Comparative immune responses of corals to stressors associated with offshore reef-based tourist platforms

Structural and Evolutionary Relationships of Melanin Cascade Proteins in Cnidarian Innate Immunity

Melanin is an essential product that plays an important role in innate immunity in a variety of organisms across the animal kingdom. Melanin synthesis is performed by many organisms using the tyrosine metabolism pathway, a general pathway that utilizes a type-three copper oxidase protein, called PO-candidates (phenoloxidase candidates). While melanin synthesis is well-characterized in organisms like arthropods and humans, it is not as well-understood in non-model organisms such as cnidarians. With the rising anthropomorphic climate change influence on marine ecosystems, cnidarians, specifically corals, are under an increased threat of bleaching and disease. Understanding innate immune pathways, such as melanin synthesis, is vital for gaining insights into how corals may be able to fight these threats. In this study, we use comparative bioinformatic approaches to provide a comprehensive analysis of genes involved in tyrosine-mediated melanin synthesis in cnidarians. Eighteen PO-candidates representing five phyla were studied to identify their evolutionary relationship. Cnidarian species were most similar to chordates due to domain presents in the amino acid sequences. From there, functionally conserved domains in coral proteins were identified in a coral disease dataset. Five stony corals exposed to stony coral tissue loss disease were leveraged to identify 18 putative tyrosine metabolism genes, genes with functionally conserved domains to their Homo sapiens counterpart. To put this pathway in the context of coral health, putative genes were correlated to melanin concentration from tissues of stony coral species in the disease exposure dataset. In this study, tyrosinase was identified in stony corals as correlated to melanin concentrations and likely plays a key role in immunity as a resistance trait. In addition, stony coral genes were assigned to all modules within the tyrosine metabolism pathway, indicating an evolutionary conservation of this pathway across phyla. Overall, this study provides a comprehensive analysis of the genes involved in tyrosine-mediated melanin synthesis in cnidarians.

Photosymbiont Density Is Correlated with Constitutive and Induced Immunity in the Facultatively Symbiotic Coral, Astrangia poculata

Scleractinian corals, essential ecosystem engineers that form the base of coral reef ecosystems, have faced unprecedented mortality in recent decades due to climate change-related stressors, including disease outbreaks. Despite this emergent threat to corals, many questions still remain regarding mechanisms underlying observed variation in disease susceptibility. Recent data suggest at least some degree of variation in disease response may be linked to variability in the relationship between host corals and their algal photosymbionts (Family Symbiodiniaceae). Still, the nuances of connections between symbiosis and immunity in cnidarians, including scleractinian corals, remain poorly understood. Here, we leveraged an emergent model species, the facultatively symbiotic, temperate, scleractinian coral Astrangia poculata, to investigate associations between symbiont density and both constitutive and induced immunity. We used a combination of controlled immune challenges with heat-inactivated pathogens and transcriptomic analyses. Our results demonstrate that A. poculata mounts a robust initial response to pathogenic stimuli that is highly similar to responses documented in tropical corals. We document positive associations between symbiont density and both constitutive and induced immune responses, in agreement with recent preliminary studies in A. poculata. A suite of immune genes, including those coding for antioxidant peroxiredoxin biosynthesis, are positively associated with symbiont density in A. poculata under constitutive conditions. Furthermore, variation in symbiont density is associated with distinct patterns of immune response; low symbiont density corals induce preventative immune mechanisms, whereas high symbiont density corals mobilize energetic resources to fuel humoral immune responses. In summary, our study reveals the need for more nuanced study of symbiosis-immune interplay across diverse scleractinian corals, preferably including quantitative energy budget analysis for full disentanglement of these complex associations and their effects on host pathogen susceptibility.

Characterization of the microbiome and immune response in corals with chronic Montipora white syndrome

Coral diseases have increased in frequency and intensity around the tropics worldwide. However, in many cases, little is known about their etiology. Montipora white syndrome (MWS) is a common disease affecting the coral Montipora capitata, a major reef builder in Hawai'i. Chronic Montipora white syndrome (cMWS) is a slow-moving form of the disease that affects M. capitata throughout the year. The effects of this chronic disease on coral immunology and microbiology are currently unknown. In this study, we use prophenoloxidase immune assays and 16S rRNA gene amplicon sequencing to characterize the microbiome and immunological response associated with cMWS. Our results show that immunological and microbiological responses are highly localized. Relative to diseased samples, apparently healthy portions of cMWS corals differed in immune activity and in the relative abundance of microbial taxa. Coral tissues with cMWS showed decreased tyrosinase-type catecholase and tyrosinase-type cresolase activity and increased laccase-type activity. Catecholase and cresolase activity were negatively correlated across all tissue types with microbiome richness. The localized effect of cMWS on coral microbiology and immunology is probably an important reason for the slow progression of the disease. This local confinement may facilitate interventions that focus on localized treatments on tissue types. This study provides an important baseline to understand the interplay between the microbiome and immune system and the mechanisms used by corals to manage chronic microbial perturbations associated with white syndrome.

Innate immune gene expression in Acropora palmata is consistent despite variance in yearly disease events

Coral disease outbreaks are expected to increase in prevalence, frequency and severity due to climate change and other anthropogenic stressors. This is especially worrying for the Caribbean branching coral Acropora palmata which has already seen an 80% decrease in cover primarily due to disease. Despite the importance of this keystone species, there has yet to be a characterization of its transcriptomic response to disease exposure. In this study we provide the first transcriptomic analysis of 12 A. palmata genotypes and their symbiont Symbiodiniaceae exposed to disease in 2016 and 2017. Year was the primary driver of gene expression variance for A. palmata and the Symbiodiniaceae. We hypothesize that lower expression of ribosomal genes in the coral, and higher expression of transmembrane ion transport genes in the Symbiodiniaceae indicate that a compensation or dysbiosis may be occurring between host and symbiont. Disease response was the second driver of gene expression variance for A. palmata and included a core set of 422 genes that were significantly differentially expressed. Of these, 2 genes (a predicted cyclin-dependent kinase 11b and aspartate 1-decarboxylase) showed negative Log2 fold changes in corals showing transmission of disease, and positive Log2 fold changes in corals showing no transmission of disease, indicating that these may be important in disease resistance. Co-expression analysis identified two modules positively correlated to disease exposure, one enriched for lipid biosynthesis genes, and the other enriched in innate immune genes. The hub gene in the immune module was identified as D-amino acid oxidase, a gene implicated in phagocytosis and microbiome homeostasis. The role of D-amino acid oxidase in coral immunity has not been characterized but could be an important enzyme for responding to disease. Our results indicate that A. palmata mounts a core immune response to disease exposure despite differences in the disease type and virulence between 2016 and 2017. These identified genes may be important for future biomarker development in this Caribbean keystone species.

Examining the Effect of Heat Stress on Montastraea cavernosa (Linnaeus 1767) from a Mesophotic Coral Ecosystem (MCE)

Coral reefs are under increasing pressure from global warming. Little knowledge, however, exists regarding heat induced stress on deeper mesophotic coral ecosystems (MCEs). Here, we examined the effect of acute (72 h) and chronic (480 h) heat stress on the host coral Montastraea cavernosa (Linnaeus 1767) collected from an upper MCE (~30 m) in Florida, USA. We examined six immune/stress-related genes: ribosomal protein L9 (RpL9), ribosomal protein S7 (RpS7), B-cell lymphoma 2 apoptosis regulator (BCL-2), heat shock protein 90 (HSP90), catalase, and cathepsin L1, as a proxy for coral response to heat stress. Quantitative real-time polymerase chain reaction (qRT-PCR) was performed to evaluate the gene expression. Overall, both acute and chronic heat stress treatments elicited a response in gene expression relative to control samples. Acute heat exposure resulted in up-regulation of catalase, BCL-2, and HSP90 at all time points from hour 24 to 48, suggesting the activation of an oxidative protective enzyme, molecular chaperone, and anti-apoptotic protein. Fewer genes were up-regulated in the chronic experiment until hour 288 (30 °C) where catalase, RpL9, and RpS7 were significantly up-regulated. Chronic heat exposure elicited a physiological response at 30 °C, which we propose as a heat-stress threshold for Montastraea cavernosa (M. cavernosa) collected from an MCE.

Reduced diversity and stability of coral-associated bacterial communities and suppressed immune function precedes disease onset in corals

Disease is an emerging threat to coral reef ecosystems worldwide, highlighting the need to understand how environmental conditions interact with coral immune function and associated microbial communities to affect holobiont health. Increased coral disease incidence on reefs adjacent to permanently moored platforms on Australia's Great Barrier Reef provided a unique case study to investigate environment-host-microbe interactions in situ. Here, we evaluate coral-associated bacterial community (16S rRNA amplicon sequencing), immune function (protein-based prophenoloxidase-activating system), and water quality parameters before, during and after a disease event. Over the course of the study, 31% of tagged colonies adjacent to platforms developed signs of white syndrome (WS), while all control colonies on a platform-free reef remained visually healthy. Corals adjacent to platforms experienced significant reductions in coral immune function. Additionally, the corals at platform sites that remained visually healthy throughout the study had reduced bacterial diversity compared to healthy colonies at the platform-free site. Interestingly, prior to the observation of macroscopic disease, corals that would develop WS had reduced bacterial diversity and significantly greater community heterogeneity between colonies compared to healthy corals at the same location. These results suggest that activities associated with offshore marine infrastructure impacts coral immunocompetence and associated bacterial community, which affects the susceptibility of corals to disease.

Transcriptional response of the heat shock gene hsp70 aligns with differences in stress susceptibility of shallow-water corals from the Mediterranean Sea

Shallow-water corals of the Mediterranean Sea are facing a dramatic increase in water temperature due to climate change, predicted to increase the frequency of bleaching and mass mortality events. However, supposedly not all corals are affected equally, as they show differences in stress susceptibility, as suggested by physiological outputs of corals along temperature gradients and under controlled conditions in terms of reproduction, demography, growth, calcification, and photosynthetic efficiency. In this study, gene expression and induction of a 70-kDa heat shock protein (HSP70) was analyzed in five common shallow-water hard corals in the Mediterranean Sea, namely Astroides calycularis, Balanophyllia europaea, Caryophyllia inornata, Cladocora caespitosa, and Leptopsammia pruvoti. The main aim was to assess the contribution of this evolutionary conserved cytoprotective mechanism to the physiological plasticity of these species that possess different growth modes (solitary vs colonial) and trophic strategies (zooxanthellate vs azooxanthellate). Using quantitative real-time PCR, in situ hsp70 baseline levels and expression profiles after a heat-shock exposure were assessed. Levels of hsp70 and heat stress induction were higher in zooxanthellate than in azooxanthellate species, and different heat stress transcriptional profiles were observed between colonial and solitary zooxanthellate corals. On the whole, the hsp70 transcriptional response to heat stress aligns with stress susceptibility of the species and suggests a contribution of trophic strategy and morphology in shaping coral resilience to stress. Understanding these molecular processes may contribute to assess the potential effects and relative resilience of Mediterranean corals under climate change.

Coral tumor-like growth anomalies induce an immune response and reduce fecundity

Coral growth anomalies (GAs) are chronic diseases that adversely affect organism health and fitness. We investigated immunity and fecundity within and among GA-affected and visually healthy control colonies of the reef-building coral Acropora hyacinthus. Compared to controls, GAs had higher activity of the key immunity enzyme phenoloxidase (PO), suggesting a localised immune response within the GA. Both GAs and healthy tissue of GA-affected colonies had significantly greater total potential PO (tpPO)-PO activity inclusive of the activated latent PO, prophenoloxidase-than control colonies. Higher tpPO activity in GA-affected corals suggests elevated constitutive immunity compared to visually healthy controls. Additionally, fewer GA-affected colonies produced gametes, fewer polyps had oocytes (p < 0.001) and the number of oocytes per polyp was lower. Therefore, GAs in A. hyacinthus might induce, or represent a shift in resource investment towards immunity and away from reproduction. While the effect on population growth is likely to be small, reduced fecundity in GA-affected colonies does suggest a selective pressure against GAs.

Cnidarian Interaction with Microbial Communities: From Aid to Animal's Health to Rejection Responses

The phylum Cnidaria is an ancient branch in the tree of metazoans. Several species exert a remarkable longevity, suggesting the existence of a developed and consistent defense mechanism of the innate immunity capable to overcome the potential repeated exposure to microbial pathogenic agents. Increasing evidence indicates that the innate immune system in Cnidarians is not only involved in the disruption of harmful microorganisms, but also is crucial in structuring tissue-associated microbial communities that are essential components of the Cnidarian holobiont and useful to the animal's health for several functions, including metabolism, immune defense, development, and behavior. Sometimes, the shifts in the normal microbiota may be used as "early" bio-indicators of both environmental changes and/or animal disease. Here the Cnidarians relationships with microbial communities and the potential biotechnological applications are summarized and discussed.

Antimicrobial and stress responses to increased temperature and bacterial pathogen challenge in the holobiont of a reef-building coral

Global increases in coral disease prevalence have been linked to ocean warming through changes in coral-associated bacterial communities, pathogen virulence and immune system function. However, the interactive effects of temperature and pathogens on the coral holobiont are poorly understood. Here, we assessed three compartments of the holobiont (host, Symbiodinium and bacterial community) of the coral Montipora aequituberculata challenged with the pathogen Vibrio coralliilyticus and the commensal bacterium Oceanospirillales sp. under ambient (27°C) and elevated (29.5 and 32°C) seawater temperatures. Few visual signs of bleaching and disease development were apparent in any of the treatments, but responses were detected in the holobiont compartments. V. coralliilyticus acted synergistically and negatively impacted the photochemical efficiency of Symbiodinium at 32°C, while Oceanospirillales had no significant effect on photosynthetic efficiency. The coral, however, exhibited a minor response to the bacterial challenges, with the response towards V. coralliilyticus being significantly more pronounced, and involving the prophenoloxidase-activating system and multiple immune system-related genes. Elevated seawater temperatures did not induce shifts in the coral-associated bacterial community, but caused significant gene expression modulation in both Symbiodinium and the coral host. While Symbiodinium exhibited an antiviral response and upregulated stress response genes, M. aequituberculata showed regulation of genes involved in stress and innate immune response processes, including immune and cytokine receptor signalling, the complement system, immune cell activation and phagocytosis, as well as molecular chaperones. These observations show that M. aequituberculata is capable of maintaining a stable bacterial community under elevated seawater temperatures and thereby contributes to preventing disease development.

Seasonal Stability in the Microbiomes of Temperate Gorgonians and the Red Coral Corallium rubrum Across the Mediterranean Sea

Populations of key benthic habitat-forming octocoral species have declined significantly in the Mediterranean Sea due to mass mortality events caused by microbial disease outbreaks linked to high summer seawater temperatures. Recently, we showed that the microbial communities of these octocorals are relatively structured; however, our knowledge on the seasonal dynamics of these microbiomes is still limited. To investigate their seasonal stability, we collected four soft gorgonian species (Eunicella singularis, Eunicella cavolini, Eunicella verrucosa and Leptogorgia sarmentosa) and the precious red coral (Corallium rubrum) from two coastal locations with different terrestrial impact levels in the Mediterranean Sea, and used next-generation amplicon sequencing of the 16S rRNA gene. The microbiomes of all soft gorgonian species were dominated by the same 'core microbiome' bacteria belonging to the Endozoicomonas and the Cellvibrionales clade BD1-7, whereas the red coral microbiome was primarily composed of 'core' Spirochaetes, Oceanospirillales ME2 and Parcubacteria. The associations with these bacterial taxa were relatively consistent over time at each location for each octocoral species. However, differences in microbiome composition and seasonal dynamics were observed between locations and could primarily be attributed to locally variant bacteria. Overall, our data provide further evidence of the intricate symbiotic relationships that exist between Mediterranean octocorals and their associated microbes, which are ancient and highly conserved over both space and time, and suggest regulation of the microbiome composition by the host, depending on local conditions.

Benthic community structure on coral reefs exposed to intensive recreational snorkeling

Chronic anthropogenic disturbances on coral reefs in the form of overfishing and pollution can shift benthic community composition away from stony corals and toward macroalgae. The use of reefs for recreational snorkeling and diving potentially can lead to similar ecological impacts if not well-managed, but impacts of snorkeling on benthic organisms are not well understood. We quantified variation in benthic community structure along a gradient of snorkeling frequency in an intensively-visited portion of the Mesoamerican Barrier Reef. We determined rates of snorkeling in 6 water sections and rates of beach visitation in 4 adjacent land sections at Akumal Bay, Mexico. For each in-water section at 1–3 m depth, we also assessed the percent cover of benthic organisms including taxa of stony corals and macroalgae. Rates of recreational snorkeling varied from low in the southwestern to very high (>1000 snorkelers d^-1) in the northeastern sections of the bay. Stony coral cover decreased and macroalgal cover increased significantly with levels of snorkeling, while trends varied among taxa for other organisms such as gorgonians, fire corals, and sea urchins. We conclude that benthic organisms appear to exhibit taxon-specific variation with levels of recreational snorkeling. To prevent further degradation, we recommend limitation of snorkeler visitation rates, coupled with visitor education and in-water guides to reduce reef-damaging behaviors by snorkelers in high-use areas. These types of management activities, integrated with reef monitoring and subsequent readjustment of management, have the potential to reverse the damage potentially inflicted on coral reefs by the expansion of reef-based recreational snorkeling.

The c-Jun N-terminal kinase prevents oxidative stress induced by UV and thermal stresses in corals and human cells

Coral reefs are of major ecological and socio-economic interest. They are threatened by global warming and natural pressures such as solar ultraviolet radiation. While great efforts have been made to understand the physiological response of corals to these stresses, the signalling pathways involved in the immediate cellular response exhibited by corals remain largely unknown. Here, we demonstrate that c-Jun N-terminal kinase (JNK) activation is involved in the early response of corals to thermal and UV stress. Furthermore, we found that JNK activity is required to repress stress-induced reactive oxygen species (ROS) accumulation in both the coral Stylophora pistillata and human skin cells. We also show that inhibiting JNK activation under stress conditions leads to ROS accumulation, subsequent coral bleaching and cell death. Taken together, our results suggest that an ancestral response, involving the JNK pathway, is remarkably conserved from corals to human, protecting cells from the adverse environmental effects.

Comparative Assessment of Mediterranean Gorgonian-Associated Microbial Communities Reveals Conserved Core and Locally Variant Bacteria

Gorgonians are key habitat-forming species of Mediterranean benthic communities, but their populations have suffered from mass mortality events linked to high summer seawater temperatures and microbial disease. However, our knowledge on the diversity, dynamics and function of gorgonian-associated microbial communities is limited. Here, we analysed the spatial variability of the microbiomes of five sympatric gorgonian species (Eunicella singularis, Eunicella cavolini, Eunicella verrucosa, Leptogorgia sarmentosa and Paramuricea clavata), collected from the Mediterranean Sea over a scale of ∼1100 km, using next-generation amplicon sequencing of the 16S rRNA gene. The microbiomes of all gorgonian species were generally dominated by members of the genus Endozoicomonas, which were at very low abundance in the surrounding seawater. Although the composition of the core microbiome (operational taxonomic units consistently present in a species) was found to be unique for each host species, significant overlap was observed. These spatially consistent associations between gorgonians and their core bacteria suggest intricate symbiotic relationships and regulation of the microbiome composition by the host. At the same time, local variations in microbiome composition were observed. Functional predictive profiling indicated that these differences could be attributed to seawater pollution. Taken together, our data indicate that gorgonian-associated microbiomes are composed of spatially conserved bacteria (core microbiome members) and locally variant members, and that local pollution may influence these local associations, potentially impacting gorgonian health.