Evidence of enhanced microbial activity in the interstitial space of branched corals: possible implications for coral metabolism

Water samples from the interstitial space of 4 Indo-Pacific coral species (Acropora sp., Echinopora horrida, Psammocora digita and Pavona clavus) and a Mediterranean coral (Cladocora cespitosa) were analysed for NO ₃ ^- +NO ₂ ^- , NH ₄ ⁺ , molybdate reactive phosphorus, bacterial and flagellate biomass and dissolved organic matter (DOM) and compared with ambient water concentrations. Higher values of NO ₃ ^- +NO ₂ ^- , bacterial and flagellate biomass were observed within the interstitial space of the corals. The lower DOM pool in the interstitium in combination with the high bacterial biomass suggests high bacterial activity and efficient substrate utilization, necessary to compensate for nanoflagellate predation. Since corals may be able to feed on bacteria, the high microbial biomass (bacterial and flagellate) may be utilized either directly as an additional heterotrophic food source, or indirectly in that microbes may act as attractants for microbe-feeding zooplankters, which in turn serve as food for the corals. The combined effect of reduced flow velocities between the coral branches and its associated fauna are probably the main factors in creating a specific environment more or less independent of the nutritive stage of the surrounding water.     

Nutrient cycling in early coral life stages: Pocillopora damicornis larvae provide their algal symbiont (Symbiodinium) with nitrogen acquired from bacterial associates

The waters surrounding coral reef ecosystems are generally poor in nutrients, yet their levels of primary production are comparable with those reported from tropical rain forests. One explanation of this paradox is the efficient cycling of nutrients between the coral host, its endosymbiotic alga Symbiodinium and a wide array of microorganisms. Despite their importance for the animals' fitness, the cycling of nutrients in early coral life stages and the initial establishment of partnerships with the microbes involved in these processes has received little scrutiny to date. Nitrogen is an essential but limited nutrient in coral reef ecosystems. In order to assess the early nutrient exchange between bacteria and corals, coral larvae of the species Pocillopora damicornis were incubated with two coral‐associated bacteria (Alteromonas sp., or Vibrio alginolyticus), prelabeled with the stable nitrogen isotope ¹⁵N. The incorporation and translocation of nitrogen from Vibrio‐ and Alteromonas bacteria into P. damicornis coral larvae and specifically into the coral‐symbiotic Symbiodinium were detected by nanoscale secondary ion mass spectrometry (NanoSIMS). A significant increase in the amount of enriched ¹⁵N (two to threefold compared to natural abundance) was observed in P. damicornis larvae within 8 h of incubation for both bacterial treatments (one‐way ANOVA, F_5,53 = 18.03, P = 0.004 for Alteromonas sp. and F_5,53 = 18.03, P = 0.0001 for V. alginolyticus). These findings reveal that coral larvae acquire nutrients previously taken up from the environment by bacteria. The additional nitrogen may increase the survival rate and fitness of the developing coral and therefore contribute to the successful maintenance of coral reefs.     

Fine-scale mapping of physicochemical and microbial landscapes of the coral skeleton

The coral skeleton harbours a diverse community of bacteria and microeukaryotes exposed to light, O₂ and pH gradients, but how such physicochemical gradients affect the coral skeleton microbiome remains unclear. In this study, we employed chemical imaging of O₂ and pH, hyperspectral reflectance imaging and spatially resolved taxonomic and inferred functional microbiome characterization to explore links between the skeleton microenvironment and microbiome in the reef-building corals Porites lutea and Paragoniastrea benhami. The physicochemical environment was more stable in the deep skeleton, and the diversity and evenness of the bacterial community increased with skeletal depth, suggesting that the microbiome was stratified along the physicochemical gradients. The bulk of the coral skeleton was in a low O₂ habitat, whereas pH varied from pH 6–9 with depth. Physicochemical gradients of O₂ and pH of the coral skeleton explained the β-diversity of the bacterial communities, and skeletal layers that showed O₂ peaks had a higher relative abundance of endolithic algae, reflecting a link between the abiotic environment and the microbiome composition. Our study links the physicochemical, microbial and functional landscapes of the coral skeleton and provides new insights into the involvement of skeletal microbes in the coral holobiont metabolism.     

Benthic community composition affects O2 availability and variability in a Northern Red Sea fringing reef

Many coral reef ecosystems experience shifts in benthic community composition from scleractinian corals to algae. However, consequences of such phase shifts on O₂ availability, important for many reef organisms, are unresolved. This study therefore comparatively investigated potential in situ effects of different benthic cover by reef macroalgae and scleractinian corals on water column O₂ concentrations in a Northern Red Sea fringing reef. Findings revealed that mean daily O₂ concentrations at algae-dominated sites were significantly lower compared to coral-dominated sites. Minimum O₂ concentrations were significantly negatively correlated, while diurnal variability in O₂ concentration was significantly positively correlated, with increasing benthic cover by algae. In contrast, no correlation with coral cover was found. These results indicate that shifts from corals to benthic algae may likely affect both in situ O₂ availability and variability. This may be particularly pronounced in reef systems with low water exchange (e.g. closed lagoons) or under calm weather conditions and suggests potential O₂-mediated effects on reef organisms.     

Removal of bacteria and nutrient dynamics within the coral reef framework of Curaçao (Netherlands Antilles)

The authors studied removal rates of bacteria and the regeneration of inorganic nutrients in coral reef cavities in the reef slope of Curaçao, Netherlands Antilles. We found that in cavities the hard substratum surface area (=ca 68% of cavity surface area) is 65% covered with sessile filter feeders. The cryptic cavity surface area exceeds the projected surface area of the reef by 1.5–8 times. Consequently, the organisms living in these cryptic habitats have potentially a large impact on pico- and nano-plankton densities and are important in reef water nutrient dynamics. We closed cavities (±70 l volume, 15 m depth) in seven experiments to study changes in bacterial densities and dissolved inorganic nutrients (DIN, DIP, and silicate) over time. Water samples were taken from the middle of the cavity at 5-min intervals, for 30 min, and analyzed for heterotrophic bacterial abundance and nutrient concentrations. After closure, bacterial abundance dropped rapidly. Of the initial bacterial concentration in the cavities, 50–60% had disappeared after 30 min, an average disappearance rate of 1.43×10⁴ bacteria ml^–1 min^–1 (0.62 mg C l^–1 d^–1; or 30.1 mg C m^–2 cavity surface area d^–1). NO_x concentrations increased significantly during the time of closure. Efflux rates varied between 1.02–9.77 mmol m^–2 cavity surface area d^–1. NH₄⁺ and PO₄^3– concentrations were variable and did not show a consistent change over time in the experiments. Comparison of bacterial organic nitrogen disappearance rates and DIN (NO_x+NH₄⁺) release rates suggests that on average only 30–40% of additional sources of N besides bacteria were required to balance the nitrogen budget. This highlights the importance of heterotrophic bacterioplankton as food for cryptic filter feeders on coral reefs. Silicate concentrations significantly decreased after closure with 0.50 mmol m^–2 cavity surface area d^–1, suggesting the net deposition of SiO₄^2– in spicules of cryptic filter feeding sponges. We conclude that coral reef cavities are a major sink for heterotrophic bacteria, a sink for dissolved silicon (DSi), and a source for NO_x. That reef cavities are a source for NO_x suggests strong remineralization and nitrification in cavities with a potential role for sponge-symbiotic microbial nitrification.Communicated by K.S. Sealey     

Phylogenetically Diverse Denitrifying and Ammonia-Oxidizing Bacteria in Corals Alcyonium gracillimum and Tubastraea coccinea

To date, the association of coral–bacteria and the ecological roles of bacterial symbionts in corals remain largely unknown. In particular, little is known about the community components of bacterial symbionts of corals involved in the process of denitrification and ammonia oxidation. In this study, the nitrite reductase (nirS and nirK) and ammonia monooxygenase subunit A (amoA) genes were used as functional markers. Diverse bacteria with the potential to be active as denitrifiers and ammonia-oxidizing bacteria (AOB) were found in two East China Sea corals: stony coral Alcyonium gracillimum and soft coral Tubastraea coccinea. The 16S rRNA gene library analysis demonstrated different communities of bacterial symbionts in these two corals of the same location. Nitrite reductase nirK gene was found only in T. coccinea, while both nirK and nirS genes were detected in A. gracillimum, which might be the result of the presence of different bacterial symbionts in these two corals. AOB rather than ammonia-oxidizing archaea were detected in both corals, suggesting that AOB might play an important role in the ammonia oxidation process of the corals. This study indicates that the coral bacterial symbionts with the potential for nitrite reduction and ammonia oxidation might have multiple ecological roles in the coral holobiont, which promotes our understanding of bacteria-mediated nitrogen cycling in corals. To our knowledge, this study is the first assessment of the community structure and phylogenetic diversity of denitrifying bacteria and AOB in corals based on nirK, nirS, and amoA gene library analysis.     

Utilization of Hydrocarbons and Vitamin B12 Production by Bacillus badius

The accumulation of vitamin B₁₂ by Bacillus badins grown on hydrocarbon was investigated. The bacterium could assimilate n-alkanes of C₁₁–C₁₈, ethanol, fumarate, α-ketoglutarate and malate. n-Alkanes of C₁₆–C₁₈, were the best for vitamin B₁₂ production. The bacterium utilized well all of the nitrogen sources tested. Above all, ammonium dihydrogen phosphate was the best for the bacteria] growth and vitamin B₁₂ production. Addition of organic nutrients such as malt extract and meat extract, and addition of metal ions such as ferrous and cobalt promoted the growth and vitamin B₁₂ production. Interestingly, vitamin B₁₂ was produced mostly in the supernatant. The cyanoform of the corrinoid predominantly formed in the supernatant would confirm the identity with cobalamin.     

Relationship of Vibrio Species Infection and Elevated Temperatures to Yellow Blotch/Band Disease in Caribbean Corals

The bacterial and temperature factors leading to yellow blotch/band disease (YBD), which affects the major reef-building Caribbean corals Montastrea spp., have been investigated. Groups of bacteria isolated from affected corals and inoculated onto healthy corals caused disease signs similar to those of YBD. The 16S rRNA genes from these bacteria were sequenced and found to correspond to four Vibrio spp. Elevating the water temperature notably increased the rate of spread of YBD on inoculated corals and induced greater coral mortality. YBD-infected corals held at elevated water temperatures had 50% lower zooxanthella densities, 80% lower division rates, and a 75% decrease in chlorophyll a and c₂ pigments compared with controls. Histological sections indicated that the algal pyrenoid was fragmented into separate segments, along with a reconfiguration and swelling of the zooxanthellae, as well as vacuolization. YBD does not appear to produce the same physiological response formerly observed in corals undergoing temperature-related bleaching. Evidence indicates that YBD affects primarily the symbiotic algae rather than coral tissue.     

Different responses of scleractinian coral Acropora pruinosa from Weizhou Island during extreme high temperature events

Ecological surveys observe coral “winners” and “losers” in global coral bleaching events. However, the key contributors to holobiont tolerance and interactions between symbionts remain unclear. Herein, we compared bleaching and unbleaching Acropora pruinosa corals from Weizhou Island, during an extreme high-temperature event in the northern South China Sea in 2020. We found the dominant Symbiodiniaceae subclade in the bleaching and unbleaching corals to be C1; however, the density of Symbiodiniaceae in the latter was significantly higher than that in the former. Additionally, the symbiotic bacteria α diversity in the unbleaching coral was significantly higher than that in the bleaching coral, with a reorganized bacterial community structure. Core microbiome analyses revealed 55 bacterial core operational taxonomic units (OTUs), of which 10 were significantly differentially enriched between the two coral groups. The significantly enriched bacterial core OTUs in the unbleaching coral were primarily nitrogen cycling related, while those enriched in the bleaching coral were associated with antimicrobial activity. RNA-Seq analyses revealed that significantly upregulated genes in the bleaching coral were primarily associated with diseases and autophagy, while those in the unbleaching coral were associated with immune defense and maintenance of the symbiotic relationship between corals and symbionts. We propose that the differences in tolerance of A. pruinosa result from the cooperation between coral host, Symbiodiniaceae, and symbiotic bacteria. In extreme high-temperature events, unbleaching corals may maintain stable symbiotic relationships by increasing the diversity of symbiotic bacteria, regulating the structure of the symbiotic bacteria community, improving the interaction between coral host and symbiont and enhancing host immunity, thus avoiding coral bleaching. This study illuminates the relationship between the coral symbiont and tolerance differences of coral holobionts, providing new insights for further exploration into the adaptability of scleractinian corals in the context of global warming.

     

Macroalgal Extracts Induce Bacterial Assemblage Shifts and Sublethal Tissue Stress in Caribbean Corals

Benthic macroalgae can be abundant on present-day coral reefs, especially where rates of herbivory are low and/or dissolved nutrients are high. This study investigated the impact of macroalgal extracts on both coral-associated bacterial assemblages and sublethal stress response of corals. Crude extracts and live algal thalli from common Caribbean macroalgae were applied onto the surface of Montastraea faveolata and Porites astreoides corals on reefs in both Florida and Belize. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene amplicons was used to examine changes in the surface mucus layer (SML) bacteria in both coral species. Some of the extracts and live algae induced detectable shifts in coral-associated bacterial assemblages. However, one aqueous extract caused the bacterial assemblages to shift to an entirely new state (Lobophora variegata), whereas other organic extracts had little to no impact (e.g. Dictyota sp.). Macroalgal extracts more frequently induced sublethal stress responses in M. faveolata than in P. astreoides corals, suggesting that cellular integrity can be negatively impacted in selected corals when comparing co-occurring species. As modern reefs experience phase-shifts to a higher abundance of macroalgae with potent chemical defenses, these macroalgae are likely impacting the composition of microbial assemblages associated with corals and affecting overall reef health in unpredicted and unprecedented ways.     

Intergenerational Transfer of Specific Bacteria in Corals and Possible Implications for Offspring Fitness

Diverse and abundant bacterial populations play important functional roles in the multi-partite association of the coral holobiont. The specificity of coral-associated assemblages remains unclear, and little is known about the inheritance of specific bacteria from the parent colony to their offspring. This study investigated if broadcast spawning and brooding corals release specific and potentially beneficial bacteria with their offspring to secure maintenance across generations. Two coral species, Acropora tenuis and Pocillopora damicornis, were maintained in 0.2 μm filtered seawater during the release of their gametes and planulae, respectively. Water samples, excluding gametes and planulae, were subsequently collected, and bacterial diversity was assessed through a pyrosequencing approach amplifying a 470-bp region of the 16S rRNA gene including the variable regions 1–3. Compared to the high bacterial diversity harboured by corals, only a few taxa of bacteria were released by adult corals. Both A. tenuis and P. damicornis released similar bacteria, and the genera Alteromonas and Roseobacter were abundant in large proportions in the seawater of both species after reproduction. This study suggests that adult corals may release bacteria with their offspring to benefit the fitness in early coral life stages.     

Effect of inorganic and organic nutrient addition on coral–algae assemblages from the Northern Red Sea

Previous studies in fringing reefs of the Northern Red Sea demonstrated that the in-situ competition of corals and algae in natural assemblages is highly variable between seasons displaying fast overgrowth of corals by benthic reef algae in fall that follows close to equilibrium between both groups of organisms in summer. This may be caused by up to 5-fold higher inorganic nutrient and 6-fold higher organic nutrient concentrations in fall and winter, thereby potentially promoting algae and cyanobacteria growth with concomitant phase shift. A long term mesocosm experiment (duration: 90 days) was conducted in order to study the effect of dissolved inorganic (ammonium, phosphate, nitrate, and mix of all three) and organic (glucose) nutrient addition onto the competitive process in the dominant coral–algae assemblages of the Northern Red Sea involving branching corals of the genus Acropora and a typical consortium of benthic turf algae. Nutrients were added in 3-fold higher concentrations compared to the annual averages, and the parameters algal growth, extension of bleached area on corals, tissue colour change and chlorophyll a concentrations were monitored at regular intervals over experimental duration. This revealed that elevated ammonium concentrations and elevated organic nutrient concentrations stimulate algal growth, while coral tissue pigmentation and chlorophyll a content were significantly decreased. But only in the elevated organic nutrient treatment all effects on corals were significantly pronounced when assembled with benthic turf algae. Supplementary logger measurements revealed that O₂ water concentrations were significantly lower in the elevated organic nutrient mesocosm compared to all other treatments, confirming side-effects on microbial activity. These findings indicate that organic nutrient input into coral reefs can affect physiology and metabolism of both corals and benthic turf algae. Reinforcing interaction between both groups of organisms along with involvement of microbes may facilitate phase shifts in coral reef ecosystems.     

Allelochemicals produced by Caribbean macroalgae and cyanobacteria have species-specific effects on reef coral microorganisms

Coral populations have precipitously declined on Caribbean reefs while algal abundance has increased, leading to enhanced competitive damage to corals, which likely is mediated by the potent allelochemicals produced by both macroalgae and benthic cyanobacteria. Allelochemicals may affect the composition and abundance of coral-associated microorganisms that control host responses and adaptations to environmental change, including susceptibility to bacterial diseases. Here, we demonstrate that extracts of six Caribbean macroalgae and two benthic cyanobacteria have both inhibitory and stimulatory effects on bacterial taxa cultured from the surfaces of Caribbean corals, macroalgae, and corals exposed to macroalgal extracts. The growth of 54 bacterial isolates was monitored in the presence of lipophilic and hydrophilic crude extracts derived from Caribbean macroalgae and cyanobacteria using 96-well plate bioassays. All 54 bacterial cultures were identified by ribotyping. Lipophilic extracts from two species of Dictyota brown algae inhibited >50% of the reef coral bacteria assayed, and hydrophilic compounds from Dictyota menstrualis particularly inhibited Vibrio bacteria, a genus associated with several coral diseases. In contrast, both lipo- and hydrophilic extracts from 2 species of Lyngbya cyanobacteria strongly stimulated bacterial growth. The brown alga Lobophora variegata produced hydrophilic compounds with broad-spectrum antibacterial effects, which inhibited 93% of the bacterial cultures. Furthermore, bacteria cultured from different locations (corals vs. macroalgae vs. coral surfaces exposed to macroalgal extracts) responded differently to algal extracts. These results reveal that extracts from macroalgae and cyanobacteria have species-specific effects on the composition of coral-microbial assemblages, which in turn may increase coral host susceptibility to disease and result in coral mortality.     

Identification of Candidate Coral Pathogens on White Band Disease-Infected Staghorn Coral

Bacterial diseases affecting scleractinian corals pose an enormous threat to the health of coral reefs, yet we still have a limited understanding of the bacteria associated with coral diseases. White band disease is a bacterial disease that affects the two Caribbean acroporid corals, the staghorn coral Acropora cervicornis and the elkhorn coral A. palmate. Species of Vibrio and Rickettsia have both been identified as putative WBD pathogens. Here we used Illumina 16S rRNA gene sequencing to profile the bacterial communities associated with healthy and diseased A. cervicornis collected from four field sites during two different years. We also exposed corals in tanks to diseased and healthy (control) homogenates to reduce some of the natural variation of field-collected coral bacterial communities. Using a combination of multivariate analyses, we identified community-level changes between diseased and healthy corals in both the field-collected and tank-exposed datasets. We then identified changes in the abundances of individual operational taxonomic units (OTUs) between diseased and healthy corals. By comparing the diseased and healthy-associated bacteria in field-collected and tank-exposed corals, we were able to identify 16 healthy-associated OTUs and 106 consistently disease-associated OTUs, which are good candidates for putative WBD pathogens. A large percentage of these disease-associated OTUs belonged to the order Flavobacteriales. In addition, two of the putative pathogens identified here belong to orders previously suggested as WBD pathogens: Vibronales and Rickettsiales.     

Regulation of Bacterial Communities Through Antimicrobial Activity by the Coral Holobiont

Interactions between corals and associated bacteria and amongst these bacterial groups are likely to play a key role in coral health. However, the complexity of these interactions is poorly understood. We investigated the functional role of specific coral-associated bacteria in maintaining microbial communities on the coral Acropora millepora (Ehrenberg 1834) and the ability of coral mucus to support or inhibit bacterial growth. Culture-independent techniques were used to assess bacterial community structures whilst bacterial culture was employed to assess intra- and inter-specific antimicrobial activities of bacteria. Members of Pseudoalteromonas and ribotypes closely related to Vibrio coralliilyticus displayed potent antimicrobial activity against a range of other cultured isolates and grew readily on detached coral mucus. Although such bacterial ribotypes would be expected to have a competitive advantage, they were rare or absent on intact and healthy coral colonies growing in situ (analysed using denaturing gradient gel electrophoresis and 16S rRNA gene sequencing). The most abundant bacterial ribotypes found on healthy corals were Gammaproteobacteria, previously defined as type A coral associates. Our results indicate that this group of bacteria and specific members of the Alphaproteobacteria described here as ‘type B associates’ may be important functional groups for coral health. We suggest that bacterial communities on coral are kept in check by a combination of host-derived and microbial interactions and that the type A associates in particular may play a key role in maintaining stability of microbial communities on healthy coral colonies.     

Bacterial Communities of Two Ubiquitous Great Barrier Reef Corals Reveals Both Site- and Species-Specificity of Common Bacterial Associates

Background

Coral-associated bacteria are increasingly considered to be important in coral health, and altered bacterial community structures have been linked to both coral disease and bleaching. Despite this, assessments of bacterial communities on corals rarely apply sufficient replication to adequately describe the natural variability. Replicated data such as these are crucial in determining potential roles of bacteria on coral.

Methodology/Principal Findings

Denaturing Gradient Gel Electrophoresis (DGGE) of the V3 region of the 16S ribosomal DNA was used in a highly replicated approach to analyse bacterial communities on both healthy and diseased corals. Although site-specific variations in the bacterial communities of healthy corals were present, host species-specific bacterial associates within a distinct cluster of gamma-proteobacteria could be identified, which are potentially linked to coral health. Corals affected by “White Syndrome” (WS) underwent pronounced changes in their bacterial communities in comparison to healthy colonies. However, the community structure and bacterial ribotypes identified in diseased corals did not support the previously suggested theory of a bacterial pathogen as the causative agent of the syndrome.

Conclusions/Significance

This is the first study to employ large numbers of replicated samples to assess the bacterial communities of healthy and diseased corals, and the first culture-independent assessment of bacterial communities on WS affected Acroporid corals on the GBR. Results indicate that a minimum of 6 replicate samples are required in order to draw inferences on species, spatial or health-related changes in community composition, as a set of clearly distinct bacterial community profiles exist in healthy corals. Coral bacterial communities may be both site and species specific. Furthermore, a cluster of gamma-proteobacterial ribotypes may represent a group of specific common coral and marine invertebrate associates. Finally, the results did not support the contention that a single bacterial pathogen may be the causative agent of WS Acroporids on the GBR.     

The impact of reduced pH on the microbial community of the coral Acropora eurystoma

Rising concentrations of atmospheric carbon dioxide are acidifying the world''s oceans. Surface seawater pH is 0.1 units lower than pre-industrial values and is predicted to decrease by up to 0.4 units by the end of the century. This change in pH may result in changes in the physiology of ocean organisms, in particular, organisms that build their skeletons/shells from calcium carbonate, such as corals. This physiological change may also affect other members of the coral holobiont, for example, the microbial communities associated with the coral, which in turn may affect the coral physiology and health. In the present study, we examined changes in bacterial communities in the coral mucus, tissue and skeleton following exposure of the coral Acropora eurystoma to two different pH conditions: 7.3 and 8.2 (ambient seawater). The microbial community was different at the two pH values, as determined by denaturing gradient gel electrophoresis and 16S rRNA gene sequence analysis. Further analysis of the community in the corals maintained at the lower pH revealed an increase in bacteria associated with diseased and stressed corals, such as Vibrionaceae and Alteromonadaceae. In addition, an increase in the number of potential antibacterial activity was recorded among the bacteria isolated from the coral maintained at pH 7.3. Taken together, our findings highlight the impact that changes in the pH may have on the coral-associated bacterial community and their potential contribution to the coral host.     

Regulation of release of antibacterials from stressed scleractinian corals

Recently, we showed that mechanical stress on scleractinian (stony) corals caused a rapid release of antibacterial material (referred to as coral antibacterial activity, or CAA), which killed various bacterial species, including the coral pathogen Vibrio coralliilyticus . We now report on studies on the regulation of CAA release from stressed scleractinian corals. Corals can repeatedly release highly active CAA as a result of sequential stress inductions. Coral fragments were transferred 19 times from one beaker into another with a stress induction each time after 10 min. There was a decrease in the level of antibacterial activity released during the first four to five transfers. After the fifth transfer, the corals kept releasing CAA for the rest of the experiment with no significant decrease. Apparently, the release of CAA is downregulated by feedback inhibition, depending on the concentration of CAA in the surrounding water. By separating CAA-treated V. coralliilyticus from the surrounding water, it was shown that CAA was bound irreversibly to bacterial cells in a stoichiometric manner. Approximately 4 × 10² bacterial cells were sufficient to bind 1 U of CAA. Resident coral bacteria were more resistant to CAA than bacteria isolated from seawater, suggesting an ecological role for CAA. CAA release was obtained from corals after removal of the mucus layer, and the mucus itself contained antibacterial activity.     

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.     

Assessing the effects of iron enrichment across holobiont compartments reveals reduced microbial nitrogen fixation in the Red Sea coral Pocillopora verrucosa

The productivity of coral reefs in oligotrophic tropical waters is sustained by an efficient uptake and recycling of nutrients. In reef‐building corals, the engineers of these ecosystems, this nutrient recycling is facilitated by a constant exchange of nutrients between the animal host and endosymbiotic photosynthetic dinoflagellates (zooxanthellae), bacteria, and other microbes. Due to the complex interactions in this so‐called coral holobiont, it has proven difficult to understand the environmental limitations of productivity in corals. Among others, the micronutrient iron has been proposed to limit primary productivity due to its essential role in photosynthesis and bacterial processes. Here, we tested the effect of iron enrichment on the physiology of the coral Pocillopora verrucosa from the central Red Sea during a 12‐day experiment. Contrary to previous reports, we did not see an increase in zooxanthellae population density or gross photosynthesis. Conversely, respiration rates were significantly increased, and microbial nitrogen fixation was significantly decreased. Taken together, our data suggest that iron is not a limiting factor of primary productivity in Red Sea corals. Rather, increased metabolic demands in response to iron enrichment, as evidenced by increased respiration rates, may reduce carbon (i.e., energy) availability in the coral holobiont, resulting in reduced microbial nitrogen fixation. This decrease in nitrogen supply in turn may exacerbate the limitation of other nutrients, creating a negative feedback loop. Thereby, our results highlight that the effects of iron enrichment appear to be strongly dependent on local environmental conditions and ultimately may depend on the availability of other nutrients.