Coral microbial community dynamics in response to anthropogenic impacts near a major city in the central Red Sea

Inherent differential microbial assemblages and functions associated with corals exhibiting different thermal phenotypes

Certain coral individuals exhibit enhanced resistance to thermal bleaching, yet the specific microbial assemblages and their roles in these phenotypes remain unclear. We compared the microbial communities of thermal bleaching-resistant (TBR) and thermal bleaching-sensitive (TBS) corals using metabarcoding and metagenomics. Our multidomain approach revealed stable distinct microbial compositions between thermal phenotypes. Notably, TBR corals were inherently enriched with microbial eukaryotes, particularly Symbiodiniaceae, linked to photosynthesis, and the biosynthesis of antibiotic and antitumor compounds and glycosylphosphatidylinositol-anchor proteins, crucial for cell wall regulation and metabolite exchange. In contrast, TBS corals were dominated by bacterial metabolic genes related to nitrogen, amino acid, and lipid metabolism. The inherent microbiome differences between TBR and TBS corals, already observed before thermal stress, point to distinct holobiont phenotypes associated to thermal bleaching resistance, offering insights into mechanisms underlying coral response to climate-induced stress.

Quorum sensing regulates virulence factors in the coral pathogen Vibrio coralliilyticus

The bacterial pathogen Vibrio coralliilyticus causes disease in coral species worldwide. The mechanisms of V. coralliilyticus coral colonization, coral microbiome interactions, and virulence factor production are understudied. In other model Vibrio species, virulence factors like biofilm formation, toxin secretion, and protease production are controlled through a density-dependent communication system called quorum sensing (QS). Comparative genomics indicated that V. coralliilyticus genomes share high sequence identity for most of the QS signaling and regulatory components identified in other Vibrio species. Here, we identify an active QS signaling pathway in two V. coralliilyticus strains with distinct infection etiologies: type strain BAA-450 and coral isolate OCN008. In V. coralliilyticus, the inter-species AI-2 autoinducer signaling pathway in both strains controls expression of the master QS transcription factor and LuxR/HapR homolog VcpR to regulate >300 genes, including protease production, biofilm formation, and two conserved type VI secretion systems (T6SSs). Activation of T6SS1 by QS results in the secretion of effectors and enables interbacterial competition and killing of prey bacteria. We conclude that the QS system in V. coralliilyticus is functional and controls the expression of genes involved in relevant bacterial behaviors typically associated with host infection.IMPORTANCEVibrio coralliilyticus infects many marine organisms, including multiple species of corals, and is a primary causative agent of tissue loss diseases and bacterial-induced bleaching. Here, we investigated a common cell-cell communication mechanism called quorum sensing, which is known to be intimately connected to virulence in other Vibrio species. Our genetic and chemical studies of V. coralliilyticus quorum sensing uncovered an active pathway that directly regulates the following key virulence factors: proteases, biofilms, and secretion systems. These findings connect bacterial signaling in communities to the infection of corals, which may lead to novel treatments and earlier diagnoses of coral diseases in reefs.

Invasive macroalgae shape chemical and microbial waterscapes on coral reefs

Over the past decades, human impacts have changed the structure of tropical benthic reef communities towards coral depletion and macroalgal proliferation. However, how these changes have modified chemical and microbial waterscapes is poorly known. Here, we assessed how the experimental removal of macroalgal assemblages influences the chemical and microbial composition of two reef boundary layers, the benthic and the momentum. Chemical and microbial waterscapes were spatially structured, both horizontally and vertically, according to macroalgal dominance and boundary layers. Microbes associated with reef degradation were enriched in the boundary layers surrounding macroalgal-dominated substrata. Dominant macroalgae were surrounded by a distinct chemical pool of diverse lipid classes (e.g., diterpenoids and glycerolipids) and labile organic matter (e.g., organooxygen compounds), which diffused from algal tissues to boundary layers according to their polarity. Finally, our results highlighted strong co-variations between specific algal-derived metabolites and planktonic microbes, giving insight into their roles in coral reef functioning and resilience.

Influence of reef habitat on coral microbial associations

Corals have complex symbiotic associations that can be influenced by the environment. We compare symbiotic dinoflagellate (family: Symbiodiniaceae) associations and the microbiome of five scleractinian coral species from three different reef habitats in Palau, Micronesia. Although pH and temperature corresponded with specific host-Symbiodiniaceae associations common to the nearshore and offshore habitats, bacterial community dissimilarity analyses indicated minimal influence of these factors on microbial community membership for the corals Coelastrea aspera, Psammocora digitata, and Pachyseris rugosa. However, coral colonies sampled close to human development exhibited greater differences in microbial community diversity compared to the nearshore habitat for the coral species Coelastrea aspera, Montipora foliosa, and Pocillopora acuta, and the offshore habitat for Coelastrea aspera, while also showing less consistency in Symbiodiniaceae associations. These findings indicate the influence that habitat location has on the bacterial and Symbiodiniaceae communities comprising the coral holobiont and provide important considerations for the conservation of coral reef communities, especially for island nations with increasing human populations and development.

Coral high molecular weight carbohydrates support opportunistic microbes in bacterioplankton from an algae-dominated reef

High molecular weight (HMW; >1 kDa) carbohydrates are a major component of dissolved organic matter (DOM) released by benthic primary producers. Despite shifts from coral to algae dominance on many reefs, little is known about the effects of exuded carbohydrates on bacterioplankton communities in reef waters. We compared the monosaccharide composition of HMW carbohydrates exuded by hard corals and brown macroalgae and investigated the response of the bacterioplankton community of an algae-dominated Caribbean reef to the respective HMW fractions. HMW coral exudates were compositionally distinct from the ambient, algae-dominated reef waters and similar to coral mucus (high in arabinose). They further selected for opportunistic bacterioplankton taxa commonly associated with coral stress (i.e., Rhodobacteraceae, Phycisphaeraceae, Vibrionaceae, and Flavobacteriales) and significantly increased the predicted energy-, amino acid-, and carbohydrate-metabolism by 28%, 44%, and 111%, respectively. In contrast, HMW carbohydrates exuded by algae were similar to those in algae tissue extracts and reef water (high in fucose) and did not significantly alter the composition and predicted metabolism of the bacterioplankton community. These results confirm earlier findings of coral exudates supporting efficient trophic transfer, while algae exudates may have stimulated microbial respiration instead of biomass production, thereby supporting the microbialization of reefs. In contrast to previous studies, HMW coral and not algal exudates selected for opportunistic microbes, suggesting that a shift in the prevalent DOM composition and not the exudate type (i.e., coral vs algae) per se, may induce the rise of opportunistic microbial taxa.

IMPORTANCE

Dissolved organic matter (DOM) released by benthic primary producers fuels coral reef food webs. Anthropogenic stressors cause shifts from coral to algae dominance on many reefs, and resulting alterations in the DOM pool can promote opportunistic microbes and potential coral pathogens in reef water. To better understand these DOM-induced effects on bacterioplankton communities, we compared the carbohydrate composition of coral- and macroalgae-DOM and analyzed the response of bacterioplankton from an algae-dominated reef to these DOM types. In line with the proposed microbialization of reefs, coral-DOM was efficiently utilized, promoting energy transfer to higher trophic levels, whereas macroalgae-DOM likely stimulated microbial respiration over biomass production. Contrary to earlier findings, coral- and not algal-DOM selected for opportunistic microbial taxa, indicating that a change in the prevalent DOM composition, and not DOM type, may promote the rise of opportunistic microbes. Presented results may also apply to other coastal marine ecosystems undergoing benthic community shifts.

Seasonal dynamics and environmental drivers of tissue and mucus microbiomes in the staghorn coral Acropora pulchra

Background

Rainfall-induced coastal runoff represents an important environmental impact in near-shore coral reefs that may affect coral-associated bacterial microbiomes. Shifts in microbiome community composition and function can stress corals and ultimately cause mortality and reef declines. Impacts of environmental stress may be site specific and differ between coral microbiome compartments (e.g., tissue versus mucus). Coastal runoff and associated water pollution represent a major stressor for near-shore reef-ecosystems in Guam, Micronesia.

Methods

Acropora pulchra colonies growing on the West Hagåtña reef flat in Guam were sampled over a period of 8 months spanning the 2021 wet and dry seasons. To examine bacterial microbiome diversity and composition, samples of A. pulchra tissue and mucus were collected during late April, early July, late September, and at the end of December. Samples were collected from populations in two different habitat zones, near the reef crest (farshore) and close to shore (nearshore). Seawater samples were collected during the same time period to evaluate microbiome dynamics of the waters surrounding coral colonies. Tissue, mucus, and seawater microbiomes were characterized using 16S DNA metabarcoding in conjunction with Illumina sequencing. In addition, water samples were collected to determine fecal indicator bacteria (FIB) concentrations as an indicator of water pollution. Water temperatures were recorded using data loggers and precipitation data obtained from a nearby rain gauge. The correlation structure of environmental parameters (temperature and rainfall), FIB concentrations, and A. pulchra microbiome diversity was evaluated using a structural equation model. Beta diversity analyses were used to investigate spatio-temporal trends of microbiome composition.

Results

Acropora pulchra microbiome diversity differed between tissues and mucus, with mucus microbiome diversity being similar to the surrounding seawater. Rainfall and associated fluctuations of FIB concentrations were correlated with changes in tissue and mucus microbiomes, indicating their role as drivers of A. pulchra microbiome diversity. A. pulchra tissue microbiome composition remained relatively stable throughout dry and wet seasons; tissues were dominated by Endozoicomonadaceae, coral endosymbionts and putative indicators of coral health. In nearshore A. pulchra tissue microbiomes, Simkaniaceae, putative obligate coral endosymbionts, were more abundant than in A. pulchra colonies growing near the reef crest (farshore). A. pulchra mucus microbiomes were more diverse during the wet season than the dry season, a distinction that was also associated with drastic shifts in microbiome composition. This study highlights the seasonal dynamics of coral microbiomes and demonstrates that microbiome diversity and composition may differ between coral tissues and the surface mucus layer.

Frenemies on the reef? Resolving the coral-Endozoicomonas association

Stony corals are poster child holobionts due to their intimate association with diverse microorganisms from all domains of life. We are only beginning to understand the diverse functions of most of these microbial associates, including potential main contributors to holobiont health and resilience. Among these, bacteria of the elusive genus Endozoicomonas are widely perceived as beneficial symbionts based on their genomic potential and their high prevalence and ubiquitous presence in coral tissues. Simultaneously, evidence of pathogenic and parasitic Endozoicomonas lineages in other marine animals is emerging. Synthesizing the current knowledge on the association of Endozoicomonas with marine holobionts, we challenge the perception of a purely mutualistic coral-Endozoicomonas relationship and propose directions to elucidate its role along the symbiotic spectrum.

The microbiome dynamics and interaction of endosymbiotic Symbiodiniaceae and fungi are associated with thermal bleaching susceptibility of coral holobionts

The thermal bleaching percentage of coral holobionts shows interspecific differences under heat-stress conditions, which are closely related to the coral-associated microbiome. However, the ecological effects of community dynamics and interactions between Symbiodiniaceae and fungi on coral thermal bleaching susceptibility remain unclear. In this study, we analyzed the diversity, community structure, functions, and potential interaction of Symbiodiniaceae and fungi among 18 coral species from a high thermal bleaching risk atoll using next-generation sequencing. The results showed that heat-tolerant C3u sub-clade and Durusdinium dominated the Symbiodiniaceae community of corals and that there were no core amplicon sequence variants in the coral-associated fungal community. Fungal richness and the abundance of confirmed functional animal-plant pathogens were significantly positively correlated with the coral thermal bleaching percentage. Fungal indicators, including Didymellaceae, Chaetomiaceae, Schizophyllum, and Colletotrichum, were identified in corals. Each coral species had a complex Symbiodiniaceae-fungi interaction network (SFIN), which was driven by the dominant Symbiodiniaceae sub-clades. The SFINs of coral holobionts with low thermal bleaching susceptibility exhibited low complexity and high betweenness centrality. These results indicate that the extra heat tolerance of coral in Huangyan Island may be linked to the high abundance of heat-tolerant Symbiodiniaceae. Fungal communities have high interspecific flexibility, and the increase of fungal diversity and pathogen abundance was correlated with higher thermal bleaching susceptibility of corals. Moreover, fungal indicators were associated with the degrees of coral thermal bleaching susceptibility, including both high and intermediate levels. The topological properties of SFINs suggest that heat-tolerant coral have limited fungal parasitism and strong microbial network resilience.IMPORTANCEGlobal warming and enhanced marine heatwaves have led to a rapid decline in coral reef ecosystems worldwide. Several studies have focused on the impact of coral-associated microbiomes on thermal bleaching susceptibility in corals; however, the ecological functions and interactions between Symbiodiniaceae and fungi remain unclear. We investigated the microbiome dynamics and potential interactions of Symbiodiniaceae and fungi among 18 coral species in Huangyan Island. Our study found that the Symbiodiniaceae community of corals was mainly composed of heat-tolerant C3u sub-clade and Durusdinium. The increase in fungal diversity and pathogen abundance has close associations with higher coral thermal bleaching susceptibility. We first constructed an interaction network between Symbiodiniaceae and fungi in corals, which indicated that restricting fungal parasitism and strong interaction network resilience would promote heat acclimatization of corals. Accordingly, this study provides insights into the role of microorganisms and their interaction as drivers of interspecific differences in coral thermal bleaching.

Probiotics reshape the coral microbiome in situ without detectable off-target effects in the surrounding environment

Beneficial microorganisms for corals (BMCs), or probiotics, can enhance coral resilience against stressors in laboratory trials. However, the ability of probiotics to restructure the coral microbiome in situ is yet to be determined. As a first step to elucidate this, we inoculated putative probiotic bacteria (pBMCs) on healthy colonies of Pocillopora verrucosa in situ in the Red Sea, three times per week, during 3 months. pBMCs significantly influenced the coral microbiome, while bacteria of the surrounding seawater and sediment remained unchanged. The inoculated genera Halomonas, Pseudoalteromonas, and Bacillus were significantly enriched in probiotic-treated corals. Furthermore, the probiotic treatment also correlated with an increase in other beneficial groups (e.g., Ruegeria and Limosilactobacillus), and a decrease in potential coral pathogens, such as Vibrio. As all corals (treated and non-treated) remained healthy throughout the experiment, we could not track health improvements or protection against stress. Our data indicate that healthy, and therefore stable, coral microbiomes can be restructured in situ, although repeated and continuous inoculations may be required in these cases. Further, our study provides supporting evidence that, at the studied scale, pBMCs have no detectable off-target effects on the surrounding microbiomes of seawater and sediment near inoculated corals.

Investigating the outcomes of a threatened gorgonian in situ transplantation: Survival and microbiome diversity in Paramuricea clavata (Risso, 1827)

Gorgonian octocorals are threatened by global and local stressors that can act synergistically to affect their health. In recent years, mass mortality events triggered by marine heatwaves have caused demographic declines in Mediterranean gorgonian populations that may lead to their collapse. Potential changes in microbiome composition under stressful conditions may further increase the susceptibility of the gorgonian holobiont to disease. Given the low recovery capacity of gorgonians, restoration approaches using transplantation are becoming an increasingly attractive option to counteract their decline. Here, we compared the survival and microbiome diversity of Paramuricea clavata colonies transplanted to sites differing in depth and local environmental conditions. Gorgonians sampled at a greater depth than the transplantation site were more likely to suffer necrosis after 1 year of monitoring. Gorgonian transplantation into environments disturbed by an anthropogenic source of pollution resulted in an imbalance of the microbiome with potential consequences on the success of restoration initiatives.

Levels, spatial distributions, and provision of petroleum hydrocarbons and phthalates in sediments from Obhur lagoon, Red Sea coast of Saudi Arabia

The levels, spatial distribution, and sources of petroleum hydrocarbons and phthalates were assessed in surface sediment samples from the urban lagoon of Obhur near Jeddah, the largest city on the Red Sea coast of Saudi Arabia. The lagoon was divided into the inner zone, middle zone, and outer zone based on its geomorphological features and developmental activities. n-Alkanes, hopane and sterane biomarkers, and unresolved complex mixture were the major petroleum hydrocarbon compounds of the total extractable organic matter. Phthalates were also measured in the sediment samples. In the three zones, n-alkanes ranged from 89.3 ± 88.5 to 103.2 ± 114.9 ng/g, whereas the hopane and sterane biomarkers varied from 69.4 ± 75.3 to 77.7 ± 69.9 ng/g and 72.5 ± 77.9-89.5 ± 82.2 ng/g, respectively. The UCM concentrations ranged from 821 ± 1119 to 1297 ± 1684 ng/g and phthalates from 37.4 ± 34.5 65 ± 68 ng/g. The primary origins of these anthropogenic hydrocarbons in the lagoon sediments were petroleum products (boat engine discharges, boat washing, lubricants, and wastewater flows) and plasticizers (plastic waste and litter). The proportions of anthropogenic hydrocarbons derived from petroleum products in the sediment's TEOM ranged from 43 ± 33 to 62 ± 15%, while the percentages for plasticizers varied from 2.9 ± 1.2 to 4.0 ± 1.6%. The presence and inputs of these contaminants from petroleum and plastic wastes in the lagoon's sediments will eventually have an impact on its habitats, including the benthic nursery and spawning areas.

Interactions between quorum sensing/quorum quenching and virulence genes may affect coral health by regulating symbiotic bacterial community

Quorum sensing (QS) and quorum quenching (QQ) are two antagonistic processes that may regulate the composition, function and structure of bacterial community. In coral holobiont, autoinducers signaling mediate the communication pathways between interspecies and intraspecies bacteria, which regulate the expression of the virulence factors that can damage host health. However, under environmental stressors, the interaction between the QS/QQ gene and virulence factors and their role in the bacterial communities and coral bleaching is still not fully clear. To address this question, here, metagenomics method was used to examine the profile of QS/QQ and virulence genes from a deeply sequenced microbial database, obtained from three bleached and non-bleached corals species. The prediction of bacterial genes of bleached samples involved in functional metabolic pathways were remarkably decreased, and the bacterial community structure on bleached samples was significantly different compared to non-bleached samples. The distribution and significant difference in QS/QQ and virulence genes were also carried out. We found that Proteobacteria was dominant bacteria among all samples, and AI-1 system is widespread within this group of bacteria. The identified specific genes consistently exhibited a trend of increased pathogenicity in bleached corals relative to non-bleached corals. The abundance of pathogenicity-associated QS genes, including bapA, pfoA and dgcB genes, were significantly increased in bleached corals and can encode the protein of biofilm formation and the membrane damaging toxins promoting pathogenic adhesion and infection. Similarly, the virulence genes, such as superoxide dismutase (Mn-SOD gene), metalloproteinase (yme1, yydH and zmpB), glycosidases (malE, malF, malG, and malK) and LodAB (lodB) genes significantly increased. Conversely, QQ genes that inhibit QS activity and virulence factors to defense the pathogens, including blpA, lsrK, amiE, aprE and gmuG showed a significant decrease in bleached groups. Furthermore, the significant correlations were found among virulence, QS/QQ genes, and coral associated bacterial community, and the virulence genes interact with key QS/QQ genes, directly or indirectly influence symbiotic bacterial communities homeostasis, thereby impacting coral health. It suggested that the functional and structural divergence in the symbiont bacteria may be partially attribute to the interplay, involving interactions among the host, bacterial communication signal systems, and bacterial virulence factors. In conclusion, these data helped to reveal the characteristic behavior of coral symbiotic bacteria, and facilitated a better understanding of bleaching mechanism from a chemical ecological perspective.

Short-term ocean acidification decreases pulsation and growth of the widespread soft coral Xenia umbellata

Coral reefs may experience lower pH values as a result of ocean acidification (OA), which has negative consequences, particularly for calcifying organisms. Thus far, the effects of this global factor have been mainly investigated on hard corals, while the effects on soft corals remain relatively understudied. We therefore carried out a manipulative aquarium experiment for 21 days to study the response of the widespread pulsating soft coral Xenia umbellata to simulated OA conditions. We gradually decreased the pH from ambient (~8.3) to three consecutive 7-day long pH treatments of 8.0, 7.8, and 7.6, using a CO2 dosing system. Monitored response variables included pulsation rate, specific growth rate, visual coloration, survival, Symbiodiniaceae cell densities and chlorophyll a content, photosynthesis and respiration, and finally stable isotopes of carbon (C) and nitrogen (N) as well as CN content. Pulsation decreased compared to controls with each consecutive lowering of the pH, i.e., 17% at pH 8.0, 26% at pH 7.8 and 32% at pH 7.6, accompanied by an initial decrease in growth rates of ~60% at pH 8.0, not decreasing further at lower pH. An 8.3 ‰ decrease of δ13C confirmed that OA exposed colonies had a higher uptake and availability of atmospheric CO2. Coral productivity, i.e., photosynthesis, was not affected by higher dissolved inorganic C availability and none of the remaining response variables showed any significant differences. Our findings suggest that pulsation is a phenotypically plastic mechanism for X. umbellata to adjust to different pH values, resulting in reduced growth rates only, while maintaining high productivity. Consequently, pulsation may allow X. umbellata to inhabit a broad pH range with minimal effects on its overall health. This resilience may contribute to the competitive advantage that soft corals, particularly X. umbellata, have over hard corals.

Microorganisms and dissolved metabolites distinguish Florida's Coral Reef habitats

As coral reef ecosystems experience unprecedented change, effective monitoring of reef features supports management, conservation, and intervention efforts. Omic techniques show promise in quantifying key components of reef ecosystems including dissolved metabolites and microorganisms that may serve as invisible sensors for reef ecosystem dynamics. Dissolved metabolites are released by reef organisms and transferred among microorganisms, acting as chemical currencies and contributing to nutrient cycling and signaling on reefs. Here, we applied four omic techniques (taxonomic microbiome via amplicon sequencing, functional microbiome via shotgun metagenomics, targeted metabolomics, and untargeted metabolomics) to waters overlying Florida's Coral Reef, as well as microbiome profiling on individual coral colonies from these reefs to understand how microbes and dissolved metabolites reflect biogeographical, benthic, and nutrient properties of this 500-km barrier reef. We show that the microbial and metabolite omic approaches each differentiated reef habitats based on geographic zone. Further, seawater microbiome profiling and targeted metabolomics were significantly related to more reef habitat characteristics, such as amount of hard and soft coral, compared to metagenomic sequencing and untargeted metabolomics. Across five coral species, microbiomes were also significantly related to reef zone, followed by species and disease status, suggesting that the geographic water circulation patterns in Florida also impact the microbiomes of reef builders. A combination of differential abundance and indicator species analyses revealed metabolite and microbial signatures of specific reef zones, which demonstrates the utility of these techniques to provide new insights into reef microbial and metabolite features that reflect broader ecosystem processes.

Bacterial dysbiosis and epithelial status of the California sea lion (Zalophus californianus) in the Gulf of California

Despite the high incidence of urogenital carcinoma (UGC) in California sea lions stranded along California, no UGC has been reported in other areas of their distribution; however, cell morphologies typical of premalignant states have been found. Risk factors for UGC include high of organochlorines and infection with a gammaherpesvirus, OtHV-1, but the importance of the bacteriome for epithelial status remains unknown. We characterized the genital bacteriome of adult female California sea lions along their distribution in the Gulf of California and examined whether the diversity and abundance of the bacteriome varied spatially, whether there were detectable differences in the bacteriome between healthy and altered epithelia, and whether the bacteriome was different in California sea lions infected with OtHV-1 or papillomavirus. We detected 2270 ASVs in the genital samples, of which 35 met the criteria for inclusion in the core bacteriome. Fusobacteriia and Clostridia were present in all samples, at high abundances, and Actinobacteria, Alphaproteobacteria, and Campylobacteria were also well-represented. Alpha diversity and abundance of the California sea lion genital bacteriome varied geographically. The abundance of bacterial ASVs varied depending on the genital epithelial status and inflammation, with differences driven by classes Fusobacteriia, Clostridia, Campylobacteria and Alphaproteobacteria. Alpha diversity and abundance were lowest in samples in which OtHV-1 was detected, and highest those with papillomavirus. Our study is the first investigation of how the bacteriome is related to epithelial status in a wild marine species prone to developing cancer.

Bacterial Community Shifts during Polyp Bail-Out Induction in Pocillopora Corals

Polyp bail-out constitutes both a stress response and an asexual reproductive strategy that potentially facilitates dispersal of some scleractinian corals, including several dominant reef-building taxa in the family Pocilloporidae. Recent studies have proposed that microorganisms may be involved in onset and progression of polyp bail-out. However, changes in the coral microbiome during polyp bail-out have not been investigated. In this study, we induced polyp bail-out in Pocillopora corals using hypersaline and hyperthermal methods. Bacterial community dynamics during bail-out induction were examined using the V5-V6 region of the 16S-rRNA gene. From 70 16S-rRNA gene libraries constructed from coral tissues, 1,980 OTUs were identified. Gammaproteobacteria and Alphaproteobacteria consistently constituted the dominant bacterial taxa in all coral tissue samples. Onset of polyp bail-out was characterized by increased relative abundance of Alphaproteobacteria and decreased abundance of Gammaproteobacteria in both induction experiments, with the shift being more prominent in response to elevated temperature than to elevated salinity. Four OTUs, affiliated with Thalassospira, Marisediminitalea, Rhodobacteraceae, and Myxococcales, showed concurrent abundance increases at the onset of polyp bail-out in both experiments, suggesting potential microbial causes of this coral stress response. IMPORTANCE Polyp bail-out represents both a stress response and an asexual reproductive strategy with significant implications for reshaping tropical coral reefs in response to global climate change. Although earlier studies have suggested that coral-associated microbiomes likely contribute to initiation of polyp bail-out in scleractinian corals, there have been no studies of coral microbiome shifts during polyp bail-out. In this study, we present the first investigation of changes in bacterial symbionts during two experiments in which polyp bail-out was induced by different environmental stressors. These results provide a background of coral microbiome dynamics during polyp bail-out development. Increases in abundance of Thalassospira, Marisediminitalea, Rhodobacteraceae, and Myxococcales that occurred in both experiments suggest that these bacteria are potential microbial causes of polyp bail-out, shedding light on the proximal triggering mechanism of this coral stress response.

Warming and nutrient enrichment can trigger seaweed loss by dysregulation of the microbiome structure and predicted function

Warming and nutrient enrichment are key pervasive drivers of ecological shifts in both aquatic and terrestrial ecosystems, impairing the physiology and survival of a wide range of foundation species. But the underlying mechanisms often remain unclear, and experiments have overlooked the potential effects mediated by changes in the microbial communities. We experimentally tested in the field orthogonal stress combinations from simulated air warming and nutrient enrichment on the intertidal foundation seaweed Cystoseira compressa, and its associated bacterial communities. A total of 523 Amplicon Sequence Variance (ASVs) formed the bacterial community on C. compressa, with 222 ASVs assigned to 69 taxa at the genus level. Most bacteria taxa experienced changes in abundance as a result of additive (65 %) and antagonistic (30 %) interactions between the two stressors, with synergies (5 %) occurring less frequently. The analysis of the predicted bacterial functional profile identified 160 metabolic pathways, and showed that these were mostly affected by additive interactions (74 %) between air warming and nutrient enrichment, while antagonisms (20 %) and synergisms (6 %) were less frequent. Overall, the two stressors combined increased functions associated with seaweed disease or degradation of major cell-wall polymers and other algicidal processes, and decreased functions associated with Quorum Quenching and photosynthetic response. We conclude that warming and nutrient enrichment can dysregulate the microbiome of seaweeds, providing a plausible mechanism for their ongoing loss, and encourage more research into the effects of human impacts on crucial but yet largely unstudied host-microbiome relationships in different aquatic and terrestrial species.

Decadal stability in coral cover could mask hidden changes on reefs in the East Asian Seas

Coral reefs in the Central Indo-Pacific region comprise some of the most diverse and yet threatened marine habitats. While reef monitoring has grown throughout the region in recent years, studies of coral reef benthic cover remain limited in spatial and temporal scales. Here, we analysed 24,365 reef surveys performed over 37 years at 1972 sites throughout East Asia by the Global Coral Reef Monitoring Network using Bayesian approaches. Our results show that overall coral cover at surveyed reefs has not declined as suggested in previous studies and compared to reef regions like the Caribbean. Concurrently, macroalgal cover has not increased, with no indications of phase shifts from coral to macroalgal dominance on reefs. Yet, models incorporating socio-economic and environmental variables reveal negative associations of coral cover with coastal urbanisation and sea surface temperature. The diversity of reef assemblages may have mitigated cover declines thus far, but climate change could threaten reef resilience. We recommend prioritisation of regionally coordinated, locally collaborative long-term studies for better contextualisation of monitoring data and analyses, which are essential for achieving reef conservation goals.

Variations in epilithic microbial biofilm composition and recruitment of a canopy-forming alga between pristine and urban rocky shores

Brown algae of the genus Ericaria are habitat formers on Mediterranean rocky shores supporting marine biodiversity and ecosystem functioning. Their population decline has prompted attempts for restoration of threatened populations. Although epilithic microbial biofilms (EMBs) are determinant for macroalgal settlement, their role in regulating the recovery of populations through the recruitment of new thalli is yet to be explored. In this study, we assessed variations in microbial biofilms composition on the settlement of Ericaria amentacea at sites exposed to different human pressures. Artificial fouling surfaces were deployed in two areas at each of three study sites in the Ligurian Sea (Capraia Island, Secche della Meloria and the mainland coast of Livorno), to allow bacterial biofilm colonization. In the laboratory, zygotes of E. amentacea were released on these surfaces to evaluate the survival of germlings. The EMB's composition was assessed through DNA metabarcoding analysis, which revealed a difference between the EMB of Capraia Island and that of Livorno. Fouling surfaces from Capraia Island had higher rates of zygote settlement than the other two sites. This suggests that different environmental conditions can influence the EMB composition on substrata, possibly influencing algal settlement rate. Assessing the suitability of rocky substrata for E. amentacea settlement is crucial for successful restoration.

Ecology of Endozoicomonadaceae in three coral genera across the Pacific Ocean

Health and resilience of the coral holobiont depend on diverse bacterial communities often dominated by key marine symbionts of the Endozoicomonadaceae family. The factors controlling their distribution and their functional diversity remain, however, poorly known. Here, we study the ecology of Endozoicomonadaceae at an ocean basin-scale by sampling specimens from three coral genera (Pocillopora, Porites, Millepora) on 99 reefs from 32 islands across the Pacific Ocean. The analysis of 2447 metabarcoding and 270 metagenomic samples reveals that each coral genus harbored a distinct new species of Endozoicomonadaceae. These species are composed of nine lineages that have distinct biogeographic patterns. The most common one, found in Pocillopora, appears to be a globally distributed symbiont with distinct metabolic capabilities, including the synthesis of amino acids and vitamins not produced by the host. The other lineages are structured partly by the host genetic lineage in Pocillopora and mainly by the geographic location in Porites. Millepora is more rarely associated to Endozoicomonadaceae. Our results show that different coral genera exhibit distinct strategies of host-Endozoicomonadaceae associations that are defined at the bacteria lineage level.

Improving the assessment of ecosystem and wildlife health: microbiome as an early indicator

Human activities are causing dramatic declines in ecosystem health, compromising the functioning of the life-support system, economic activity, and animal and human health. In this context, monitoring the health of ecosystems and wildlife populations is crucial for determining ecological dynamics and assessing management interventions. A growing body of evidence indicates that microbiome provides a meaningful early indicator of ecosystem and wildlife health. Microbiome is ubiquitous and both environmental and host-associated microbiomes rapidly reflect anthropogenic disturbances. However, we still need to overcome current limitations such as nucleic acid degradation, sequencing depth, and the establishment of baseline data to maximize the potential of microbiome studies.

Response of coral bacterial composition and function to water quality variations under anthropogenic influence

Microbial communities play key roles in the adaptation of corals living in adverse environments, as the microbiome flexibility can enhance environmental plasticity of coral holobiont. However, the ecological association of coral microbiome and related function to locally deteriorating water quality remains underexplored. In this work, we used 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) to investigate the seasonal changes of bacterial communities, particularly their functional genes related to carbon (C), nitrogen (N), phosphorus (P) and sulfur (S) cycle, of the scleractinian coral Galaxea fascicularis from nearshore reefs exposed anthropogenic influence. We used nutrient concentrations as the indicator of anthropogenic activities in coastal reefs, and found a higher nutrient pressure in spring than summer. The bacterial diversity, community structure and dominant bacteria of coral shifted significantly due to seasonal variations dominated by nutrient concentrations. Additionally, the network structure and nutrient cycling gene profiles in summer under low nutrient stress was distinct from that under poor environmental conditions in spring, with lower network complexity and abundance of CNPS cycling genes in summer compared with spring. We further identified significant correlations between microbial community (taxonomic composition and co-occurrence network) and geochemical functions (abundance of multiple functional genes and functional community). Nutrient enrichment was proved to be the most important environmental fluctuation in controlling the diversity, community structure, interactional network and functional genes of the coral microbiome. These results highlight that seasonal shifts in coral-associated bacteria due to anthropogenic activities alter the functional potentials, and provide novel insight about the mechanisms of coral adaptation to locally deteriorating environments.

The coral microbiome: towards an understanding of the molecular mechanisms of coral-microbiota interactions

Corals live in a complex, multipartite symbiosis with diverse microbes across kingdoms, some of which are implicated in vital functions, such as those related to resilience against climate change. However, knowledge gaps and technical challenges limit our understanding of the nature and functional significance of complex symbiotic relationships within corals. Here, we provide an overview of the complexity of the coral microbiome focusing on taxonomic diversity and functions of well-studied and cryptic microbes. Mining the coral literature indicate that while corals collectively harbour a third of all marine bacterial phyla, known bacterial symbionts and antagonists of corals represent a minute fraction of this diversity and that these taxa cluster into select genera, suggesting selective evolutionary mechanisms enabled these bacteria to gain a niche within the holobiont. Recent advances in coral microbiome research aimed at leveraging microbiome manipulation to increase coral's fitness to help mitigate heat stress-related mortality are discussed. Then, insights into the potential mechanisms through which microbiota can communicate with and modify host responses are examined by describing known recognition patterns, potential microbially derived coral epigenome effector proteins and coral gene regulation. Finally, the power of omics tools used to study corals are highlighted with emphasis on an integrated host-microbiota multiomics framework to understand the underlying mechanisms during symbiosis and climate change-driven dysbiosis.

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.

Effects of light intensity and photoperiod on the cultivation of the soft coral Sarcophyton trocheliophorum

Coral reefs are one of the most diverse, productive ecosystems in the world, and light plays crucial role in its survival. Notably, the effects of light conditions on soft coral and its adaptive mechanism were unclear. Thus, the present study aimed to investigate and evaluate the effects of different light intensities (30, 80 and 130 μmol m^-2 s^-1) and photoperiods (18D:6L, 12D:12L and 6D:18L) on cultivation of soft coral Sarcophyton trocheliophorum. During two 50-day of the experiments, we monitored the zooxanthellae density, Chl a content, enzyme activities (SOD, CAT and GST) and microbial diversity of S. trocheliophorum. Our study's outcomes found that, at the end of the experiment, the 80 μmol m^-2 s^-1 light intensity group and 12D:12L photoperiod group both possessed the highest zooxanthellae density (2.54 × 10⁸ ± 0.14 × 10⁸ cells g^-1 DW and 2.40 × 10⁸ ± 0.07 × 10⁸ cells g^-1 DW, respectively), Chl a content (295.01 ± 14.13 μg g^-1 DW and 287.78 ± 16.13 μg g^-1 DW, respectively) and microbial diversity and relatively stable enzyme activities level. Besides, we speculated that the reason for the decline of zooxanthellae density, Chl a content and microbial diversity under other light conditions might be that it induced light stress and caused oxidative damage. The main bacterial composition of S. trocheliophorum in different light conditions was similar at the phylum level, showing the stability of microbial community structure. Proteobacteria, Actinobacteria and Firmicutes were dominant under all light conditions, so we hypothesized that these bacteria phylum play a crucial role in coral growth and survival. In conclusion, compared with the other treatments, 80 μmol m^-2 s^-1 light intensity and 12D:12L photoperiod were more beneficial to the growth performance of S. trocheliophorum and could be recommended for its cultivation condition. Our study could provide helpful information for sustainable management plans for the cultivation and conservation of soft corals, which was especially important to the protection and restoration of degraded coral reefs.

Diversity and Distribution of Microbial Communities Associated with Reef Corals of the Malay Peninsula

Coral-associated bacteria play critical roles in the regulation of coral health and function. Environmental perturbations that alter the bacterial community structure can render the coral holobiont more susceptible and less resilient to disease. Understanding the natural variation of the coral microbiome across space and host species provides a baseline that can be used to distinguish shifts in community structure. Using a 16S rRNA gene metabarcoding approach, this study examines bacterial community structure across three scleractinian coral hosts. Our results show that corals of three regions-eastern and western Peninsular Malaysia and Singapore-host distinct bacterial communities; despite these differences, we were able to identify a core microbiome shared across all three species. This core microbiome was also present in samples previously collected in Thailand, suggesting that these core microbes play an important role in promoting and maintaining host health. For example, several have been identified as dimethylsulfoniopropionate (DMSP) metabolizers that have roles in sulfur cycling and the suppression of bacterial pathogens. Pachyseris speciosa has the most variable microbiome, followed by Porites lutea, with the composition of the Diploastrea heliopora microbiome the least variable throughout all locations. Microbial taxa associated with each region or site are likely shaped by local environmental conditions. Taken together, host identity is a major driver of differences in microbial community structure, while environmental heterogeneity shapes communities at finer scales.

Bacterial and Fungal Co-Occurrence in the Nudibranch, Pteraeolidia semperi

Despite the increasing recognition and importance surrounding bacterial and fungal interactions, and their critical contributions to ecosystem functioning and host fitness, studies examining their co-occurrence remain in their infancy. Similarly, studies have yet to characterise the bacterial and fungal communities associated with nudibranchs or their core microbial members. Doing this can advance our understanding of how the microbiome helps a host adapt and persist in its environment. In this study, we characterised the bacterial and fungal communities associated with 46 Pteraeolidia semperi nudibranch individuals collected from four offshore islands in Singapore. We found no distinct spatial structuring of microbial community, richness, or diversity across sampling locations. The bacterial genera Mycoplasma and Endozoicomonas were found across all samples and islands. The fungal genus Leucoagaricus was found with the highest occurrence, but was not found everywhere, and this is the first record of its reported presence in marine environments. The co-occurrence network suggests that bacterial and fungal interactions are limited, but we identified the bacterial family Colwelliaceae as a potential keystone taxon with its disproportionately high number of edges. Furthermore, Colwelliaceae clusters together with other bacterial families such as Pseudoalteromonadaceae and Alteromonadaceae, all of which have possible roles in the digestion of food.

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.

Adaptive changes of coral Galaxea fascicularis holobiont in response to nearshore stress

Global change and local stressors are simultaneously affecting the nearshore corals, and microbiome flexibility may assist corals in thriving under such multiple stressors. Here, we investigated the effects of various environmental variables on Galaxea fascicularis holobiont from nearshore and offshore reefs. These nearshore reefs were more turbid, eutrophic, and warm than offshore reefs. However, coral physiological parameters did not differ significantly. Corals under stressful nearshore environments had low symbiont diversity and selected more tolerant Symbiodiniaceae. The bacterial diversity of offshore corals was significantly higher, and their community composition varied obviously. Diffusion limitations and environmental heterogeneity were essential in structuring microbial communities. Functional annotation analysis demonstrated significant differences between nearshore and offshore corals in bacterial functional groups. Environmental stress significantly reduced the complexity and connectivity of bacterial networks, and the abundances of keystone taxa altered considerably. These results indicated that corals could thrive nearshore through holobiont plasticity to cope with multiple environmental stresses.

The pulsating soft coral Xenia umbellata shows high resistance to warming when nitrate concentrations are low

The resistance of hard corals to warming can be negatively affected by nitrate eutrophication, but related knowledge for soft corals is scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to different levels of nitrate eutrophication (control = 0.6, medium = 6, high = 37 μM nitrate) in a laboratory experiment, with additional warming (27.7 to 32.8 °C) from days 17 to 37. High nitrate eutrophication enhanced cellular chlorophyll a content of Symbiodiniaceae by 168%, while it reduced gross photosynthesis by 56%. After additional warming, polyp pulsation rate was reduced by 100% in both nitrate eutrophication treatments, and additional polyp loss of 7% d⁻¹ and total fragment mortality of 26% was observed in the high nitrate eutrophication treatment. Warming alone did not affect any of the investigated response parameters. These results suggest that X. umbellata exhibits resistance to warming, which may facilitate ecological dominance over some hard corals as ocean temperatures warm, though a clear negative physiological response occurs when combined with nitrate eutrophication. This study thus confirms the importance of investigating combinations of global and local factors to understand and manage changing coral reefs.

Full-Length Transcriptome Maps of Reef-Building Coral Illuminate the Molecular Basis of Calcification, Symbiosis, and Circa-Dian Genes

Coral transcriptomic data largely rely on short-read sequencing, which severely limits the understanding of coral molecular mechanisms and leaves many important biological questions unresolved. Here, we sequence the full-length transcriptomes of four common and frequently dominant reef-building corals using the PacBio Sequel II platform. We obtain information on reported gene functions, structures, and expression profiles. Among them, a comparative analysis of biomineralization-related genes provides insights into the molecular basis of coral skeletal density. The gene expression profiles of the symbiont Symbiodiniaceae are also isolated and annotated from the holobiont sequence data. Finally, a phylogenetic analysis of key circadian clock genes among 40 evolutionarily representative species indicates that there are four key members in early metazoans, including cry genes; Clock or Npas2; cyc or Arntl; and tim, while per, as the fifth member, occurs in Bilateria. In summary, this work provides a foundation for further work on the manipulation of skeleton production or symbiosis to promote the survival of these important organisms.

Microbiome Restructuring: Dominant Coral Bacterium Endozoicomonas Species Respond Differentially to Environmental Changes

Bacteria in the coral microbiome play a crucial role in determining coral health and fitness, and the coral host often restructures its microbiome composition in response to external factors. An important but often neglected factor determining this microbiome restructuring is the ability of microbiome members to respond to changes in the environment. To address this issue, we examined how the microbiome structure of Acropora muricata corals changed over 9 months following a reciprocal transplant experiment. Using a combination of metabarcoding, genomics, and comparative genomics approaches, we found that coral colonies separated by a small distance harbored different dominant Endozoicomonas-related phylotypes belonging to two different species, including a novel species, “Candidatus Endozoicomonas penghunesis” 4G, whose chromosome-level (complete) genome was also sequenced in this study. Furthermore, the two dominant Endozoicomonas species had different potentials to scavenge reactive oxygen species, suggesting potential differences in responding to the environment. Differential capabilities of dominant members of the microbiome to respond to environmental change can (i) provide distinct advantages or disadvantages to coral hosts when subjected to changing environmental conditions and (ii) have positive or negative implications for future reefs.

IMPORTANCE The coral microbiome has been known to play a crucial role in host health. In recent years, we have known that the coral microbiome changes in response to external stressors and that coral hosts structure their microbiome in a host-specific manner. However, an important internal factor, the ability of microbiome members to respond to change, has been often neglected. In this study, we combine metabarcoding, culturing, and genomics to delineate the differential ability of two dominant Endozoicomonas species, including a novel “Ca. Endozoicomonas penghunesis” 4G, to respond to change in the environment following a reciprocal transplant experiment.

Temporal Variation in the Microbiome of Tropical and Temperate Octocorals

Bacterial members of the coral holobiont play an important role in determining coral fitness. However, most knowledge of the coral microbiome has come from reef-building scleractinian corals, with far less known about the nature and importance of the microbiome of octocorals (subclass Octocorallia), which contribute significantly to reef biodiversity and functional complexity. We examined the diversity and structure of the bacterial component of octocoral microbiomes over summer and winter, with a focus on two temperate (Erythropodium hicksoni, Capnella gaboensis; Sydney Harbour) and two tropical (Sinularia sp., Sarcophyton sp.; Heron Island) species common to reefs in eastern Australia. Bacterial communities associated with these octocorals were also compared to common temperate (Plesiastrea versipora) and tropical (Acropora aspera) hard corals from the same reefs. Using 16S rRNA amplicon sequencing, bacterial diversity was found to be heterogeneous among octocorals, but we observed changes in composition between summer and winter for some species (C. gaboensis and Sinularia sp.), but not for others (E. hicksoni and Sarcophyton sp.). Bacterial community structure differed significantly between all octocoral species within both the temperate and tropical environments. However, on a seasonal basis, those differences were less pronounced. The microbiomes of C. gaboensis and Sinularia sp. were dominated by bacteria belonging to the genus Endozoicomonas, which were a key conserved feature of their core microbiomes. In contrast to previous studies, our analysis revealed that Endozoicomonas phylotypes are shared across different octocoral species, inhabiting different environments. Together, our data demonstrates that octocorals harbour a broad diversity of bacterial partners, some of which comprise 'core microbiomes' that potentially impart important functional roles to their hosts.

Long-Term Heat Selection of the Coral Endosymbiont Cladocopium C1acro (Symbiodiniaceae) Stabilizes Associated Bacterial Communities

Heat-tolerant strains of the coral endosymbiont, Cladocopium C1^acro (Symbiodiniaceae), have previously been developed via experimental evolution. Here, we examine physiological responses and bacterial community composition (using 16S rRNA gene metabarcoding) in cultures of 10 heat-evolved (SS) and 9 wild-type (WT) strains, which had been exposed for 6 years to 31 °C and 27 °C, respectively. We also examine whether the associated bacterial communities were affected by a three-week reciprocal transplantation to both temperatures. The SS strains had bacterial communities with lower diversities that showed more stability and lower variability when exposed to elevated temperatures compared with the WT strains. Amplicon sequence variants (ASVs) of the bacterial genera Labrenzia, Algiphilus, Hyphobacterium and Roseitalea were significantly more associated with the SS strains compared with the WT strains. WT strains showed higher abundance of ASVs assigned to the genera Fabibacter and Tropicimonas. We hypothesize that these compositional differences in associated bacterial communities between SS and WT strains also contribute to the thermal tolerance of the microalgae. Future research should explore functional potential between bacterial communities using metagenomics to unravel specific genomic adaptations.

Spatial and interspecific differences in coral-associated bacterial diversity in Hainan, China

Coral reefs are suffering from environmental change and anthropogenic disturbances. It is well known that microbes play an indispensable role in the stable state of coral reef health. Furthermore, the coral reef microbial database helps to understand the connections among microbiomes shifts and ecosystem stress. Hainan Province is the main coral reef distribution area in China. Therefore, targeted microbial reference information from Hainan, including several coral microbiomes, was generated by 16S rRNA gene sequencing in this study. This study focused on a small range of coral-associated bacterial information and found a relationship between microbes and the surrounding environment based on coral interspecific and environmental factors. Interestingly, compared with species, the differences of bacterial community structures are best explained by site. It seems that various environmental factors contribute more to the microbial structure of corals than interspecific influences.

Bacterial Indicators Are Ubiquitous Members of Pelagic Microbiome in Anthropogenically Impacted Coastal Ecosystem

Coastal zones are exposed to various anthropogenic impacts, such as different types of wastewater pollution, e.g., treated wastewater discharges, leakage from sewage systems, and agricultural and urban runoff. These various inputs can introduce allochthonous organic matter and microbes, including pathogens, into the coastal marine environment. The presence of fecal bacterial indicators in the coastal environment is usually monitored using traditional culture-based methods that, however, fail to detect their uncultured representatives. We have conducted a year-around in situ survey of the pelagic microbiome of the dynamic coastal ecosystem, subjected to different anthropogenic pressures to depict the seasonal and spatial dynamics of traditional and alternative fecal bacterial indicators. To provide an insight into the environmental conditions under which bacterial indicators thrive, a suite of environmental factors and bacterial community dynamics were analyzed concurrently. Analyses of 16S rRNA amplicon sequences revealed that the coastal microbiome was primarily structured by seasonal changes regardless of the distance from the wastewater pollution sources. On the other hand, fecal bacterial indicators were not affected by seasons and accounted for up to 34% of the sequence proportion for a given sample. Even more so, traditional fecal indicator bacteria (Enterobacteriaceae) and alternative wastewater-associated bacteria (Lachnospiraceae, Ruminococcaceae, Arcobacteraceae, Pseudomonadaceae and Vibrionaceae) were part of the core coastal microbiome, i.e., present at all sampling stations. Microbial source tracking and Lagrangian particle tracking, which we employed to assess the potential pollution source, revealed the importance of riverine water as a vector for transmission of allochthonous microbes into the marine system. Further phylogenetic analysis showed that the Arcobacteraceae in our data set was affiliated with the pathogenic Arcobacter cryaerophilus, suggesting that a potential exposure risk for bacterial pathogens in anthropogenically impacted coastal zones remains. We emphasize that molecular analyses combined with statistical and oceanographic models may provide new insights for environmental health assessment and reveal the potential source and presence of microbial indicators, which are otherwise overlooked by a cultivation approach.

Contrasting microbiome dynamics of putative denitrifying bacteria in two octocoral species exposed to dissolved organic carbon (DOC) and warming

Mutualistic nutrient cycling in the coral-algae symbiosis depends on limited nitrogen (N) availability for algal symbionts. Denitrifying prokaryotes capable of reducing nitrate or nitrite to dinitrogen could thus support coral holobiont functioning by limiting N availability. Octocorals show some of the highest denitrification rates among reef organisms; however, little is known about the community structures of associated denitrifiers and their response to environmental fluctuations. Combining 16S rRNA gene amplicon sequencing with nirS in-silico PCR and quantitative PCR, we found differences in bacterial community dynamics between two octocorals exposed to excess dissolved organic carbon (DOC) and concomitant warming. Although bacterial communities of the gorgonian Pinnigorgia flava remained largely unaffected by DOC and warming, the soft coral Xenia umbellata exhibited a pronounced shift toward Alphaproteobacteria dominance under excess DOC. Likewise, the relative abundance of denitrifiers was not altered in P. flava but decreased by 1 order of magnitude in X. umbellata under excess DOC, likely due to decreased proportions of Ruegeria spp. Given that holobiont C:N ratios remained stable in P. flava but showed a pronounced increase with excess DOC in X. umbellata, our results suggest that microbial community dynamics may reflect the nutritional status of the holobiont. Hence, denitrifier abundance may be directly linked to N availability. This suggests a passive regulation of N cycling microbes based on N availability, which could help stabilize nutrient limitation in the coral-algal symbiosis and thereby support holobiont functioning in a changing environment.

IMPORTANCE Octocorals are important members of reef-associated benthic communities that can rapidly replace scleractinian corals as the dominant ecosystem engineers on degraded reefs. Considering the substantial change in the (a)biotic environment that is commonly driving reef degradation, maintaining a dynamic and metabolically diverse microbial community might contribute to octocoral acclimatization. Nitrogen (N) cycling microbes, in particular denitrifying prokaryotes, may support holobiont functioning by limiting internal N availability, but little is known about the identity and (a)biotic drivers of octocoral-associated denitrifiers. Here, we show contrasting dynamics of bacterial communities associated with two common octocoral species, the soft coral Xenia umbellata and the gorgonian Pinnigorgia flava after a 6-week exposure to excess dissolved organic carbon under concomitant warming conditions. The specific responses of denitrifier communities of the two octocoral species aligned with the nutritional status of holobiont members. This suggests a passive regulation based on N availability in the coral holobiont.

Rapid Shifts in Bacterial Communities and Homogeneity of Symbiodiniaceae in Colonies of Pocillopora acuta Transplanted Between Reef and Mangrove Environments

It has been proposed that an effective approach for predicting whether and how reef-forming corals persist under future climate change is to examine populations thriving in present day extreme environments, such as mangrove lagoons, where water temperatures can exceed those of reef environments by more than 3°C, pH levels are more acidic (pH < 7.9, often below 7.6) and O₂ concentrations are regularly considered hypoxic (<2 mg/L). Defining the physiological features of these “extreme” corals, as well as their relationships with the, often symbiotic, organisms within their microbiome, could increase our understanding of how corals will persist into the future. To better understand coral-microbe relationships that potentially underpin coral persistence within extreme mangrove environments, we therefore conducted a 9-month reciprocal transplant experiment, whereby specimens of the coral Pocillopora acuta were transplanted between adjacent mangrove and reef sites on the northern Great Barrier Reef. Bacterial communities associated with P. acuta specimens native to the reef environment were dominated by Endozoicomonas, while Symbiodiniaceae communities were dominated by members of the Cladocopium genus. In contrast, P. acuta colonies native to the mangrove site exhibited highly diverse bacterial communities with no dominating members, and Symbiodiniaceae communities dominated by Durusdinium. All corals survived for 9 months after being transplanted from reef-to-mangrove, mangrove-to-reef environments (as well as control within environment transplants), and during this time there were significant changes in the bacterial communities, but not in the Symbiodiniaceae communities or their photo-physiological functioning. In reef-to-mangrove transplanted corals, there were varied, but sometimes rapid shifts in the associated bacterial communities, including a loss of “core” bacterial members after 9 months where coral bacterial communities began to resemble those of the native mangrove corals. Bacterial communities associated with mangrove-to-reef P. acuta colonies also changed from their original composition, but remained different to the native reef corals. Our data demonstrates that P. acuta associated bacterial communities are strongly influenced by changes in environmental conditions, whereas Symbiodiniaceae associated communities remain highly stable.

The Eco-Evo Mandala: Simplifying Bacterioplankton Complexity into Ecohealth Signatures

The microbiome emits informative signals of biological organization and environmental pressure that aid ecosystem monitoring and prediction. Are the many signals reducible to a habitat-specific portfolio that characterizes ecosystem health? Does an optimally structured microbiome imply a resilient microbiome? To answer these questions, we applied our novel Eco-Evo Mandala to bacterioplankton data from four habitats within the Great Barrier Reef, to explore how patterns in community structure, function and genetics signal habitat-specific organization and departures from theoretical optimality. The Mandala revealed communities departing from optimality in habitat-specific ways, mostly along structural and functional traits related to bacterioplankton abundance and interaction distributions (reflected by ϵ and λ as power law and exponential distribution parameters), which are not linearly associated with each other. River and reef communities were similar in their relatively low abundance and interaction disorganization (low ϵ and λ) due to their protective structured habitats. On the contrary, lagoon and estuarine inshore reefs appeared the most disorganized due to the ocean temperature and biogeochemical stress. Phylogenetic distances (D) were minimally informative in characterizing bacterioplankton organization. However, dominant populations, such as Proteobacteria, Bacteroidetes, and Cyanobacteria, were largely responsible for community patterns, being generalists with a large functional gene repertoire (high D) that increases resilience. The relative balance of these populations was found to be habitat-specific and likely related to systemic environmental stress. The position on the Mandala along the three fundamental traits, as well as fluctuations in this ecological state, conveys information about the microbiome's health (and likely ecosystem health considering bacteria-based multitrophic dependencies) as divergence from the expected relative optimality. The Eco-Evo Mandala emphasizes how habitat and the microbiome's interaction network topology are first- and second-order factors for ecosystem health evaluation over taxonomic species richness. Unhealthy microbiome communities and unbalanced microbes are identified not by macroecological indicators but by mapping their impact on the collective proportion and distribution of interactions, which regulates the microbiome's ecosystem function.

Characterisation and distribution of the bacterial genus Endozoicomonas in a threatened surf clam

The toheroa Paphies ventricosa is a large Aotearoa New Zealand (ANZ) endemic surf clam of cultural importance to many Māori, the Indigenous people of ANZ. Extensive commercial and recreational harvesting in the 20th century dramatically reduced populations, leading to the collapse and closure of the fishery. Despite being protected for >40 yr, toheroa have inexplicably failed to recover. In 2017, intracellular microcolonies (IMCs) of bacteria were detected in 'sick' toheroa in northern ANZ. Numerous mass mortality events (MMEs) have recently been recorded in ANZ shellfish, with many events linked by the presence of IMCs resembling Rickettsia-like organisms (RLOs). While similar IMCs have been implicated in MMEs in surf clams elsewhere, the impact of these IMCs on the health or recovery of toheroa is unknown. A critical first step towards understanding the significance of a pathogen in a host population is pathogen identification and characterisation. To begin this process, we examined 16S rRNA gene sequences of the putative IMCs from 4 toheroa populations that showed 97% homology to Endozoicomonas spp. sequences held in GenBank. Phylogenetic analysis identified closely related Endozoicomonas strains from the North and South Island, ANZ, and in situ hybridization, using 16S rRNA gene probes, confirmed the presence of the sequenced IMC gene in the gill and digestive gland tissues of toheroa. Quantitative PCR revealed site-specific and seasonal abundance patterns of Endozoicomonas spp. in toheroa populations. Although implicated in disease outbreaks elsewhere, the role of Endozoicomonas spp. within the ANZ shellfish mortality landscape remains uncertain.

The diversity and metabolic potential of the microbial functional gene associated with Porites pukoensis

Coral reef ecosystems usually distribute in oligotrophic tropical and subtropical marine environments, but they possess great biodiversity and high productivity. It may attribute to its efficient internal nutrient cycle system. However, the knowledge of functional microbial community structure is still limited. In this study, both functional gene array (Geochip 5.0) and nifH Illumina sequencing were used to profile the overall functional genes and diazotrophic communities associated with coral Porites pukoensis. More than 7500 microbial functional genes were detected from archaea, bacteria, and fungi. Most of these genes are related to the transformation of carbon, nitrogen, sulfur, and phosphorus, providing evidence that microbes in the coral holobiont play important roles in the biogeochemical cycle of coral reef ecosystems. Our results indicated a high diversity of diazotrophs associated with corals. The dominant diazotrophic groups were related to phyla Alphaproteobacteria, Deltaproteobacteria, Cyanobacteria, and Gammaproteobacteria. And the dominant diazotrophic communities were divided into four clusters. They were affiliated with nifH sequences from genera Zymomonas, Halorhodospira, Leptolyngbya, Trichormus, and Desulfovibrio, indicating these groups may play a more important role in the nitrogen-fixing process in the coral holobiont. This study revealed functional gene diversity and suggested the roles they played in the biogeochemical cycling of the coral holobiont.

Taxonomic profiling of Symbiodiniaceae and bacterial communities associated with Indo-Pacific corals in the Gulf of Thailand using PacBio sequencing of full-length ITS and 16S rRNA genes

Corals live with complex assemblages of microbes including bacteria, the dinoflagellate Symbiodiniaceae, fungi and viruses in a coral holobiont. These coral-associated microorganisms play an important role in their host fitness and survival. Here, we investigated the structure and diversity of algal and bacterial communities associated with five Indo-Pacific coral species, using full-length 16S rRNA and internal transcribed spacer sequences. While the dinoflagellate communities associated with Poriteslutea were dominated with Symbiodiniaceae genus Cladocopium, the other four coral hosts were associated mainly with members of the Durusdinium genus, suggesting that host species was one of the underlying factors influencing the structure and composition of dinoflagellate communities associated with corals in the Gulf of Thailand. Alphaproteobacteria dominated the microbiomes of Pocillopora spp. while Pavonafrondifera and P. lutea were associated primarily with Gammaproteobacteria. Finally, we demonstrated a superior performance of full-length 16S rRNA sequences in achieving species-resolution taxonomic classification of coral-associated microbiota.

Multi-domain probiotic consortium as an alternative to chemical remediation of oil spills at coral reefs and adjacent sites

BACKGROUND

Beginning in the last century, coral reefs have suffered the consequences of anthropogenic activities, including oil contamination. Chemical remediation methods, such as dispersants, can cause substantial harm to corals and reduce their resilience to stressors. To evaluate the impacts of oil contamination and find potential alternative solutions to chemical dispersants, we conducted a mesocosm experiment with the fire coral Millepora alcicornis, which is sensitive to environmental changes. We exposed M. alcicornis to a realistic oil-spill scenario in which we applied an innovative multi-domain bioremediator consortium (bacteria, filamentous fungi, and yeast) and a chemical dispersant (Corexit® 9500, one of the most widely used dispersants), to assess the effects on host health and host-associated microbial communities.

RESULTS

The selected multi-domain microbial consortium helped to mitigate the impacts of the oil, substantially degrading the polycyclic aromatic and n-alkane fractions and maintaining the physiological integrity of the corals. Exposure to Corexit 9500 negatively impacted the host physiology and altered the coral-associated microbial community. After exposure, the abundances of certain bacterial genera such as Rugeria and Roseovarius increased, as previously reported in stressed or diseased corals. We also identified several bioindicators of Corexit 9500 in the microbiome. The impact of Corexit 9500 on the coral health and microbial community was far greater than oil alone, killing corals after only 4 days of exposure in the flow-through system. In the treatments with Corexit 9500, the action of the bioremediator consortium could not be observed directly because of the extreme toxicity of the dispersant to M. alcicornis and its associated microbiome.

CONCLUSIONS

Our results emphasize the importance of investigating the host-associated microbiome in order to detect and mitigate the effects of oil contamination on corals and the potential role of microbial mitigation and bioindicators as conservation tools. Chemical dispersants were far more damaging to corals and their associated microbiome than oil, and should not be used close to coral reefs. This study can aid in decision-making to minimize the negative effects of oil and dispersants on coral reefs. Video abstract.

Shifts in abundance and network complexity of coral bacteria in response to elevated ammonium stress

Coral bacteria are highly dynamic and acutely affected by host health and environmental conditions. However, there is limited knowledge of how the dynamics of coral-associated bacterial communities and interactions among bacterial members change in response to dissolved inorganic nutrient stressors. Here, we used high-throughput sequencing of the 16S rRNA gene to examine dynamic changes in coral-associated bacterial communities under elevated ammonium stress. Short-term exposure to high levels of ammonium does not significantly harm coral holobiont. Physiological parameters such as carbohydrate, chlorophyll a, and lipid content of coral holobiont were not affected. After three weeks of elevated ammonium stress, however, the coral-associated bacterial community changed significantly. The abundance of certain bacterial populations increased significantly, with enrichment of pathogenic and opportunistic bacteria and a decrease in defensive and core bacteria. Keystone bacterial species in the co-occurrence network changed considerably. Under elevated ammonium stress, the abundance of keystone species associated with corals was lower and the complexity of keystone bacterial relationships decreased significantly. Our results indicate that bacteria respond to elevated ammonium stress through changes in abundance and co-occurrence among bacterial members. This precedes visual symptoms of changes in coral physiological conditions and could be used as an early warning indicator of elevated ammonium stress in coastal coral reef management.

Towards enhancing coral heat tolerance: a "microbiome transplantation" treatment using inoculations of homogenized coral tissues

BACKGROUND

Microbiome manipulation could enhance heat tolerance and help corals survive the pressures of ocean warming. We conducted coral microbiome transplantation (CMT) experiments using the reef-building corals, Pocillopora and Porites, and investigated whether this technique can benefit coral heat resistance while modifying the bacterial microbiome. Initially, heat-tolerant donors were identified in the wild. We then used fresh homogenates made from coral donor tissues to inoculate conspecific, heat-susceptible recipients and documented their bleaching responses and microbiomes by 16S rRNA gene metabarcoding.

RESULTS

Recipients of both coral species bleached at lower rates compared to the control group when exposed to short-term heat stress (34 °C). One hundred twelve (Pocillopora sp.) and sixteen (Porites sp.) donor-specific bacterial species were identified in the microbiomes of recipients indicating transmission of bacteria. The amplicon sequence variants of the majority of these transmitted bacteria belonged to known, putatively symbiotic bacterial taxa of corals and were linked to the observed beneficial effect on the coral stress response. Microbiome dynamics in our experiments support the notion that microbiome community evenness and dominance of one or few bacterial species, rather than host-species identity, were drivers for microbiome stability in a holobiont context.

CONCLUSIONS

Our results suggest that coral recipients likely favor the uptake of putative bacterial symbionts, recommending to include these taxonomic groups in future coral probiotics screening efforts. Our study suggests a scenario where these donor-specific bacterial symbionts might have been more efficient in supporting the recipients to resist heat stress compared to the native symbionts present in the control group. These findings urgently call for further experimental investigation of the mechanisms of action underlying the beneficial effect of CMT and for field-based long-term studies testing the persistence of the effect. Video abstract.

Mapping environmental impact assessment research landscapes in the Arab world using visualization and bibliometric techniques

Interests and concerns on environmental issues have attracted much attention over the past few decades. This is in harmony with the growing understanding of environmental impacts associated with human activities and their role in degrading ecosystems. In line with these concerns, considerable advances in science and technology to assess, mitigate, or lessen these adverse impacts have emerged (i.e., environmental impact assessment (EIA) methodologies). The involvement of EIA in sustainable development has become a prevalent topic in research in either developed and developing countries. The present work investigated the research status, development trends, and hotspots of EIA in a region with massive environmental challenges; the Arab world. Bibliometric analysis and visualization mapping were utilized with an objective of revealing and evaluating the developments in knowledge on EIA from the Arab world. A sum of 595 documents was the productivity of the Arab world on EIA (2.1% of total global productivity). Most of the studies were performed by scholars in Egypt (143 documents; 24.0%), followed by Saudi Arabia (96 documents; 16.1%), and Tunisia (68 documents; 11.4%). France, the USA, and the UK were, respectively, the most collaborated countries with the Arab world on EIA. Most of the publications on EIA were in prestigious journals in relation to environmental sciences. King Abdulaziz University, Saudi Arabia and University of Kuwait were the most productive institutions (24 documents/institution). Topics in relation to assessing different environmental impacts on the quality and quantity of water will continue to be vital themes of research. While, the utilization of remote sensing, geographic information systems, risk assessment, life cycle assessment, bioaccumulation, and biomarkers techniques in assessing environmental impacts will continue to be dominant as efficient tools in conducting EIA related research. The outcomes displayed, in general, a rapidly and steadily rising interests on EIA. However, the development of regional experience, increasing of funds and advancing of competencies will further promote research activities on EIA.

Microbiome community and complexity indicate environmental gradient acclimatisation and potential microbial interaction of endemic coral holobionts in the South China Sea

Regional acclimatisation and microbial interactions significantly influence the resilience of reef-building corals facing anthropogenic climate change, allowing them to adapt to environmental stresses. However, the connections between community structure and microbial interactions of the endemic coral microbiome and holobiont acclimatisation remain unclear. Herein, we used generation sequencing of internal transcribed spacer (ITS2) and 16S rRNA genes to investigate the microbiome composition (Symbiodiniaceae and bacteria) and associated potential interactions of endemic dominant coral holobionts (Pocillopora verrucosa and Turbinaria peltata) in the South China Sea (SCS). We found that shifts in Symbiodiniaceae and bacterial communities of P. verrucosa were associated with latitudinal gradient and climate zone changes, respectively. The C1 sub-clade consistently dominated the Symbiodiniaceae community in T. peltata; yet, the bacterial community structure was spatially heterogeneous. The relative abundance of the core microbiome among P. verrucosa holobionts was reduced in the biogeographical transition zone, while bacterial taxa associated with anthropogenic activity (Escherichia coli and Sphingomonas) were identified in the core microbiomes. Symbiodiniaceae and bacteria potentially interact in microbial co-occurrence networks. Further, increased bacterial, and Symbiodiniaceae α-diversity was associated with increased and decreased network complexity, respectively. Hence, Symbiodiniaceae and bacteria demonstrated different flexibility in latitudinal or climatic environmental regimes, which correlated with holobiont acclimatisation. Core microbiome analysis has indicated that the function of core bacterial microbiota might have changed in distinct environmental regimes, implying potential human activity in the coral habitats. Increased bacterial α diversity may lead to a decline in the stability of coral-microorganism symbioses, whereas rare Symbiodiniaceae may help to retain symbioses. Cladocopium, γ-proteobacteria, while α-proteobacteria may have been the primary drivers in the Symbiodiniaceae-bacterial interactions (SBIs). Our study highlights the association between microbiome shift in distinct environmental regimes and holobiont acclimatisation, while providing insights into the impact of SBIs on holobiont health and acclimatisation during climate change.

Metagenomic insights into the taxonomy, function, and dysbiosis of prokaryotic communities in octocorals

BACKGROUND

In octocorals (Cnidaria Octocorallia), the functional relationship between host health and its symbiotic consortium has yet to be determined. Here, we employed comparative metagenomics to uncover the distinct functional and phylogenetic features of the microbiomes of healthy Eunicella gazella, Eunicella verrucosa, and Leptogorgia sarmentosa tissues, in contrast with the microbiomes found in seawater and sediments. We further explored how the octocoral microbiome shifts to a pathobiome state in E. gazella.

RESULTS

Multivariate analyses based on 16S rRNA genes, Clusters of Orthologous Groups of proteins (COGs), Protein families (Pfams), and secondary metabolite-biosynthetic gene clusters annotated from 20 Illumina-sequenced metagenomes each revealed separate clustering of the prokaryotic communities of healthy tissue samples of the three octocoral species from those of necrotic E. gazella tissue and surrounding environments. While the healthy octocoral microbiome was distinguished by so-far uncultivated Endozoicomonadaceae, Oceanospirillales, and Alteromonadales phylotypes in all host species, a pronounced increase of Flavobacteriaceae and Alphaproteobacteria, originating from seawater, was observed in necrotic E. gazella tissue. Increased abundances of eukaryotic-like proteins, exonucleases, restriction endonucleases, CRISPR/Cas proteins, and genes encoding for heat-shock proteins, inorganic ion transport, and iron storage distinguished the prokaryotic communities of healthy octocoral tissue regardless of the host species. An increase of arginase and nitric oxide reductase genes, observed in necrotic E. gazella tissues, suggests the existence of a mechanism for suppression of nitrite oxide production by which octocoral pathogens may overcome the host's immune system.

CONCLUSIONS

This is the first study to employ primer-less, shotgun metagenome sequencing to unveil the taxonomic, functional, and secondary metabolism features of prokaryotic communities in octocorals. Our analyses reveal that the octocoral microbiome is distinct from those of the environmental surroundings, is host genus (but not species) specific, and undergoes large, complex structural changes in the transition to the dysbiotic state. Host-symbiont recognition, abiotic-stress response, micronutrient acquisition, and an antiviral defense arsenal comprising multiple restriction endonucleases, CRISPR/Cas systems, and phage lysogenization regulators are signatures of prokaryotic communities in octocorals. We argue that these features collectively contribute to the stabilization of symbiosis in the octocoral holobiont and constitute beneficial traits that can guide future studies on coral reef conservation and microbiome therapy. Video Abstract.