Cellular responses in sea fan corals: granular amoebocytes react to pathogen and climate stressors

Exploring different methods of Exaiptasia diaphana infection to follow Vibrio parahaemolyticus dissemination in the whole animal

An increase in wastewater rejection and rising seawater temperature are the two main causes of the spreading of pathogenic bacteria in the ocean that present a risk to the health of marine organisms, i.e., corals. Deciphering the infectious mechanism is of interest to better disease management. The quantity of infecting bacteria as well as method of pathogen administration is an important parameter in studying host-pathogen interactions. In this study, we have tested two models of infection (bathing or injection) of Exaiptasia diaphana (E. diaphana) with a clinically isolated strain of Vibrio parahaemolyticus expressing constitutively a Green Fluorescent Protein (Vp-GFP). We followed Vp-GFP dissemination over time with confocal microscopy at 6, 24, and 30 h. During the early time of infection, bacteria were observed adhering to the ectoderm in both infection methods. In later stages of the infection, Vp-GFP were lost from the ectoderm and appeared in the gastroderm. Compared to bathing, the injection method was supposed to provide better control of the bacteria quantity introduced inside the animal. However, injection induced a stress response with contraction and rejection of bacteria thus making it impossible to control the number of infecting bacteria. In conclusion, we recommended using the bathing technique that is closer to the infection route found in the environment and, moreover, did not cause injury to the animal. We also demonstrated, by using Vp-GFP, that we could track pathogenic bacteria in different tissues of E. diaphana over the time of infection and quantify them in the whole animal, thus opening a technical approach for developing new strategies to fight infection disease.

Oxylipin Receptors and Their Role in Inter-Partner Signalling in a Model Cnidarian-Dinoflagellate Symbiosis

Oxylipin signalling is central in biology, mediating processes such as cellular homeostasis, inflammation and molecular signalling. It may also facilitate inter-partner communication in the cnidarian-dinoflagellate symbiosis, though this aspect remains understudied. In this study, four oxylipin receptors were characterised using immunohistochemistry and immunoblotting in the sea anemone Exaiptasia diaphana ('Aiptasia'): Prostaglandin E2 receptor 2 (EP2) and 4 (EP4), Transient Receptor Potential cation channel A1 (TRPA1) and Glutamate Receptor Ionotropic, Kainate 2 (GRIK2). Receptor abundance and localisation were compared between aposymbiotic anemones and symbiotic anemones hosting either native Breviolum minutum or non-native Durusdinium trenchii. All receptors were localised to the putative symbiosome of freshly isolated symbionts, suggesting a role in host-symbiont crosstalk. EP2, EP4 and TRPA1 abundance decreased in the gastrodermis of anemones hosting B. minutum, indicating potential downregulation of pathways mediated by these receptors. In contrast, GRIK2 abundance increased in anemones hosting D. trenchii in both the epidermis and gastrodermis; GRIK2 acts as a chemosensor of potential pathogens in other systems and could play a similar role here given D. trenchii's reputation as a sub-optimal partner for Aiptasia. This study contributes to the understanding of oxylipin signalling in the cnidarian-dinoflagellate symbiosis and supports further exploration of host-symbiont molecular signalling.

Pathology of tissue loss in three key gorgonian species in the Mediterranean Sea

The Mediterranean is known for its marine biodiversity, especially gorgonian forests. Unfortunately, these are experiencing rapid declines due to climate change, manifested by repeated marine heat waves resulting in mass mortality events since the early 1990 s. To better understand why gorgonians are declining, more systematic approaches to investigate the exact causes are needed, and pathology may aid in this goal. We described gross and microscopic pathology of tissue loss in three key gorgonian species in the Mediterranean region, Paramuricea clavata, Eunicella cavolini, and Leptogorgia sarmentosa, that were all experiencing various degrees of acute to subacute tissue loss characterized by exposed axial skeleton sometimes partly colonized by epibionts and thinning of adjacent tissues. The most significant variety of lesions was seen in P. clavata followed by L. sarmentosa and E. cavolini. For all species, dissociation of gastrodermal cells was the dominant microscopic lesion followed by necrosis of the gastrodermis. Ciliates invading gastrodermis and associated with necrosis of polyps were seen only in E. cavolini. Epidermal tissue loss was seen only in L. sarmentosa, while P. clavata was distinguished by a prominent inflammatory response and unidentified dark round structures within the tentacle epidermis and gastrodermis with no host response. Further work to understand the cause of death in gorgonians is needed, particularly to elucidate the role of ciliates and environmental co-factors or infectious agents not visible on light microscopy, as well as applications of additional tools such as cytology.

Global warming-related response after bacterial challenge in Astroides calycularis, a Mediterranean thermophilic coral

A worldwide increase in the prevalence of coral diseases and mortality has been linked to ocean warming due to changes in coral-associated bacterial communities, pathogen virulence, and immune system function. In the Mediterranean basin, the worrying upward temperature trend has already caused recurrent mass mortality events in recent decades. To evaluate how elevated seawater temperatures affect the immune response of a thermophilic coral species, colonies of Astroides calycularis were exposed to environmental (23 °C) or elevated (28 °C) temperatures, and subsequently challenged with bacterial lipopolysaccharides (LPS). Using immunolabeling with specific antibodies, we detected the production of Toll-like receptor 4 (TLR4) and nuclear factor kappa B (NF-kB), molecules involved in coral immune responses, and heat shock protein 70 (HSP70) activity, involved in general responses to thermal stress. A histological approach allowed us to characterize the tissue sites of activation (epithelium and/or gastroderm) under different experimental conditions. The activity patterns of the examined markers after 6 h of LPS stimulation revealed an up-modulation at environmental temperature. Under warmer conditions plus LPS-challenge, TLR4-NF-kB activation was almost completely suppressed, while constituent elevated values were recorded under thermal stress only. An HSP70 up-regulation appeared in both treatments at elevated temperature, with a significantly higher activation in LPS-challenge colonies. Such an approach is useful for further understanding the molecular pathogen-defense mechanisms in corals in order to disentangle the complex interactive effects on the health of these ecologically relevant organisms related to global climate change.

Host starvation and in hospite degradation of algal symbionts shape the heat stress response of the Cassiopea-Symbiodiniaceae symbiosis

BACKGROUND

Global warming is causing large-scale disruption of cnidarian-Symbiodiniaceae symbioses fundamental to major marine ecosystems, such as coral reefs. However, the mechanisms by which heat stress perturbs these symbiotic partnerships remain poorly understood. In this context, the upside-down jellyfish Cassiopea has emerged as a powerful experimental model system.

RESULTS

We combined a controlled heat stress experiment with isotope labeling and correlative SEM-NanoSIMS imaging to show that host starvation is a central component in the chain of events that ultimately leads to the collapse of the Cassiopea holobiont. Heat stress caused an increase in catabolic activity and a depletion of carbon reserves in the unfed host, concurrent with a reduction in the supply of photosynthates from its algal symbionts. This state of host starvation was accompanied by pronounced in hospite degradation of algal symbionts, which may be a distinct feature of the heat stress response of Cassiopea. Interestingly, this loss of symbionts by degradation was concealed by body shrinkage of the starving animals, resulting in what could be referred to as "invisible" bleaching.

CONCLUSIONS

Overall, our study highlights the importance of the nutritional status in the heat stress response of the Cassiopea holobiont. Compared with other symbiotic cnidarians, the large mesoglea of Cassiopea, with its structural sugar and protein content, may constitute an energy reservoir capable of delaying starvation. It seems plausible that this anatomical feature at least partly contributes to the relatively high stress tolerance of these animals in rapidly warming oceans. Video Abstract.

Impact of rising seawater temperature on a phagocytic cell population during V. parahaemolyticus infection in the sea anemone E. pallida

Climate change is increasing ocean temperatures and consequently impacts marine life (e.g., bacterial communities). In this context, studying host-pathogen interactions in marine organisms is becoming increasingly important, not only for ecological conservation, but also to reduce economic loss due to mass mortalities in cultured species. In this study, we used Exaiptasia pallida (E. pallida), an anemone, as an emerging marine model to better understand the effect of rising temperatures on the infection induced by the pathogenic marine bacterium Vibrio parahaemolyticus. The effect of temperature on E. pallida was examined at 6, 24, or 30 h after bath inoculation with 108 CFU of V. parahaemolyticus expressing GFP (Vp-GFP) at 27°C (husbandry temperature) or 31°C (heat stress). Morphological observations of E. pallida and their Hsps expression demonstrated heat stress induced increasing damage to anemones. The kinetics of the infections revealed that Vp-GFP were localized on the surface of the ectoderm and in the mucus during the first hours of infection and in the mesenterial filaments thereafter. To better identify the E. pallida cells targeted by Vp-GFP infection, we used spectral flow cytometry. E. pallida cell types were identified based on their autofluorescent properties. corresponding to different cell types (algae and cnidocytes). We identified an AF10 population whose autofluorescent spectrum was identical to that of human monocytes/macrophage, suggesting that this spectral print could be the hallmark of phagocytic cells called "amebocytes''. AF10 autofluorescent cells had a high capacity to phagocytize Vp-GFP, suggesting their possible role in fighting infection. This was confirmed by microscopy using sorted AF10 and GFP-positive cells (AF10+/GFP+). The number of AF10+/GFP+ cells were reduced at 31°C, demonstrating that increased temperature not only damages tissue but also affects the immune response of E. pallida. In conclusion, our study provides a springboard for more comprehensive studies of immune defense in marine organisms and paves the way for future studies of the dynamics, activation patterns, and functional responses of immune cells when encountering pathogens.

An introduction to lesions and histology of scleractinian corals

Stony corals (Scleractinia) are in the Phylum Cnidaria (cnidae referring to various types of stinging cells). They may be solitary or colonial, but all secrete an external, supporting aragonite skeleton. Large, colonial members of this phylum are responsible for the accretion of coral reefs in tropical and subtropical waters that form the foundations of the most biodiverse marine ecosystems. Coral reefs worldwide, but particularly in the Caribbean, are experiencing unprecedented levels of disease, resulting in reef degradation. Most coral diseases remain poorly described and lack clear case definitions, while the etiologies and pathogenesis are even more elusive. This introductory guide is focused on reef-building corals and describes basic gross and microscopic lesions in these corals in order to serve as an invitation to other veterinary pathologists to play a critical role in defining and advancing the field of coral pathology.

A combined diagnostic approach for the investigation of lesions resembling aspergillosis in Caribbean sea fans (Gorgonia spp.)

Aspergillosis of gorgonian sea fans is a Caribbean-wide disease characterized by focal, annular purple pigmentation with central tissue loss. We applied a holistic diagnostic approach including histopathology and a combination of culture and direct molecular identification of fungi to evaluate these lesions with the goal of determining the diversity of associated micro-organisms and pathology. Biopsies were collected from 14 sea fans without gross lesions and 44 sea fans with lesions grossly consistent with aspergillosis in shallow fringing reefs of St. Kitts. Histologically, the tissue loss margin had exposure of the axis and amoebocyte encapsulation with abundant mixed micro-organisms. Polyp loss, gastrodermal necrosis, and coenenchymal amoebocytosis were at the lesion interface (purpled area transitioning to grossly normal tissue) with algae (n = 21), fungus-like hyphae (n = 20), ciliate protists (n = 16), cyanobacteria (n = 15), labyrinthulomycetes (n = 5), or no micro-organisms (n = 8). Slender, septate hyaline hyphae predominated over other morphological categories, but were confined to the axis with little host response other than periaxial melanization. Hyphae were absent in 6 lesioned sea fans and present in 5 control biopsies, questioning their pathogenicity and necessary role in lesion causation. From cultivation, different fungi were isolated and identified by sequencing of the nuclear ribosomal internal transcribed spacer region. In addition, 2 primer pairs were used in a nested format to increase the sensitivity for direct amplification and identification of fungi from lesions, thereby circumventing cultivation. Results suggest mixed and opportunistic infections in sea fans with these lesions, requiring longitudinal or experimental studies to better determine the pathogenesis.

Step-by-Step Regeneration of Tentacles after Injury in Anemonia viridis-Morphological and Structural Cell Analyses

Benthic marine invertebrates, such as corals, are often subjected to injury caused by several sources. Here, the differences and characteristics in injured and health tissues in terms of cellular components are shown through a histological investigation of the soft coral Anemonia viridis at 0 h, 6 h, 24 h, and 7 days after injury caused by tentacle amputation. In addition, a new tool was used for the first time in invertebrates, positron emission tomography, in order to investigate the events that occur during regeneration within a longer time period (0 h, 24 h, and 14 days after the tentacles were cut). Higher integrated density values were measured through a densitometric analysis in sections stained with Fontana-Masson at 24 h after the tentacles were cut. This suggests an increase in melanin-like containing cells and a subsequent increase in fibroblast-like cells differentiated by amoebocytes that converge to the lesion site in the early stages of inflammation and regeneration. This work provides, for the first time, an elucidation of the events that occur during wound-healing and regeneration in basal metazoan, focusing on the characterisation of immune cells and their role. Our results indicate that Mediterranean anthozoan proves to be a valuable model for studying regeneration. Many events highlighted in this research occur in different phyla, suggesting that they are highly conserved.

Integrative biology of injury in animals

Mechanical injury is a prevalent challenge in the lives of animals with myriad potential consequences for organisms, including reduced fitness and death. Research on animal injury has focused on many aspects, including the frequency and severity of wounding in wild populations, the short- and long-term consequences of injury at different biological scales, and the variation in the response to injury within or among individuals, species, ontogenies, and environmental contexts. However, relevant research is scattered across diverse biological subdisciplines, and the study of the effects of injury has lacked synthesis and coherence. Furthermore, the depth of knowledge across injury biology is highly uneven in terms of scope and taxonomic coverage: much injury research is biomedical in focus, using mammalian model systems and investigating cellular and molecular processes, while research at organismal and higher scales, research that is explicitly comparative, and research on invertebrate and non-mammalian vertebrate species is less common and often less well integrated into the core body of knowledge about injury. The current state of injury research presents an opportunity to unify conceptually work focusing on a range of relevant questions, to synthesize progress to date, and to identify fruitful avenues for future research. The central aim of this review is to synthesize research concerning the broad range of effects of mechanical injury in animals. We organize reviewed work by four broad and loosely defined levels of biological organization: molecular and cellular effects, physiological and organismal effects, behavioural effects, and ecological and evolutionary effects of injury. Throughout, we highlight the diversity of injury consequences within and among taxonomic groups while emphasizing the gaps in taxonomic coverage, causal understanding, and biological endpoints considered. We additionally discuss the importance of integrating knowledge within and across biological levels, including how initial, localized responses to injury can lead to long-term consequences at the scale of the individual animal and beyond. We also suggest important avenues for future injury biology research, including distinguishing better between related yet distinct injury phenomena, expanding the subjects of injury research to include a greater variety of species, and testing how intrinsic and extrinsic conditions affect the scope and sensitivity of injury responses. It is our hope that this review will not only strengthen understanding of animal injury but will contribute to building a foundation for a more cohesive field of 'injury biology'.

Heat stress increases immune cell function in Hexacorallia

Climate change induced heat stress has increased coral bleaching events worldwide. Differentially regulated immune genes are one of the primary responses to heat stress suggesting that immune activation is critical. However, the cellular immune mechanisms of coral bleaching is currently unknown, and it is still not known if the immune response documented during heat stress is a consequence of bleaching or is directly caused by the heat stress itself. To address this question, we have used two model system sea anemones (Order: Actiniaria): Exaiptasia diaphana and Nematostella vectensis. E. diaphana is an established sea anemone model for algal symbiont interaction, while N. vectensis is an established sea anemone model that lacks the algal symbiont. Here, we examined the effect of increased temperature on phagocytic activity, as an indication of immune function. Our data shows that immune cell activity increases during heat stress, while small molecule pinocytosis remains unaffected. We observed an increase in cellular production of reactive oxygen species with increasing temperatures. We also found that the cellular immune activity was not affected by the presence of the Symbiodiniaceae. Our results suggest that the immune activity observed in heat-stress induced bleaching in corals is a fundamental and basic response independent of the bleaching effect. These results establish a foundation for improving our understanding of hexacorallian immune cell biology, and its potential role in coral bleaching.

Indications of symbiotic state influencing melanin-synthesis immune response in the facultative coral Astrangia poculata

Increased ocean warming is causing detrimental impacts to tropical corals worldwide. Compounding the effects of heat stress, incidences of tropical coral disease have risen concurrently. While tropical coral responses to these impacts are well studied, temperate coral responses remain largely unknown. The present study focused on the immune response of the temperate coral Astrangia poculata to increased temperature and disease. Symbiotic and aposymbiotic A. poculata were collected from Narragansett Bay, Rhode Island (USA) in summer and winter seasons and exposed to control (18°C) versus elevated temperatures (26°C) in the presence of an immune stimulant (i.e. lipopolysaccharide) for a 12 h period. Prophenoloxidase (PPO) and melanin concentrations from the melanin-synthesis pathway were assessed via spectrophotometry to examine immune responses. While PPO measurements were higher on average in symbiotic corals compared with aposymbiotic corals, temperature and season did not significantly affect this metric. Melanin was significantly higher in symbiotic compared to aposymbiotic corals, implying that symbiotic state may be important for melanin-synthesis response. Conversely, melanin as an immune response may be of less importance in aposymbiotic A. poculata due to the potential capacity of other immune responses in this species. In addition, differences in resource allocation to immune investment as a result of symbiosis is plausible given melanin production observed within the present study. However, thermal stressors may reduce the overall influence of symbiosis on melanin production. Future studies should build upon these results to further understand the entirety of innate immunity responses in temperate coral species.

A health survey of the reef forming scleractinian cold-water corals Lophelia pertusa and Madrepora oculata in a remote submarine canyon on the European continental margin, NE Atlantic

Monitoring of cold-water corals (CWCs) for pathogens and diseases is limited due to the environment, protected nature of the corals and their habitat and as well as the challenging and sampling effort required. It is recognised that environmental factors such as temperature and pH can expedite the ability of pathogens to cause diseases in cold-water corals therefore the characterisation of pathogen diversity, prevalence and associated pathologies is essential. The present study combined histology and polymerase chain reaction (PCR) diagnostic techniques to screen for two significant pathogen groups (bacteria of the genus Vibrio and the protozoan Haplosporidia) in the dominant NE Atlantic deep-water framework corals Lophelia pertusa (13 colonies) and Madrepora oculata (2 colonies) at three sampling locations (canyon head, south branch and the flank) in the Porcupine Bank Canyon (PBC), NE Atlantic. One M. oculata colony and four L. pertusa colonies were collected from both the canyon flank and the south branch whilst five L. pertusa colonies were collected from the canyon head. No pathogens were detected in the M. oculata samples. Neither histology nor PCR detected Vibrio spp. in L. pertusa, although Illumina technology used in this study to profile the CWCs microbiome, detected V. shilonii (0.03%) in a single L. pertusa individual, from the canyon head, that had also been screened in this study. A macroborer was observed at a prevalence of 0.07% at the canyon head only. Rickettsiales-like organisms (RLOs) were visualised with an overall prevalence of 40% and with a low intensity of 1 to 4 (RLO) colonies per individual polyp by histology. L. pertusa from the PBC canyon head had an RLO prevalence of 13.3% with the highest detection of 26.7% recorded in the south branch corals. Similarly, unidentified cells observed in L. pertusa from the south branch (20%) were more common than those observed in L. pertusa from the canyon head (6.7%). No RLOs or unidentified cells were observed in corals from the flank. Mean particulate organic matter concentration is highest in the south branch (2,612 μg l^-1) followed by the canyon head (1,065 μg l^-1) and lowest at the canyon flank (494 μg l^-1). Although the route of pathogen entry and the impact of RLO infection on L. pertusa is unclear, particulate availability and the feeding strategies employed by the scleractinian corals may be influencing their exposure to pathogens. The absence of a pathogen in M. oculata may be attributed to the smaller number of colonies screened or the narrower diet in M. oculata compared to the unrestricted diet exhibited in L. pertusa, if ingestion is a route of entry for pathogen groups. The findings of this study also shed some light on how environmental conditions experienced by deep sea organisms and their life strategies may be limiting pathogen diversity and prevalence.

The Skeleton and Biomineralization Mechanism as Part of the Innate Immune System of Stony Corals

Stony corals are among the most important calcifiers in the marine ecosystem as they form the coral reefs. Coral reefs have huge ecological importance as they constitute the most diverse marine ecosystem, providing a home to roughly a quarter of all marine species. In recent years, many studies have shed light on the mechanisms underlying the biomineralization processes in corals, as characterizing the calicoblast cell layer and genes involved in the formation of the calcium carbonate skeleton. In addition, considerable advancements have been made in the research field of coral immunity as characterizing genes involved in the immune response to pathogens and stressors, and the revealing of specialized immune cells, including their gene expression profile and phagocytosis capabilities. Yet, these two fields of corals research have never been integrated. Here, we discuss how the coral skeleton plays a role as the first line of defense. We integrate the knowledge from both fields and highlight genes and proteins that are related to biomineralization and might be involved in the innate immune response and help the coral deal with pathogens that penetrate its skeleton. In many organisms, the immune system has been tied to calcification. In humans, immune factors enhance ectopic calcification which causes severe diseases. Further investigation of coral immune genes which are involved in skeleton defense as well as in biomineralization might shed light on our understanding of the correlation and the interaction of both processes as well as reveal novel comprehension of how immune factors enhance calcification.

Whole-Genome Sequencing Highlights Conservative Genomic Strategies of a Stress-Tolerant, Long-Lived Scleractinian Coral, Porites australiensis Vaughan, 1918

Massive corals of the genus Porites, common, keystone reef builders in the Indo-Pacific Ocean, are distinguished by their relative stress tolerance and longevity. In order to identify genetic bases of these attributes, we sequenced the complete genome of a massive coral, Porites australiensis. We developed a genome assembly and gene models of comparable quality to those of other coral genomes. Proteome analysis identified 60 Porites skeletal matrix protein genes, all of which show significant similarities to genes from other corals and even to those from a sea anemone, which has no skeleton. Nonetheless, 30% of its skeletal matrix proteins were unique to Porites and were not present in the skeletons of other corals. Comparative genomic analyses showed that genes widely conserved among other organisms are selectively expanded in Porites. Specifically, comparisons of transcriptomic responses of P. australiensis and Acropora digitifera, a stress-sensitive coral, reveal significant differences in regard to genes that respond to increased water temperature, and some of the genes expanded exclusively in Porites may account for the different thermal tolerances of these corals. Taken together, widely shared genes may have given rise to unique biological characteristics of Porites, massive skeletons and stress tolerance.

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.

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.

Light modulated cnidocyte discharge predates the origins of eyes in Cnidaria

Complex biological traits often originate by integrating previously separate parts, but the organismal functions of these precursors are challenging to infer. If we can understand the ancestral functions of these precursors, it could help explain how they persisted and how they facilitated the origins of complex traits. Animal eyes are some of the best studied complex traits, and they include many parts, such as opsin-based photoreceptor cells, pigment cells, and lens cells. Eye evolution is understood through conceptual models that argue these parts gradually came together to support increasingly sophisticated visual functions. Despite the well-accepted logic of these conceptual models, explicit comparative studies to identify organismal functions of eye precursors are lacking. Here, we investigate how precursors functioned before they became part of eyes in Cnidaria, a group formed by sea anemones, corals, and jellyfish. Specifically, we test whether ancestral photoreceptor cells regulated the discharge of cnidocytes, the expensive single-use cells with various functions including prey capture, locomotion, and protection. Similar to a previous study of Hydra, we show an additional four distantly related cnidarian groups discharge significantly more cnidocytes when exposed to dim blue light compared with bright blue light. Our comparative analyses support the hypothesis that the cnidarian ancestor was capable of modulating cnidocyte discharge with light, which we speculate uses an opsin-based phototransduction pathway homologous to that previously described in Hydra. Although eye precursors might have had other functions like regulating timing of spawning, our findings are consistent with the hypothesis that photoreceptor cells which mediate cnidocyte discharge predated eyes, perhaps facilitating the prolific origination of eyes in Cnidaria.

Transcription Profiling of Cultured Acropora digitifera Adult Cells Reveals the Existence of Ancestral Genome Regulatory Modules Underlying Pluripotency and Cell Differentiation in Cnidaria

Due to their pluripotent nature and unlimited cell renewal, stem cells have been proposed as an ideal material for establishing long-term cnidarian cell cultures. However, the lack of unifying principles associated with "stemness" across the phylum complicates stem cells' identification and isolation. Here, we for the first time report gene expression profiles for cultured coral cells, focusing on regulatory gene networks underlying pluripotency and differentiation. Cultures were initiated from Acropora digitifera tip fragments, the fastest growing tissue in Acropora. Overall, in vitro transcription resembled early larvae, overexpressing orthologs of premetazoan and Hydra stem cell markers, and transcripts with roles in cell division, migration, and differentiation. Our results suggest the presence of pluripotent cell types in cultures and indicate the existence of ancestral genome regulatory modules underlying pluripotency and cell differentiation in cnidaria. Cultured cells appear to be synthesizing protein, differentiating, and proliferating.

Ecological Factors Mediate Immunity and Parasitic Co-Infection in Sea Fan Octocorals

The interplay among environment, demography, and host-parasite interactions is a challenging frontier. In the ocean, fundamental changes are occurring due to anthropogenic pressures, including increased disease outbreaks on coral reefs. These outbreaks include multiple parasites, calling into question how host immunity functions in this complex milieu. Our work investigates the interplay of factors influencing co-infection in the Caribbean sea fan octocoral, Gorgonia ventalina, using metrics of the innate immune response: cellular immunity and expression of candidate immune genes. We used existing copepod infections and live pathogen inoculation with the Aspergillus sydowii fungus, detecting increased expression of the immune recognition gene Tachylectin 5A (T5A) in response to both parasites. Cellular immunity increased by 8.16% in copepod infections compared to controls and single Aspergillus infections. We also detected activation of cellular immunity in reef populations, with a 13.6% increase during copepod infections. Cellular immunity was similar in the field and in the lab, increasing with copepod infections and not the fungus. Amoebocyte density and the expression of T5A and a matrix metalloproteinase (MMP) gene were also positively correlated across all treatments and colonies, irrespective of parasitic infection. We then assessed the scaling of immune metrics to population-level disease patterns and found random co-occurrence of copepods and fungus across 15 reefs in Puerto Rico. The results suggest immune activation by parasites may not alter parasite co-occurrence if factors other than immunity prevail in structuring parasite infection. We assessed non-immune factors in the field and found that sea fan colony size predicted infection by the copepod parasite. Moreover, the effect of infection on immunity was small relative to that of site differences and live coral cover, and similar to the effect of reproductive status. While additional immune data would shed light on the extent of this pattern, ecological factors may play a larger role than immunity in controlling parasite patterns in the wild. Parsing the effects of immunity and ecological factors in octocoral co-infection shows how disease depends on more than one host and one parasite and explores the application of co-infection research to a colonial marine organism.

Deciphering Coral Disease Dynamics: Integrating Host, Microbiome, and the Changing Environment

Diseases of tropical reef organisms is an intensive area of study, but despite significant advances in methodology and the global knowledge base, identifying the proximate causes of disease outbreaks remains difficult. The dynamics of infectious wildlife diseases are known to be influenced by shifting interactions among the host, pathogen, and other members of the microbiome, and a collective body of work clearly demonstrates that this is also the case for the main foundation species on reefs, corals. Yet, among wildlife, outbreaks of coral diseases stand out as being driven largely by a changing environment. These outbreaks contributed not only to significant losses of coral species but also to whole ecosystem regime shifts. Here we suggest that to better decipher the disease dynamics of corals, we must integrate more holistic and modern paradigms that consider multiple and variable interactions among the three major players in epizootics: the host, its associated microbiome, and the environment. In this perspective, we discuss how expanding the pathogen component of the classic host-pathogen-environment disease triad to incorporate shifts in the microbiome leading to dysbiosis provides a better model for understanding coral disease dynamics. We outline and discuss issues arising when evaluating each component of this trio and make suggestions for bridging gaps between them. We further suggest that to best tackle these challenges, researchers must adjust standard paradigms, like the classic one pathogen-one disease model, that, to date, have been ineffectual at uncovering many of the emergent properties of coral reef disease dynamics. Lastly, we make recommendations for ways forward in the fields of marine disease ecology and the future of coral reef conservation and restoration given these observations.

Energy depletion and opportunistic microbial colonisation in white syndrome lesions from corals across the Indo-Pacific

Corals are dependent upon lipids as energy reserves to mount a metabolic response to biotic and abiotic challenges. This study profiled lipids, fatty acids, and microbial communities of healthy and white syndrome (WS) diseased colonies of Acropora hyacinthus sampled from reefs in Western Australia, the Great Barrier Reef, and Palmyra Atoll. Total lipid levels varied significantly among locations, though a consistent stepwise decrease from healthy tissues from healthy colonies (HH) to healthy tissue on WS-diseased colonies (HD; i.e. preceding the lesion boundary) to diseased tissue on diseased colonies (DD; i.e. lesion front) was observed, demonstrating a reduction in energy reserves. Lipids in HH tissues were comprised of high energy lipid classes, while HD and DD tissues contained greater proportions of structural lipids. Bacterial profiling through 16S rRNA gene sequencing and histology showed no bacterial taxa linked to WS causation. However, the relative abundance of Rhodobacteraceae-affiliated sequences increased in DD tissues, suggesting opportunistic proliferation of these taxa. While the cause of WS remains inconclusive, this study demonstrates that the lipid profiles of HD tissues was more similar to DD tissues than to HH tissues, reflecting a colony-wide systemic effect and provides insight into the metabolic immune response of WS-infected Indo-Pacific corals.

Pathology of multifocal purple spots, a nonspecific lesion morphology of Caribbean sea fans Gorgonia spp

Disease is contributing to the decline of coral reefs globally, but the cause and pathogenesis of most coral diseases are poorly understood. Using Gorgonia ventalina and G. flabellum as a model for coral disease diagnosis, we histologically and microbiologically examined 45 biopsies of lesions resembling Gorgonia multifocal purple spots (MFPS) with the aim of forming a comprehensive case definition based on gross and microscopic morphologic descriptions and associated etiologies. Macroscopically, all lesions were small circular areas of purple pigmentation. Gross morphologies included pigmentation only (4/45, 9%), or pigmentation with branchlet expansion and fusion (19/45, 22%), sessile masses (17/45, 38%), or hard nodules (5/45, 9%). Histological morphologic diagnoses included amoebocyte encapsulation (9/45, 20%), coenenchymal amoebocytosis (6/45, 13%), melanin (17/45, 38%), and gorgonin deposition (13/45, 29%). Sixty-four percent of instances of fungi and 86% of labyrinthulomycetes were localized to grossly normal portions of the biopsy, whereas barnacles were only within lesions, and 87% of instances of algae and 82% of cyanobacteria were within lesioned area of the biopsy. Penicillium (n = 12) was the predominant genus of fungi isolated from biopsies. Barnacles were identified as Conopea sp. using molecular techniques. The pathology and etiology underlying MFPS lesions are diverse, consistent with a highly nonspecific lesion pattern rather than a specific disease. This study demonstrates the importance of microscopic examination of tissues for accurate classification of coral diseases and lesion patterns.

Cnidarian Immunity and the Repertoire of Defense Mechanisms in Anthozoans

Anthozoa is the most specious class of the phylum Cnidaria that is phylogenetically basal within the Metazoa. It is an interesting group for studying the evolution of mutualisms and immunity, for despite their morphological simplicity, Anthozoans are unexpectedly immunologically complex, with large genomes and gene families similar to those of the Bilateria. Evidence indicates that the Anthozoan innate immune system is not only involved in the disruption of harmful microorganisms, but is also 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. Here, we report on the current state of the art of Anthozoan immunity. Like other invertebrates, Anthozoans possess immune mechanisms based on self/non-self-recognition. Although lacking adaptive immunity, they use a diverse repertoire of immune receptor signaling pathways (PRRs) to recognize a broad array of conserved microorganism-associated molecular patterns (MAMP). The intracellular signaling cascades lead to gene transcription up to endpoints of release of molecules that kill the pathogens, defend the self by maintaining homeostasis, and modulate the wound repair process. The cells play a fundamental role in immunity, as they display phagocytic activities and secrete mucus, which acts as a physicochemical barrier preventing or slowing down the proliferation of potential invaders. Finally, we describe the current state of knowledge of some immune effectors in Anthozoan species, including the potential role of toxins and the inflammatory response in the Mediterranean Anthozoan Anemonia viridis following injection of various foreign particles differing in type and dimensions, including pathogenetic bacteria.

Investigating the roles of transforming growth factor-beta in immune response of Orbicella faveolata, a scleractinian coral

Symbiotic relationships range from parasitic to mutualistic, yet all endosymbionts face similar challenges, including evasion of host immunity. Many symbiotic organisms have evolved similar mechanisms to face these challenges, including manipulation of the host's transforming growth factor-beta (TGFβ) pathway. Here we investigate the TGFβ pathway in scelaractinian corals which are dependent on symbioses with dinoflagellates from the family Symbiodiniaceae. Using the Caribbean coral, Orbicella faveolata, we explore the effects of enhancement and inhibition of the TGFβ pathway on host gene expression. Following transcriptomic analyses, we demonstrated limited effects of pathway manipulation in absence of immune stimulation. However, manipulation of the TGFβ pathway significantly affects the subsequent ability of host corals to mount an immune response. Enhancement of the TGFβ pathway eliminates transcriptomic signatures of host coral immune response, while inhibition of the pathway maintains the response. This is, to our knowledge, the first evidence of an immunomodulatory role for TGFβ in a scelaractinian coral. These findings suggest variation in TGFβ signaling may have implications in the face of increasing disease prevelance. Our results suggest that the TGFβ pathway can modulate tradeoffs between symbiosis and immunity. Further study of links between symbiosis, TGFβ, and immunity is needed to better understand the ecological implications of these findings.

Nodular-like growth and axial thickening in gorgonians are a defensive response to endolithic cyanobacteria, involving amyloid deposition

An accurate approach to coral disease study is critical for understanding the global decline of coral populations. Such an approach should involve the proper use of medical concepts and terminology to avoid confusion and promote clarity in the coral disease literature. Inflammatory and neoplastic disorders have been frequently confused in corals. They are both reported as 'growth anomalies' because of their possible gross similarity, but in fact they are very different types of lesions and pathologic phenomena. In this work, we assessed the distribution and prevalence of growth anomalies, externally visible as nodular-like lesions, in the soft corals Eunicella cavolinii and E. singularis in 2008-2009 in 3 different areas along the Campanian coastline of Italy. Histopathology revealed them as chronic inflammatory lesions, resembling chronic inflammatory lesions of vertebrates, encapsulating an unidentified pathogen. Congo red and Masson Fontana histochemistry highlighted an amoebocyte infiltration with the presence of new apposition of melanin coupled with amyloid sheets intended as part of the defensive response, as reported in other invertebrates. A parallel molecular analysis of 16S rRNA of the lesions suggested that the causative agent is an endolithic cyanobacterium belonging to the order Nostocales. This is the first study assessing the presence of amyloid fibrils in corals.

Warming and pollutants interact to modulate octocoral immunity and shape disease outcomes

Warming environments can alter the outcome of host-parasite relationships with important consequences for biodiversity. Warming often increases disease risk, and interactions with other environmental factors can intensify impacts by modifying the underlying mechanisms, such as host immunity. In coastal ecosystems, metal pollution is a pervasive stressor that influences disease and immunity in many organisms. Despite the crisis facing coral reefs, which stems in part from warming-associated disease outbreaks, the impacts of metal pollutants on scleractinian and octocoral disease are largely unknown. We investigated how warming oceans and copper pollution affect host immunity and disease risk for two diseases of the abundant Caribbean octocoral, the sea fan Gorgonia ventalina. Field surveys across a sediment copper concentration gradient in Puerto Rico, USA revealed that cellular immunity of sea fans increased by 12.6% at higher sediment copper concentrations, while recovery from multifocal purple spots disease (MFPS) tended to decrease. MFPS severity in the field increased at warmer sites. In a controlled laboratory experiment, sea fans were inoculated with live cultures of a labyrinthulid parasite to test the interactive effects of temperature and copper on immune activation. As in the field, higher copper induced greater immunity, but the factorial design of the experiment revealed that copper and temperature interacted to modulate the immune response to the parasite: immune cell densities increased with elevated temperature at lower copper concentrations, but not with high copper concentrations. Tissue damage was also greater in treatments with higher copper and warmer temperatures. Field and lab evidence confirm that elevated copper hinders sea fan immune defenses against damaging parasites. Temperature and copper influenced host-pathogen interactions in octocorals by modulating immunity, disease severity, and disease recovery. This is the first evidence that metal pollution affects processes influencing disease in octocorals and highlights the importance of immune mechanisms in environmentally mediated disease outbreaks. Although coral conservation efforts must include a focus on global factors, such as rapid warming, reducing copper and other pollutants that compromise coral health on a local scale may help corals fight disease in a warming ocean.

Characterization and expression of tyrosinase-like genes in the anemone Exaiptasia pallida as a function of health and symbiotic state

Coral disease is a major threat to reef ecosystems and therefore, understanding the cellular pathways underlying disease progression and resistance is critical to mitigating future outbreaks. This study focused on tyrosinase-like proteins in cnidarians, which contribute to melanin synthesis, an invertebrate innate immune defense. Specifically, characterization and phylogenetic analysis of cnidarian tyrosinases were performed, and their role in symbiosis and a "mystery disease" in the anemone Exaiptasia pallida was investigated using qPCR. The results reveal a diversity of tyrosinase-like proteins in cnidarians that separate into two major clades on a phylogenetic tree, suggesting functional divergence. Two E. pallida sequences, Ep_Tyr1 and Ep_Tyr2, were further investigated, and qPCR results revealed no gene expression differences as a function of symbiotic state, but decreased expression in late disease stages. Overall this work provides evidence for the participation of tyrosinases in the cnidarian immune response.

Proteomic Investigation of a Diseased Gorgonian Coral Indicates Disruption of Essential Cell Function and Investment in Inflammatory and Other Immune Processes

As scleractinian coral cover declines in the face of increased frequency in disease outbreaks, future reefs may become dominated by octocorals. Understanding octocoral disease responses and consequences is therefore necessary if we are to gain insight into the future of ecosystem services provided by coral reefs. In Florida, populations of the octocoral Eunicea calyculata infected with Eunicea black disease (EBD) were observed in the field in the fall of 2011. This disease was recognized by a stark, black pigmentation caused by heavy melanization. Histological preparations of E. calyculata infected with EBD demonstrated granular amoebocyte (GA) mobilization, melanin granules in much of the GA population, and the presence of fungal hyphae penetrating coral tissue. Previous transcriptomic analysis also identified immune trade-offs evidenced by increased immune investment at the expense of growth. Our investigation utilized proteogenomic techniques to reveal decreased investment in general cell signaling while increasing energy production for immune responses. Inflammation was also prominent in diseased E. calyculata and sheds light on factors driving the extreme phenotype observed with EBD. With disease outbreaks continuing to increase in frequency, our results highlight new targets within the cnidarian immune system and provide a framework for understanding transcriptomics in the context of an organismal disease phenotype and its protein expression.

Heat stress compromises epithelial integrity in the coral, Acropora hyacinthus

It is well understood that heat stress causes bleaching in corals. Much work has focused on the way heat stress disrupts corals’ symbiotic relationship with endosymbiotic algal dinoflagellate, Symbiodiniaceae, a process called bleaching. However, the damage to the coral tissue that occurs during the bleaching process and, importantly, the factors that contribute to subsequent recovery, are not well understood. I hypothesize that the host tissue damage created by heat stress initiates cascades of wound healing factors that maintain epithelial integrity. These factors may be found to contribute to the coral’s potential capacity to recover. In this study, I present evidence that heat stress causes damage to the coral host tissue and that collagen is present in the gastrodermis of heat-stressed corals. I found that, during the early stages of bleaching, an important transcription factor for wound healing, Grainyhead, is expressed throughout the gastrodermis, where the cellular and tissue rearrangements occur. Lastly, using phylogenetics, I found that cnidarian Grainyhead proteins evolved three distinct groups and that evolution of this protein family likely happened within each taxonomic group. These findings have important implications for our study of coral resiliency in the face of climate change.

Local confinement of disease-related microbiome facilitates recovery of gorgonian sea fans from necrotic-patch disease

Microbiome disruptions triggering disease outbreaks are increasingly threatening corals worldwide. In the Tropical Eastern Pacific, a necrotic-patch disease affecting gorgonian corals (sea fans, Pacifigorgia spp.) has been observed in recent years. However, the composition of the microbiome and its disease-related disruptions remain unknown in these gorgonian corals. Therefore, we analysed 16S rRNA gene amplicons from tissues of healthy colonies (n = 19) and from symptomatic-asymptomatic tissues of diseased colonies (n = 19) of Pacifigorgia cairnsi (Gorgoniidae: Octocorallia) in order to test for disease-related changes in the bacterial microbiome. We found that potential endosymbionts (mostly Endozoicomonas spp.) dominate the core microbiome in healthy colonies. Moreover, healthy tissues differed in community composition and functional profile from those of the symptomatic tissues but did not show differences to asymptomatic tissues of the diseased colonies. A more diverse set of bacteria was observed in symptomatic tissues, together with the decline in abundance of the potential endosymbionts from the healthy core microbiome. Furthermore, according to a comparative taxonomy-based functional profiling, these symptomatic tissues were characterized by the increase in heterotrophic, ammonia oxidizer and dehalogenating bacteria and by the depletion of nitrite and sulphate reducers. Overall, our results suggest that the bacterial microbiome associated with the disease behaves opportunistically and is likely in a state of microbial dysbiosis. We also conclude that the confinement of the disease-related consortium to symptomatic tissues may facilitate colony recovery.

Cellular stress responses to chronic heat shock and shell damage in temperate Mya truncata

Acclimation, via phenotypic flexibility, is a potential means for a fast response to climate change. Understanding the molecular mechanisms underpinning phenotypic flexibility can provide a fine-scale cellular understanding of how organisms acclimate. In the last 30 years, Mya truncata populations around the UK have faced an average increase in sea surface temperature of 0.7 °C and further warming of between 1.5 and 4 °C, in all marine regions adjacent to the UK, is predicted by the end of the century. Hence, data are required on the ability of M. truncata to acclimate to physiological stresses, and most notably, chronic increases in temperature. Animals in the present study were exposed to chronic heat-stress for 2 months prior to shell damage and subsequently, only 3, out of 20 damaged individuals, were able to repair their shells within 2 weeks. Differentially expressed genes (between control and damaged animals) were functionally enriched with processes relating to cellular stress, the immune response and biomineralisation. Comparative transcriptomics highlighted genes, and more broadly molecular mechanisms, that are likely to be pivotal in this lack of acclimation. This study demonstrates that discovery-led transcriptomic profiling of animals during stress-response experiments can shed light on the complexity of biological processes and changes within organisms that can be more difficult to detect at higher levels of biological organisation.

Responses of an oyster host (Crassostrea virginica) and its protozoan parasite (Perkinsus marinus) to increasing air temperature

Background

Changes in climate are predicted to influence parasite and pathogen infection patterns in terrestrial and marine environments. Increases in temperature in particular may greatly alter biological processes, such as host-parasite interactions. For example, parasites could differentially benefit from increased reproduction and transmission or hosts could benefit from elevated immune responses that may mediate or even eliminate infections. In the southeastern United States, the Eastern oyster, Crassostrea virginica, is infected by the lethal protozoan parasite, Perkinsus marinus. Under field conditions, intertidal (air-exposed) oysters have been found to have significantly higher P. marinus infection intensity and marginally higher infection prevalence than subtidal (submerged) oysters. During summer, air temperatures are much warmer than water and this exposure of intertidal oysters to higher temperatures is a suggested mechanism for increased infection intensity.

Methods

We simulated intertidal exposure using controlled laboratory experiments to determine how host traits (survival and immune response) and parasite infection intensity will respond to elevated air temperature ranging from 27 °C to 53 °C during emersion at low tide. In Georgia, where our work was conducted, the average summer water temperature is 29 °C and the average maximum high air temperature in July is 33 °C (though oysters have been shown to survive at much higher air temperatures).

Results

Host survival declined as temperature increased, with a definitive drop-off between 39–43 °C. Negative effects of air temperature on host immune response (phagocytic activity) were detectable only at extremely high temperatures (47–50 °C) when hosts were suffering acute mortality. Parasite infection intensity peaked at 35 °C.

Discussion

Our results suggest that an increase in average summer air temperature to 35 °C or higher could affect oyster survival directly through temperature-related impacts in the short-term and indirectly through increased P. marinus infection intensity over the long-term.

Transcriptional analyses provide new insight into the late-stage immune response of a diseased Caribbean coral

Increasing global temperatures due to climate change have resulted in respective increases in the severity and frequency of epizootics around the globe. Corals in particular have faced rapid declines due to disease outbreaks. Understanding immune responses and associated potential life-history trade-offs is therefore a priority. In the autumn of 2011, a novel disease of octocorals of the genus Eunicea was first documented in the Florida Keys. Termed Eunicea Black Disease (EBD), the disease is easily identified by the dark appearance of affected tissue, caused by a strong melanization response on the part of the host. In order to better understand the response of corals to EBD, we conducted full transcriptome analysis of 3 healthy and 3 diseased specimens of Eunicea calyculata collected from offshore southeast Florida. Differential expression and protein analyses revealed a strong, diverse immune response to EBD characterized by phagocytosis, adhesion and melanization on the part of the host. Furthermore, coexpression network analyses suggested this might come at the cost of reduced cell cycle progression and growth. This is in accordance with past histological studies of naturally infected hard corals, suggesting that potential trade-offs during infection may affect post-outbreak recovery of reef ecosystems by reducing both organismal growth and fecundity. Our findings highlight the importance of considering factors beyond mortality when estimating effects of disease outbreaks on ecosystems.

Host-microbe interactions in octocoral holobionts - recent advances and perspectives

Octocorals are one of the most ubiquitous benthic organisms in marine ecosystems from the shallow tropics to the Antarctic deep sea, providing habitat for numerous organisms as well as ecosystem services for humans. In contrast to the holobionts of reef-building scleractinian corals, the holobionts of octocorals have received relatively little attention, despite the devastating effects of disease outbreaks on many populations. Recent advances have shown that octocorals possess remarkably stable bacterial communities on geographical and temporal scales as well as under environmental stress. This may be the result of their high capacity to regulate their microbiome through the production of antimicrobial and quorum-sensing interfering compounds. Despite decades of research relating to octocoral-microbe interactions, a synthesis of this expanding field has not been conducted to date. We therefore provide an urgently needed review on our current knowledge about octocoral holobionts. Specifically, we briefly introduce the ecological role of octocorals and the concept of holobiont before providing detailed overviews of (I) the symbiosis between octocorals and the algal symbiont Symbiodinium; (II) the main fungal, viral, and bacterial taxa associated with octocorals; (III) the dominance of the microbial assemblages by a few microbial species, the stability of these associations, and their evolutionary history with the host organism; (IV) octocoral diseases; (V) how octocorals use their immune system to fight pathogens; (VI) microbiome regulation by the octocoral and its associated microbes; and (VII) the discovery of natural products with microbiome regulatory activities. Finally, we present our perspectives on how the field of octocoral research should move forward, and the recognition that these organisms may be suitable model organisms to study coral-microbe symbioses.

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.

Octocoral co-infection as a balance between host immunity and host environment

Co-infection is the reality in natural populations, but few studies incorporate the players that matter in the wild. We integrate the environment, host demography, two parasites, and host immunity in a study of co-infection to determine the drivers of parasite interactions. Here, we use an ecologically important Caribbean sea fan octocoral, Gorgonia ventalina, that is co-infected by a copepod and a labyrinthulid protist. We first expanded upon laboratory studies by showing that immune suppression is associated with the labyrinthulid in a natural setting. Histological analyses revealed that immune cells (amoebocytes) were significantly suppressed in both labyrinthulid infections and co-infections relative to healthy sea fans, but remained unchanged in copepod infections. However, surveys of natural coral populations demonstrated a critical role for the environment and host demography in this co-infection: the prevalence of copepod infections increased with sea fan size while labyrinthulid prevalence increased with water depth. Although we predicted that immune suppression by the labyrinthulid would facilitate copepod infection, the two parasites did not co-occur in the sea fans more often than expected by chance. These results suggest that the distinct ecological drivers for each parasite overwhelm the role of host immune suppression in determining the distribution of parasites among hosts. This interplay of the environment and parasite-mediated immune suppression in sea fan co-infection provides insights into the factors underlying co-occurrence patterns in wild co-infections. Moving forward, simultaneous consideration of co-occurring parasites, host traits, and the environmental context will improve the understanding of host - parasite interactions and their consequences.

Comparison of chemical compounds associated with sclerites from healthy and diseased sea fan corals (Gorgonia ventalina)

BACKGROUND

The roles of gorgonian sclerites as structural components and predator deterrents have been widely studied. Yet their role as barriers against microbes has only recently been investigated, and even less is known about the diversity and roles of the chemical compounds associated with sclerites.

METHODS

Here, we examine the semi-volatile organic compound fraction (SVOCs) associated with sclerites from healthy and diseased Gorgonia ventalina sea fan corals to understand their possible role as a stress response or in defense of infection. We also measured the oxidative potential of compounds from diseased and healthy G. ventalina colonies.

RESULTS

The results showed that sclerites harbor a great diversity of SVOCs. Overall, 70 compounds were identified, the majority of which are novel with unknown biological roles. The majority of SVOCs identified exhibit multiple immune-related roles including antimicrobial and radical scavenging functions. The free radical activity assays further confirmed the anti-oxidative potential of some these compounds. The anti-oxidative activity was, nonetheless, similar across sclerites regardless of the health condition of the colony, although sclerites from diseased sea fans display slightly higher anti-oxidative activity than the healthy ones.

DISCUSSION

Sclerites harbor great SVOCs diversity, the majority of which are novel to sea fans or any other corals. Yet the scientific literature consulted showed that the roles of compounds found in sclerites vary from antioxidant to antimicrobial compounds. However, this study fell short in determine the origin of the SVOCs identified, undermining our capacity to determine the biological roles of the SVOCs on sclerites and sea fans.

Associations between transcriptional changes and protein phenotypes provide insights into immune regulation in corals

Disease outbreaks in marine ecosystems have driven worldwide declines of numerous taxa, including corals. Some corals, such as Orbicella faveolata, are particularly susceptible to disease. To explore the mechanisms contributing to susceptibility, colonies of O. faveolata were exposed to immune challenge with lipopolysaccharides. RNA sequencing and protein activity assays were used to characterize the response of corals to immune challenge. Differential expression analyses identified 17 immune-related transcripts that varied in expression post-immune challenge. Network analyses revealed several groups of transcripts correlated to immune protein activity. Several transcripts, which were annotated as positive regulators of immunity were included in these groups, and some were downregulated following immune challenge. Correlations between expression of these transcripts and protein activity results further supported the role of these transcripts in positive regulation of immunity. The observed pattern of gene expression and protein activity may elucidate the processes contributing to the disease susceptibility of species like O. faveolata.

RNA-Seq of the Caribbean reef-building coral Orbicella faveolata (Scleractinia-Merulinidae) under bleaching and disease stress expands models of coral innate immunity

Climate change-driven coral disease outbreaks have led to widespread declines in coral populations. Early work on coral genomics established that corals have a complex innate immune system, and whole-transcriptome gene expression studies have revealed mechanisms by which the coral immune system responds to stress and disease. The present investigation expands bioinformatic data available to study coral molecular physiology through the assembly and annotation of a reference transcriptome of the Caribbean reef-building coral, Orbicella faveolata. Samples were collected during a warm water thermal anomaly, coral bleaching event and Caribbean yellow band disease outbreak in 2010 in Puerto Rico. Multiplex sequencing of RNA on the Illumina GAIIx platform and de novo transcriptome assembly by Trinity produced 70,745,177 raw short-sequence reads and 32,463 O. faveolata transcripts, respectively. The reference transcriptome was annotated with gene ontologies, mapped to KEGG pathways, and a predicted proteome of 20,488 sequences was generated. Protein families and signaling pathways that are essential in the regulation of innate immunity across Phyla were investigated in-depth. Results were used to develop models of evolutionarily conserved Wnt, Notch, Rig-like receptor, Nod-like receptor, and Dicer signaling. O. faveolata is a coral species that has been studied widely under climate-driven stress and disease, and the present investigation provides new data on the genes that putatively regulate its immune system.

In situ hybridisation detects pro-apoptotic gene expression of a Bcl-2 family member in white syndrome-affected coral

White syndrome has been described as one of the most prolific diseases on the Great Barrier Reef. Previously, apoptotic cell death has been described as the mechanism driving the characteristic rapid tissue loss associated with this disease, but the molecular mechanisms controlling apoptotic cell death in coral disease have yet to be investigated. In situ methods were used to study the expression patterns of 2 distinct regulators of apoptosis in Acropora hyacinthus tissues undergoing white syndrome and apoptotic cell death. Apoptotic genes within the Bcl-2 family were not localized in apparently healthy coral tissues. However, a Bcl-2 family member (bax-like) was found to localize to cells and tissues affected by white syndrome and those with morphological evidence for apoptosis. A potential up-regulation of pro-apoptotic or bax-like gene expression in tissues with apoptotic cell death adjacent to disease lesions is consistent with apoptosis being the primary cause of rapid tissue loss in coral affected by white syndrome. Pro-apoptotic (bax-like) expression in desmocytes and the basal tissue layer, the calicodermis, distant from the disease lesion suggests that apoptosis may also underlie the sloughing of healthy tissues associated with the characteristic, rapid spread of tissue loss, evident of this disease. This study also shows that in situ hybridisation is an effective tool for studying gene expression in adult corals, and wider application of these methods should allow a better understanding of many aspects of coral biology and disease pathology.

Identification of unstimulated constitutive immunocytes, by enzyme histochemistry, in the coenenchyme of the octocoral Swiftia exserta

Most animals rely on circulating hemocytes as cellular effectors of immunity. These cells traditionally have been characterized by morphology, function, and cellular contents. Morphological descriptions use granule differences and cell shapes; functional descriptions rely on phagocytic ability and oxygen transport; and cellular content descriptions include cytochemical features and key enzymes. Key enzymes used to identify phagocytes in tissues include hydrolytic enzymes, peroxidase, and--in invertebrates--phenoloxidase. Cnidaria such as Swiftia exserta lack a circulatory system, thereby complicating the identification of immune effector cells. As a first step in identifying immunocytes, this study focused on basic enzymes used during phagocytosis and encapsulation; both processes have been reported in octocorals such as S. exserta. Earlier work suggested that there are two populations of phagocytic cells: a constitutive population and an induced population following a trauma-associated challenge. To identify the constitutive immune effector cells in S. exserta in a nonactivated state, we used cryosections of unstimulated animals and the following enzymes to serve as identifying proxies due to their roles in phagocytosis and encapsulation: (1) acid phosphatase, (2) alkaline phosphatase, (3) non-specific esterase, (4) β-glucuronidase, (5) peroxidase, and (6) phenoloxidase. Our results indicate that in unstimulated animals, two distinct cell populations could function as immunocytes. These cell types were differentiated by their enzyme reactivity and their location within the mesoglea of S. exserta, and have been described as either "oblong granular cells" or "granular amoebocytes."

The coral immune response facilitates protection against microbes during tissue regeneration

Increasing physical damage on coral reefs from predation, storms and anthropogenic disturbances highlights the need to understand the impact of injury on the coral immune system. In this study, we examined the regulation of the coral immune response over 10 days following physical trauma artificially inflicted on in situ colonies of the coral Acropora aspera, simultaneously with bacterial colonization of the lesions. Corals responded to injury by increasing the expression of immune system-related genes involved in the Toll-like and NOD-like receptor signalling pathways and the lectin-complement system in three phases (<2, 4 and 10 days post-injury). Phenoloxidase activity was also significantly upregulated in two phases (<3 and 10 days post-injury), as were levels of non-fluorescent chromoprotein. In addition, green fluorescent protein expression was upregulated in response to injury from 4 days post-injury, while cyan fluorescent protein expression was reduced. No shifts in the composition of coral-associated bacterial communities were evident following injury based on 16S rRNA gene amplicon pyrosequencing. Bacteria-specific fluorescence in situ hybridization also showed no evidence of bacterial colonization of the wound or regenerating tissues. Coral tissues showed near-complete regeneration of lesions within 10 days. This study demonstrates that corals exhibit immune responses that support rapid recovery following physical injury, maintain coral microbial homeostasis and prevent bacterial infestation that may compromise coral fitness.