Symbiodiniaceae-bacteria interactions: rethinking metabolite exchange in reef-building corals as multi-partner metabolic networks

The intimate relationship between scleractinian corals and their associated microorganisms is fundamental to healthy coral reef ecosystems. Coral-associated microbes (Symbiodiniaceae and other protists, bacteria, archaea, fungi and viruses) support coral health and resilience through metabolite transfer, inter-partner signalling, and genetic exchange. However, much of our understanding of the coral holobiont relationship has come from studies that have investigated either coral-Symbiodiniaceae or coral-bacteria interactions in isolation, while relatively little research has focused on other ecological and metabolic interactions potentially occurring within the coral multi-partner symbiotic network. Recent evidences of intimate coupling between phytoplankton and bacteria have demonstrated that obligate resource exchange between partners fundamentally drives their ecological success. Here, we posit that similar associations with bacterial consortia regulate Symbiodiniaceae productivity and are in turn central to the health of corals. Indeed, we propose that this bacteria-Symbiodiniaceae-coral relationship underpins the coral holobiont's nutrition, stress tolerance and potentially influences the future survival of coral reef ecosystems under changing environmental conditions. Resolving Symbiodiniaceae-bacteria associations is therefore a logical next step towards understanding the complex multi-partner interactions occurring in the coral holobiont.     

Chronic low-level nutrient enrichment benefits coral thermal performance in a fore reef habitat

Global- and local-scale anthropogenic stressors have been the main drivers of coral reef decline, causing shifts in coral reef community composition and ecosystem functioning. Excess nutrient enrichment can make corals more vulnerable to ocean warming by suppressing calcification and reducing photosynthetic performance. However, in some environments, corals can exhibit higher growth rates and thermal performance in response to nutrient enrichment. In this study, we measured how chronic nutrient enrichment at low concentrations affected coral physiology, including endosymbiont and coral host response variables, and holobiont metabolic responses of Pocillopora spp. colonies in Mo'orea, French Polynesia. We experimentally enriched corals with dissolved inorganic nitrogen and phosphate for 15 months on an oligotrophic fore reef in Mo'orea. We first characterized symbiont and coral physiological traits due to enrichment and then used thermal performance curves to quantify the relationship between metabolic rates and temperature for experimentally enriched and control coral colonies. We found that endosymbiont densities and total tissue biomass were 54% and 22% higher in nutrient-enriched corals, respectively, relative to controls. Algal endosymbiont nitrogen content cell⁻¹ was 44% lower in enriched corals relative to the control colonies. In addition, thermal performance metrics indicated that the maximal rate of performance for gross photosynthesis was 29% higher and the rate of oxygen evolution at a reference temperature (26.8 °C) for gross photosynthesis was 33% higher in enriched colonies compared to the control colonies. These differences were not attributed to symbiont community composition between corals in different treatments, as C42, a symbiont type in the Cladocopium genus, was the dominant endosymbiont type found in all corals. Together, our results show that in an oligotrophic fore reef environment, nutrient enrichment can cause changes in coral endosymbiont physiology that increase the performance of the coral holobiont.

     

Stable isotopes as tracers of trophic interactions in marine mutualistic symbioses

Mutualistic nutritional symbioses are widespread in marine ecosystems. They involve the association of a host organism (algae, protists, or marine invertebrates) with symbiotic microorganisms, such as bacteria, cyanobacteria, or dinoflagellates. Nutritional interactions between the partners are difficult to identify in symbioses because they only occur in intact associations. Stable isotope analysis (SIA) has proven to be a useful tool to highlight original nutrient sources and to trace nutrients acquired by and exchanged between the different partners of the association. However, although SIA has been extensively applied to study different marine symbiotic associations, there is no review taking into account of the different types of symbiotic associations, how they have been studied via SIA, methodological issues common among symbiotic associations, and solutions that can be transferred from one type of association with another. The present review aims to fill such gaps in the scientific literature by summarizing the current knowledge of how isotopes have been applied to key marine symbioses to unravel nutrient exchanges between partners, and by describing the difficulties in interpreting the isotopic signal. This review also focuses on the use of compound‐specific stable isotope analysis and on statistical advances to analyze stable isotope data. It also highlights the knowledge gaps that would benefit from future research.     

Discovery and quantification of anaerobic nitrogen metabolisms among oxygenated tropical Cuban stony corals

Coral reef health depends on an intricate relationship among the coral animal, photosynthetic algae, and a complex microbial community. The holobiont can impact the nutrient balance of their hosts amid an otherwise oligotrophic environment, including by cycling physiologically important nitrogen compounds. Here we use ¹⁵N-tracer experiments to produce the first simultaneous measurements of ammonium oxidation, nitrate reduction, and nitrous oxide (N₂O) production among five iconic species of reef-building corals (Acropora palmata, Diploria labyrinthiformis, Orbicella faveolata, Porites astreoides, and Porites porites) in the highly protected Jardines de la Reina reefs of Cuba. Nitrate reduction is present in most species, but ammonium oxidation is low potentially due to photoinhibition and assimilatory competition. Coral-associated rates of N₂O production indicate a widespread potential for denitrification, especially among D. labyrinthiformis, at rates of ~1 nmol cm⁻² d⁻¹. In contrast, A. palmata displays minimal active nitrogen metabolism. Enhanced rates of nitrate reduction and N₂O production are observed coincident with dark net respiration periods. Genomes of bacterial cultures isolated from multiple coral species confirm that microorganisms with the ability to respire nitrate anaerobically to either dinitrogen gas or ammonium exist within the holobiont. This confirmation of anaerobic nitrogen metabolisms by coral-associated microorganisms sheds new light on coral and reef productivity.Subject terms: Biogeochemistry, Biogeochemistry     

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.     

Unfamiliar partnerships limit cnidarian holobiont acclimation to warming

Enhancing the resilience of corals to rising temperatures is now a matter of urgency, leading to growing efforts to explore the use of heat tolerant symbiont species to improve their thermal resilience. The notion that adaptive traits can be retained by transferring the symbionts alone, however, challenges the holobiont concept, a fundamental paradigm in coral research. Holobiont traits are products of a specific community (holobiont) and all its co‐evolutionary and local adaptations, which might limit the retention or transference of holobiont traits by exchanging only one partner. Here we evaluate how interchanging partners affect the short‐ and long‐term performance of holobionts under heat stress using clonal lineages of the cnidarian model system Aiptasia (host and Symbiodiniaceae strains) originating from distinct thermal environments. Our results show that holobionts from more thermally variable environments have higher plasticity to heat stress, but this resilience could not be transferred to other host genotypes through the exchange of symbionts. Importantly, our findings highlight the role of the host in determining holobiont productivity in response to thermal stress and indicate that local adaptations of holobionts will likely limit the efficacy of interchanging unfamiliar compartments to enhance thermal tolerance.     

High‐resolution imaging sheds new light on a multi‐tier symbiotic partnership between a “walking” solitary coral,a sipunculan,and a bivalve from East Africa

Marine symbioses are integral to the persistence of ecosystem functioning in coral reefs. Solitary corals of the species Heteropsammia cochlea and Heterocyathus aequicostatus have been observed to live in symbiosis with the sipunculan worm Aspidosiphon muelleri muelleri, which inhabits a cavity within the coral, in Zanzibar (Tanzania). The symbiosis of these photosymbiotic corals enables the coral holobiont to move, in fine to coarse unconsolidated substrata, a process termed as “walking.” This allows the coral to escape sediment cover in turbid conditions which is crucial for these light‐dependent species. An additional commensalistic symbiosis of this coral‐worm holobiont is found between the Aspidosiphon worm and the cryptoendolithic bivalve Jousseaumiella sp., which resides within the cavity of the coral skeleton. To understand the morphological alterations caused by these symbioses, interspecific relationships, with respect to the carbonate structures between these three organisms, are documented using high‐resolution imaging techniques (scanning electron microscopy and µCT scanning). Documenting multi‐layered symbioses can shed light on how morphological plasticity interacts with environmental conditions to contribute to species persistence.     

Habitat-specific environmental conditions primarily control the microbiomes of the coral Seriatopora hystrix

Reef-building corals form complex relationships with a range of microorganisms including bacteria, archaea, fungi and the unicellular microalgae of the genus Symbiodinium, which together form the coral holobiont. These symbionts are known to have both beneficial and deleterious effects on their coral host, but little is known about what the governing factors of these relationships are, or the interactions that exist between the different members of the holobiont and their environment. Here we used 16S ribosomal RNA gene amplicon sequencing to investigate how archaeal and bacterial communities associated with the widespread scleractinian coral Seriatopora hystrix are influenced by extrinsic (reef habitat and geographic location) and intrinsic (host genotype and Symbiodinium subclade) factors. Bacteria dominate the microbiome of S. hystrix, with members of the Alphaproteobacteria, Gammaproteobacteria and Bacteriodetes being the most predominant in all samples. The richness and evenness of these communities varied between reef habitats, but there was no significant difference between distinct coral host lineages or corals hosting distinct Symbiodinium subclades. The coral microbiomes correlated to reef habitat (depth) and geographic location, with a negative correlation between Alpha- and Gammaproteobacteria, driven by the key members of both groups (Rhodobacteraceae and Hahellaceae, respectively), which showed significant differences between location and depth. This study suggests that the control of microbial communities associated with the scleractinian coral S. hystrix is driven primarily by external environmental conditions rather than by those directly associated with the coral holobiont.     

Conservation genetics and the resilience of reef-building corals

Coral reefs have suffered long-term decline due to a range of anthropogenic disturbances and are now also under threat from climate change. For appropriate management of these vulnerable and valuable ecosystems it is important to understand the factors and processes that determine their resilience and that of the organisms inhabiting them, as well as those that have led to existing patterns of coral reef biodiversity. The scleractinian (stony) corals deposit the structural framework that supports and promotes the maintenance of biological diversity and complexity of coral reefs, and as such, are major components of these ecosystems. The success of reef-building corals is related to their obligate symbiotic association with dinoflagellates of the genus Symbiodinium. These one-celled algal symbionts (zooxanthellae) live in the endodermal tissues of their coral host, provide most of the host's energy budget and promote rapid calcification. Furthermore, zooxanthellae are the main primary producers on coral reefs due to the oligotrophic nature of the surrounding waters. In this review paper, we summarize and critically evaluate studies that have employed genetics and/or molecular biology in examining questions relating to the evolution and ecology of reef-building corals and their algal endosymbionts, and that bear relevance to coral reef conservation. We discuss how these studies can focus future efforts, and examine how these approaches enhance our understanding of the resilience of reef-building corals.     

海洋酸化对珊瑚礁生态系统的影响研究进展

目前,大气CO2浓度的升高已导致海水pH值比工业革命前下降了约0.1,海水碳酸盐平衡体系随之变化,进而影响珊瑚礁生态系统的健康。近年来的研究表明海洋酸化导致造礁石珊瑚幼体补充和群落恢复更加困难,造礁石珊瑚和其它造礁生物(Reef-building organisms)钙化率降低甚至溶解,乃至影响珊瑚礁鱼类的生命活动。虽然海洋酸化对造礁石珊瑚光合作用的影响不显著,但珊瑚-虫黄藻共生体系会受到一定影响。建议选择典型海区进行长期系统监测,结合室内与原位模拟试验,从个体、种群、群落到系统不同层面,运用生理学和分子生物学技术,结合生态学研究手段,综合研究珊瑚的相应响应,以期深入认识海洋酸化对珊瑚礁生态系统健康(例如珊瑚白化)的影响及其效应。     

The susceptibility and resilience of corals to thermal stress: adaptation,acclimatization or both?

Coral reefs are threatened with worldwide decline from multiple factors, chief among them climate change ( Hughes et al. 2003 ; Hoegh‐Guldberg et al. 2007 ). The foundation of coral reefs is an endosymbiosis between coral hosts and their resident photosynthetic dinoflagellates (genus Symbiodinium) and this partnership (or holobiont) is exquisitely sensitive to temperature stress. The primary response to hyperthermic stress is coral bleaching, which is the loss of symbionts from coral tissues—the collapse of the symbiosis ( Weis 2008 ). Bleaching can result in increased coral mortality which can ultimately lead to severely compromised reef health ( Hoegh‐Guldberg et al. 2007 ). Despite this grim picture of coral bleaching and reef degradation, coral susceptibility to stress and bleaching is highly variable ( Coles & Brown 2003 ). There is enormous interest in discovering the factors that determine susceptibility in order to help us predict if and how corals will survive a period of rapid global warming. In this issue, Barshis et al. (2010) examine the ecophysiological and genetic basis for differential responses to stress in Porites lobata in American Samoa. They combine a reciprocal transplant experimental design between two neighbouring, but very different reef environments with state‐of‐the‐art physiological biomarkers and molecular genetic markers for both partners to tease apart the contribution of environmental and fixed influences on stress susceptibility. Their results suggest the presence of a fixed, rather than environmental effect on expression of ubiquitin conjugates, one key marker for physiological stress response. In addition, the authors show genetic differentiation in host populations between the two sites suggesting strong selection for physiological adaptation to differing environments across small geographic distances. These conclusions point the study of coral resilience and susceptibility in a new direction.     

Experimental biology of coral reef ecosystems

Coral reef ecosystems are at the crossroads. While significant gaps still exist in our understanding of how “normal” reefs work, unprecedented changes in coral reef systems have forced the research community to change its focus from basic research to understand how one of the most diverse ecosystems in the world works to basic research with strong applied implications to alleviate damage, save, or restore coral reef ecosystems. A wide range of stressors on local, regional, and global spatial scales including over fishing, diseases, large-scale disturbance events, global climate change (e.g., ozone depletion, global warming), and over population have all contributed to declines in coral cover or phase shifts in community structure on time scales never observed before. Many of these changes are directly or indirectly related to anthropogenically induced changes in the global support network that affects all ecosystems. This review focuses on some recent advances in the experimental biology of coral reef ecosystems, and in particular scleractinian corals, at all levels of biological organization. Many of the areas of interest and techniques discussed reflect a progression of technological advances in biology and ecology but have found unique and timely application in the field of experimental coral reef biology. The review, by nature, will not be exhaustive and reflects the author's interests to a large degree. Because of the voluminous literature available, an attempt has been made to capture the essential elements and references for each topic discussed.     

<Emphasis Type="Italic">Megaviridae</Emphasis>-like particles associated with <Emphasis Type="Italic">Symbiodinium</Emphasis> spp. from the endemic coral <Emphasis Type="Italic">Mussismilia braziliensis</Emphasis>

Coral reefs are one of the most dynamic and productive marine ecosystems. The coral holobiont consists of the coral animal and a variety of associated microorganisms that include symbiotic dinoflagellates of the genus Symbiodinium, bacteria, archaea, fungi and viruses. The interactions among these components are crucial for coral health and, consequently, to the coral reef resilience to disturbance. Environmental stressors such as elevated temperature, high irradiance and ultraviolet (UV) radiation can lead to the breakdown of the coral-Symbiodinium symbiosis in a phenomenon known as “coral bleaching”. The present study provides evidence for virus-like particles (VLPs) induced in UV-irradiated Symbiodinium spp. cultures (clades A and C) that were isolated from the coral Mussismilia braziliensis, suggesting a latent viral infection in these strains. Scanning and transmission electron microscopy images of the UV stressed cultures revealed the presence of giant (ca. 450 nm) and small (ca. 40 nm) VLPs. Morphological features link the giant VLPs to the family Megaviridae. Symbiodinium spp. Megaviridae giant viruses and other associated viruses may represent dynamic forces driving and influencing health of the coral holobiont.     

Mass coral bleaching causes biotic homogenization of reef fish assemblages

Global climate change is altering community composition across many ecosystems due to nonrandom species turnover, typically characterized by the loss of specialist species and increasing similarity of biological communities across spatial scales. As anthropogenic disturbances continue to alter species composition globally, there is a growing need to identify how species responses influence the establishment of distinct assemblages, such that management actions may be appropriately assigned. Here, we use trait‐based analyses to compare temporal changes in five complementary indices of reef fish assemblage structure among six taxonomically distinct coral reef habitats exposed to a system‐wide thermal stress event. Our results revealed increased taxonomic and functional similarity of previously distinct reef fish assemblages following mass coral bleaching, with changes characterized by subtle, but significant, shifts toward predominance of small‐bodied, algal‐farming habitat generalists. Furthermore, while the taxonomic or functional richness of fish assemblages did not change across all habitats, an increase in functional originality indicated an overall loss of functional redundancy. We also found that prebleaching coral composition better predicted changes in fish assemblage structure than the magnitude of coral loss. These results emphasize how measures of alpha diversity can mask important changes in the structure and functioning of ecosystems as assemblages reorganize. Our findings also highlight the role of coral species composition in structuring communities and influencing the diversity of responses of reef fishes to disturbance. As new coral species configurations emerge, their desirability will hinge upon the composition of associated species and their capacity to maintain key ecological processes in spite of ongoing disturbances.     

Interwoven Biology of the Tsetse Holobiont

Microbial symbionts can be instrumental to the evolutionary success of their hosts. Here, we discuss medically significant tsetse flies (Diptera: Glossinidae), a group comprised of over 30 species, and their use as a valuable model system to study the evolution of the holobiont (i.e., the host and associated microbes). We first describe the tsetse microbiota, which, despite its simplicity, harbors a diverse range of associations. The maternally transmitted microbes consistently include two Gammaproteobacteria, the obligate mutualists Wigglesworthia spp. and the commensal Sodalis glossinidius, along with the parasitic Alphaproteobacteria Wolbachia. These associations differ in their establishment times, making them unique and distinct from previously characterized symbioses, where multiple microbial partners have associated with their host for a significant portion of its evolution. We then expand into discussing the functional roles and intracommunity dynamics within this holobiont, which enhances our understanding of tsetse biology to encompass the vital functions and interactions of the microbial community. Potential disturbances influencing the tsetse microbiome, including salivary gland hypertrophy virus and trypanosome infections, are highlighted. While previous studies have described evolutionary consequences of host association for symbionts, the initial steps facilitating their incorporation into a holobiont and integration of partner biology have only begun to be explored. Research on the tsetse holobiont will contribute to the understanding of how microbial metabolic integration and interdependency initially may develop within hosts, elucidating mechanisms driving adaptations leading to cooperation and coresidence within the microbial community. Lastly, increased knowledge of the tsetse holobiont may also contribute to generating novel African trypanosomiasis disease control strategies.     

Small-scale environmental variation influences whether coral-dwelling fish promote or impede coral growth

Mutualistic symbioses are ubiquitous in nature and facilitate high biodiversity and productivity of ecosystems by enhancing the efficiency of energy and nutrient use within ecological communities. For example, small groups of fish that inhabit coral colonies in reef ecosystems potentially enhance coral growth through defense from coral predators, aeration of coral tissue and nutrient provisioning. This study examines whether the prevalence and consequences of fish-coral interactions vary among sites with different environmental conditions in a coral reef lagoon, using the humbug damselfish Dascyllus aruanus and its preferred coral host Pocillopora damicornis as a study system. Using a field experiment, we tested the site-specific effects of D. aruanus on coral growth, and show that the cost-benefit ratio for corals hosting fish varies with local environmental variation. Results of this study also demonstrate that fish prefer to inhabit coral colonies with particular branch-spacing characteristics, and that the local abundance of D. aruanus influences the proportion of coral colonies within a site that are occupied by fish rather than increasing the number of fish per colony. We also show that corals consistently benefit from hosting D. aruanus via defense from predation by corallivorous butterflyfish, regardless of local environmental conditions. These findings highlight the need to consider the potential for multiple scale- and state-dependent interaction effects when examining the ecology of fish-coral associations. We suggest that fluctuating cost-benefit ratios for species interactions may contribute to the maintenance of different colony phenotypes within coral populations.     

Modelling the acclimation capacity of coral reefs to a warming ocean

The symbiotic relationship between corals and photosynthetic algae is the foundation of coral reef ecosystems. This relationship breaks down, leading to coral death, when sea temperature exceeds the thermal tolerance of the coral-algae complex. While acclimation via phenotypic plasticity at the organismal level is an important mechanism for corals to cope with global warming, community-based shifts in response to acclimating capacities may give valuable indications about the future of corals at a regional scale. Reliable regional-scale predictions, however, are hampered by uncertainties on the speed with which coral communities will be able to acclimate. Here we present a trait-based, acclimation dynamics model, which we use in combination with observational data, to provide a first, crude estimate of the speed of coral acclimation at the community level and to investigate the effects of different global warming scenarios on three iconic reef ecosystems of the tropics: Great Barrier Reef, South East Asia, and Caribbean. The model predicts that coral acclimation may confer some level of protection by delaying the decline of some reefs such as the Great Barrier Reef. However, the current rates of acclimation will not be sufficient to rescue corals from global warming. Based on our estimates of coral acclimation capacities, the model results suggest substantial declines in coral abundances in all three regions, ranging from 12% to 55%, depending on the region and on the climate change scenario considered. Our results highlight the importance and urgency of precise assessments and quantitative estimates, for example through laboratory experiments, of the natural acclimation capacity of corals and of the speed with which corals may be able to acclimate to global warming.     

Will increased storm disturbance affect the biodiversity of intertidal,nonscleractinian sessile fauna on coral reefs?

Relatively little is known about how the future effects of climatic change, including increases in sea level, temperature and storm severity and frequency, will impact on patterns of biodiversity on coral reefs, with the notable exception of recent work on corals and fish in tropical reef ecosystems. Sessile invertebrates such as ascidians, sponges and bryozoans occupying intertidal rubble habitats on coral reefs contribute significantly to the overall biodiversity and ecosystem function, but there is little or no information available on the likely impacts on these species from climate change. The existing strong physical gradients in these intertidal habitats will be exacerbated under predicted climatic change. By examining the distribution and abundance of nonscleractinian, sessile invertebrate assemblages exposed to different levels of wave action and at different heights on the shore around a coral reef, we show that coral reef intertidal biodiversity is particularly sensitive to physical disturbance. As physical disturbance regimes increase due to more intense storms and wave action associated with global warming, we can expect to see a corresponding decrease in the diversity of these cryptic sessile assemblages. This could impact negatively on the future health and productivity of coral reef ecosystems, given the ecosystem services these organisms provide.     

Ectohydrolytic enzyme activities of bacteria associated with Orbicella annularis coral

Ectohydrolytic enzyme activity (EEA) potential of 37 bacterial isolates derived from Orbicella annularis coral and 2 coral pathogens (Vibrio shilonii and V. coralliilyticus) was measured as model to infer the role of bacteria in organic matter processing within coral reef ecosystems. Bacterial cell-specific activities of eight enzyme types were measured after incubation in organic matter enriched and unenriched filtered seawater. Max value of activities of alkaline phosphatase, oleate-lipase, stearate-lipase and proteinase were 769.3, 327.6, 82.9 and 36.7 amol cell⁻¹ h⁻¹, respectively. Chitinase, α-mannosidase, α-glucosidase and β-glucosidase were generally lower by comparison (max 4.7–20.7 amol cell⁻¹ h⁻¹). No “super” isolates (bacteria expressing high levels of all ectohydrolases) were found suggesting a “specialization” among individual bacterial strains. Cumulatively, the 39 isolates tested displayed a broad range of cell-specific enzyme activities in both organic matter conditions. Culture-independent measurement of coral mucus layer EEA in O. annularis off a Panama reef showed comparable EEA patterns and diversity as the isolates. Volume-specific EEAs of all enzymes except alkaline phosphatase were 8–48 times higher in mucus than in surrounding seawater (SSW) samples. However, cell-specific EEAs in mucus were generally lower than in the SSW partly due to more abundant cells in the mucus than in SSW. For field samples, ≥ 85% of proteinase was cell-bound, while lipase was preferentially dissolved (40–96%). In general, the production of dissolved EEAs varied among measurements depending on sample source and enzyme types, suggesting a potential role of ectoenzyme size distribution in linking the whole reef ecosystem. Our findings support that the cumulative ectoenzyme expression (“ectoenzymome”) of the coral microbiome has the potential to maintain the functional resilience of the coral holobiont and response to stress through its contribution to organic matter processing within coral reef ecosystems.

     

Heat stress reduces the contribution of diazotrophs to coral holobiont nitrogen cycling

Efficient nutrient cycling in the coral-algal symbiosis requires constant but limited nitrogen availability. Coral-associated diazotrophs, i.e., prokaryotes capable of fixing dinitrogen, may thus support productivity in a stable coral-algal symbiosis but could contribute to its breakdown when overstimulated. However, the effects of environmental conditions on diazotroph communities and their interaction with other members of the coral holobiont remain poorly understood. Here we assessed the effects of heat stress on diazotroph diversity and their contribution to holobiont nutrient cycling in the reef-building coral Stylophora pistillata from the central Red Sea. In a stable symbiotic state, we found that nitrogen fixation by coral-associated diazotrophs constitutes a source of nitrogen to the algal symbionts. Heat stress caused an increase in nitrogen fixation concomitant with a change in diazotroph communities. Yet, this additional fixed nitrogen was not assimilated by the coral tissue or the algal symbionts. We conclude that although diazotrophs may support coral holobiont functioning under low nitrogen availability, altered nutrient cycling during heat stress abates the dependence of the coral host and its algal symbionts on diazotroph-derived nitrogen. Consequently, the role of nitrogen fixation in the coral holobiont is strongly dependent on its nutritional status and varies dynamically with environmental conditions.Subject terms: Microbial ecology, Climate-change ecology