapt in an evolutionary sense to warming oceans is unknown. Using a quantitative genetics approach, we estimated the proportion of the variance in thermal tolerance traits that has a genetic basis (i.e. heritability) as a proxy for their adaptive potential in the widespread Indo-Pacific reef-building coral Acropora millepora. We chose two physiologically different populations that associate respectively with one thermo-tolerant (Symbiodinium clade D) and one less tolerant symbiont type (Symbiodinium C2). In both symbiont types, pulse amplitude modulated (PAM) fluorometry and high performance liquid chromatography (HPLC) analysis revealed significant heritabilities for traits related to both photosynthesis and photoprotective pigment profile. However, quantitative real-time polymerase chain reaction (qRT-PCR) assays showed a lack of heritability in both coral host populations for their own expression of fundamental stress genes. Coral colony growth, contributed to by both symbiotic partners, displayed heritability. High heritabilities for functional key traits of algal symbionts, along with their short clonal generation time and high population sizes allow for their rapid thermal adaptation. However, the low overall heritability of coral host traits, along with the corals'' long generation time, raise concern about the timely adaptation of the coral-algal symbiosis in the face of continued rapid climate warming.     

Apoptosis and autophagy as mechanisms of dinoflagellate symbiont release during cnidarian bleaching: every which way you lose

Cnidarian bleaching results from the breakdown in the symbiosis between the host cnidarian and its dinoflagellate symbiont. Coral bleaching in recent years has increasingly caused degradation and mortality of coral reefs on a global scale. Although much is understood about the environmental causes of bleaching, the underlying cellular mechanisms of symbiont release that drive the process are just beginning to be described. In this study, we investigated the roles of two cellular pathways, host cell apoptosis and autophagy, in the bleaching process of the symbiotic anemone Aiptasia pallida. Host cell apoptosis was experimentally manipulated using gene knockdown of an anemone caspase by RNA interference, chemical inhibition of caspase using ZVAD-fmk and an apoptosis-inducer wortmannin. Autophagy was manipulated by chemical inhibition using wortmannin or induction using rapamycin. The applications of multiple single treatments resulted in some increased bleaching in anemones under control conditions but no significant drop in bleaching in individuals subjected to a hyperthermic stress. These results indicated that no single pathway is responsible for symbiont release during bleaching. However, when multiple inhibitors were applied simultaneously to block both apoptosis and autophagy, there was a significant reduction in bleaching in heat-stressed anemones. Our results allow us to formulate a model for cellular processes involved in the control of cnidarian bleaching where apoptosis and autophagy act together in a see-saw mechanism such that if one is inhibited the other is induced. Similar interconnectivity between apoptosis and autophagy has previously been shown in vertebrates including involvement in an innate immune response to pathogens and parasites. This suggests that the bleaching response could be a modified immune response that recognizes and removes dysfunctional symbionts.     

<Superscript>13</Superscript>C metabolomics reveals widespread change in carbon fate during coral bleaching

Introduction

Rising seawater temperatures are threatening the persistence of coral reefs; where above critical thresholds, thermal stress results in a breakdown of the coral-dinoflagellate symbiosis and the loss of algal symbionts (coral bleaching). As symbiont-derived organic products typically form a major portion of host energy budgets, this has major implications for the fitness and persistence of symbiotic corals.

Objectives

We aimed to determine change in autotrophic carbon fate within individual compounds and downstream metabolic pathways in a coral symbiosis exposed to varying degrees of thermal stress and bleaching.

Methods

We applied gas chromatography–mass spectrometry coupled to a stable isotope tracer (¹³C), to track change in autotrophic carbon fate, in symbiont and host individually, following exposure to elevated water temperature.

Results

Thermal stress resulted in partner-specific changes in carbon fate, which progressed with heat stress duration. We detected modifications to carbohydrate and fatty acid metabolism, lipogenesis, and homeostatic responses to thermal, oxidative and osmotic stress. Despite pronounced photodamage, remaining in hospite symbionts continued to produce organic products de novo and translocate to the coral host. However as bleaching progressed, we observed minimal ¹³C enrichment of symbiont long-chain fatty acids, also reflected in ¹³C enrichment of host fatty acid pools.

Conclusion

These data have major implications for our understanding of coral symbiosis function during bleaching. Our findings suggest that during early stage bleaching, remaining symbionts continue to effectively translocate a variety of organic products to the host, however under prolonged thermal stress there is likely a reduction in the quality of these products.

     

Patterns of Gene Expression in a Scleractinian Coral Undergoing Natural Bleaching

Coral bleaching is a major threat to coral reefs worldwide and is predicted to intensify with increasing global temperature. This study represents the first investigation of gene expression in an Indo-Pacific coral species undergoing natural bleaching which involved the loss of algal symbionts. Quantitative real-time polymerase chain reaction experiments were conducted to select and evaluate coral internal control genes (ICGs), and to investigate selected coral genes of interest (GOIs) for changes in gene expression in nine colonies of the scleractinian coral Acropora millepora undergoing bleaching at Magnetic Island, Great Barrier Reef, Australia. Among the six ICGs tested, glyceraldehyde 3-phosphate dehydrogenase and the ribosomal protein genes S7 and L9 exhibited the most constant expression levels between samples from healthy-looking colonies and samples from the same colonies when severely bleached a year later. These ICGs were therefore utilised for normalisation of expression data for seven selected GOIs. Of the seven GOIs, homologues of catalase, C-type lectin and chromoprotein genes were significantly up-regulated as a result of bleaching by factors of 1.81, 1.46 and 1.61 (linear mixed models analysis of variance, P < 0.05), respectively. We present these genes as potential coral bleaching response genes. In contrast, three genes, including one putative ICG, showed highly variable levels of expression between coral colonies. Potential variation in microhabitat, gene function unrelated to the stress response and individualised stress responses may influence such differences between colonies and need to be better understood when designing and interpreting future studies of gene expression in natural coral populations.     

Host Coenzyme Q Redox State Is an Early Biomarker of Thermal Stress in the Coral Acropora millepora

Bleaching episodes caused by increasing seawater temperatures may induce mass coral mortality and are regarded as one of the biggest threats to coral reef ecosystems worldwide. The current consensus is that this phenomenon results from enhanced production of harmful reactive oxygen species (ROS) that disrupt the symbiosis between corals and their endosymbiotic dinoflagellates, Symbiodinium. Here, the responses of two important antioxidant defence components, the host coenzyme Q (CoQ) and symbiont plastoquinone (PQ) pools, are investigated for the first time in colonies of the scleractinian coral, Acropora millepora, during experimentally-induced bleaching under ecologically relevant conditions. Liquid chromatography-mass spectrometry (LC-MS) was used to quantify the states of these two pools, together with physiological parameters assessing the general state of the symbiosis (including photosystem II photochemical efficiency, chlorophyll concentration and Symbiodinium cell densities). The results show that the responses of the two antioxidant systems occur on different timescales: (i) the redox state of the Symbiodinium PQ pool remained stable until twelve days into the experiment, after which there was an abrupt oxidative shift; (ii) by contrast, an oxidative shift of approximately 10% had occurred in the host CoQ pool after 6 days of thermal stress, prior to significant changes in any other physiological parameter measured. Host CoQ pool oxidation is thus an early biomarker of thermal stress in corals, and this antioxidant pool is likely to play a key role in quenching thermally-induced ROS in the coral-algal symbiosis. This study adds to a growing body of work that indicates host cellular responses may precede the bleaching process and symbiont dysfunction.     

Differential nitric oxide synthesis and host apoptotic events correlate with bleaching susceptibility in reef corals

Coral bleaching poses a threat to coral reefs worldwide. As a consequence of the temperature-induced breakdown in coral–dinoflagellate symbiosis, bleaching can have extensive effects on reef communities. However, our understanding of bleaching at a cellular level is limited, and this is particularly true regarding differential susceptibility among coral species. Recent work suggests that bleaching may represent a host innate immune-like response to symbiont dysfunction that involves synthesis of the signalling compound nitric oxide (NO) and the induction of host apoptotic-like cell death. In this study, we examined the activity of apoptosis-regulating enzymes alongside oxidised NO accumulation (a proxy for NO synthesis) in the reef corals Acropora millepora, Montipora digitata, and Pocillopora damicornis during experimental thermal stress. P. damicornis was the most sensitive species, suffering mortality (tissue sloughing) after 5 days at 33 °C but non-lethal bleaching after 9 days at 31.5 °C. A. millepora bleached at 33 °C but remained structurally intact, while M. digitata showed little evidence of bleaching. P. damicornis and A. millepora both exhibited evidence of temperature-induced NO synthesis and, after 5 days of heating, levels of oxidised NO in both species were fivefold higher than in controls maintained at 28.5 °C. These responses preceded bleaching by a number of days and may have occurred before symbiont dysfunction (measured as chlorophyll a degradation and oxidised NO accumulation). In A. millepora, apparent NO synthesis correlated with the induction of host apoptotic-like pathways, while in P. damicornis, the upregulation of apoptotic pathways occurred later. No evidence of elevated NO production or apoptosis was observed in M. digitata at 33 °C and baseline activity of apoptosis-regulating enzymes was negligible in this species. These findings provide important physiological data in the context of the responses of corals to global change and suggest that early events in the host may be important in the collapse of the coral–dinoflagellate symbiosis.     

Are Niemann‐Pick type C proteins key players in cnidarian–dinoflagellate endosymbioses?

The symbiotic interaction between cnidarians, such as corals and sea anemones, and the unicellular algae Symbiodinium is regulated by yet poorly understood cellular mechanisms, despite the ecological importance of coral reefs. These mechanisms, including host–symbiont recognition and metabolic exchange, control symbiosis stability under normal conditions, but also lead to symbiosis breakdown (bleaching) during stress. This study describes the repertoire of the sterol‐trafficking proteins Niemann‐Pick type C (NPC1 and NPC2) in the symbiotic sea anemone Anemonia viridis. We found one NPC1 gene in contrast to the two genes (NPC1 and NPC1L1) present in vertebrate genomes. While only one NPC2 gene is present in many metazoans, this gene has been duplicated in cnidarians, and we detected four NPC2 genes in A. viridis. However, only one gene (AvNPC2‐d) was upregulated in symbiotic relative to aposymbiotic sea anemones and displayed higher expression in the gastrodermis (symbiont‐containing tissue) than in the epidermis. We performed immunolabelling experiments on tentacle cross sections and demonstrated that the AvNPC2‐d protein was closely associated with symbiosomes. In addition, AvNPC1 and AvNPC2‐d gene expression was strongly downregulated during stress. These data suggest that AvNPC2‐d is involved in both the stability and dysfunction of cnidarian–dinoflagellate symbioses.     

Resistance to thermal stress in corals without changes in symbiont composition

Discovering how corals can adjust their thermal sensitivity in the context of global climate change is important in understanding the long-term persistence of coral reefs. In this study, we showed that short-term preconditioning to higher temperatures, 3°C below the experimentally determined bleaching threshold, for a period of 10 days provides thermal tolerance for the symbiosis stability between the scleractinian coral, Acropora millepora and Symbiodinium. Based on genotypic analysis, our results indicate that the acclimatization of this coral species to thermal stress does not come down to simple changes in Symbiodinium and/or the bacterial communities that associate with reef-building corals. This suggests that the physiological plasticity of the host and/or symbiotic components appears to play an important role in responding to ocean warming. The further study of host and symbiont physiology, both of Symbiodinium and prokaryotes, is of paramount importance in the context of global climate change, as mechanisms for rapid holobiont acclimatization will become increasingly important to the long-standing persistence of coral reefs.     

Different responses of scleractinian coral Acropora pruinosa from Weizhou Island during extreme high temperature events

Ecological surveys observe coral “winners” and “losers” in global coral bleaching events. However, the key contributors to holobiont tolerance and interactions between symbionts remain unclear. Herein, we compared bleaching and unbleaching Acropora pruinosa corals from Weizhou Island, during an extreme high-temperature event in the northern South China Sea in 2020. We found the dominant Symbiodiniaceae subclade in the bleaching and unbleaching corals to be C1; however, the density of Symbiodiniaceae in the latter was significantly higher than that in the former. Additionally, the symbiotic bacteria α diversity in the unbleaching coral was significantly higher than that in the bleaching coral, with a reorganized bacterial community structure. Core microbiome analyses revealed 55 bacterial core operational taxonomic units (OTUs), of which 10 were significantly differentially enriched between the two coral groups. The significantly enriched bacterial core OTUs in the unbleaching coral were primarily nitrogen cycling related, while those enriched in the bleaching coral were associated with antimicrobial activity. RNA-Seq analyses revealed that significantly upregulated genes in the bleaching coral were primarily associated with diseases and autophagy, while those in the unbleaching coral were associated with immune defense and maintenance of the symbiotic relationship between corals and symbionts. We propose that the differences in tolerance of A. pruinosa result from the cooperation between coral host, Symbiodiniaceae, and symbiotic bacteria. In extreme high-temperature events, unbleaching corals may maintain stable symbiotic relationships by increasing the diversity of symbiotic bacteria, regulating the structure of the symbiotic bacteria community, improving the interaction between coral host and symbiont and enhancing host immunity, thus avoiding coral bleaching. This study illuminates the relationship between the coral symbiont and tolerance differences of coral holobionts, providing new insights for further exploration into the adaptability of scleractinian corals in the context of global warming.

     

Symbiophagy as a cellular mechanism for coral bleaching

Coral bleaching is a major contributor to the global declines of coral reefs. This phenomenon is characterized by the loss of symbiotic algae, their pigments or both. Despite wide scientific interest, the mechanisms by which bleaching occurs is still poorly understood. Here we report that the removal of the symbiont during light and temperature stress is achieved using the host's cellular autophagic-associated machinery. Host cellular and sub-cellular morphologies showed increased vacuolization and appearance of autophagic membranes surrounding a variety of organelles and surrounding the symbiotic algae. Markers of autophagy (Rab 7 and LAS) corroborate these observations. Results showed that during stress the symbiont vacuolar membrane is transformed from a conduit of nutrient exchange to a digestive organelle resulting in the consumption of the symbiont, a process we term symbiophagy. We posit that during a stress event, the mechanism maintaining symbiosis is destabilized and symbiophagy is activated, ultimately resulting in the phenomenon of bleaching. Symbiophagy may have evolved from a more general primordial innate intracellular protective pathway termed xenophagy.     

Bleaching and stress in coral reef ecosystems: hsp70 expression by the giant barrel sponge Xestospongia muta

Sponges are a prominent component of coral reef ecosystems. Like reef-building corals, some sponges have been reported to bleach and die. The giant barrel sponge Xestospongia muta is one of the largest and most important components of Caribbean coral reef communities. Tissues of X. muta contain cyanobacterial symbionts of the Synechococcus group. Two types of bleaching have been described: cyclic bleaching, from which sponges recover, and fatal bleaching, which usually results in sponge death. We quantified hsp70 gene expression as an indicator of stress in X. muta undergoing cyclic and fatal bleaching and in response to thermal and salinity variability in both field and laboratory settings. Chlorophyll a content of sponge tissue was estimated to determine whether hsp70 expression was related to cyanobacterial abundance. We found that fatally bleached sponge tissue presented significantly higher hsp70 gene expression, but cyclically bleached tissue did not, yet both cyclic and fatally bleached tissues had lower chlorophyll a concentrations than nonbleached tissue. These results corroborate field observations suggesting that cyclic bleaching is a temporary, nonstressful state, while fatal bleaching causes significant levels of stress, leading to mortality. Our results support the hypothesis that Synechococcus symbionts are commensals that provide no clear advantage to their sponge host. In laboratory experiments, sponge pieces incubated at 30 °C exhibited significantly higher hsp70 expression than control pieces after 1.5 h, with sponge mortality after less than 15 h. In contrast, sponges at different salinities were not significantly stressed after the same period of time. Stress associated with increasing seawater temperatures may result in declining sponge populations in coral reef ecosystems.     

Major cellular and physiological impacts of ocean acidification on a reef building coral

As atmospheric levels of CO(2) increase, reef-building corals are under greater stress from both increased sea surface temperatures and declining sea water pH. To date, most studies have focused on either coral bleaching due to warming oceans or declining calcification due to decreasing oceanic carbonate ion concentrations. Here, through the use of physiology measurements and cDNA microarrays, we show that changes in pH and ocean chemistry consistent with two scenarios put forward by the Intergovernmental Panel on Climate Change (IPCC) drive major changes in gene expression, respiration, photosynthesis and symbiosis of the coral, Acropora millepora, before affects on biomineralisation are apparent at the phenotype level. Under high CO(2) conditions corals at the phenotype level lost over half their Symbiodinium populations, and had a decrease in both photosynthesis and respiration. Changes in gene expression were consistent with metabolic suppression, an increase in oxidative stress, apoptosis and symbiont loss. Other expression patterns demonstrate upregulation of membrane transporters, as well as the regulation of genes involved in membrane cytoskeletal interactions and cytoskeletal remodeling. These widespread changes in gene expression emphasize the need to expand future studies of ocean acidification to include a wider spectrum of cellular processes, many of which may occur before impacts on calcification.