Phosphorus metabolism of reef organisms with algal symbionts

Phosphorus nutrition strategies in a Symbiodiniacean species: Implications in coral-alga symbiosis facing increasing phosphorus deficiency in future warmer oceans

Coral reefs thrive in the oligotrophic ocean and rely on symbiotic algae to acquire nutrients. Global warming is projected to intensify surface ocean nutrient deficiency and anthropogenic discharge of wastes with high nitrogen (N): phosphorus (P) ratios can exacerbate P nutrient limitation. However, our understanding on how symbiotic algae cope with P deficiency is limited. Here, we investigated the responses of a coral symbiotic species of Symbiodiniaceae, Cladocopium goreaui, to P-limitation by examining its physiological performance and transcriptomic profile. Under P stress, C. goreaui exhibited decreases in algal growth, photosynthetic efficiency, and cellular P content but enhancement in carbon fixation, N assimilation, N:P ratio, and energy metabolism, with downregulated expression of carbohydrate exporter genes. Besides, C. goreaui showed flexible mechanisms of utilizing different dissolved organic phosphorus to relieve P deficiency. When provided glycerol phosphate, C. goreaui hydrolyzed it extracellularly to produce phosphate for uptake. When grown on phytate, in contrast, C. goreaui upregulated the endocytosis pathway while no dissolved inorganic phosphorus was released into the medium, suggesting that phytate was transported into the cell, potentially via the endocytosis pathway. This study sheds light on the survival strategies of C. goreaui and potential weakening of its role as an organic carbon supplier in P-limited environments, underscoring the importance of more systematic investigation on future projections of such effects.

Possible beneficial interactions of ciliated protozoans with coral health and resilience

Microbial interactions contribute significantly to coral health in the marine environment. Most beneficial associations have been described with their bacterial communities, but knowledge of beneficial associations between protozoan ciliates and corals is still lacking. Ciliates are important bacterial predators and provide nutrition to higher trophic-level organisms. The mucus secreted by corals and the microenvironment of the coral surface layer attract ciliates based on their food preferences. The mixotrophic and heterotrophic ciliates play a major role in nutrient cycling by increasing nitrogen, phosphorus, and extractable sulfur, which can enhance the proliferation of coral beneficial microbe. Besides, bacterial predator ciliates reduce the pathogenic bacterial population that infects the coral and also act as bioindicators for assessing the toxicity of the reef ecosystem. Thus, these ciliates can be used as a beneficial partner in influencing coral health and resilience under various stress conditions. Herein, we explore the urgent need to understand the complex beneficial interactions of ciliates that may occur in the coral reef ecosystem.

Cellular traits regulate fluorescence-based light-response phenotypes of coral photosymbionts living in-hospite

Diversity across algal family Symbiodiniaceae contributes to the environmental resilience of certain coral species. Chlorophyll-a fluorescence measurements are frequently used to determine symbiont health and resilience, but more work is needed to refine these tools and establish how they relate to underlying cellular traits. We examined trait diversity in symbionts from the generas Cladocopium and Durusdinium, collected from 12 aquacultured coral species. Photophysiological metrics (ΦPSII, σPSII, ρ, τ1, τ2, antenna bed quenching, non-photochemical quenching, and qP) were assessed using a prototype multi-spectral fluorometer over a variable light protocol which yielded a total of 1,360 individual metrics. Photophysiological metrics were then used to establish four unique light-response phenotypic variants. Corals harboring C15 were predominantly found within a single light-response phenotype which clustered separately from all other coral fragments. The majority of Durusdinium dominated colonies also formed a separate light-response phenotype which it shared with a few C1 dominated corals. C15 and D1 symbionts appear to differ in which mechanisms they use to dissipate excess light energy. Spectrally dependent variability is also observed across light-response phenotypes that may relate to differences in photopigment utilization. Symbiont cell biochemical and structural traits (atomic C:N:P, cell size, chlorophyll-a, neutral lipid content) was also assessed within each sample and differ across light-response phenotypes, linking photophysiological metrics with underlying primary cellular traits. Strong correlations between first- and second-order traits, such as Quantum Yield and cellular N:P content, or light dissipation pathways (qP and NPQ) and C:P underline differences across symbiont types and may also provide a means for using fluorescence-based metrics as biomarkers for certain primary-cellular traits.

Reef-building corals farm and feed on their photosynthetic symbionts

Coral reefs are highly diverse ecosystems that thrive in nutrient-poor waters, a phenomenon frequently referred to as the Darwin paradox1. The energy demand of coral animal hosts can often be fully met by the excess production of carbon-rich photosynthates by their algal symbionts2,3. However, the understanding of mechanisms that enable corals to acquire the vital nutrients nitrogen and phosphorus from their symbionts is incomplete4-9. Here we show, through a series of long-term experiments, that the uptake of dissolved inorganic nitrogen and phosphorus by the symbionts alone is sufficient to sustain rapid coral growth. Next, considering the nitrogen and phosphorus budgets of host and symbionts, we identify that these nutrients are gathered through symbiont 'farming' and are translocated to the host by digestion of excess symbiont cells. Finally, we use a large-scale natural experiment in which seabirds fertilize some reefs but not others, to show that the efficient utilization of dissolved inorganic nutrients by symbiotic corals established in our laboratory experiments has the potential to enhance coral growth in the wild at the ecosystem level. Feeding on symbionts enables coral animals to tap into an important nutrient pool and helps to explain the evolutionary and ecological success of symbiotic corals in nutrient-limited waters.

Species-Specific Response of Corals to Imbalanced Ratios of Inorganic Nutrients

Dissolved inorganic phosphorus (DIP) is a limiting nutrient in the physiology of scleractinian corals. Anthropogenic addition of dissolved inorganic nitrogen (DIN) to coastal reefs increases the seawater DIN:DIP ratio and further increases P limitation, which is detrimental to coral health. The effects of imbalanced DIN:DIP ratios on coral physiology require further investigation in coral species other than the most studied branching corals. Here we investigated the nutrient uptake rates, elemental tissue composition and physiology of a foliose stony coral, Turbinaria reniformis, and a soft coral, Sarcophyton glaucum, exposed to four different DIN: DIP ratios (0.5:0.2, 0.5:1, 3:0.2, 3:1). The results show that T. reniformis had high uptake rates of DIN and DIP, proportional to the seawater nutrient concentrations. DIN enrichment alone led to an increase in tissue N content, shifting the tissue N:P ratio towards P limitation. However, S. glaucum had 5 times lower uptake rates and only took up DIN when the seawater was simultaneously enriched with DIP. This double uptake of N and P did not alter tissue stoichiometry. This study allows us to better understand the susceptibility of corals to changes in the DIN:DIP ratio and predict how coral species will respond under eutrophic conditions in the reef.

Antiscalants used in the desalination industry impact the physiology of the coral Montipora capricornis

Many coral reefs are found in arid and semi-arid regions that often face severe water scarcity and depend on seawater desalination for freshwater supply. Alongside freshwater production, desalination plants discharge brine waste into the sea. Brine includes various chemicals (e.g., antiscalants) that may harm the coastal environment. Although widely used, little is known about the ecotoxicological effects of antiscalants (AS) on hard corals. This study compared the impacts of polyphosphonate-based and polymer-based ASs on the coral Montipora capricornis. After two weeks of exposure, we determined the effects of AS on coral physiology, symbiotic microalgae, and associated bacteria, using various analytical approaches such as optical coherence tomography, pulse amplitude modulated fluorometry, and oxidative stress biomarkers. Both ASs reduced polyp activity (∼25%) and caused tissue damage (30% and 41% for polymer and polyphosphonate based AS, respectively). In addition, exposure to polyphosphonate-based AS decreased the abundance of endosymbiotic algae (39%) and upregulated the antioxidant capacity of the animal host (45%). The microalgal symbionts were under oxidative stress, with increased levels of antioxidant capacity and oxidative damage (a 2-fold increase compared to the control). Interestingly, exposure to AS enhanced the numbers of associated bacteria (∼40% compared to the control seawater) regardless of the AS type. Our results introduce new insights into the effects of brine on the physiology of hard corals, highlighting that choosing AS type must be examined according to the receiving ecosystem.

Positive interactions between corals and damselfish increase coral resistance to temperature stress

By the century's end, many tropical seas will reach temperatures exceeding most coral species' thermal tolerance on an annual basis. The persistence of corals in these regions will, therefore, depend on their abilities to tolerate recurrent thermal stress. Although ecologists have long recognized that positive interspecific interactions can ameliorate environmental stress to expand the realized niche of plants and animals, coral bleaching studies have largely overlooked how interactions with community members outside of the coral holobiont shape the bleaching response. Here, we subjected a common coral, Pocillopora grandis, to 10 days of thermal stress in aquaria with and without the damselfish Dascyllus flavicaudus (yellowtail dascyllus), which commonly shelter within these corals, to examine how interactions with damselfish impacted coral thermal tolerance. Corals often benefit from nutrients excreted by animals they interact with and prior to thermal stress, corals grown with damselfish showed improved photophysiology (F_v /F_m ) and developed larger endosymbiont populations. When exposed to thermal stress, corals with fish performed as well as control corals maintained at ambient temperatures without fish. In contrast, corals exposed to thermal stress without fish experienced photophysiological impairment, a more than 50% decline in endosymbiont density, and a 36% decrease in tissue protein content. At the end of the experiment, thermal stress caused average calcification rates to decrease by over 80% when damselfish were absent but increase nearly 25% when damselfish were present. Our study indicates that damselfish-derived nutrients can increase coral thermal tolerance and are consistent with the Stress Gradient Hypothesis, which predicts that positive interactions become increasingly important for structuring communities as environmental stress increases. Because warming of just a few degrees can exceed corals' temperature tolerance to trigger bleaching and mortality, positive interactions could play a critical role in maintaining some coral species in warming regions until climate change is aggressively addressed.

Biogeochemical dynamics of particulate organic phosphorus and its potential environmental implication in a typical "algae-type" eutrophic lake

Organic phosphorus (P_o) plays a very important role in the process of lake eutrophication, but there is still a lack of knowledge about the internal cycle of P_o in suspended particulate matter (SPM) dominated by algal debris. In this study, the characterization of bioavailable P_o by sequential extraction and enzymatic hydrolysis showed that 45% of extracted TP was P_o in SPM of Lake Dianchi, and 43-98% of total P_o in H₂O, NaHCO₃ and NaOH fractions was enzymatically hydrolyzable P_o (EHP, H₂O-EHP: 31-53%). Importantly, labile monoester P was the main organic form (68%) of EHP, and its potential bioavailability was higher than that of diester P and phytate-like P. According to the estimation of P pools in SPM of the whole lake, the total load of P_i plus EHP in the H₂O extract of SPM was 74.9 t and had great potential risk to enhance eutrophication in the lake water environment. Accordingly, reducing the amount of SPM in the water during the algal blooming period is likely to be a necessary measure that can successfully interfere with or block the continuous stress of unhealthy levels of P on the aquatic ecosystem.

The diversity and ecology of Symbiodiniaceae: A traits-based review

Among the most successful microeukaryotes to form mutualisms with animals are dinoflagellates in the family Symbiodiniaceae. These photosynthetic symbioses drive significant primary production and are responsible for the formation of coral reef ecosystems but are particularly sensitive when environmental conditions become extreme. Annual episodes of widespread coral bleaching (disassociation of the mutualistic partnership) and mortality are forecasted from the year 2060 under current trends of ocean warming. However, host cnidarians and dinoflagellate symbionts display exceptional genetic and functional diversity, and meaningful predictions of the future that embrace this biological complexity are difficult to make. A recent move to trait-based biology (and an understanding of how traits are shaped by the environment) has been adopted to move past this problem. The aim of this review is to: (1) provide an overview of the major cnidarian lineages that are symbiotic with Symbiodiniaceae; (2) summarise the symbiodiniacean genera associated with cnidarians with reference to recent changes in taxonomy and systematics; (3) examine the knowledge gaps in Symbiodiniaceae life history from a trait-based perspective; (4) review Symbiodiniaceae trait variation along three abiotic gradients (light, nutrients, and temperature); and (5) provide recommendations for future research of Symbiodiniaceae traits. We anticipate that a detailed understanding of traits will further reveal basic knowledge of the evolution and functional diversity of these mutualisms, as well as enhance future efforts to model stability and change in ecosystems dependent on cnidarian-dinoflagellate organisms.

Contaminant Discharge From Outfalls and Subsequent Aquatic Ecological Risks in the River Systems in Dhaka City: Extent of Waste Load Contribution in Pollution

Dhaka, the capital city, which is the nerve center of Bangladesh, is crisscrossed by six different rivers. A network of peripheral rivers connects the city and functions as a natural drainage system for a massive amount of wastewater and sewage by the increased number of inhabitants impacting the overall environmental soundness and human health. This study intended to identify and characterize the outfalls along the peripheral rivers of Dhaka city with the assessment of different pollution indices such as comprehensive pollution index (CPI), organic pollution index (OPI), and ecological risk indices (E_RI). The study evaluated the status of the pollution in the aquatic system in terms of ambient water quality parameters along the peripheral rivers due to discharge from outfalls with a particular focus on waste load contribution. Among the identified outfalls, the majority are industrial discharge (60%), and some are originated from municipal (30%), or domestic sewers (10%). Water quality parameters such as suspended solids (SS), 5-day biochemical oxygen demand (BOD₅), and Ammoniacal Nitrogen (NH₃-N) for most of the peripheral rivers deviated by as much as 40–50% from industrial discharge standards by the environment conservation rules, Bangladesh, 1997. Based on the CPI, the rivers Buriganga, Dhaleshwari, and Turag could be termed as severely polluted (CPI > 2.0), while the OPI indicated heavy organic pollutant (OPI > 4) contamination in the Dhaleshwari and Buriganga rivers. The associated pollution indices demonstrate a trend for each subsequent peripheral river with significant pollution toward the downstream areas. The demonstrated waste loading map from the outfalls identified sources of significant environmental contaminants in different rivers leading to subsequent ecological risks. The study outcomes emphasize the necessity of systematic investigation and monitoring while controlling the point and non-point urban pollution sources discharging into the peripheral rivers of Dhaka city.

Impact of nitrogen (N) and phosphorus (P) enrichment and skewed N:P stoichiometry on the skeletal formation and microstructure of symbiotic reef corals

Reported divergent responses of coral growth and skeletal microstructure to the nutrient environment complicate knowledge-based management of water quality in coral reefs. By re-evaluating published results considering the taxonomy of the studied corals and the N:P stoichiometry of their nutrient environment, we could resolve some of the major apparent contradictions. Our analysis suggests that Acroporids behave differently to several other common genera and show distinct responses to specific nutrient treatments. We hypothesised that both the concentrations of dissolved inorganic N and P in the water and their stoichiometry shape skeletal growth and microstructure. We tested this hypothesis by exposing Acropora polystoma fragments to four nutrient treatments for > 10 weeks: high nitrate/high phosphate (HNHP), high nitrate/low phosphate (HNLP), low nitrate/high phosphate (LNHP) and low nitrate/low phosphate (LNLP). HNHP corals retained high zooxanthellae densities and their linear extension and calcification rates were up to ten times higher than in the other treatments. HNLP and LNLP corals bleached through loss of symbionts. The photochemical efficiency (Fv/Fm) of residual symbionts in HNLP corals was significantly reduced, indicating P-starvation. Micro-computed tomography (µCT) of the skeletal microstructure revealed that reduced linear extension in nutrient limited or nutrient starved conditions (HNLP, LNHP, LNLP) was associated with significant thickening of skeletal elements and reduced porosity. These changes can be explained by the strongly reduced linear extension rate in combination with a smaller reduction in the calcification rate. Studies using increased skeletal density as a proxy for past thermal bleaching events should consider that such an increase in density may also be associated with temperature-independent response to the nutrient environment. Furthermore, the taxonomy of corals and seawater N:P stoichiometry should be considered when analysing and managing the impacts of nutrient pollution.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00338-022-02223-0.

Eutrophication on Coral Reefs: What Is the Evidence for Phase Shifts, Nutrient Limitation and Coral Bleaching

Coral reefs continue to experience extreme environmental pressure from climate change stressors, but many coral reefs are also exposed to eutrophication. It has been proposed that changes in the stoichiometry of ambient nutrients increase the mortality of corals, whereas eutrophication may facilitate phase shifts to macroalgae-dominated coral reefs when herbivory is low or absent. But are corals ever nutrient limited, and can eutrophication destabilize the coral symbiosis making it more sensitive to environmental stress because of climate change? The effects of eutrophication are confounded not just by the effects of climate change but by the presence of chemical pollutants in industrial, urban, and agricultural wastes. Because of these confounding effects, the increases in nutrients or changes in their stoichiometry in coastal environments, although they are important at the organismal and community level, cannot currently be disentangled from each other or from the more significant effects of climate change stressors on coral reefs.

The role of three-tandem Pho Boxes in the control of the C-P lyase operon in a thermophilic cyanobacterium

Cyanobacteria have an inherited advantage in phosphonate phytoremediation. However, studies on phosphonate metabolism in cyanobacteria are rare and mostly focus on physiology and ecology. Here, C-P lyase gene cluster regulation in an undomesticated thermophilic Synechococcus OS-B' was examined in Synechocystis sp. PCC6803, a unicellular cyanobacterial model. Phylogenetic and cluster synteny analysis of C-P lyase genes revealed a closer relationship between Syn OS-B' and Thermus thermophilus, than with other cyanobacteria. Pho boxes were identified in the 5'-end-flanking region of the C-P lyase gene cluster, through which the downstream gene expression was regulated in a phosphate concentration-dependent manner. Unexpectedly, the phosphate concentration that thoroughly inhibited Pho boxes was almost two orders of magnitude higher than that of any natural or anthropogenic wastewater reported so far. The Pho boxes mediated regulation was achieved through the Pho regulon two-component system, and the absence of either SphS or SphR ablated the cell's ability to sense ambient phosphate changes. The three tandems of Pho boxes maintained inequivalent roles, of which the third tandem was not essential; however, it played a role in adjusting Pho boxes response in both positive and negative manner under phosphorus limitation.

High plasticity of nitrogen fixation and denitrification of common coral reef substrates in response to nitrate availability

Nitrogen cycling in coral reefs may be affected by nutrient availability, but knowledge about concentration-dependent thresholds that modulate dinitrogen fixation and denitrification is missing. We determined the effects of different nitrate concentrations (ambient, 1, 5, 10 μM nitrate addition) on both processes under two light scenarios (i.e., light and dark) using a combined acetylene assay for two common benthic reef substrates, i.e., turf algae and coral rubble. For both substrates, dinitrogen fixation rates peaked at 5 μM nitrate addition in light, whereas denitrification was highest at 10 μM nitrate addition in the dark. At 10 μm nitrate addition in the dark, a near-complete collapse of dinitrogen fixation concurrent with a 76-fold increase in denitrification observed for coral rubble, suggesting potential threshold responses linked to the nutritional state of the community. We conclude that dynamic nitrogen cycling activity may help stabilise nitrogen availability in microbial communities associated with coral reef substrates.

Asymmetric physiological response of a reef-building coral to pulsed versus continuous addition of inorganic nutrients

Coral reefs, especially those located near-shore, are increasingly exposed to anthropogenic, eutrophic conditions that are often chronic. Yet, corals under unperturbed conditions may frequently receive natural and usually temporary nutrient supplementation through biological sources such as fishes. We compared physiological parameters indicative of long- and short-term coral health (day and night calcification, fragment surface area, productivity, energy reserves, and tissue stoichiometry) under continuous and temporary nutrient enrichment. The symbiotic coral Acropora intermedia was grown for 7 weeks under continuously elevated (press) levels of ammonium (14 µmol L^-1) and phosphate (10 µmol L^-1) as separate and combined treatments, to discern the individual and interactive nutrient effects. Another treatment exposed A. intermedia twice-daily to an ammonium and phosphate pulse of the same concentrations as the press treatments to simulate natural biotic supplementation. Press exposure to elevated ammonium or phosphate produced mixed effects on physiological responses, with little interaction between the nutrients in the combined treatment. Overall, corals under press exposure transitioned resources away from calcification. However, exposure to nutrient pulses often enhanced physiological responses. Our findings indicate that while continuous nutrient enrichment may pose a threat to coral health, episodic nutrient pulses that resemble natural nutrient supplementation may significantly benefit coral health and physiology.

Nitrogen fixation and denitrification activity differ between coral- and algae-dominated Red Sea reefs

Coral reefs experience phase shifts from coral- to algae-dominated benthic communities, which could affect the interplay between processes introducing and removing bioavailable nitrogen. However, the magnitude of such processes, i.e., dinitrogen (N2) fixation and denitrification levels, and their responses to phase shifts remain unknown in coral reefs. We assessed both processes for the dominant species of six benthic categories (hard corals, soft corals, turf algae, coral rubble, biogenic rock, and reef sands) accounting for > 98% of the benthic cover of a central Red Sea coral reef. Rates were extrapolated to the relative benthic cover of the studied organisms in co-occurring coral- and algae-dominated areas of the same reef. In general, benthic categories with high N2 fixation exhibited low denitrification activity. Extrapolated to the respective reef area, turf algae and coral rubble accounted for > 90% of overall N2 fixation, whereas corals contributed to more than half of reef denitrification. Total N2 fixation was twice as high in algae- compared to coral-dominated areas, whereas denitrification levels were similar. We conclude that algae-dominated reefs promote new nitrogen input through enhanced N2 fixation and comparatively low denitrification. The subsequent increased nitrogen availability could support net productivity, resulting in a positive feedback loop that increases the competitive advantage of algae over corals in reefs that experienced a phase shift.

Mesoglea Extracellular Matrix Reorganization during Regenerative Process in Anemonia viridis (Forskål, 1775)

Given the anatomical simplicity and the extraordinary ability to regenerate missing parts of the body, Cnidaria represent an excellent model for the study of the mechanisms regulating regenerative processes. They possess the mesoglea, an amorphous and practically acellular extracellular matrix (ECM) located between the epidermis and the gastrodermis of the body and tentacles and consists of the same molecules present in the ECM of vertebrates, such as collagen, laminin, fibronectin and proteoglycans. This feature makes cnidarians anthozoans valid models for understanding the ECM role during regenerative processes. Indeed, it is now clear that its role in animal tissues is not just tissue support, but instead plays a key role during wound healing and tissue regeneration. This study aims to explore regenerative events after tentacle amputation in the Mediterranean anemone Anemonia viridis, focusing in detail on the reorganization of the ECM mesoglea. In this context, both enzymatic, biometric and histological experiments reveal how this gelatinous connective layer plays a fundamental role in the correct restoration of the original structures by modifying its consistency and stiffness. Indeed, through the deposition of collagen I, it might act as a scaffold and as a guide for the reconstruction of missing tissues and parts, such as amputated tentacles.

Characterization of the microbiome and immune response in corals with chronic Montipora white syndrome

Coral diseases have increased in frequency and intensity around the tropics worldwide. However, in many cases, little is known about their etiology. Montipora white syndrome (MWS) is a common disease affecting the coral Montipora capitata, a major reef builder in Hawai'i. Chronic Montipora white syndrome (cMWS) is a slow-moving form of the disease that affects M. capitata throughout the year. The effects of this chronic disease on coral immunology and microbiology are currently unknown. In this study, we use prophenoloxidase immune assays and 16S rRNA gene amplicon sequencing to characterize the microbiome and immunological response associated with cMWS. Our results show that immunological and microbiological responses are highly localized. Relative to diseased samples, apparently healthy portions of cMWS corals differed in immune activity and in the relative abundance of microbial taxa. Coral tissues with cMWS showed decreased tyrosinase-type catecholase and tyrosinase-type cresolase activity and increased laccase-type activity. Catecholase and cresolase activity were negatively correlated across all tissue types with microbiome richness. The localized effect of cMWS on coral microbiology and immunology is probably an important reason for the slow progression of the disease. This local confinement may facilitate interventions that focus on localized treatments on tissue types. This study provides an important baseline to understand the interplay between the microbiome and immune system and the mechanisms used by corals to manage chronic microbial perturbations associated with white syndrome.

The biotransformation of soil phosphorus in the water level fluctuation zone could increase eutrophication in reservoirs

The massive amounts of phosphorus (P) entering into rivers and reservoirs may induce eutrophication. However, the link between the transport and transformation of soil P and the dynamics of P availability in reservoir regions are not well demonstrated. The present study selected the Pengxi River suffering the anti-seasonal water level fluctuation of the Three Gorgers Reservoir as the study area. Soil nutrients along the longitudinal and lateral gradients of the Pengxi River were investigated to illustrate the spatial distribution patterns, analyzed by the Hedley extraction schemes. The effects of biotic and abiotic factors on soil P transformation and the dynamics of bioavailable P were evaluated via determinations of enzymatic hydrolysis phosphorus (EHP) with and without ultraviolet (UV) irradiation. The results indicated that soil nutrients varied significantly between the water level fluctuation zone (WLFZ) and upland along the river longitudinal gradients, where the trends of the extracted OP were the same in H₂O, NaHCO₃ and NaOH extractions. The EHP accounted for 33.67 ± 15.87% of the total extracted OP, of which Monoester P, Phytate-like P and NHOP were determined at all extracts but Diester P was mainly found at H₂O and NaOH extracts. UV irradiation significantly increased P bioavailability up to 24.44%. These results could demonstrate the mechanism of soil P transformation via UV irradiation and enzymatic hydrolysis. Therefore, the bioavailable P enters the water body during the submergence period may lead to eutrophication in the Pengxi River, which could pose a risk to the reservoir ecosystem.

Iron Availability Modulates the Response of Endosymbiotic Dinoflagellates to Heat Stress

Warming and nutrient limitation are stressors known to weaken the health of microalgae. In situations of stress, access to energy reserves can minimize physiological damage. Because of its widespread requirements in biochemical processes, iron is an important trace metal, especially for photosynthetic organisms. Lowered iron availability in oceans experiencing rising temperatures may contribute to the thermal sensitivity of reef-building corals, which rely on mutualisms with dinoflagellates to survive. To test the influence of iron concentration on thermal sensitivity, the physiological responses of cultured symbiotic dinoflagellates (genus Breviolum; family Symbiodiniaceae) were evaluated when exposed to increasing temperatures (26 to 30°C) and iron concentrations ranging from replete (500 pM Fe') to limiting (50 pM Fe') under a diurnal light cycle with saturating radiance. Declines in photosynthetic efficiency at elevated temperatures indicated sensitivity to heat stress. Furthermore, five times the amount of iron was needed to reach exponential growth during heat stress (50 pM Fe' at 26-28°C vs. 250 pM Fe' at 30°C). In treatments where exponential growth was reached, Breviolum psygmophilum grew faster than B.minutum, possibly due to greater cellular contents of iron and other trace metals. The metal composition of B.psygmophilum shifted only at the highest temperature (30°C), whereas changes in B.minutum were observed at lower temperatures (28°C). The influence of iron availability in modulating each alga's response to thermal stress suggests the importance of trace metals to the health of coral-algal mutualisms. Ultimately, a greater ability to acquire scarce metals may improve the tolerance of corals to physiological stressors and contribute to the differences in performance associated with hosting one symbiont species over another.

Tracing the Trophic Plasticity of the Coral-Dinoflagellate Symbiosis Using Amino Acid Compound-Specific Stable Isotope Analysis

The association between corals and photosynthetic dinoflagellates is one of the most well-known nutritional symbioses, but nowadays it is threatened by global changes. Nutritional exchanges are critical to understanding the performance of this symbiosis under stress conditions. Here, compound-specific δ¹⁵N and δ¹³C values of amino acids (δ¹⁵N_AA and δ¹³C_AA) were assessed in autotrophic, mixotrophic and heterotrophic holobionts as diagnostic tools to follow nutritional interactions between the partners. Contrary to what was expected, heterotrophy was mainly traced through the δ¹⁵N of the symbiont’s amino acids (AAs), suggesting that symbionts directly profit from host heterotrophy. The trophic index (TP) ranged from 1.1 to 2.3 from autotrophic to heterotrophic symbionts. In addition, changes in TP across conditions were more significant in the symbionts than in the host. The similar δ¹³C-AAs signatures of host and symbionts further suggests that symbiont-derived photosynthates are the main source of carbon for AAs synthesis. Symbionts, therefore, appear to be a key component in the AAs biosynthetic pathways, and might, via this obligatory function, play an essential role in the capacity of corals to withstand environmental stress. These novel findings highlight important aspects of the nutritional exchanges in the coral–dinoflagellates symbiosis. In addition, they feature δ¹⁵N_AA as a useful tool for studies regarding the nutritional exchanges within the coral–symbiodiniaceae symbiosis.

Influences of phosphorus concentration and porewater advection on phosphorus dynamics in carbonate sands around the Weizhou Island, northern South China Sea

A series of flow-through reactor experiments were undertaken to assess the potential effect of porewater advection and dissolved inorganic phosphorus (DIP) concentration on benthic DIP dynamics in permeable sediments collected from the Weizhou Island, northern South China Sea. The flux of DIP ranged from -0.13 to 0.05 mmol m^-2 h^-1, and the reversal from DIP efflux to influx occurred when the DIP concentration reached a threshold. DIP release from the sediment into the seawater peaked at intermediate advection rate, which perhaps provide optimum conditions for DIP release related to CaCO₃ dissolution. Phosphorus limitation in seawater could be relieved by DIP release from the sediment, and CaCO₃-bound P in carbonate sands may play a major role in benthic DIP release and decrease in the molar nitrogen/phosphorus ratio in seawater around the Weizhou Island.

Optimization of medium using response surface methodology to enhance the growth of Effrenium voratum (Symbiodiniaceae, Dinophyceae)

Survival of coral reef-associated Symbiodiniaceae is vital to maintain the healthy coral community in coral reefs. However, knowledge about cultivation of free-living or symbiotic Symbiodiniaceae has been limited. In this study, the response surface methodology was applied to optimize the medium for Effrenium voratum. The results showed that the impacts of nutrient components on algal growth were: FeCl₃ > NaH₂ PO₄ >MnSO₄ > MgSO₄ /CoSO₄ > KCl>ZnSO₄ > CaCl₂ /NaNO₃ , among which NaH₂ PO₄ and FeCl₃ significantly affected algal growth. The optimal medium was: natural seawater supplemented with NaH₂ PO₄ ·2H₂ O 0.25 mM,FeCl₃ ·6H₂ O 14.24 μM, NaNO₃ 0.94 mM, MgSO₄ ·7H₂ O 40.63 mM, KCl 5.37 mM, CaCl₂ ·2H₂ O 4.08 mM, ZnSO₄ ·7H₂ O 0.35 μM, MnSO₄ 9.93 μM, and CoSO₄ 0.36 μM. The use of the optimized medium resulted in an increase of biomass yield (0.76 g dry weight · L^-1 ) by 46% over that using the initial medium, which agreed with the predicted value (0.71 g · L^-1 ). Additionally, fatty acids, mainly consisting of palmitic acid (C16:0) and ethyl carbonate (C20:0), accounted for approximately 50% of the total fatty acids in E. voratum. Interestingly, docosahexaenoic acid (DHA) accounted for 6% of total fatty acids, a high proportion that makes E. voratum a potential candidate feedstock in aquaculture for DHA production.

Sub-cellular imaging shows reduced photosynthetic carbon and increased nitrogen assimilation by the non-native endosymbiont Durusdinium trenchii in the model cnidarian Aiptasia

Hosting different symbiont species can affect inter-partner nutritional fluxes within the cnidarian-dinoflagellate symbiosis. Using nanoscale secondary ion mass spectrometry (NanoSIMS), we measured the spatial incorporation of photosynthetically fixed ¹³ C and heterotrophically derived ¹⁵ N into host and symbiont cells of the model symbiotic cnidarian Aiptasia (Exaiptasia pallida) when colonized with its native symbiont Breviolum minutum or the non-native Durusdinium trenchii. Breviolum minutum exhibited high photosynthetic carbon assimilation per cell and translocation to host tissue throughout symbiosis establishment, whereas D. trenchii assimilated significantly less carbon, but obtained more host nitrogen. These findings suggest that D. trenchii has less potential to provide photosynthetically fixed carbon to the host despite obtaining considerable amounts of heterotrophically derived nitrogen. These sub-cellular events help explain previous observations that demonstrate differential effects of D. trenchii compared to B. minutum on the host transcriptome, proteome, metabolome and host growth and asexual reproduction. Together, these differential effects suggest that the non-native host-symbiont pairing is sub-optimal with respect to the host's nutritional benefits under normal environmental conditions. This contributes to our understanding of the ways in which metabolic integration impacts the benefits of a symbiotic association, and the potential evolution of novel host-symbiont pairings.

A multiomic analysis of in situ coral-turf algal interactions

Viruses, microbes, and host macroorganisms form ecological units called holobionts. Here, a combination of metagenomic sequencing, metabolomic profiling, and epifluorescence microscopy was used to investigate how the different components of the holobiont including bacteria, viruses, and their associated metabolites mediate ecological interactions between corals and turf algae. The data demonstrate that there was a microbial assemblage unique to the coral-turf algae interface displaying higher microbial abundances and larger microbial cells. This was consistent with previous studies showing that turf algae exudates feed interface and coral-associated microbial communities, often at the detriment of the coral. Further supporting this hypothesis, when the metabolites were assigned a nominal oxidation state of carbon (NOSC), we found that the turf algal metabolites were significantly more reduced (i.e., have higher potential energy) compared to the corals and interfaces. The algae feeding hypothesis was further supported when the ecological outcomes of interactions (e.g., whether coral was winning or losing) were considered. For example, coral holobionts losing the competition with turf algae had higher Bacteroidetes-to-Firmicutes ratios and an elevated abundance of genes involved in bacterial growth and division. These changes were similar to trends observed in the obese human gut microbiome, where overfeeding of the microbiome creates a dysbiosis detrimental to the long-term health of the metazoan host. Together these results show that there are specific biogeochemical changes at coral-turf algal interfaces that predict the competitive outcomes between holobionts and are consistent with algal exudates feeding coral-associated microbes.

Nitrogen eutrophication particularly promotes turf algae in coral reefs of the central Red Sea

While various sources increasingly release nutrients to the Red Sea, knowledge about their effects on benthic coral reef communities is scarce. Here, we provide the first comparative assessment of the response of all major benthic groups (hard and soft corals, turf algae and reef sands—together accounting for 80% of the benthic reef community) to in-situ eutrophication in a central Red Sea coral reef. For 8 weeks, dissolved inorganic nitrogen (DIN) concentrations were experimentally increased 3-fold above environmental background concentrations around natural benthic reef communities using a slow release fertilizer with 15% total nitrogen (N) content. We investigated which major functional groups took up the available N, and how this changed organic carbon (C_org) and N contents using elemental and stable isotope measurements. Findings revealed that hard corals (in their tissue), soft corals and turf algae incorporated fertilizer N as indicated by significant increases in δ¹⁵N by 8%, 27% and 28%, respectively. Among the investigated groups, C_org content significantly increased in sediments (+24%) and in turf algae (+33%). Altogether, this suggests that among the benthic organisms only turf algae were limited by N availability and thus benefited most from N addition. Thereby, based on higher C_org content, turf algae potentially gained competitive advantage over, for example, hard corals. Local management should, thus, particularly address DIN eutrophication by coastal development and consider the role of turf algae as potential bioindicator for eutrophication.

Trait-Based Comparison of Coral and Sponge Microbiomes

Corals and sponges harbor diverse microbial communities that are integral to the functioning of the host. While the taxonomic diversity of their microbiomes has been well-established for corals and sponges, their functional roles are less well-understood. It is unclear if the similarities of symbiosis in an invertebrate host would result in functionally similar microbiomes, or if differences in host phylogeny and environmentally driven microhabitats within each host would shape functionally distinct communities. Here we addressed this question, using metatranscriptomic and 16S rRNA gene profiling techniques to compare the microbiomes of two host organisms from different phyla. Our results indicate functional similarity in carbon, nitrogen, and sulfur assimilation, and aerobic nitrogen cycling. Additionally, there were few statistical differences in pathway coverage or abundance between the two hosts. For example, we observed higher coverage of phosphonate and siderophore metabolic pathways in the star coral, Montastraea cavernosa, while there was higher coverage of chloroalkane metabolism in the giant barrel sponge, Xestospongia muta. Higher abundance of genes associated with carbon fixation pathways was also observed in M. cavernosa, while in X. muta there was higher abundance of fatty acid metabolic pathways. Metagenomic predictions based on 16S rRNA gene profiling analysis were similar, and there was high correlation between the metatranscriptome and metagenome predictions for both hosts. Our results highlight several metabolic pathways that exhibit functional similarity in these coral and sponge microbiomes despite the taxonomic differences between the two microbiomes, as well as potential specialization of some microbially based metabolism within each host.

Assessing the impacts of phosphate mining on coral reef communities and reef development

Phosphate mining activities on Christmas Island began in the late 1800's providing a unique, long-term case study in which to assess the impacts of mining on coral reef development. Watershed modelling was used to identify potential "hotspots" of mining runoff on to adjacent reefs. Pollution hotspots were also confirmed by analysis of reef sediment. Phosphate rich mining runoff flowed from local watersheds onto nearshore coral reefs with levels of up to 54,000 mg/kg of total phosphate recorded in reef sediment at the Dryers reef site adjacent to the main phosphate storage facility. Using this combination of watershed modelling and in-situ sediment contamination data we identified six coral reef sites along an environmental impact gradient. In-situ benthic transects were paired with a new rubble-encruster method enabling the analysis to combine large scale transect information alongside fine-scale data on epibenthic and encruster assemblages. Results demonstrate that phosphate rich sediment loading negatively impacted coral reef building communities, in particular, branching corals and calcareous encrusting organisms, critical to the future survival of coral reef ecosystems. These findings highlight the importance of curtailing runoff and pollution from catchment based mining activities and protecting reefs for the future.

Nutrient Availability and Metabolism Affect the Stability of Coral-Symbiodiniaceae Symbioses

Coral reefs rely upon the highly optimized coral-Symbiodiniaceae symbiosis, making them sensitive to environmental change and susceptible to anthropogenic stress. Coral bleaching is predominantly attributed to photo-oxidative stress, yet nutrient availability and metabolism underpin the stability of symbioses. Recent studies link symbiont proliferation under nutrient enrichment to bleaching; however, the interactions between nutrients and symbiotic stability are nuanced. Here, we demonstrate how bleaching is regulated by the forms and ratios of available nutrients and their impacts on autotrophic carbon metabolism, rather than algal symbiont growth. By extension, historical nutrient conditions mediate host-symbiont compatibility and bleaching tolerance over proximate and evolutionary timescales. Renewed investigations into the coral nutrient metabolism will be required to truly elucidate the cellular mechanisms leading to coral bleaching.

Seabird nutrients are assimilated by corals and enhance coral growth rates

Nutrient subsidies across ecotone boundaries can enhance productivity in the recipient ecosystem, especially if the nutrients are transferred from a nutrient rich to an oligotrophic ecosystem. This study demonstrates that seabird nutrients from islands are assimilated by endosymbionts in corals on fringing reefs and enhance growth of a dominant reef-building species, Acropora formosa. Nitrogen stable isotope ratios (δ¹⁵N) of zooxanthellae were enriched in corals near seabird colonies and decreased linearly with distance from land, suggesting that ornithogenic nutrients were assimilated in corals. In a one-year reciprocal transplant experiment, A. formosa fragments grew up to four times faster near the seabird site than conspecifics grown without the influence of seabird nutrients. The corals influenced by elevated ornithogenic nutrients were located within a marine protected area with abundant herbivorous fish populations, which kept nuisance macroalgae to negligible levels despite high nutrient concentrations. In this pristine setting, seabird nutrients provide a beneficial nutrient subsidy that increases growth of the ecologically important branching corals. The findings highlight the importance of catchment–to–reef management, not only for ameliorating negative impacts from land but also to maintain beneficial nutrient subsidies, in this case seabird guano.

Studies on phosphorus deficiency in the Qianbei-Pockmarked goat

Objective

Qianbei-Pockmarked goats are affected by a disorder locally referred to as ‘Ruanguzheng Disorder’, which is characterized by emaciation, lameness, muscular relaxation, stiffness of the extremities, and abnormal curvatures of the long bones. Our objective was to determine the relationship between the disorder and phosphorus deficiency.

Methods

Tissue samples were collected from affected and healthy animals, while soil and herbage samples were collected from affected and healthy pastures. Biochemical parameters were determined using an automatic biochemical analyzer (OLYMPUS AU 640, Olympus Optical Co., Tokyo, Japan). Mineral contents in soil, forage, and tissue were determined using a Perkin-Elmer AAS5000 atomic absorption spectrophotometer (Perkin-Elmer, Norwalk, CT, USA).

Results

The results showed that phosphorus contents in herbages from affected pastures were markedly lower than those from healthy areas (p<0.01), and the ratio of calcium to phosphorus in the affected herbages was 12.93:1. The phosphorus contents of wool, blood, tooth, and bone from affected animals were also markedly lower than those from healthy animals (p<0.01). Serum phosphorus values in affected animals were much lower than those in healthy animals, while serum alkaline phosphatase values from affected animals were markedly higher than those from healthy animals (p<0.01). Inorganic phosphorus values from affected animals were approximately half of that in the control group. Supplementation of disodium hydrogen phosphate prevented and cured the disorder.

Conclusion

This study demonstrates that Ruanguzheng disorder in Qianbei-Pockmarked goats is primarily caused by phosphorus deficiencies in herbage due to fenced pastures and natural habitat fragmentation.

Possible roles of glutamine synthetase in responding to environmental changes in a scleractinian coral

Glutamine synthetase is an enzyme that plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine. In this study, the activity and responses of glutamine synthetase towards environmental changes were investigated in the scleractinian coral Pocillopora damicornis. The identified glutamine synthetase (PdGS) was comprised of 362 amino acids and predicted to contain one Gln-synt_N and one Gln-synt_C domain. Expression of PdGS mRNA increased significantly after 12 h (1.28-fold, p < 0.05) of exposure to elevated ammonium, while glutamine synthetase activity increased significantly from 12 to 24 h, peaking at 12 h (54.80 U mg^-1, p < 0.05). The recombinant protein of the mature PdGS (rPdGS) was expressed in E. coli BL21, and its activities were detected under different temperature, pH and glufosinate levels. The highest levels of rPdGS activity were observed at 25 °C and pH 8 respectively, but decreased significantly at lower temperature, and higher or lower pH. Furthermore, the level of rPdGS activities was negatively correlated with the concentration of glufosinate, specifically decreasing at 10^-5 mol L^-1 glufosinate to be less than 50% (p < 0.05) of that in the blank. These results collectively suggest that PdGS, as a homologue of glutamine synthetase, was involved in the nitrogen assimilation in the scleractinian coral. Further, its physiological functions could be suppressed by high temperature, ocean acidification and residual glufosinate, which might further regulate the coral-zooxanthella symbiosis via the nitrogen metabolism in the scleractinian coral P. damicornis.

Intra-colonial diversity in the scleractinian coral, Acropora millepora: identifying the nutritional gradients underlying physiological integration and compartmentalised functioning

Scleractinian corals are colonial organisms comprising multiple physiologically integrated polyps and branches. Colonialism in corals is highly beneficial, and allows a single colony to undergo several life processes at once through physiological integration and compartmentalised functioning. Elucidating differences in the biochemical composition of intra-colonial branch positions will provide valuable insight into the nutritional reserves underlying different regions in individual coral colonies. This will also ascertain prudent harvesting strategies of wild donor-colonies to generate coral stock with high survival and vigour prospects for reef-rehabilitation efforts and captive husbandry. This study examined the effects of colony branch position on the nutritional profile of two different colony sizes of the common scleractinian, Acropora millepora. For smaller colonies, branches were sampled at three locations: the colony centre (S-centre), 50% of the longitudinal radius length (LRL) (S-50), and the colony edge (S-edge). For larger colonies, four locations were sampled: the colony centre (L-centre), 33.3% of the LRL (L-33), 66.6% of the LRL (L-66), and the edge (L-edge). Results demonstrate significant branch position effects, with the edge regions containing higher protein, likely due to increased tissue synthesis and calcification. Meanwhile, storage lipid and total fatty acid concentrations were lower at the edges, possibly reflecting catabolism of high-energy nutrients to support proliferating cells. Results also showed a significant effect of colony size in the two classes examined. While the major protein and structural lipid sink was exhibited at the edge for both sizes, the major sink for high-energy lipids and fatty acids appeared to be the L-66 position of the larger colonies and the S-centre and S-50 positions for the smaller colonies. These results confirm that the scleractinian coral colony is not nutritionally homogeneous, and while different regions of the coral colony are functionally specialised, so too are their nutritional profiles geared toward meeting specific energetic demands.

Coral Food, Feeding, Nutrition, and Secretion: A Review

Tropical scleractinian corals are dependent to varying degrees on their photosymbiotic partners. Under normal levels of temperature and irradiance, they can provide most, but not all, of the host's nutritional requirements. Heterotrophy is required to adequately supply critical nutrients, especially nitrogen and phosphorus. Scleractinian corals are known as mesozooplankton predators, and most employ tentacle capture. The ability to trap nano- and picoplankton has been demonstrated by several coral species and appears to fulfill a substantial proportion of their daily metabolic requirements. The mechanism of capture likely involves mucociliary activity or extracoelenteric digestion, but the relative contribution of these avenues have not been evaluated. Many corals employ mesenterial filaments to procure food in various forms, but the functional morphology and chemical activities of these structures have been poorly documented. Corals are capable of acquiring nutrition from particulate and dissolved organic matter, although the degree of reliance on these sources generally has not been established. Corals, including tropical, deep- and cold-water species, are known as a major source of carbon and other nutrients for benthic communities through the secretion of mucus, despite wide variation in chemical composition. Mucus is cycled through the planktonic microbial loop, the benthos, and the microbial community within the sediments. The consensus indicates that the dissolved organic fraction of mucus usually exceeds the insoluble portion, and both serve as sources for the growth of nano- and picoplankton. As many corals employ mucus to trap food, a portion is taken back during feeding. The net gain or loss has not been evaluated, although production is generally thought to exceed consumption. The same is true for the net uptake and loss of dissolved organic matter by mucus secretion. Octocorals are thought not to employ mucus capture or mesenterial filaments during feeding and generally rely on tentacular filtration of weakly swimming mesozooplankton, particulates, dissolved organic matter, and picoplankton. Nonsymbiotic species in the tropics favor phytoplankton and weakly swimming zooplankton. Azooxanthellate soft corals are opportunistic feeders and shift their diet according to the season from phyto- and nanoplankton in summer to primarily particulate organic matter (POM) in winter. Cold-water species favor POM, phytodetritus, microplankton, and larger zooplankton when available. Antipatharians apparently feed on mesozooplankton but also use mucus nets, possibly for capture of POM. Feeding modes in this group are poorly known.

Sugar enrichment provides evidence for a role of nitrogen fixation in coral bleaching

The disruption of the coral-algae symbiosis (coral bleaching) due to rising sea surface temperatures has become an unprecedented global threat to coral reefs. Despite decades of research, our ability to manage mass bleaching events remains hampered by an incomplete mechanistic understanding of the processes involved. In this study, we induced a coral bleaching phenotype in the absence of heat and light stress by adding sugars. The sugar addition resulted in coral symbiotic breakdown accompanied by a fourfold increase of coral-associated microbial nitrogen fixation. Concomitantly, increased N:P ratios by the coral host and algal symbionts suggest excess availability of nitrogen and a disruption of the nitrogen limitation within the coral holobiont. As nitrogen fixation is similarly stimulated in ocean warming scenarios, here we propose a refined coral bleaching model integrating the cascading effects of stimulated microbial nitrogen fixation. This model highlights the putative role of nitrogen-fixing microbes in coral holobiont functioning and breakdown.

Trophic and stoichiometric consequences of nutrification for the intertidal tropical zoanthid Zoanthus sociatus

Zoanthids are conspicuous and abundant members of intertidal environments, where they are exposed to large environmental fluctuations and subject to increasing loads of anthropogenic nutrients. Here we assess the trophic ecology and stoichiometric consequences of nutrient loading for symbiotic zoanthids inhabiting different intertidal habitats. More specifically, we analysed the stable isotope signature (δ¹³C and δ¹⁵N), elemental composition (C, N and P) and stoichiometry (C:N, C:P, N:P) of Zoanthus sociatus differently exposed to nutrification. Results suggest that autotrophy is the main feeding mode of zoanthids and that the effect water nutrient content differently affects the elemental phenotype of zoanthids depending on tidal habitat. Additionally, habitat effects on Z. sociatus P-related stoichiometric traits highlight functional differences likely associated with variation in Symbiodinium density. These findings provide an innovative approach to assess how cnidarian-dinoflagellate symbioses response to ecosystem changes in environmentally dynamic reef flats, particularly nutrient loading.

Phosphate deficiency promotes coral bleaching and is reflected by the ultrastructure of symbiotic dinoflagellates

Enrichment of reef environments with dissolved inorganic nutrients is considered a major threat to the survival of corals living in symbiosis with dinoflagellates (Symbiodinium sp.). We argue, however, that the direct negative effects on the symbiosis are not necessarily caused by the nutrient enrichment itself but by the phosphorus starvation of the algal symbionts that can be caused by skewed nitrogen (N) to phosphorus (P) ratios. We exposed corals to imbalanced N:P ratios in long-term experiments and found that the undersupply of phosphate severely disturbed the symbiosis, indicated by the loss of coral biomass, malfunctioning of algal photosynthesis and bleaching of the corals. In contrast, the corals tolerated an undersupply with nitrogen at high phosphate concentrations without negative effects on symbiont photosynthesis, suggesting a better adaptation to nitrogen limitation. Transmission electron microscopy analysis revealed that the signatures of ultrastructural biomarkers represent versatile tools for the classification of nutrient stress in symbiotic algae. Notably, high N:P ratios in the water were clearly identified by the accumulation of uric acid crystals.

Virus-host interactions and their roles in coral reef health and disease

Coral reefs occur in nutrient-poor shallow waters, constitute biodiversity and productivity hotspots, and are threatened by anthropogenic disturbance. This Review provides an introduction to coral reef virology and emphasizes the links between viruses, coral mortality and reef ecosystem decline. We describe the distinctive benthic-associated and water-column- associated viromes that are unique to coral reefs, which have received less attention than viruses in open-ocean systems. We hypothesize that viruses of bacteria and eukaryotes dynamically interact with their hosts in the water column and with scleractinian (stony) corals to influence microbial community dynamics, coral bleaching and disease, and reef biogeochemical cycling. Last, we outline how marine viruses are an integral part of the reef system and suggest that the influence of viruses on reef function is an essential component of these globally important environments.