The role of microorganisms in coral bleaching

A gauge of coral physiology: re-examining temporal changes in Endozoicomonas abundance correlated with natural coral bleaching

Bacteria contribute to many physiological functions of coral holobionts, including responses to bleaching. The bacterial genus, Endozoicomonas, dominates the microbial flora of many coral species and its abundance appears to be correlated with coral bleaching. However, evidences for decoupling of bleaching and Endozoicomonas abundance changes have also been reported. In 2020, a severe bleaching event was recorded at reefs in Taiwan, providing a unique opportunity to re-examine bleaching-Endozoicomonas association using multiple stony corals in natural environments. In this study, we monitored tissue color and microbiome changes in three coral species (Montipora sp., Porites sp., and Stylophora pistillata) in Kenting National Park, following the bleaching event. All tagged Montipora sp. and Porites sp. recovered from bleaching within 1 year, while high mortality occurred in S. pistillata. Microbiome analysis found no correlation of Endozoicomonas relative abundance and bleaching severity during the sampling period, but found a stronger correlation when the month in which bleaching occurred was excluded. Moreover, Endozoicomonas abundance increased during recovery months in Montipora sp. and Porites sp., whereas in S. pistillata it was nearly depleted. These results suggest that Endozoicomonas abundance may represent a gauge of coral health and reflect recovery of some corals from stress. Interestingly, even though different Endozoicomonas strains predominated in the three corals, these Endozoicomonas strains were also shared among coral taxa. Meanwhile, several Endozoicomonas strains showed secondary emergence during coral recovery, suggesting possible symbiont switching in Endozoicomonas. These findings indicate that it may be possible to introduce Endozoicomonas to non-native coral hosts as a coral probiotic.

Recent advancements in coral health, microbiome interactions and climate change

Corals are the visible indicators of the disasters induced by global climate change and anthropogenic activities and have become a highly vulnerable ecosystem on the verge of extinction. Multiple stressors could act individually or synergistically which results in small to large scale tissue degradation, reduced coral covers, and makes the corals vulnerable to various diseases. The coralline diseases are like the Chicken pox in humans because they spread hastily throughout the coral ecosystem and can devastate the coral cover formed over centuries in an abbreviated time. The extinction of the entire reef ecosystem will alter the ocean and earth's amalgam of biogeochemical cycles causing a threat to the entire planet. The current manuscript provides an overview of the recent advancement in coral health, microbiome interactions and climate change. Culture dependent and independent approaches in studying the microbiome of corals, the diseases caused by microorganisms, and the reservoirs of coral pathogens are also discussed. Finally, we discuss the possibilities of protecting the coral reefs from diseases through microbiome transplantation and the capabilities of remote sensing in monitoring their health status.

Prokaryotic and eukaryotic microbial communities associated with coral species have high host specificity in the South China Sea

Reef-building corals are well known for their obligate association with Symbiodiniaceae, and an array of other microbes, including bacteria, fungi, and symbiotic algae (i.e., total microbiome), which together form the coral holobiont. The total microbiome plays an intricate part in maintaining the homeostasis of the coral holobiont and is closely associated with host health. However, the composition of the coral associated microbiome and interaction between its different members remains elusive because few analyses have bridged taxonomically disparate groups. This research gaps have prevented a holistic understanding of the total microbiome. Thus, to simultaneously characterize the bacterial, fungal and symbiotic algal communities associated with different coral species, and explore the relationship between these symbionts and coral health, healthy and bleached tissues from four coral species, Acropora muricata, Galaxea fascicularis, Platygyra daedalea, and Pavona explanulata, were collected from the Xisha Islands of the South China Sea. Using high throughput sequencing, a high degree of host-specificity was observed among bacterial, fungal, and algal groups across coral species. There were no obvious changes in the microbial community structure of apparently healthy and bleached corals, but host bleaching allowed colonization of the holobionts by diverse opportunistic microbes, resulting in a significant elevation in the α-diversity of microbial communities. In addition, co-occurrence analysis of the coral microbiota also identified more complex microbial interactions in bleached corals than in healthy ones. In summary, this study characterized the structure of coral-associated microbiomes across four coral species, and systematically studied microbiome differences between healthy and bleached corals. The findings improve our understanding of the heterogeneity of symbiotic microorganisms and the impact of coral's physiological status on its associated microbial communities composition.

Identification of quorum sensing-regulated Vibrio fortis as potential pathogenic bacteria for coral bleaching and the effects on the microbial shift

Coastal pollution, global warming, ocean acidification, and other reasons lead to the imbalance of the coral reef ecosystem, resulting in the increasingly serious problem of coral degradation. Coral bleaching is often accompanied by structural abnormalities of coral symbiotic microbiota, among which Vibrio is highly concerned. In this study, Vibrio fortis S10-1 (MCCC 1H00104), isolated from sea cucumber, was used for the bacterial infection on coral Seriatopora guttatus and Pocillopora damicornis. The infection of S10-1 led to coral bleaching and a significant reduction of photosynthetic function in coral holobiont, and the pathogenicity of V. fortis was regulated by quorum sensing. Meanwhile, Vibrio infection also caused a shift of coral symbiotic microbial community, with significantly increased abundant Proteobacteria and Actinobacteria and significantly reduced abundant Firmicutes; on genus level, the abundance of Bacillus decreased significantly and the abundance of Rhodococcus, Ralstonia, and Burkholderia-Caballeronia-Paraburkholderia increased significantly; S10-1 infection also significantly impacted the water quality in the micro-ecosystem. In contrast, S10-1 infection showed less effect on the microbial community of the live stone, which reflected that the microbes in the epiphytic environment of the live stone might have a stronger ability of self-regulation; the algal symbionts mainly consisted of Cladocopium sp. and showed no significant effect by the Vibrio infection. This study verified that V. fortis is the primary pathogenic bacterium causing coral bleaching, revealed changes in the microbial community caused by its infection, provided strong evidence for the "bacterial bleaching" hypothesis, and provided an experimental experience for the exploration of the interaction mechanism among microbial communities, especially coral-associated Vibrio in the coral ecosystem, and potential probiotic strategy or QS regulation on further coral disease control.

Microbiota of healthy and bleached corals of the species Siderastrea stellata in response to river influx and seasonality in Brazilian northeast

Although coral bleaching is increasing worldwide due to warming oceans exacerbated by climate change, there has been a growing recognition that local stressors may play an additional role. Important stressors include the physicochemical and microbiological influences that are related to river runoff. Here, we investigated the microbiota associated to mucus and tissue of endemic coral Siderastrea stellata, collected from Brazilian northeast coral reefs of Barra de Santo Antônio (subject to river runoff) and Maragogi (minimal river runoff) during both the rainy and dry seasons. We sequenced the V4 region of 16S rDNA and used multiple R packages to process raw data and performed statistical analysis to reveal the microbial community structure composition and functional predictions. Major dissimilarities between microbial communities were related to seasonality, while healthy and bleached specimens were mainly associated with the enrichment of several less abundant taxa involved in specific metabolic functions, mainly related to the nitrogen cycle. We were not able to observe the dominance of groups that has been previously associated with bleachings, such as Vibrionaceae or Burkholderiaceae. The influx of freshwater appears to increase the homogeneity between individuals in Barra de Santo Antonio, especially during the rainy season. By contrast, we observed an increased homogeneity between samples in Maragogi during the dry season. Understanding the dynamics of the coral microbiota and how bleaching appears in response to specific environmental variables, in addition to determining the conditions that lead to a more robust coral microbiota, is essential for choosing the most appropriate area and conservation methods, for example.

Diversity of the holopelagic Sargassum microbiome from the Great Atlantic Sargassum Belt to coastal stranding locations

The holopelagic brown macroalgae Sargassum natans and Sargassum fluitans form essential habitats for attached and mobile fauna which contributes to a unique biodiversity in the Atlantic Ocean. However, holopelagic Sargassum natans (genotype I & VIII) and Sargassum fluitans (genotype III) have begun forming large accumulations with subsequent strandings on the western coast of Africa, the Caribbean and northern Brazil, threatening local biodiversity of coastal ecosystems and triggering economic losses. Moreover, stranded masses of holopelagic Sargassum may introduce or facilitate growth of bacteria that are not normally abundant in coastal regions where Sargassum is washing ashore. Hitherto, it is not clear how the holopelagic Sargassum microbiome varies across its growing biogeographic range and what factors drive the microbial composition. We determined the microbiome associated with holopelagic Sargassum from the Great Atlantic Sargassum Belt to coastal stranding sites in Mexico and Florida. We characterized the Sargassum microbiome via amplicon sequencing of the 16S V4 region hypervariable region of the rRNA gene. The microbial community of holopelagic Sargassum was mainly composed of photo(hetero)trophs, organic matter degraders and potentially pathogenic bacteria from the Pseudomonadaceae, Rhodobacteraceae and Vibrionaceae. Sargassum genotypes S. natans I, S. natans VIII and S. fluitans III contained similar microbial families, but relative abundances and diversity varied. LEfSE analyses further indicated biomarker genera that were indicative of Sargassum S. natans I/VIII and S. fluitans III. The holopelagic Sargassum microbiome showed biogeographic patterning with high relative abundances of Vibrio spp., but additional work is required to determine whether that represents health risks in coastal environments. Our study informs coastal management policy, where the adverse sanitary effects of stranded Sargassum might impact the health of coastal ecosystems.

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

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

Ciliated protozoan occurrence and association in the pathogenesis of coral disease

Various microbial infections have significantly contributed to disease progression leading to the mortality of corals. However, the holobiont and the external surfaces of coral, including the secreted mucus, provide a varied microenvironment that attracts ciliates based on their feeding preferences. Besides, some ciliates (e.g., Philasterine scuticociliate) may enter through the injuries or lesions on corals or through their indirect interactions with other types of microbes that influence coral health. Thus, ciliates occurrence and association are described with 12 different diseases worldwide. White syndrome disease lesions have diverse ciliate associations, and higher ciliate diversity was observed with diseased genera Acropora. Also, it was described, about sixteen ciliate species ingest coral Symbiodiniaceae and histophagous ciliates for coral tissue loss as secondary invaders. However, the ciliates nature of association with the coral disease remains unclear for primary or opportunistic secondary pathogenicity. Herein, we explore the urgent need to understand the complex interactions of ciliates in coral health.

Microbial community structure shifts and potential Symbiodinium partner bacterial groups of bleaching coral Pocillopora verrucosa in South China Sea

The community structure of coral associated microorganisms will change greatly in coral bleaching. However, the relationship between specific bacteria groups and Symbiodinium, which is easy to be found in the bleaching process, has been ignored for a long time. In this study, the changes of coral microbial community during a natural bleaching event in the South China Sea were studied by 16S rRNA gene high-throughput sequencing. The microbial community composition of bleached corals was significantly different from that of normal corals (P < 0.001). OTUs belong to Bacillus, Exiguobacterium, Oceanobacillus, Saccharibacteria and Ostreobiaceae was significantly increased in the bleaching corals. The relative abundance of 30.9% OTUS changed significantly during coral bleaching. The relative abundance of potential coral pathogenic groups was not significantly different between normal and bleaching corals. Symbiodinium positively correlated bacterial groups accounted for 6.9% and 4.3% in the normal corals and bleached corals, respectively. The dominated groups of potential Symbiodinium-partner bacteria are Lactococcus and Bacillus. The potential Symbiodinium-partner bacterial groups in bleached corals were significantly lower than that in the normal corals, which further showed their coexistence with Symbiodinium. This study provides insight into the role of potential Symbiodinium-partner bacterial groups in the coral bleaching process and supports the theory of beneficial microorganisms for corals.

Heat stress destabilizes symbiotic nutrient cycling in corals

Ocean warming is causing repeated mass coral bleaching, leading to catastrophic losses of coral reefs worldwide. Our ability to slow or revert this decline is hampered by an incomplete understanding of the processes underlying the breakdown of the coral–algal symbiosis. Here, we show that heat stress destabilizes the nutrient cycling between corals and their endosymbiotic algae long before bleaching becomes apparent. Notably, increased metabolic energy demands shift the coral–algal symbiosis from a nitrogen- to a carbon-limited state, reducing translocation and recycling of photosynthetic carbon. This effectively undermines the ecological advantage of harboring algal symbionts and directly contributes to the breakdown of the coral–algal symbiosis during heat stress.

Recurrent mass bleaching events are pushing coral reefs worldwide to the brink of ecological collapse. While the symptoms and consequences of this breakdown of the coral–algal symbiosis have been extensively characterized, our understanding of the underlying causes remains incomplete. Here, we investigated the nutrient fluxes and the physiological as well as molecular responses of the widespread coral Stylophora pistillata to heat stress prior to the onset of bleaching to identify processes involved in the breakdown of the coral–algal symbiosis. We show that altered nutrient cycling during heat stress is a primary driver of the functional breakdown of the symbiosis. Heat stress increased the metabolic energy demand of the coral host, which was compensated by the catabolic degradation of amino acids. The resulting shift from net uptake to release of ammonium by the coral holobiont subsequently promoted the growth of algal symbionts and retention of photosynthates. Together, these processes form a feedback loop that will gradually lead to the decoupling of carbon translocation from the symbiont to the host. Energy limitation and altered symbiotic nutrient cycling are thus key factors in the early heat stress response, directly contributing to the breakdown of the coral–algal symbiosis. Interpreting the stability of the coral holobiont in light of its metabolic interactions provides a missing link in our understanding of the environmental drivers of bleaching and may ultimately help uncover fundamental processes underpinning the functioning of endosymbioses in general.

Microscale tracking of coral-vibrio interactions

To improve our understanding of coral infection and disease, it is important to study host-pathogen interactions at relevant spatio-temporal scales. Here, we provide a dynamic microscopic view of the interaction between a coral pathogen, Vibrio coralliilyticus and its coral host Pocillopora damicornis. This was achieved using a microfluidics-based system facilitating control over flow, light and temperature conditions. Combined with time-resolved biochemical and microbial analyses of the system exudates, this approach provides novel insights into the early phases of a coral infection at unprecedented spatio-temporal resolution. We provide evidence that infection may occur through ingestion of the pathogen by the coral polyps, or following pathogen colonization of small tissue lesions on the coral surface. Pathogen ingestion invariably induced the release of pathogen-laden mucus from the gastrovascular cavity. Despite the high bacterial load used in our experiments, approximately one-third of coral fragments tested did not develop further symptoms. In the remaining two-thirds, mucus spewing was followed by the severing of calicoblastic connective tissues (coenosarc) and subsequently necrosis of most polyps. Despite extensive damage to symptomatic colonies, we frequently observed survival of individual polyps, often accompanied by polyp bail-out. Biochemical and microbial analyses of exudates over the course of symptomatic infections revealed that severing of the coenosarc was followed by an increase in matrix metaloprotease activity, and subsequent increase in both pathogen and total bacterial counts. Combined, these observations provide a detailed description of a coral infection, bringing us a step closer to elucidating the complex interactions underlying coral disease.

Gene expression associated with disease resistance and long-term growth in a reef-building coral

Rampant coral disease, exacerbated by climate change and other anthropogenic stressors, threatens reefs worldwide, especially in the Caribbean. Physically isolated yet genetically connected reefs such as Flower Garden Banks National Marine Sanctuary (FGBNMS) may serve as potential refugia for degraded Caribbean reefs. However, little is known about the mechanisms and trade-offs of pathogen resistance in reef-building corals. Here, we measure pathogen resistance in Montastraea cavernosa from FGBNMS. We identified individual colonies that demonstrated resistance or susceptibility to Vibrio spp. in a controlled laboratory environment. Long-term growth patterns suggest no trade-off between disease resistance and calcification. Predictive (pre-exposure) gene expression highlights subtle differences between resistant and susceptible genets, encouraging future coral disease studies to investigate associations between resistance and replicative age and immune cell populations. Predictive gene expression associated with long-term growth underscores the role of transmembrane proteins involved in cell adhesion and cell-cell interactions, contributing to the growing body of knowledge surrounding genes that influence calcification in reef-building corals. Together these results demonstrate that coral genets from isolated sanctuaries such as FGBNMS can withstand pathogen challenges and potentially aid restoration efforts in degraded reefs. Furthermore, gene expression signatures associated with resistance and long-term growth help inform strategic assessment of coral health parameters.

Coral gasdermin triggers pyroptosis

Gasdermins are executioners of the inflammatory cell death pathway pyroptosis that has so far been studied exclusively in vertebrates. In this study, we identified gasdermin E (GSDME) homologs in several invertebrate species including corals. We report that coral GSDME was cleaved by caspase 3 at two sites, yielding two active isoforms of GSDME N-terminal domain that were capable of inducing pyroptosis. Ectopic coexpression of coral GSDME and caspase 3 in human cells promoted pyroptosis. Corals infected with Vibrio coralliilyticus, a bacterial pathogen causing rapid tissue necrosis of corals worldwide, exhibited necrotic death with elevated caspase 3 activity and GSDME cleavage, whereas inhibition of caspase 3 blocked GSDME cleavage and protected corals from necrotic death. These results indicate that functional gasdermin exists in invertebrates and that coral gasdermin is involved in pathogen-induced coral death. Furthermore, our studies also suggest that mediation of pyroptosis is an evolutionarily conserved function of gasdermins.

Differential expression of Exaiptasia pallida GIMAP genes upon induction of apoptosis and autophagy suggests a potential role in cnidarian symbiosis and disease

Coral reefs, one of the world's most productive and diverse ecosystems, are currently threatened by a variety of stressors that result in increased prevalence of both bleaching and disease. Therefore, understanding the molecular mechanisms involved in these responses is critical to mitigate future damage to the reefs. One group of genes that is potentially involved in cnidarian immunity and symbiosis is GTPases of immunity associated proteins (GIMAP). In vertebrates, this family of proteins is involved in regulating the fate of developing lymphocytes and interacts with proteins involved in apoptosis and autophagy. As apoptosis, autophagy and immunity have previously been shown to be involved in cnidarian symbiosis and disease, the goal of this research was to determine the role of cnidarian GIMAPs in these processes using the anemone Exaiptasia pallida To do so, GIMAP genes were characterized in the E. pallida genome and changes in gene expression were measured using qPCR in response to chemical induction of apoptosis, autophagy and treatment with the immune stimulant lipopolysaccharide (LPS) in both aposymbiotic and symbiotic anemones. The results revealed four GIMAP-like genes in E. pallida, referred to as Ep_GIMAPs Induction of apoptosis and autophagy resulted in a general downregulation of Ep_GIMAPs, but no significant changes were observed in response to LPS treatment. This indicates that Ep_GIMAPs may be involved in the regulation of apoptosis and autophagy, and therefore could play a role in cnidarian-dinoflagellate symbiosis. Overall, these results increase our knowledge on the function of GIMAPs in a basal metazoan.

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.

Variable effects of local management on coral defenses against a thermally regulated bleaching pathogen

Bleaching and disease are decimating coral reefs especially when warming promotes bleaching pathogens, such as Vibrio coralliilyticus. We demonstrate that sterilized washes from three common corals suppress V. coralliilyticus but that this defense is compromised when assays are run at higher temperatures. For a coral within the ecologically critical genus Acropora, inhibition was 75 to 154% greater among colonies from coral-dominated marine protected areas versus adjacent fished areas that were macroalgae-dominated. Acropora microbiomes were more variable within fished areas, suggesting that reef degradation may also perturb coral microbial communities. Defenses of a robust poritid coral and a weedy pocilloporid coral were not affected by reef degradation, and microbiomes were unaltered for these species. For some ecologically critical, but bleaching-susceptible, corals such as Acropora, local management to improve reef state may bolster coral resistance to global change, such as bacteria-induced coral bleaching during warming events.

The effect of bacteria on planula-larvae settlement and metamorphosis in the octocoral Rhytisma fulvum fulvum

While increasing evidence supports a key role of bacteria in coral larvae settlement and development, the relative importance of environmentally-acquired versus vertically-transferred bacterial population is not clear. Here we have attempted to elucidate the role of post-brooding-acquired bacteria on the development of planula-larvae of the octocoral Rhytisma f. fulvum, in an in vitro cultivation system employing different types of filtered (FSW) and autoclaved (ASW) seawater and with the addition of native bacteria. A good development of larvae was obtained in polystyrene 6-well cell culture plates in the absence of natural reef substrata, achieving a 60–80% of larvae entering metamorphosis after 32 days, even in bacteria-free seawater, indicating that the bacteria acquired during the brooding period are sufficient to support planulae development. No significant difference in planulae attachment and development was observed when using 0.45 μm or 0.22 μm FSW, although autoclaving the 0.45 μm FSW negatively affected larval development, indicating the presence of beneficial bacteria. Autoclaving the different FSW homogenized the development of the larvae among the different treatments. The addition of bacterial strains isolated from the different FSW did not cause any significant effect on planulae development, although some specific strains of the genus Alteromonas seem to be beneficial for larvae development. Light was beneficial for planulae development after day 20, although no Symbiodinium cells could be observed, indicating either that light acts as a positive cue for larval development or the presence of beneficial phototrophic bacteria in the coral microbiome. The feasibility of obtaining advanced metamorphosed larvae in sterilized water provides an invaluable tool for studying the physiological role of the bacterial symbionts in the coral holobiont and the specificity of bacteria-coral interactions.

Host-microbiota interactions shed light on mortality events in the striped venus clam Chamelea gallina

Mass mortalities due to disease outbreaks have recently affected a number of major taxa in marine ecosystems. Climate- and pollution-induced stress may compromise host immune defenses, increasing the risk of opportunistic diseases. Despite growing evidence that mass mortality events affecting marine species worldwide are strongly influenced by the interplay of numerous environmental factors, the reductionist approaches most frequently used to investigate these factors hindered the interpretation of these multifactorial pathologies. In this study, we propose a broader approach based on the combination of RNA-sequencing and 16S microbiota analyses to decipher the factors underlying mass mortality in the striped venus clam, Chamelea gallina, along the Adriatic coast. On one hand, gene expression profiling and functional analyses of microbial communities showed the over-expression of several genes and molecular pathways involved in xenobiotic metabolism, suggesting potential chemical contamination in mortality sites. On the other hand, the down-regulation of several genes involved in immune and stress response, and the over-representation of opportunistic pathogens such as Vibrio and Photobacterium spp. indicates that these microbial species may take advantage of compromised host immune pathways and defense mechanisms that are potentially affected by chemical exposure, resulting in periodic mortality events. We propose the application of our approach to interpret and anticipate the risks inherent in the combined effects of pollutants and microbes on marine animals in today's rapidly changing environment.

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.

Global Networks of Symbiodinium-Bacteria Within the Coral Holobiont

Scleractinian corals form the framework of coral reefs and host abundant and diverse microbial communities that are fundamental to their success. A very limited number of studies have examined the co-occurrence of multiple partners within the coral 'holobiont' and their pattern of specificity over different geographical scales. In this study, we explored two molecular sequence datasets representing associations between corals and dinoflagellates in the genus Symbiodinium and between corals and bacteria, across the globe. Through a network theory approach, we characterised patterns of co-occurrences between bacteria and Symbiodinium with 13 coral genera across six water basins. The majority of the bacteria-Symbiodinium co-occurrences were specific to either a coral genus or water basin, emphasising both coral host and environment as important factors driving the diversity of coral assemblages. Yet, results also identified bacteria and Symbiodinium that were shared by multiple coral genera across several water basins. The analyses indicate that shared co-occurrences are independent of the phylogenetic and biogeographic relationship of coral hosts.

Ruegeria sp. Strains Isolated from the Reef-Building Coral Galaxea fascicularis Inhibit Growth of the Temperature-Dependent Pathogen Vibrio coralliilyticus

The coral microbiome has attracted increased attention because of its potential roles in host protection against deadly diseases. However, little is known about the role of coral-associated bacteria against the temperature-dependent opportunistic pathogen Vibrio coralliilyticus. In this study, we tested whether bacteria associated with the reef-building coral Galaxea fascicularis could inhibit the growth of V. coralliilyticus. Twenty-nine cultivable bacteria were successfully isolated from a healthy colony of G. fascicularis kept in an aquarium. Among the bacterial isolates, three Ruegeria sp. strains inhibited the growth of V. coralliilyticus P1 as a reference strain and Vibrio sp. isolated in this study. Ruegeria sp. strains were also detected from other G. fascicularis colonies in the aquarium and in previous field studies by 16S rRNA amplicon sequencing, suggesting that Ruegeria sp. strains are common among G. fascicularis colonies. These results illuminate the potential role of Ruegeria sp. in protecting corals against pathogenic Vibrio species.

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.

A Newly Designed Primer Revealed High Phylogenetic Diversity of Endozoicomonas in Coral Reefs

Endozoicomonas bacteria are commonly regarded as having a potentially symbiotic relationship with their coral hosts. However, their diversity and phylogeny in samples collected from various sources remain unclear. Therefore, we designed an Endozoicomonas-specific primer paired with a bacterial universal primer to detect the 16S ribosomal RNA (rRNA) genes of this taxon and conducted an in-depth investigation of the Endozoicomonas community structure in reef-building corals. The primer had high specificity in the V3-V4 region (95.6%) and its sensitivity was high, particularly when Endozoicomonas was rare in samples (e.g., in seawater, which had a higher alpha diversity of Endozoicomonas than corals). In coral samples, predominant V3-V4 ribotypes had greater divergence than predominant V1-V2 ribotypes, and were grouped into at least 9 novel clades in a phylogenetic tree, indicating Endozoicomonas had high phylogenetic diversity. Divergence within this genus was potentially higher than that among 7 outgroup genera based on the phylogenetic distances of partial 16S rDNA sequences, suggesting that the taxonomy of this genus needs to be revised. In conclusion, dominant Endozoicomonas populations had variable phylogenies; furthermore, the newly designed primers may be useful molecular tools for the reliable detection of the Endozoicomonas community in marine environments.

Predicted Bacterial Interactions Affect in Vivo Microbial Colonization Dynamics in Nematostella

The maintenance and resilience of host-associated microbiota during development is a fundamental process influencing the fitness of many organisms. Several host properties were identified as influencing factors on bacterial colonization, including the innate immune system, mucus composition, and diet. In contrast, the importance of bacteria–bacteria interactions on host colonization is less understood. Here, we use bacterial abundance data of the marine model organism Nematostella vectensis to reconstruct potential bacteria–bacteria interactions through co-occurrence networks. The analysis indicates that bacteria–bacteria interactions are dynamic during host colonization and change according to the host’s developmental stage. To assess the predictive power of inferred interactions, we tested bacterial isolates with predicted cooperative or competitive behavior for their ability to influence bacterial recolonization dynamics. Within 3 days of recolonization, all tested bacterial isolates affected bacterial community structure, while only competitive bacteria increased bacterial diversity. Only 1 week after recolonization, almost no differences in bacterial community structure could be observed between control and treatments. These results show that predicted competitive bacteria can influence community structure for a short period of time, verifying the in silico predictions. However, within 1 week, the effects of the bacterial isolates are neutralized, indicating a high degree of resilience of the bacterial community.

Corallivory and the microbial debacle in two branching scleractinians

The grazing activity by specific marine organisms represents a growing threat to the survival of many scleractinian species. For example, the recent proliferation of the corallivorous gastropod Drupella now constitutes a critical case in all South-East Asian waters. If the damaging effects caused by this marine snail on coral polyps are relatively well known, the indirect incidence of predation on coral microbial associates is still obscure and might also potentially impair coral health. In this study, we compared the main ecological traits of coral-associated bacterial and viral communities living in the mucus layer of Acropora formosa and Acropora millepora, of healthy and predated individuals (i.e., colonized by Drupella rugosa), in the Bay of Van Phong (Vietnam). Our results show a substantial impact of the gastropod on a variety of microbiological markers. Colonized corals harbored much more abundant and active epibiotic bacteria whose community composition shifted toward more pathogenic taxa (belonging to the Vibrionales, Clostridiales, Campylobacterales, and Alteromonadales orders), together with their specific phages. Viral epibionts were also greatly influenced by Drupella corallivory with spectacular modifications in their concentrations, life strategies, genotype richness, and diversity. Novel and abundant circular Rep-encoding ssDNA viruses (CRESS-DNA viruses) were detected and characterized in grazed corals and we propose that their occurrence may serve as indicator of the coral health status. Finally, our results reveal that corallivory can cause severe dysbiosis by altering virus-bacteria interactions in the mucus layer, and ultimately favoring the development of local opportunistic infections.

Shifting white pox aetiologies affecting Acropora palmata in the Florida Keys, 1994-2014

We propose 'the moving target hypothesis' to describe the aetiology of a contemporary coral disease that differs from that of its historical disease state. Hitting the target with coral disease aetiology is a complex pursuit that requires understanding of host and environment, and may lack a single pathogen solution. White pox disease (WPX) affects the Caribbean coral Acropora palmata. Acroporid serratiosis is a form of WPX for which the bacterial pathogen (Serratia marcescens) has been established. We used long-term (1994-2014) photographic monitoring to evaluate historical and contemporary epizootiology and aetiology of WPX affecting A. palmata at eight reefs in the Florida Keys. Ranges of WPX prevalence over time (0-71.4%) were comparable for the duration of the 20-year study. Whole colony mortality and disease severity were high in historical (1994-2004), and low in contemporary (2008-2014), outbreaks of WPX. Acroporid serratiosis was diagnosed for some historical (1999, 2003) and contemporary (2012, 2013) outbreaks, but this form of WPX was not confirmed for all WPX cases. Our results serve as a context for considering aetiology as a moving target for WPX and other coral diseases for which pathogens are established and/or candidate pathogens are identified. Coral aetiology investigations completed to date suggest that changes in pathogen, host and/or environment alter the disease state and complicate diagnosis.

Microbial dinitrogen fixation in coral holobionts exposed to thermal stress and bleaching

Coral holobionts (i.e., coral-algal-prokaryote symbioses) exhibit dissimilar thermal sensitivities that may determine which coral species will adapt to global warming. Nonetheless, studies simultaneously investigating the effects of warming on all holobiont members are lacking. Here we show that exposure to increased temperature affects key physiological traits of all members (herein: animal host, zooxanthellae and diazotrophs) of both Stylophora pistillata and Acropora hemprichii during and after thermal stress. S. pistillata experienced severe loss of zooxanthellae (i.e., bleaching) with no net photosynthesis at the end of the experiment. Conversely, A. hemprichii was more resilient to thermal stress. Exposure to increased temperature (+ 6°C) resulted in a drastic increase in daylight dinitrogen (N2 ) fixation, particularly in A. hemprichii (threefold compared with controls). After the temperature was reduced again to in situ levels, diazotrophs exhibited a reversed diel pattern of activity, with increased N2 fixation rates recorded only in the dark, particularly in bleached S. pistillata (twofold compared to controls). Concurrently, both animal hosts, but particularly bleached S. pistillata, reduced both organic matter release and heterotrophic feeding on picoplankton. Our findings indicate that physiological plasticity by coral-associated diazotrophs may play an important role in determining the response of coral holobionts to ocean warming.

Microbial Dysbiosis: Rethinking Disease in Marine Ecosystems

With growing environmental pressures placed on our marine habitats there is concern that the prevalence and severity of diseases affecting marine organisms will increase. Yet relative to terrestrial systems, we know little about the underlying causes of many of these diseases. Moreover, factors such as saprophytic colonizers and a lack of baseline data on healthy individuals make it difficult to accurately assess the role of specific microbial pathogens in disease states. Emerging evidence in the field of medicine suggests that a growing number of human diseases result from a microbiome imbalance (or dysbiosis), questioning the traditional view of a singular pathogenic agent. Here we discuss the possibility that many diseases seen in marine systems are, similarly, the result of microbial dysbiosis and the rise of opportunistic or polymicrobial infections. Thus, understanding and managing disease in the future will require us to also rethink definitions of disease and pathogenesis for marine systems. We suggest that a targeted, multidisciplinary approach that addresses the questions of microbial symbiosis in both healthy and diseased states, and at that the level of the holobiont, will be key to progress in this area.

A Bacterial Pathogen Displaying Temperature-Enhanced Virulence of the Microalga Emiliania huxleyi

Emiliania huxleyi is a globally abundant microalga that plays a significant role in biogeochemical cycles. Over the next century, sea surface temperatures are predicted to increase drastically, which will likely have significant effects on the survival and ecology of E. huxleyi. In a warming ocean, this microalga may become increasingly vulnerable to pathogens, particularly those with temperature-dependent virulence. Ruegeria is a genus of Rhodobacteraceae whose population size tracks that of E. huxleyi throughout the alga’s bloom–bust lifecycle. A representative of this genus, Ruegeria sp. R11, is known to cause bleaching disease in a red macroalga at elevated temperatures. To investigate if the pathogenicity of R11 extends to microalgae, it was co-cultured with several cell types of E. huxleyi near the alga’s optimum (18°C), and at an elevated temperature (25°C) known to induce virulence in R11. The algal populations were monitored using flow cytometry and pulse-amplitude modulated fluorometry. Cultures of algae without bacteria remained healthy at 18°C, but lower cell counts in control cultures at 25°C indicated some stress at the elevated temperature. Both the C (coccolith-bearing) and S (scale-bearing swarming) cell types of E. huxleyi experienced a rapid decline resulting in apparent death when co-cultured with R11 at 25°C, but had no effect on N (naked) cell type at either temperature. R11 had no initial negative impact on C and S type E. huxleyi population size or health at 18°C, but caused death in older co-cultures. This differential effect of R11 on its host at 18 and 25°C suggest it is a temperature-enhanced opportunistic pathogen of E. huxleyi. We also detected caspase-like activity in dying C type cells co-cultured with R11, which suggests that programmed cell death plays a role in the death of E. huxleyi triggered by R11 – a mechanism induced by viruses (EhVs) and implicated in E. huxleyi bloom collapse. Given that E. huxleyi has recently been shown to have acquired resistance against EhVs at elevated temperature, bacterial pathogens with temperature-dependent virulence, such as R11, may become much more important in the ecology of E. huxleyi in a warming climate.

Snapshot of a Bacterial Microbiome Shift during the Early Symptoms of a Massive Sponge Die-Off in the Western Mediterranean

Ocean warming is affecting marine benthic ecosystems through mass mortality events that involve marine invertebrates, in particular bivalves, corals, and sponges. Among these events, extensive die-offs of Ircinia fasciculata sponges have been recurrently reported in western Mediterranean. The goal of our study was to test whether the temperature-related mass sponge die-offs were associated with or preceded by an early unbalanced bacterial microbiome in the sponge tissues. We took advantage of the early detection of disease and compared the microbiomes of healthy vs. early diseased I. fasciculata tissues. Our results showed a microbiome shift in early diseased tissues. The abundance of Gammaproteobacteria and Acidobacteria increased and that of Deltaproteobacteria decreased in diseased vs. healthy tissues. The change in community composition was also noticeable at the operational taxonomic unit (OTU) level. Diseased tissues contained more bacterial sequences previously identified in injured or stressed sponges and corals than healthy tissues. Bacterial diversity increased significantly in diseased tissues, which contained a higher number of low abundance OTUs. Our results do not support the hypothesis of one particular pathogen, whether a Vibrio or any other bacteria, triggering the Northwestern Mediterranean mass mortalities of I. fasciculata. Our data rather suggest an early disruption of the bacterial microbiome balance in healthy sponges through a shift in OTU abundances, and the purported consequent decline of the sponge fitness and resistance to infections. Opportunistic bacteria could colonize the sponge tissues, taking benefit of the sponge weakness, before one or more virulent pathogens might proliferate ending in the mass sponge die-off.

Host, pathogen and the environment: the case of Macrobrachium rosenbergii, Vibrio parahaemolyticus and magnesium

Macrobrachium rosenbergii is well-known as the giant freshwater prawn, and is a commercially significant source of seafood. Its production can be affected by various bacterial contaminations. Among which, the genus Vibrio shows a higher prevalence in aquatic organisms, especially M. rosenbergii, causing food-borne illnesses. Vibrio parahaemolyticus, a species of Vibrio is reported as the main causative of the early mortality syndrome. Vibrio parahaemolyticus infection in M. rosenbergii was studied previously in relation to the prawn's differentially expressed immune genes. In the current review, we will discuss the growth conditions for both V. parahaemolyticus and M. rosenbergii and highlight the role of magnesium in common, which need to be fully understood. Till date, there has not been much research on this aspect of magnesium. We postulate a model that screens a magnesium-dependent pathway which probably might take effect in connection with N-acetylglucosamine binding protein and chitin from V. parahaemolyticus and M. rosenbergii, respectively. Further studies on magnesium as an environment for V. parahaemolyticus and M. rosenbergii interaction studies will provide seafood industry with completely new strategies to employ and to avoid seafood related contaminations.

The Role of Vibrios in Diseases of Corals

The tissue, skeleton, and secreted mucus of corals supports a highly dynamic and diverse community of microbes, which play a major role in the health status of corals such as the provision of essential nutrients or the metabolism of waste products. However, members of the Vibrio genus are prominent as causative agents of disease in corals. The aim of this chapter is to review our understanding of the spectrum of disease effects displayed by coral-associated vibrios, with a particular emphasis on the few species where detailed studies of pathogenicity have been conducted. The role of Vibrio shilonii in seasonal bleaching of Oculina patagonica and the development of the coral probiotic hypothesis is reviewed, pointing to unanswered questions about this phenomenon. Detailed consideration is given to studies of V. coralliilyticus and related pathogens and changes in the dominance of vibrios associated with coral bleaching. Other Vibrio-associated disease syndromes discussed include yellow band/blotch disease and tissue necrosis in temperate gorgonian corals. The review includes analysis of the role of enzymes, resistance to oxidative stress, and quorum sensing in virulence of coral-associated vibrios. The review concludes that we should probably regard most-possibly all-vibrios as "opportunistic" pathogens which, under certain environmental conditions, are capable of overwhelming the defense mechanisms of appropriate hosts, leading to rapid growth and tissue destruction.

Draft Genome Sequence of Vibrio sp. Strain Evh12, a Bacterium Retrieved from the Gorgonian Coral Eunicella verrucosa

To shed light on the associations established between Vibrio species and soft corals in coastal ecosystems, we report here the draft genome sequence of Vibrio sp. strain Evh12, a bacterium that has been isolated from the gorgonian coral Eunicella verrucosa and that shows antagonistic activity against Escherichia coli.

Symbiodinium clades A and D differentially predispose Acropora cytherea to disease and Vibrio spp. colonization

Coral disease outbreaks have increased over the last three decades, but their causal agents remain mostly unclear (e.g., bacteria, viruses, fungi, protists). This study details a 14-month-long survey of coral colonies in which observations of the development of disease was observed in nearly half of the sampled colonies. A bimonthly qPCR method was used to quantitatively and qualitatively evaluate Symbiodinium assemblages of tagged colonies, and to detect the presence of Vibrio spp. Firstly, our data showed that predisposition to disease development in general, and, more specifically, infection by Vibrio spp. in Acropora cytherea depended on which clades of Symbiodinium were harbored. In both cases, harboring clade D rather than A was beneficial to the coral host. Secondly, the detection of Vibrio spp. in only colonies that developed disease strongly suggests opportunistic traits of the bacteria. Finally, even if sporadic cases of switching and probably shuffling were observed, this long-term survey does not suggest specific-clade recruitment in response to stressors. Altogether, our results demonstrate that the fitness of the coral holobiont depends on its initial consortium of Symbiodinium, which is distinct among colonies, rather than a temporary adaptation achieved through acquiring different Symbiodinium clades.

Histopathology of crustose coralline algae affected by white band and white patch diseases

Crustose coralline algae (CCA) are major benthic calcifiers that play crucial roles in marine ecosystems, particularly coral reefs. Over the past two decades, epizootics have been reported for several CCA species on coral reefs worldwide. However, their causes remain often unknown in part because few studies have investigated CCA pathologies at a microscopic scale. We studied the cellular changes associated with two syndromes: Coralline White Band Syndrome (CWBS) and Coralline White Patch Disease (CWPD) from samples collected in Curaçao, southern Caribbean. Healthy-looking tissue of diseased CCA did not differ from healthy tissue of healthy CCA. In diseased tissues of both pathologies, the three characteristic cell layers of CCA revealed cells completely depleted of protoplasmic content, but presenting an intact cell wall. In addition, CWBS showed a transition area between healthy and diseased tissues consisting of cells partially deprived of protoplasmic material, most likely corresponding to the white band characterizing the disease at the macroscopic level. This transition area was absent in CWPD. Regrowth at the lesion boundary were sometimes observed in both syndromes. Tissues of both healthy and diseased CCA were colonised by diverse boring organisms. Fungal infections associated with the diseased cells were not seen. However, other bioeroders were more abundant in diseased vs healthy CCA and in diseased vs healthy-looking tissues of diseased CCA. Although their role in the pathogenesis is unclear, this suggests that disease increases CCA susceptibility to bioerosion. Further investigations using an integrated approach are needed to carry out the complete diagnosis of these diseases.

Coral Mucus Is a Hot Spot for Viral Infections

There is increasing suspicion that viral communities play a pivotal role in maintaining coral health, yet their main ecological traits still remain poorly characterized. In this study, we examined the seasonal distribution and reproduction pathways of viruses inhabiting the mucus of the scleractinians Fungia repanda and Acropora formosa collected in Nha Trang Bay (Vietnam) during an 11-month survey. The strong coupling between epibiotic viral and bacterial abundance suggested that phages are dominant among coral-associated viral communities. Mucosal viruses also exhibited significant differences in their main features between the two coral species and were also remarkably contrasted with their planktonic counterparts. For example, their abundance (inferred from epifluorescence counts), lytic production rates (KCN incubations), and the proportion of lysogenic cells (mitomycin C inductions) were, respectively, 2.6-, 9.5-, and 2.2-fold higher in mucus than in the surrounding water. Both lytic and lysogenic indicators were tightly coupled with temperature and salinity, suggesting that the life strategy of viral epibionts is strongly dependent upon environmental circumstances. Finally, our results suggest that coral mucus may represent a highly favorable habitat for viral proliferation, promoting the development of both temperate and virulent phages. Here, we discuss how such an optimized viral arsenal could be crucial for coral viability by presumably forging complex links with both symbiotic and adjacent nonsymbiotic microorganisms.

A comprehensive analysis of the microbial communities of healthy and diseased marine macroalgae and the detection of known and potential bacterial pathogens

Microorganisms are increasingly being recognized as the causative agents in the diseases of marine higher organisms, such as corals, sponges, and macroalgae. Delisea pulchra is a common, temperate red macroalga, which suffers from a bleaching disease. Two bacterial strains, Nautella italica R11 and Phaeobacter gallaeciensis LSS9, have been shown in vitro to cause bleaching symptoms, but previous work has failed to detect them during a natural bleaching event. To provide a link between in vitro observations and natural occurrences of the disease, we employ here deep-sequencing of the 16S rRNA gene to comprehensively analyze the community composition of healthy and diseased D. pulchra samples from two separate locations. We observed operational taxonomic units (OTUs) with 100% identity and coverage to the 16S RNA gene sequence of both in vitro pathogens, but only the OTU with similarity to strain LSS9 showed a statistically significant higher abundance in diseased samples. Our analysis also reveals the existence of other bacterial groups within the families Rhodobacteraceae and Flavobacteriaceae that strongly contribute to difference between diseased and healthy samples and thus these groups potentially contain novel macroalgal pathogens and/or saprophytes. Together our results provide evidence for the ecological relevance of one kind of in vitro pathogen, but also highlight the possibility that multiple opportunistic pathogens are involved in the bleaching disease of D. pulchra.

High occurrence of viruses in the mucus layer of scleractinian corals

Viruses attract increasing interest from environmental microbiologists seeking to understand their function and role in coral health. However, little is known about their main ecological traits within the coral holobiont. In this study, a quantitative and qualitative characterization of viral and bacterial communities was conducted on the mucus of seven different coral species of the Van Phong Bay (Vietnam). On average, the concentrations of viruses and bacteria were, respectively, 17- and twofold higher in the mucus than in the surrounding water. The examination of bacterial community composition also showed remarkable differences between mucus and water samples. The percentage of active respiring cells was nearly threefold higher in mucus (m = 24.8%) than in water (m = 8.6%). Interestingly, a positive and highly significant correlation was observed between the proportion of active cells and viral abundance in the mucus, suggesting that the metabolism of the bacterial associates is probably a strong determinant of the distribution of viruses within the coral holobiont. Overall, coral mucus, given its unique physicochemical characteristics and sticking properties, can be regarded as a highly selective biotope for abundant, diversified and specialized symbiotic microbial and viral organisms.

Disturbance to conserved bacterial communities in the cold-water gorgonian coral Eunicella verrucosa

The bacterial communities associated with healthy and diseased colonies of the cold-water gorgonian coral Eunicella verrucosa at three sites off the south-west coast of England were compared using denaturing gradient gel electrophoresis (DGGE) and clone libraries. Significant differences in community structure between healthy and diseased samples were discovered, as were differences in the level of disturbance to these communities at each site; this correlated with depth and sediment load. The majority of cloned sequences from healthy coral tissue affiliated with the Gammaproteobacteria. The stability of the bacterial community and dominance of specific genera found across visibly healthy colonies suggest the presence of a specific microbial community. Affiliations included a high proportion of Endozoicomonas sequences, which were most similar to sequences found in tropical corals. This genus has been found in a number of invertebrates and is suggested to have a role in coral health and in the metabolisation of dimethylsulfoniopropionate (DMSP) produced by zooxanthellae. However, screening of colonies for the presence of zooxanthellae produced a negative result. Diseased colonies showed a decrease in affiliated clones and an increase in clones related to potentially harmful/transient microorganisms but no increase in a particular pathogen. This study demonstrates that a better understanding of these bacterial communities, the factors that affect them and their role in coral health and disease will be of critical importance in predicting future threats to temperate gorgonian communities.

The versatile nature of coral-associated viruses

A recent hypothesis considers that many coral pathologies are the result of a sudden structural alteration of the epibiotic bacterial communities in response to environmental disturbances. However, the ecological mechanisms that lead to shifts in their composition are still unclear. In the ocean, viruses represent a major bactericidal agent but little is known on their occurrence within the coral holobiont. Recent reports have revealed that viruses are abundant and diversified within the coral mucus and therefore could be decisive for coral health. However, their mode of action is still unknown, and there is now an urgent need to shed light on the nature of the relationships they might have with the other prokaryotic and eukaryotic members of the holobiont. In this opinion letter, we are putting forward the hypothesis that coral-associated viruses (mostly bacterial and algal viruses), depending on the environmental conditions might either reinforce coral stability or conversely fasten their decline. We propose that these processes are presumably based on an environmentally driven shift in infection strategies allowing viruses to regulate, circumstantially, both coral symbionts (bacteria or Symbiodinium) and surrounding pathogens.

Outcomes of infections of sea anemone Aiptasia pallida with Vibrio spp. pathogenic to corals

Incidents of coral disease are on the rise. However, in the absence of a surrogate animal host, understanding of the interactions between coral pathogens and their hosts remains relatively limited, compared to other pathosystems of similar global importance. A tropical sea anemone, Aiptasia pallida, has been investigated as a surrogate model to study certain aspects of coral biology. Therefore, to test whether the utility of this surrogate model can be extended to study coral diseases, in the present study, we tested its susceptibility to common coral pathogens (Vibrio coralliilyticus and Vibrio shiloi) as well as polymicrobial consortia recovered from the Caribbean Yellow Band Disease (CYBD) lesions. A. pallida was susceptible to each of the tested pathogens. A. pallida responded to the pathogens with darkening of the tissues (associated with an increased melanization) and retraction of tentacles, followed by complete disintegration of polyp tissues. Loss of zooxanthellae was not observed; however, the disease progression pattern is consistent with the behavior of necrotizing pathogens. Virulence of some coral pathogens in Aiptasia was paralleled with their glycosidase activities.

Evolution of TNF-induced apoptosis reveals 550 My of functional conservation

The Precambrian explosion led to the rapid appearance of most major animal phyla alive today. It has been argued that the complexity of life has steadily increased since that event. Here we challenge this hypothesis through the characterization of apoptosis in reef-building corals, representatives of some of the earliest animals. Bioinformatic analysis reveals that all of the major components of the death receptor pathway are present in coral with high-predicted structural conservation with Homo sapiens. The TNF receptor-ligand superfamilies (TNFRSF/TNFSF) are central mediators of the death receptor pathway, and the predicted proteome of Acropora digitifera contains more putative coral TNFRSF members than any organism described thus far, including humans. This high abundance of TNFRSF members, as well as the predicted structural conservation of other death receptor signaling proteins, led us to wonder what would happen if corals were exposed to a member of the human TNFSF (HuTNFα). HuTNFα was found to bind directly to coral cells, increase caspase activity, cause apoptotic blebbing and cell death, and finally induce coral bleaching. Next, immortalized human T cells (Jurkats) expressing a functional death receptor pathway (WT) and a corresponding Fas-associated death domain protein (FADD) KO cell line were exposed to a coral TNFSF member (AdTNF1) identified and purified here. AdTNF1 treatment resulted in significantly higher cell death (P < 0.0001) in WT Jurkats compared with the corresponding FADD KO, demonstrating that coral AdTNF1 activates the H. sapiens death receptor pathway. Taken together, these data show remarkable conservation of the TNF-induced apoptotic response representing 550 My of functional conservation.

Evidence for multiple stressor interactions and effects on coral reefs

Concern is growing about the potential effects of interacting multiple stressors, especially as the global climate changes. We provide a comprehensive review of multiple stressor interactions in coral reef ecosystems, which are widely considered to be one of the most sensitive ecosystems to global change. First, we synthesized coral reef studies that examined interactions of two or more stressors, highlighting stressor interactions (where one stressor directly influences another) and potentially synergistic effects on response variables (where two stressors interact to produce an effect that is greater than purely additive). For stressor-stressor interactions, we found 176 studies that examined at least 2 of the 13 stressors of interest. Applying network analysis to analyze relationships between stressors, we found that pathogens were exacerbated by more costressors than any other stressor, with ca. 78% of studies reporting an enhancing effect by another stressor. Sedimentation, storms, and water temperature directly affected the largest number of other stressors. Pathogens, nutrients, and crown-of-thorns starfish were the most-influenced stressors. We found 187 studies that examined the effects of two or more stressors on a third dependent variable. The interaction of irradiance and temperature on corals has been the subject of more research (62 studies, 33% of the total) than any other combination of stressors, with many studies reporting a synergistic effect on coral symbiont photosynthetic performance (n = 19). Second, we performed a quantitative meta-analysis of existing literature on this most-studied interaction (irradiance and temperature). We found that the mean effect size of combined treatments was statistically indistinguishable from a purely additive interaction, although it should be noted that the sample size was relatively small (n = 26). Overall, although in aggregate a large body of literature examines stressor effects on coral reefs and coral organisms, considerable gaps remain for numerous stressor interactions and effects, and insufficient quantitative evidence exists to suggest that the prevailing type of stressor interaction is synergistic.

Ecological Inferences from a deep screening of the Complex Bacterial Consortia associated with the coral, Porites astreoides

The functional role of the bacterial organisms in the reef ecosystem and their contribution to the coral well-being remain largely unclear. The first step in addressing this gap of knowledge relies on in-depth characterization of the coral microbial community and its changes in diversity across coral species, space and time. In this study, we focused on the exploration of microbial community assemblages associated with an ecologically important Caribbean scleractinian coral, Porites astreoides, using Illumina high-throughput sequencing of the V5 fragment of 16S rRNA gene. We collected data from a large set of biological replicates, allowing us to detect patterns of geographical structure and resolve co-occurrence patterns using network analyses. The taxonomic analysis of the resolved diversity showed consistent and dominant presence of two OTUs affiliated with the order Oceanospirillales, which corroborates a specific pattern of bacterial association emerging for this coral species and for many other corals within the genus Porites. We argue that this specific association might indicate a symbiotic association with the adult coral partner. Furthermore, we identified a highly diverse rare bacterial 'biosphere' (725 OTUs) also living along with the dominant bacterial symbionts, but the assemblage of this biosphere is significantly structured along the geographical scale. We further discuss that some of these rare bacterial members show significant association with other members of the community reflecting the complexity of the networked consortia within the coral holobiont.

16SrDNA Pyrosequencing of the Mediterranean Gorgonian Paramuricea clavata Reveals a Link among Alterations in Bacterial Holobiont Members, Anthropogenic Influence and Disease Outbreaks

Mass mortality events of benthic invertebrates in the Mediterranean Sea are becoming an increasing concern with catastrophic effects on the coastal marine environment. Sea surface temperature anomalies leading to physiological stress, starvation and microbial infections were identified as major factors triggering animal mortality. However the highest occurrence of mortality episodes in particular geographic areas and occasionally in low temperature deep environments suggest that other factors play a role as well. We conducted a comparative analysis of bacterial communities associated with the purple gorgonian Paramuricea clavata, one of the most affected species, collected at different geographic locations and depth, showing contrasting levels of anthropogenic disturbance and health status. Using massive parallel 16SrDNA gene pyrosequencing we showed that the bacterial community associated with healthy P. clavata in pristine locations was dominated by a single genus Endozoicomonas within the order Oceanospirillales which represented ∼90% of the overall bacterial community. P. clavata samples collected in human impacted areas and during disease events had higher bacterial diversity and abundance of disease-related bacteria, such as vibrios, than samples collected in pristine locations whilst showed a reduced dominance of Endozoicomonas spp. In contrast, bacterial symbionts exhibited remarkable stability in P. clavata collected both at euphotic and mesophotic depths in pristine locations suggesting that fluctuations in environmental parameters such as temperature have limited effect in structuring the bacterial holobiont. Interestingly the coral pathogen Vibrio coralliilyticus was not found on diseased corals collected during a deep mortality episode suggesting that neither temperature anomalies nor recognized microbial pathogens are solely sufficient to explain for the events. Overall our data suggest that anthropogenic influence may play a significant role in determining the coral health status by affecting the composition of the associated microbial community. Environmental stressful events and microbial infections may thus be superimposed to compromise immunity and trigger mortality outbreaks.

Coral and macroalgal exudates vary in neutral sugar composition and differentially enrich reef bacterioplankton lineages

Increasing algal cover on tropical reefs worldwide may be maintained through feedbacks whereby algae outcompete coral by altering microbial activity. We hypothesized that algae and coral release compositionally distinct exudates that differentially alter bacterioplankton growth and community structure. We collected exudates from the dominant hermatypic coral holobiont Porites spp. and three dominant macroalgae (one each Ochrophyta, Rhodophyta and Chlorophyta) from reefs of Mo'orea, French Polynesia. We characterized exudates by measuring dissolved organic carbon (DOC) and fractional dissolved combined neutral sugars (DCNSs) and subsequently tracked bacterioplankton responses to each exudate over 48 h, assessing cellular growth, DOC/DCNS utilization and changes in taxonomic composition (via 16S rRNA amplicon pyrosequencing). Fleshy macroalgal exudates were enriched in the DCNS components fucose (Ochrophyta) and galactose (Rhodophyta); coral and calcareous algal exudates were enriched in total DCNS but in the same component proportions as ambient seawater. Rates of bacterioplankton growth and DOC utilization were significantly higher in algal exudate treatments than in coral exudate and control incubations with each community selectively removing different DCNS components. Coral exudates engendered the smallest shift in overall bacterioplankton community structure, maintained high diversity and enriched taxa from Alphaproteobacteria lineages containing cultured representatives with relatively few virulence factors (VFs) (Hyphomonadaceae and Erythrobacteraceae). In contrast, macroalgal exudates selected for less diverse communities heavily enriched in copiotrophic Gammaproteobacteria lineages containing cultured pathogens with increased VFs (Vibrionaceae and Pseudoalteromonadaceae). Our results demonstrate that algal exudates are enriched in DCNS components, foster rapid growth of bacterioplankton and select for bacterial populations with more potential VFs than coral exudates.