Vibriosp. as a potentially important member of the Black Band Disease (BBD) consortium inFaviasp. corals

Evaluating the coral microbiome during cryopreservation

Coral reefs are threatened by various local and global stressors, including elevated ocean temperatures due to anthropogenic climate change. Coral cryopreservation could help secure the diversity of threatened corals. Recently, isochoric vitrification was used to demonstrate that coral fragments lived to 24 hr post-thaw; however, in this study, they were stressed post-thaw. The microbial portion of the coral holobiont has been shown to affect host fitness and the impact of cryopreservation treatment on coral microbiomes is unknown. Therefore, we examined the coral-associated bacterial communities pre- and post-cryopreservation treatments, with a view towards informing potential future stress reduction strategies. We characterized the microbiome of the Hawaiian finger coral, Porites compressa in the wild and at seven steps during the isochoric vitrification process. We observed significant changes in microbiome composition, including: 1) the natural wild microbiomes of P. compressa were dominated by Endozoicomonadaceae (76.5 % relative abundance) and consistent between samples, independent of collection location across Kāne'ohe Bay; 2) Endozoicomonadaceae were reduced to <6.9 % in captivity, and further reduced to <0.5 % relative abundance after isochoric vitrification; and 3) Vibrionaceae dominated communities post-thaw (58.5-74.7 % abundance). Thus, the capture and cryopreservation processes, are implicated as possible causal agents of dysbiosis characterized by the loss of putatively beneficial symbionts (Endozoicomonadaceae) and overgrowth of potential pathogens (Vibrionaceae). Offsetting these changes with probiotic restoration treatments may alleviate cryopreservation stress and improve post-thaw husbandry.

Microbiomes of three coral species in the Mexican Caribbean and their shifts associated with the Stony Coral Tissue Loss Disease

Stony Coral Tissue Loss Disease (SCTLD) has caused widespread coral mortality in the Caribbean Region. However, how the disease presence alters the microbiome community, their structure, composition, and metabolic functionality is still poorly understood. In this study, we characterized the microbial communities of the tissues of apparently healthy and diseased SCTLD colonies of the species Siderastrea siderea, Orbicella faveolata, and Montastraea cavernosa to explore putative changes related to the presence of SCTLD. Gammaproteobacteria, Alphaproteobacteria, and Bacteroidia were the best represented classes in the healthy tissues of all coral species, and alpha diversity did not show significant differences among the species. The microbial community structure between coral species was significantly different (PERMANOVA: F = 3.46, p = 0.001), and enriched genera were detected for each species: Vibrio and Photobacterium in S. siderea, Spirochaeta2 and Marivivens in O. faveolata and SAR202_clade and Nitrospira in M. cavernosa. Evidence of SCTLD in the microbial communities was more substantial in S. siderea, where differences in alpha diversity, beta diversity, and functional profiles were observed. In O. faveolata, differences were detected only in the community structure, while M. cavernosa samples showed no significant difference. Several microbial groups were found to have enriched abundances in tissue from SCTLD lesions from S. siderea and O. faveolata, but no dominant bacterial group was detected. Our results contribute to understanding microbial diversity associated with three scleractinian coral species and the shifts in their microbiomes associated with SCTLD in the Mexican Caribbean.

Osmotic stress response of the coral and oyster pathogen Vibrio coralliilyticus: acquisition of catabolism gene clusters for the compatible solute and signaling molecule myo-inositol

Marine bacteria experience fluctuations in osmolarity that they must adapt to, and most bacteria respond to high osmolarity by accumulating compatible solutes also known as osmolytes. The osmotic stress response and compatible solutes used by the coral and oyster pathogen Vibrio coralliilyticus were unknown. In this study, we showed that to alleviate osmotic stress V. coralliilyticus biosynthesized glycine betaine (GB) and transported into the cell choline, GB, ectoine, dimethylglycine, and dimethylsulfoniopropionate, but not myo-inositol. Myo-inositol is a stress protectant and a signaling molecule that is biosynthesized and used by algae. Bioinformatics identified myo-inositol (iol) catabolism clusters in V. coralliilyticus and other Vibrio, Photobacterium, Grimontia, and Enterovibrio species. Growth pattern analysis demonstrated that V. coralliilyticus utilized myo-inositol as a sole carbon source, with a short lag time of 3 h. An iolG deletion mutant, which encodes an inositol dehydrogenase, was unable to grow on myo-inositol. Within the iol clusters were an MFS-type (iolT1) and an ABC-type (iolXYZ) transporter and analyses showed that both transported myo-inositol. IolG and IolA phylogeny among Vibrionaceae species showed different evolutionary histories indicating multiple acquisition events. Outside of Vibrionaceae, IolG was most closely related to IolG from a small group of Aeromonas fish and human pathogens and Providencia species. However, IolG from hypervirulent A. hydrophila strains clustered with IolG from Enterobacter, and divergently from Pectobacterium, Brenneria, and Dickeya plant pathogens. The iol cluster was also present within Aliiroseovarius, Burkholderia, Endozoicomonas, Halomonas, Labrenzia, Marinomonas, Marinobacterium, Cobetia, Pantoea, and Pseudomonas, of which many species were associated with marine flora and fauna.IMPORTANCEHost associated bacteria such as Vibrio coralliilyticus encounter competition for nutrients and have evolved metabolic strategies to better compete for food. Emerging studies show that myo-inositol is exchanged in the coral-algae symbiosis, is likely involved in signaling, but is also an osmolyte in algae. The bacterial consumption of myo-inositol could contribute to a breakdown of the coral-algae symbiosis during thermal stress or disrupt the coral microbiome. Phylogenetic analyses showed that the evolutionary history of myo-inositol metabolism is complex, acquired multiple times in Vibrio, but acquired once in many bacterial plant pathogens. Further analysis also showed that a conserved iol cluster is prevalent among many marine species (commensals, mutualists, and pathogens) associated with marine flora and fauna, algae, sponges, corals, molluscs, crustaceans, and fish.

Osmotic stress response of the coral and oyster pathogen Vibrio coralliilyticus : acquisition of catabolism gene clusters for the compatible solute and signaling molecule myo -inositol

Marine bacteria experience fluctuations in osmolarity that they must adapt to, and most bacteria respond to high osmolarity by accumulating compatible solutes also known as osmolytes. The osmotic stress response and compatible solutes used by the coral and oyster pathogen Vibrio coralliilyticus were unknown. In this study, we showed that to alleviate osmotic stress V. coralliilyticus biosynthesized glycine betaine (GB) and transported into the cell choline, GB, ectoine, dimethylglycine, and dimethylsulfoniopropionate, but not myo -inositol. Myo -inositol is a stress protectant and a signaling molecule that is biosynthesized and used by algae. Bioinformatics identified myo -inositol ( iol ) catabolism clusters in V. coralliilyticus and other Vibrio, Photobacterium, Grimontia, and Enterovibrio species. Growth pattern analysis demonstrated that V. coralliilyticus utilized myo -inositol as a sole carbon source, with a short lag time of 3 h. An iolG deletion mutant, which encodes an inositol dehydrogenase, was unable to grow on myo -inositol. Within the iol clusters were an MFS-type ( iolT1) and an ABC-type ( iolXYZ) transporter and analyses showed that both transported myo -inositol. IolG and IolA phylogeny among Vibrionaceae species showed different evolutionary histories indicating multiple acquisition events. Outside of Vibrionaceae , IolG was most closely related to IolG from a small group of Aeromonas fish and human pathogens and Providencia species. However, IolG from hypervirulent A. hydrophila strains clustered with IolG from Enterobacter, and divergently from Pectobacterium, Brenneria, and Dickeya plant pathogens. The iol cluster was also present within Aliiroseovarius, Burkholderia, Endozoicomonas, Halomonas, Labrenzia, Marinomonas, Marinobacterium, Cobetia, Pantoea, and Pseudomonas, of which many species were associated with marine flora and fauna.

IMPORTANCE

Host associated bacteria such as V. coralliilyticus encounter competition for nutrients and have evolved metabolic strategies to better compete for food. Emerging studies show that myo -inositol is exchanged in the coral-algae symbiosis, is likely involved in signaling, but is also an osmolyte in algae. The bacterial consumption of myo -inositol could contribute to a breakdown of the coral-algae symbiosis during thermal stress or disrupt the coral microbiome. Phylogenetic analyses showed that the evolutionary history of myo -inositol metabolism is complex, acquired multiple times in Vibrio, but acquired once in many bacterial plant pathogens. Further analysis also showed that a conserved iol cluster is prevalent among many marine species (commensals, mutualists, and pathogens) associated with marine flora and fauna, algae, sponges, corals, molluscs, crustaceans, and fish.

Bacterial symbionts of the precious coral Corallium rubrum are differentially distributed across colony-specific compartments and differ among colormorphs

Corals engage in symbioses with micro-organisms that provide nutrients and protect the host. Where the prokaryotic microbes perform their symbiotic functions within a coral is, however, poorly understood. Here, we studied the tissue-specific microbiota of the coral Corallium rubrum by dissecting its tissues from the skeleton and separating the white polyps from the red-coloured coenenchyme, followed by 16S rRNA gene metabarcoding of the three fractions. Dissection was facilitated by incubating coral fragments in RNAlater, which caused tissues to detach from the skeleton. Our results show compartmentalisation of the microbiota. Specifically, Endozoicomonas, Parcubacteria and a Gammaproteobacteria were primarily located in polyps, whereas Nitrincolaceae and one Spirochaeta phylotype were found mainly in the coenenchyme. The skeleton-associated microbiota was distinct from the microbiota in the tissues. Given the difference in tissue colour and microbiota of the polyps and coenenchyme, we analysed the microbiota of three colormorphs of C. rubrum (red, pink, white), finding that the main difference was a very low abundance of Spirochaeta in white colormorphs. While the functions of C. rubrum's symbionts are unknown, their localisation within the colony suggests that microhabitats exist, and the presence of Spirochaeta appears to be linked to the colour of C. rubrum.

Coral Disease and Ingestion: Investigating the Role of Heterotrophy in the Transmission of Pathogenic Vibrio spp. using a Sea Anemone (Exaiptasia pallida) Model System

Understanding disease transmission in corals can be complicated given the intricacy of the holobiont and difficulties associated with ex situ coral cultivation. As a result, most of the established transmission pathways for coral disease are associated with perturbance (i.e., damage) rather than evasion of immune defenses. Here, we investigate ingestion as a potential pathway for the transmission of coral pathogens that evades the mucus membrane. Using sea anemones (Exaiptasia pallida) and brine shrimp (Artemia sp.) to model coral feeding, we tracked the acquisition of the putative pathogens, Vibrio alginolyticus, V. harveyi, and V. mediterranei using GFP-tagged strains. Vibrio sp. were provided to anemones using 3 experimental exposures (i) direct water exposure alone, (ii) water exposure in the presence of a food source (non-spiked Artemia), and (iii) through a "spiked" food source (Vibrio-colonized Artemia) created by exposing Artemia cultures to GFP-Vibrio via the ambient water overnight. Following a 3 h feeding/exposure duration, the level of acquired GFP-Vibrio was quantified from anemone tissue homogenate. Ingestion of spiked Artemia resulted in a significantly greater burden of GFP-Vibrio equating to an 830-fold, 3,108-fold, and 435-fold increase in CFU mL-1 when compared to water exposed trials and a 207-fold, 62-fold, and 27-fold increase in CFU mL-1 compared to water exposed with food trials for V. alginolyticus, V. harveyi, and V. mediterranei, respectively. These data suggest that ingestion can facilitate delivery of an elevated dose of pathogenic bacteria in cnidarians and may describe an important portal of entry for pathogens in the absence of perturbing conditions. IMPORTANCE The front line of pathogen defense in corals is the mucus membrane. This membrane coats the surface body wall creating a semi-impermeable layer that inhibits pathogen entry from the ambient water both physically and biologically through mutualistic antagonism from resident mucus microbes. To date, much of the coral disease transmission research has been focused on mechanisms associated with perturbance of this membrane such as direct contact, vector lesions (predation/biting), and waterborne exposure through preexisting lesions. The present research describes a potential transmission pathway that evades the defenses provided by this membrane allowing unencumbered entry of bacteria as in association with food. This pathway may explain an important portal of entry for emergence of idiopathic infections in otherwise healthy corals and can be used to improve management practices for coral conservation.

Large-scale impact of the 2016 Marine Heatwave on the plankton-associated microbial communities of the Great Barrier Reef (Australia)

The Great Barrier Reef (GBR) is the world's largest coral ecosystem and is threatened by climate change. This study investigated the impact of the 2016 Marine Heatwave (MHW) on plankton associated microbial communities along a ∼800 km transect in the GBR. 16S rRNA gene metabarcoding of archived plankton samples collected from November 2014 to August 2016 in this region showed a significant increase in Planctomycetes and bacteria belonging to the genus Vibrio and Synechococcus during and after the heatwave. Notably, Droplet Digital PCR and targeted metagenomic analysis applied on samples collected four months after the MHW event revealed the presence of several potential pathogenic Vibrio species previously associated with diseases in aquatic animals. Overall, the 2016 MHW significantly impacted the surface picoplankton community and fostered the spread of potentially pathogenic bacteria across the GBR providing an additional threat for marine biodiversity in this area.

Red Sea Atlas of Coral-Associated Bacteria Highlights Common Microbiome Members and Their Distribution across Environmental Gradients-A Systematic Review

The Red Sea is a suitable model for studying coral reefs under climate change due to its strong environmental gradient that provides a window into future global warming scenarios. For instance, corals in the southern Red Sea thrive at temperatures predicted to occur at the end of the century in other biogeographic regions. Corals in the Red Sea thrive under contrasting thermal and environmental regimes along their latitudinal gradient. Because microbial communities associated with corals contribute to host physiology, we conducted a systematic review of the known diversity of Red Sea coral-associated bacteria, considering geographic location and host species. Our assessment comprises 54 studies of 67 coral host species employing cultivation-dependent and cultivation-independent techniques. Most studies have been conducted in the central and northern Red Sea, while the southern and western regions remain largely unexplored. Our data also show that, despite the high diversity of corals in the Red Sea, the most studied corals were Pocillopora verrucosa, Dipsastraea spp., Pleuractis granulosa, and Stylophora pistillata. Microbial diversity was dominated by bacteria from the class Gammaproteobacteria, while the most frequently occurring bacterial families included Rhodobacteraceae and Vibrionaceae. We also identified bacterial families exclusively associated with each of the studied coral orders: Scleractinia (n = 125), Alcyonacea (n = 7), and Capitata (n = 2). This review encompasses 20 years of research in the Red Sea, providing a baseline compendium for coral-associated bacterial diversity.

Exploring the Diversity and Biotechnological Potential of Cultured and Uncultured Coral-Associated Bacteria

Coral-associated microbes are crucial for the biology of their hosts, contributing to nutrient cycling, adaptation, mitigation of toxic compounds, and biological control of pathogens. Natural products from coral-associated micro-organisms (CAM) may possess unique traits. Despite this, the use of CAM for biotechnological purposes has not yet been adequately explored. Here, we investigated the production of commercially important enzymes by 37 strains of bacteria isolated from the coral species Mussismilia braziliensis, Millepora alcicornis, and Porites astreoides. In-vitro enzymatic assays showed that up to 56% of the isolates produced at least one of the seven enzymes screened (lipase, caseinase, keratinase, cellulase, chitinase, amylase, and gelatinase); one strain, identified as Bacillus amyloliquefaciens produced all these enzymes. Additionally, coral species-specific cultured and uncultured microbial communities were identified. The phylum Firmicutes predominated among the isolates, including the genera Exiguobacterium, Bacillus, and Halomonas, among others. Next-generation sequencing and bacteria culturing produced similar but also complementary data, with certain genera detected only by one or the other method. Our results demonstrate the importance of exploring different coral species as sources of specific micro-organisms of biotechnological and industrial interest, at the same time reinforcing the economic and ecological importance of coral reefs as reservoirs of such diversity.

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.

Virulence as a Side Effect of Interspecies Interaction in Vibrio Coral Pathogens

Vibrio coralliilyticus and Vibrio mediterranei are important coral pathogens capable of inducing serious coral damage, which increases severely when they infect the host simultaneously. This has consequences related to the dispersion of these pathogens among different locations that could enhance deleterious effects on coral reefs. However, the mechanisms underlying this synergistic interaction are unknown. The work described here provides a new perspective on the complex interactions among these two Vibrio coral pathogens, suggesting that coral infection could be a collateral effect of interspecific competition. Major implications of this work are that (i) Vibrio virulence mechanisms are activated in the absence of the host as a response to interspecific competition and (ii) release of molecules by Vibrio coral pathogens produces changes in the coral microbiome that favor the pathogenic potential of the entire Vibrio community. Thus, our results highlight that social cues and competition sensing are crucial determinants of development of coral diseases.

The increase in prevalence and severity of coral disease outbreaks produced by Vibrio pathogens, and related to global warming, has seriously impacted reef-building corals throughout the oceans. The coral Oculina patagonica has been used as a model system to study coral bleaching produced by Vibrio infection. Previous data demonstrated that when two coral pathogens (Vibrio coralliilyticus and Vibrio mediterranei) simultaneously infected the coral O. patagonica, their pathogenicity was greater than when each bacterium was infected separately. Here, to understand the mechanisms underlying this synergistic effect, transcriptomic analyses of monocultures and cocultures as well as experimental infection experiments were performed. Our results revealed that the interaction between the two vibrios under culture conditions overexpressed virulence factor genes (e.g., those encoding siderophores, the type VI secretion system, and toxins, among others). Moreover, under these conditions, vibrios were also more likely to form biofilms or become motile through induction of lateral flagella. All these changes that occur as a physiological response to the presence of a competing species could favor the colonization of the host when they are present in a mixed population. Additionally, during coral experimental infections, we showed that exposure of corals to molecules released during V. coralliilyticus and V. mediterranei coculture induced changes in the coral microbiome that favored damage to coral tissue and increased the production of lyso-platelet activating factor. Therefore, we propose that competition sensing, defined as the physiological response to detection of harm or to the presence of a competing Vibrio species, enhances the ability of Vibrio coral pathogens to invade their host and cause tissue necrosis.

Vibrio coralliilyticus infection triggers a behavioural response and perturbs nutritional exchange and tissue integrity in a symbiotic coral

Under homoeostatic conditions, the relationship between the coral Pocillopora damicornis and Vibrio coralliilyticus is commensal. An increase in temperature, or in the abundance of V. coralliilyticus, can turn this association pathogenic, causing tissue lysis, expulsion of the corals’ symbiotic algae (genus Symbiodinium), and eventually coral death. Using a combination of microfluidics, fluorescence microscopy, stable isotopes, electron microscopy and NanoSIMS isotopic imaging, we provide insights into the onset and progression of V. coralliilyticus infection in the daytime and at night, at the tissue and (sub-)cellular level. The objective of our study was to connect the macro-scale behavioural response of the coral to the micro-scale nutritional interactions that occur between the host and its symbiont. In the daytime, polyps enhanced their mucus production, and actively spewed pathogens. Vibrio infection primarily resulted in the formation of tissue lesions in the coenosarc. NanoSIMS analysis revealed infection reduced ¹³C-assimilation in Symbiodinium, but increased ¹³C-assimilation in the host. In the night incubations, no mucus spewing was observed, and a mucus film was formed on the coral surface. Vibrio inoculation and infection at night showed reduced ¹³C-turnover in Symbiodinium, but did not impact host ¹³C-turnover. Our results show that both the nutritional interactions that occur between the two symbiotic partners and the behavioural response of the host organism play key roles in determining the progression and severity of host-pathogen interactions. More generally, our approach provides a new means of studying interactions (ranging from behavioural to metabolic scales) between partners involved in complex holobiont systems, under both homoeostatic and pathogenic conditions.

Ecological and molecular characterization of a coral black band disease outbreak in the Red Sea during a bleaching event

Black Band Disease (BBD) is a widely distributed and destructive coral disease that has been studied on a global scale, but baseline data on coral diseases is missing from many areas of the Arabian Seas. Here we report on the broad distribution and prevalence of BBD in the Red Sea in addition to documenting a bleaching-associated outbreak of BBD with subsequent microbial community characterization of BBD microbial mats at this reef site in the southern central Red Sea. Coral colonies with BBD were found at roughly a third of our 22 survey sites with an overall prevalence of 0.04%. Nine coral genera were infected including Astreopora, Coelastrea, Dipsastraea, Gardineroseris, Goniopora, Montipora, Pavona, Platygyra, and Psammocora. For a southern central Red Sea outbreak site, overall prevalence was 40 times higher than baseline (1.7%). Differential susceptibility to BBD was apparent among coral genera with Dipsastraea (prevalence 6.1%), having more diseased colonies than was expected based on its abundance within transects. Analysis of the microbial community associated with the BBD mat showed that it is dominated by a consortium of cyanobacteria and heterotrophic bacteria. We detected the three main indicators for BBD (filamentous cyanobacteria, sulfate-reducing bacteria (SRB), and sulfide-oxidizing bacteria (SOB)), with high similarity to BBD-associated microbes found worldwide. More specifically, the microbial consortium of BBD-diseased coral colonies in the Red Sea consisted of Oscillatoria sp. (cyanobacteria), Desulfovibrio sp. (SRB), and Arcobacter sp. (SOB). Given the similarity of associated bacteria worldwide, our data suggest that BBD represents a global coral disease with predictable etiology. Furthermore, we provide a baseline assessment of BBD disease prevalence in the Red Sea, a still understudied region.

Low pH reduces the virulence of black band disease on Orbicella faveolata

Black band is a deadly coral disease found worldwide, which may become more virulent as oceanic conditions continue to change. To determine the effects of climate change and ocean acidification on black band disease virulence, Orbicella faveolata corals with black band were exposed to different temperature and pH conditions. Results showed a significant decrease in disease progression under low pH (7.7) conditions. Low pH also altered the relative abundance of the bacterial community of the black band disease consortium. Here, there was a significant decrease in Roseofilum, the cyanobacterium that typically dominates the black band mat. These results indicate that as oceanic pH decreases so may the virulence of a worldwide coral disease.

Comparative Metagenomics of the Polymicrobial Black Band Disease of Corals

Black Band Disease (BBD), the destructive microbial consortium dominated by the cyanobacterium Roseofilum reptotaenium, affects corals worldwide. While the taxonomic composition of BBD consortia has been well-characterized, substantially less is known about its functional repertoire. We sequenced the metagenomes of Caribbean and Pacific black band mats and cultured Roseofilum and obtained five metagenome-assembled genomes (MAGs) of Roseofilum, nine of Proteobacteria, and 12 of Bacteroidetes. Genomic content analysis suggests that Roseofilum is a source of organic carbon and nitrogen, as well as natural products that may influence interactions between microbes. Proteobacteria and Bacteroidetes members of the disease consortium are suited to the degradation of amino acids, proteins, and carbohydrates. The accumulation of sulfide underneath the black band mat, in part due to a lack of sulfur oxidizers, contributes to the lethality of the disease. The presence of sulfide:quinone oxidoreductase genes in all five Roseofilum MAGs and in the MAGs of several heterotrophs demonstrates that resistance to sulfide is an important characteristic for members of the BBD consortium.

Unraveling the microbial processes of black band disease in corals through integrated genomics

Coral disease outbreaks contribute to the ongoing degradation of reef ecosystems, however, microbial mechanisms underlying the onset and progression of most coral diseases are poorly understood. Black band disease (BBD) manifests as a cyanobacterial-dominated microbial mat that destroys coral tissues as it rapidly spreads over coral colonies. To elucidate BBD pathogenesis, we apply a comparative metagenomic and metatranscriptomic approach to identify taxonomic and functional changes within microbial lesions during in-situ development of BBD from a comparatively benign stage termed cyanobacterial patches. Results suggest that photosynthetic CO₂-fixation in Cyanobacteria substantially enhances productivity of organic matter within the lesion during disease development. Photosynthates appear to subsequently promote sulfide-production by Deltaproteobacteria, facilitating the major virulence factor of BBD. Interestingly, our metagenome-enabled transcriptomic analysis reveals that BBD-associated cyanobacteria have a putative mechanism that enables them to adapt to higher levels of hydrogen sulfide within lesions, underpinning the pivotal roles of the dominant cyanobacterium within the polymicrobial lesions during the onset of BBD. The current study presents sequence-based evidence derived from whole microbial communities that unravel the mechanism of development and progression of BBD.

Bacterial community diversity of the deep-sea octocoral Paramuricea placomus

Compared to tropical corals, much less is known about deep-sea coral biology and ecology. Although the microbial communities of some deep-sea corals have been described, this is the first study to characterize the bacterial community associated with the deep-sea octocoral, Paramuricea placomus. Samples from five colonies of P. placomus were collected from Baltimore Canyon (379–382 m depth) in the Atlantic Ocean off the east coast of the United States of America. DNA was extracted from the coral samples and 16S rRNA gene amplicons were pyrosequenced using V4-V5 primers. Three samples sequenced deeply (>4,000 sequences each) and were further analyzed. The dominant microbial phylum was Proteobacteria, but other major phyla included Firmicutes and Planctomycetes. A conserved community of bacterial taxa held in common across the three P. placomus colonies was identified, comprising 68–90% of the total bacterial community depending on the coral individual. The bacterial community of P. placomus does not appear to include the genus Endozoicomonas, which has been found previously to be the dominant bacterial associate in several temperate and tropical gorgonians. Inferred functionality suggests the possibility of nitrogen cycling by the core bacterial community.

Metaproteomics reveals metabolic transitions between healthy and diseased stony coral Mussismilia braziliensis

Infectious diseases such as white plague syndrome (WPS) and black band disease (BBD) have caused massive coral loss worldwide. We performed a metaproteomic study on the Abrolhos coral Mussismilia braziliensis to define the types of proteins expressed in healthy corals compared to WPS- and BBD-affected corals. A total of 6363 MS/MS spectra were identified as 361 different proteins. Healthy corals had a set of proteins that may be considered markers of holobiont homoeostasis, including tubulin, histone, Rab family, ribosomal, peridinin-chlorophyll a-binding protein, F0F1-type ATP synthase, alpha-iG protein, calmodulin and ADP-ribosylation factor. Cnidaria proteins found in healthy M. braziliensis were associated with Cnidaria-Symbiodinium endosymbiosis and included chaperones (hsp70, hsp90 and calreticulin), structural and membrane modelling proteins (actin) and proteins with functions related to intracellular vesicular traffic (Rab7 and ADP-ribosylation factor 1) and signal transduction (14-3-3 protein and calmodulin). WPS resulted in a clear shift in the predominance of proteins, from those related to aerobic nitrogen-fixing bacteria (i.e. Rhizobiales, Sphingomonadales and Actinomycetales) in healthy corals to those produced by facultative/anaerobic sulphate-reducing bacteria (i.e. Enterobacteriales, Alteromonadales, Clostridiales and Bacteroidetes) in WPS corals. BBD corals developed a diverse community dominated by cyanobacteria and sulphur cycle bacteria. Hsp60, hsp90 and adenosylhomocysteinase proteins were produced mainly by cyanobacteria in BBD corals, which is consistent with elevated oxidative stress in hydrogen sulphide- and cyanotoxin-rich environments. This study demonstrates the usefulness of metaproteomics for gaining better comprehension of coral metabolic status in health and disease, especially in reef systems such as the Abrolhos that are suffering from the increase in global and local threatening events.

Characterisation of an atypical manifestation of black band disease on Porites lutea in the Western Indian Ocean

Recent surveys conducted on Reunion Island coral reefs revealed an atypical manifestation of black band disease on the main framework building coral, Porites lutea. This BBD manifestation (PorBBD) presented a thick lighter-colored band, which preceded the typical BBD lesion. Whilst BBD aetiology has been intensively described worldwide, it remains unclear if corals with apparently similar lesions across coral reefs are affected by the same pathogens. Therefore, a multidisciplinary approach involving field surveys, gross lesion monitoring, histopathology and 454-pyrosequencing was employed to provide the first comprehensive characterization of this particular manifestation. Surveys conducted within two geomorphological zones over two consecutive summers and winters showed spatial and seasonal patterns consistent with those found for typical BBD. Genetic analyses suggested an uncharacteristically high level of Vibrio spp. bacterial infection within PorBBD. However, microscopic analysis revealed high densities of cyanobacteria, penetrating the compromised tissue as well as the presence of basophilic bodies resembling bacterial aggregates in the living tissue, adjacent to the bacterial mat. Additionally, classical BBD-associated cyanobacterial strains, genetically related to Pseudoscillatoria coralii and Roseofilum reptotaenium were identified and isolated and the presence of sulfate-reducers or sulfide-oxidizers such as Desulfovibrio and Arcobacter, previously shown to be associated with anoxic microenvironment within typical BBD was also observed, confirming that PorBBD is a manifestation of classical BBD.

The changing dynamics of coral reef science in Arabia

Six percent of the world's coral reefs occur around the Arabian Peninsula, providing a valuable ecological, economic and scientific resource for the nations bordering its shores. We provide the first region-wide assessment of the current status and historical trends in coral reef research, focusing on research in the Red Sea, Arabian Sea, and Arabian Gulf. In total, 633 regional reef publications have been produced since the 1930s, covering a wide variety of themes and taxa. Our results show a great deal of commonality in regional reef research, but also highlight important differences in research among the various seas as well as knowledge gaps that represent opportunities for future research. A regionally-integrated approach to future research is essential. There is a growing need for large-scale research to guide management of reefs and their stressors, as these operate at much larger scales than the national borders within which most research currently occurs.

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.

Microbial transcriptome profiling of black band disease in a Faviid coral during a seasonal disease peak

The etiology of black band disease (BBD), a persistent, globally distributed coral disease characterized by a dark microbial mat, is still unclear. A metatranscriptomics approach was used to unravel the roles of the major mat constituents in the disease process. By comparing the transcriptomes of the mat constituents with those of the surface microbiota of diseased and healthy corals, we showed a shift in bacterial composition and function in BBD-affected corals. mRNA reads of Cyanobacteria, Bacteroidetes and Firmicutes phyla were prominent in the BBD mat. Cyanobacterial adenosylhomocysteinase, involved in cyanotoxin production, was the most transcribed gene in the band consortium. Pathogenic and non-pathogenic forms of Vibrio spp., mainly transcribing the thiamine ABC transporter, were abundant and highly active in both the band and surface tissues. Desulfovibrio desulfuricans was the primary producer of sulfide in the band. Members of the Bacilli class expressed high levels of rhodanese, an enzyme responsible for cyanide and sulfide detoxification. These results offer a first look at the varied functions of the microbiota in the disease mat and surrounding coral surface and enabled us to develop an improved functional model for this disease.

Microbiome shifts and the inhibition of quorum sensing by Black Band Disease cyanobacteria

Disruption of the microbiome often correlates with the appearance of disease symptoms in metaorganisms such as corals. In Black Band Disease (BBD), a polymicrobial disease consortium dominated by the filamentous cyanobacterium Roseofilum reptotaenium displaces members of the epibiotic microbiome. We examined both normal surface microbiomes and BBD consortia on Caribbean corals and found that the microbiomes of healthy corals were dominated by Gammaproteobacteria, in particular Halomonas spp., and were remarkably stable across spatial and temporal scales. In contrast, the microbial community structure in black band consortia was more variable and more diverse. Nevertheless, deep sequencing revealed that members of the disease consortium were present in every sampled surface microbiome of Montastraea, Orbicella and Pseudodiploria corals, regardless of the health status. Within the BBD consortium, we identified lyngbic acid, a cyanobacterial secondary metabolite. It strongly inhibited quorum sensing (QS) in the Vibrio harveyi QS reporters. The effects of lyngbic acid on the QS reporters depended on the presence of the CAI-1 receptor CqsS. Lyngbic acid inhibited luminescence in native coral Vibrio spp. that also possess the CAI-1-mediated QS. The effects of this naturally occurring QS inhibitor on bacterial regulatory networks potentially contribute to the structuring of the interactions within BBD consortia.

Changes in the bacterial community associated with black band disease in a Red Sea coral, Favia sp., in relation to disease phases

Changes of the black band disease (BBD)-associated microbial consortium on the surface of a Favia sp. coral colony were assessed in relation to the different disease phases. A number of highly active bacterial groups changed in numbers as the BBD disease signs changed. These included Gamma- and Epsilonproteobacteria, Bacteroidetes and Firmicutes groups. One cyanobacterium strain, BGP10_4ST (FJ210722), was constantly present in the disease interface and adjacent tissues of the affected corals, regardless of disease phase. The dynamics of the operational taxonomic units (OTUs) of this BBD-specific strain provide a marker regarding the disease phase. The disease's active phase is characterized by a wide dark band progressing along the tissue-skeleton interface and by numerous bacterial OTUs. Cyanobacterial OTUs decreased in numbers as the disease signs waned, perhaps opening a niche for additional microorganisms. Even when black band signs disappeared there was a consistent though low abundance of the BBD-specific cyanobacteria (BGP10_4ST), and the microbial community of the disease-skeleton interface remained surprisingly similar to the original band community. These results provide an indication that the persistence of even low numbers of this BBD-specific cyanobacterium in coral tissues during the non-active (or subclinical) state could facilitate reinitiation of BBD signs during the following summer. This may indicate that this bacterium is major constituent of the disease and that its persistence and ability to infiltrate the coral tissues may act to facilitate the assembly of the other BBD-specific groups of bacteria.

Quorum sensing signal production and microbial interactions in a polymicrobial disease of corals and the coral surface mucopolysaccharide layer

Black band disease (BBD) of corals is a complex polymicrobial disease considered to be a threat to coral reef health, as it can lead to mortality of massive reef-building corals. The BBD community is dominated by gliding, filamentous cyanobacteria with a highly diverse population of heterotrophic bacteria. Microbial interactions such as quorum sensing (QS) and antimicrobial production may be involved in BBD disease pathogenesis. In this study, BBD (whole community) samples, as well as 199 bacterial isolates from BBD, the surface mucopolysaccharide layer (SML) of apparently healthy corals, and SML of apparently healthy areas of BBD-infected corals were screened for the production of acyl homoserine lactones (AHLs) and for autoinducer-2 (AI-2) activity using three bacterial reporter strains. AHLs were detected in all BBD (intact community) samples tested and in cultures of 5.5% of BBD bacterial isolates. Over half of a subset (153) of the isolates were positive for AI-2 activity. AHL-producing isolates were further analyzed using LC-MS/MS to determine AHL chemical structure and the concentration of (S)-4,5-dihydroxy-2,3-pentanedione (DPD), the biosynthetic precursor of AI-2. C6-HSL was the most common AHL variant detected, followed by 3OC4-HSL. In addition to QS assays, 342 growth challenges were conducted among a subset of the isolates, with 27% of isolates eliciting growth inhibition and 2% growth stimulation. 24% of BBD isolates elicited growth inhibition as compared to 26% and 32% of the bacteria from the two SML sources. With one exception, only isolates that exhibited AI-2 activity or produced DPD inhibited growth of test strains. These findings demonstrate for the first time that AHLs are present in an active coral disease. It is possible that AI-2 production among BBD and coral SML bacteria may structure the microbial communities of both a polymicrobial infection and the healthy coral microbiome.

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.

New insights into Oculina patagonica coral diseases and their associated Vibrio spp. communities

Bleaching of Oculina patagonica has been extensively studied in the Eastern Mediterranean Sea, although no studies have been carried out in the Western basin. In 1996 Vibrio mediterranei was reported as the causative agent of bleaching in O. patagonica but it has not been related to bleached or healthy corals since 2003, suggesting that it was no longer involved in bleaching of O. patagonica. In an attempt to clarify the relationship between Vibrio spp., seawater temperature and coral diseases, as well as to investigate the putative differences between Eastern and Western Mediterranean basins, we have analysed the seasonal patterns of the culturable Vibrio spp. assemblages associated with healthy and diseased O. patagonica colonies. Two sampling points located in the Spanish Mediterranean coast were chosen for this study: Alicante Harbour and the Marine Reserve of Tabarca. A complex and dynamic assemblage of Vibrio spp. was present in O. patagonica along the whole year and under different environmental conditions and coral health status. While some Vibrio spp. were detected all year around in corals, the known pathogens V. mediteranei and V. coralliilyticus were only present in diseased specimens. The pathogenic potential of these bacteria was studied by experimental infection under laboratory conditions. Both vibrios caused diseased signs from 24 °C, being higher and faster at 28 °C. Unexpectedly, the co-inoculation of these two Vibrio species seemed to have a synergistic pathogenic effect over O. patagonica, as disease signs were readily observed at temperatures at which bleaching is not normally observed.

The possible role of cyanobacterial filaments in coral black band disease pathology

Black band disease (BBD), characterized by a black mat or line that migrates across a coral colony leaving behind it a bare skeleton, is a persistent disease affecting massive corals worldwide. Previous microscopic and molecular examination of this disease in faviid corals from the Gulf of Eilat revealed a number of possible pathogens with the most prominent being a cyanobacterium identified as Pseudoscillatoria coralii. We examined diseased coral colonies using histopathological and molecular methods in order to further assess the possible role of this cyanobacterium, its mode of entry, and pathological effects on the coral host tissues. Affected areas of colonies with BBD were sampled for examination using both light and transmission electron microscopies. Results showed that this dominant cyanobacterium was found on the coral surface, at the coral-skeletal interface, and invading the polyp tissues and gastrovascular cavity. Although tissues surrounding the invasive cyanobacterial filaments did not show gross morphological alterations, microscopic examination revealed that the coral cells surrounding the lesion were dissociated, necrotic, and highly vacuolated. No amoebocytes were evident in the mesoglea of affected tissues suggesting a possible repression of the coral immune response. Morphological and molecular similarity of the previously isolated BBD-associated cyanobacterium P. coralii to the current samples strengthens the premise that this species is involved in the disease in this coral. These results indicate that the cyanobacteria may play a pivotal role in this disease and that the mode of entry may be via ingestion, penetrating the coral via the gastrodermis, as well as through the skeletal-tissue interface.

An improved detection and quantification method for the coral pathogen Vibrio coralliilyticus

DNA- and RNA-based PCR and reverse-transcription real-time PCR assays were developed for diagnostic detection of the vcpA zinc-metalloprotease implicated in the virulence of the coral pathogen Vibrio coralliilyticus. Both PCR methods were highly specific for V. coralliilyticus and failed to amplify strains of closely-related Vibrio species. The assays correctly detected all globally occurring V. coralliilyticus isolates including a newly-described isolate [TAV24] infecting gorgonians in the Mediterranean Sea and highlighted those isolates that had been potentially misidentified, in particular V. tubiashii strains ATCC 19105 and RE22, historically described as important oyster pathogens. The real-time assay is sensitive, detecting 10 gene copies and the relationships between gene copy number and cycle threshold (C_T) were highly linear (R²≥99.7). The real-time assay was also not affected by interference from non-target DNA. These assays are useful for rapid detection of V. coralliilyticus and monitoring of virulence levels in environmental samples, allowing for implementation of timely management steps to limit and possibly prevent losses due to V. coralliilyticus infection, as well as furthering investigations of factors affecting pathogenesis of this important marine pathogen.

Comparing bacterial community composition between healthy and white plague-like disease states in Orbicella annularis using PhyloChip™ G3 microarrays

Coral disease is a global problem. Diseases are typically named or described based on macroscopic changes, but broad signs of coral distress such as tissue loss or discoloration are unlikely to be specific to a particular pathogen. For example, there appear to be multiple diseases that manifest the rapid tissue loss that characterizes 'white plague.' PhyloChip™ G3 microarrays were used to compare the bacterial community composition of both healthy and white plague-like diseased corals. Samples of lobed star coral (Orbicella annularis, formerly of the genus Montastraea[1]) were collected from two geographically distinct areas, Dry Tortugas National Park and Virgin Islands National Park, to determine if there were biogeographic differences between the diseases. In fact, all diseased samples clustered together, however there was no consistent link to Aurantimonas coralicida, which has been described as the causative agent of white plague type II. The microarrays revealed a large amount of bacterial heterogeneity within the healthy corals and less diversity in the diseased corals. Gram-positive bacterial groups (Actinobacteria, Firmicutes) comprised a greater proportion of the operational taxonomic units (OTUs) unique to healthy samples. Diseased samples were enriched in OTUs from the families Corynebacteriaceae, Lachnospiraceae, Rhodobacteraceae, and Streptococcaceae. Much previous coral disease work has used clone libraries, which seem to be methodologically biased toward recovery of Gram-negative bacterial sequences and may therefore have missed the importance of Gram-positive groups. The PhyloChip™data presented here provide a broader characterization of the bacterial community changes that occur within Orbicella annularis during the shift from a healthy to diseased state.

Pyrosequencing-based profiling of archaeal and bacterial 16S rRNA genes identifies a novel archaeon associated with black band disease in corals

Black band disease (BBD) is a microbial consortium that creates anoxic, sulfide-rich microenvironments and kills underlying coral tissues as it rapidly migrates across colonies. Although bacterial communities associated with BBD have been studied extensively, the presence and roles of archaea are unexplored. Using amplicon-pyrosequencing of 16S ribosomal RNA genes, we investigated the community structure of both archaea and bacteria within microbial lesions of BBD and the less-virulent precursor stage, 'cyanobacterial patches' (CP), affecting the coral Montipora hispida. We detected characteristic shifts in microbial communities during the development of BBD from CP, reflecting microenvironmental changes within lesions. Archaeal profiles in CP suggested a diverse assemblage affiliated with the Thaumarchaeota and Euryarchaeota, similar to communities described for oxic marine environments. In contrast, a novel ribotype, distantly affiliated to the Euryarchaeota, dominated up to 94% of archaeal sequences retrieved from BBD. The physiological characteristics of this dominant archaeal ribotype are unknown because of the novelty of its 16S ribosomal RNA gene sequences; however, their prominent associations with BBD lesions suggest the ability to thrive in the organic- and sulfide-rich anoxic microenvironment characteristic of BBD lesions. Discovery of this novel archaeal ribotype provides new insights into the microbial ecology and aetiology of BBD.

Bacterial consortium of Millepora dichotoma exhibiting unusual multifocal lesion event in the Gulf of Eilat, Red Sea

Colonies of the hydrocoral Millepora dichotoma along the Gulf of Eilat are exhibiting unusual tissue lesions in the form of white spots. The emergence and rapid establishment of these multifocal tissue lesions was the first of its kind reported in this region. A characterization of this morphological anomaly revealed bleached tissues with a significant presence of bacteria in the tissue lesion area. To ascertain possible differences in microbial biota between the lesion area and non-affected tissues, we characterized the bacterial diversity in the two areas of these hydrocorals. Both culture-independent (molecular) and culture-dependent assays showed a shift in bacterial community structure between the healthy and affected tissues. Several 16S rRNA gene sequences retrieved from the affected tissues matched sequences of bacterial clones belonging to Alphaproteobacteria and Bacteroidetes members previously associated with various diseases in scleractinian corals.

Antibacterial activity of Pseudoalteromonas in the coral holobiont

Corals harbor diverse and abundant prokaryotic populations. Bacterial communities residing in the coral mucus layer may be either pathogenic or symbiotic. Some species may produce antibiotics as a method of controlling populations of competing microbial species. The present study characterizes cultivable Pseudoalteromonas sp. isolated from the mucus layer of different coral species from the northern Gulf of Eilat, Red Sea, Israel. Six mucus-associated Pseudoalteromonas spp. obtained from different coral species were screened for antibacterial activity against 23 tester strains. Five of the six Pseudoalteromonas strains demonstrated extracellular antibacterial activity against Gram-positive-but not Gram-negative-tester strains. Active substances secreted into the cell-free supernatant are heat-tolerant and inhibit growth of Bacillus cereus, Staphylococcus aureus, and of ten endogenous Gram-positive marine bacteria isolated from corals. The Pseudoalteromonas spp. isolated from Red sea corals aligned in a phylogenetic tree with previously isolated Pseudoalteromonas spp. of marine origin that demonstrated antimicrobial activity. These results suggest that coral mucus-associated Pseudoalteromonas may play a protective role in the coral holobiont's defense against potential Gram-positive coral pathogens.

Spatial and species variations in bacterial communities associated with corals from the Red Sea as revealed by pyrosequencing

Microbial associations with corals are common and are most likely symbiotic, although their diversity and relationships with environmental factors and host species remain unclear. In this study, we adopted a 16S rRNA gene tag-pyrosequencing technique to investigate the bacterial communities associated with three stony Scleractinea and two soft Octocorallia corals from three locations in the Red Sea. Our results revealed highly diverse bacterial communities in the Red Sea corals, with more than 600 ribotypes detected and up to 1,000 species estimated from a single coral species. Altogether, 21 bacterial phyla were recovered from the corals, of which Gammaproteobacteria was the most dominant group, and Chloroflexi, Chlamydiae, and the candidate phylum WS3 were reported in corals for the first time. The associated bacterial communities varied greatly with location, where environmental conditions differed significantly. Corals from disturbed areas appeared to share more similar bacterial communities, but larger variations in community structures were observed between different coral species from pristine waters. Ordination methods identified salinity and depth as the most influential parameters affecting the abundance of Vibrio, Pseudoalteromonas, Serratia, Stenotrophomonas, Pseudomonas, and Achromobacter in the corals. On the other hand, bacteria such as Chloracidobacterium and Endozoicomonas were more sensitive to the coral species, suggesting that the host species type may be influential in the associated bacterial community, as well. The combined influences of the coral host and environmental factors on the associated microbial communities are discussed. This study represents the first comparative study using tag-pyrosequencing technology to investigate the bacterial communities in Red Sea corals.

Temperature regulation of virulence factors in the pathogen Vibrio coralliilyticus

Sea surface temperatures (SST) are rising because of global climate change. As a result, pathogenic Vibrio species that infect humans and marine organisms during warmer summer months are of growing concern. Coral reefs, in particular, are already experiencing unprecedented degradation worldwide due in part to infectious disease outbreaks and bleaching episodes that are exacerbated by increasing SST. For example, Vibrio coralliilyticus, a globally distributed bacterium associated with multiple coral diseases, infects corals at temperatures above 27 °C. The mechanisms underlying this temperature-dependent pathogenicity, however, are unknown. In this study, we identify potential virulence mechanisms using whole genome sequencing of V. coralliilyticus ATCC (American Type Culture Collection) BAA-450. Furthermore, we demonstrate direct temperature regulation of numerous virulence factors using proteomic analysis and bioassays. Virulence factors involved in motility, host degradation, secretion, antimicrobial resistance and transcriptional regulation are upregulated at the higher virulent temperature of 27 °C, concurrent with phenotypic changes in motility, antibiotic resistance, hemolysis, cytotoxicity and bioluminescence. These results provide evidence that temperature regulates multiple virulence mechanisms in V. coralliilyticus, independent of abundance. The ecological and biological significance of this temperature-dependent virulence response is reinforced by climate change models that predict tropical SST to consistently exceed 27 °C during the spring, summer and fall seasons. We propose V. coralliilyticus as a model Gram-negative bacterium to study temperature-dependent pathogenicity in Vibrio-related diseases.

Fine-structural analysis of black band disease-infected coral reveals boring cyanobacteria and novel bacteria

Examination of coral fragments infected with black band disease (BBD) at the fine- and ultrastructural levels using scanning (SEM) and transmission electron microscopy (TEM) revealed novel features of the disease. SEM images of the skeleton from the host coral investigated (Montastraea annularis species complex) revealed extensive boring underneath the BBD mat, with cyanobacterial filaments present within some of the bore holes. Cyanobacteria were observed to penetrate into the overlying coral tissue from within the skeleton and were present throughout the mesoglea between tissue layers (coral epidermis and gastrodermis). A population of novel, as yet unidentified, small filamentous bacteria was found at the leading edge of the migrating band. This population increased in number within the band and was present within degrading coral epithelium, suggesting a role in disease etiology. In coral tissue in front of the leading edge of the band, cyanobacterial filaments were observed to be emerging from bundles of sloughed-off epidermal tissue. Degraded gastrodermis that contained actively dividing zooxanthellae was observed using both TEM and SEM. The BBD mat contained cyanobacterial filaments that were twisted, characteristic of negative-tactic responses. Some evidence of boring was found in apparently healthy control coral fragments; however, unlike in BBD-infected fragments, there were no associated cyanobacteria. These results suggest the coral skeleton as a possible source of pathogenic BBD cyanobacteria. Additionally, SEM revealed the presence of a potentially important group of small, filamentous BBD-associated bacteria yet to be identified.

A meta-analysis of 16S rRNA gene clone libraries from the polymicrobial black band disease of corals

Black band disease (BBD) is a polymicrobial disease affecting corals on reefs worldwide. Since 2002, researchers have constructed clone libraries from the BBD consortium using 16S rRNA gene primers targeting a variety of phyla. In the present study, a meta-analysis was conducted of published libraries from 2002 through the present that contain bacterial sequence data associated with individual clones and BBD samples. The libraries analyzed were from 87 BBD samples, collected from 16 species of scleractinian corals in 10 different geographic locations, and included 327 unique operational taxonomic units (OTUs). One OTU (cyanobacterial) was present in 62 (71%) samples, and three (one Cytophaga-Flavobacter-Bacteriodetes and two alphaproteobacterial) were present in 11 (13%) of the samples. The remaining 323 unique OTUs were present in <10% of the samples. The Alphaproteobacteria was the most diversely represented group. Analysis of clone libraries using nonmetric multidimensional scaling indicated strong regional specificity of BBD microbial populations, but limited host coral specificity. The results of this analysis support the hypotheses that: (1) a specific cyanobacterium may be the primary pathogen of BBD; (2) additional functional groups, required for BBD pathobiology, are represented by variable opportunistic species; and (3) opportunistic BBD microorganisms are primarily derived from the environment.

Comparative analysis of bacterioplankton assemblages from Karenia brevis bloom and nonbloom water on the west Florida shelf (Gulf of Mexico, USA) using 16S rRNA gene clone libraries

The brevetoxin-producing dinoflagellate, Karenia brevis, forms nearly annual blooms off the Florida west coast, severely impacting the region's ecology and economy. Bacteria are often cited as either promoting or interfering with the development of algal blooms, and thus a detailed study of the bacterioplankton assemblages associated with K. brevis was undertaken. We developed sixteen 16S rRNA gene clone libraries from K. brevis bloom and adjacent nonbloom water to determine the bacterial groups present and assess the influence of K. brevis cell number and/or depth on bacterioplankton community composition. Most notably, bacterial groups such as Rhodobacterales (Alphaproteobacteria) and Cytophagales/Sphingobacteriales (Bacteroidetes), reported previously to be associated with other harmful algal species, were often abundant in the presence of K. brevis. Cyanobacteria frequently dominated surface samples containing no detectable K. brevis, consistent with earlier work suggesting that these photosynthetic organisms may be important in promoting the proliferation of these blooms by conditioning the water. Moreover, differences in the abundance/diversity of traditionally more rare and often undocumented phylogenetic groups (e.g. Betaproteobacteria, Deltaproteobacteria, Chloroflexus, Firmicutes) were apparent in bloom vs. nonbloom water. This is the first study to document the association of these phylogenetic groups with natural K. brevis populations and suggests a potential role for these microorganisms in K. brevis bloom dynamics.

Cyanotoxins are not implicated in the etiology of coral black band disease outbreaks on Pelorus Island, Great Barrier Reef

Cyanobacterial toxins (i.e. microcystins) produced within the microbial mat of coral black band disease (BBD) have been implicated in disease pathogenicity. This study investigated the presence of toxins within BBD lesions and other cyanobacterial patch (CP) lesions, which, in some instances ( approximately 19%), facilitated the onset of BBD, from an outbreak site at Pelorus Island on the inshore, central Great Barrier Reef (GBR). Cyanobacterial species that dominated the biomass of CP and BBD lesions were cultivated and identified, based on morphology and 16S rRNA gene sequences, as Blennothrix- and Oscillatoria-affiliated species, respectively, and identical to cyanobacterial sequences retrieved from previous molecular studies from this site. The presence of the cyanotoxins microcystin, cylindrospermopsin, saxitoxin, nodularin and anatoxin and their respective gene operons in field samples of CP and BBD lesions and their respective culture isolations was tested using genetic (PCR-based screenings), chemical (HPLC-UV, FTICR-MS and LC/MS(n)) and biochemical (PP2A) methods. Cyanotoxins and cyanotoxin synthetase genes were not detected in any of the samples. Cyanobacterial species dominant within CP and BBD lesions were phylogenetically distinct from species previously shown to produce cyanotoxins and isolated from BBD lesions. The results from this study demonstrate that cyanobacterial toxins appear to play no role in the pathogenicity of CP and BBD at this site on the GBR.