Corals maintain complex symbiotic relationships with diverse microorganisms, including fungi, which are often overlooked but represent a critical component of the coral holobiont. This study explores the fungal diversity associated with the tissue and skeleton of the red coral Corallium rubrum, a key species in Mediterranean Marine Animal Forests (MAFs). Using a culture-based approach, we recovered a broad spectrum of fungal diversity, dominated by Ascomycota such as Penicillium, Cladosporium and Aspergillus. The discovery of numerous taxa with known bioactive properties underscores the potential ecological and biotechnological relevance of coral-associated fungi. At the same time, the presence of species such as Aspergillus sydowii, which is considered pathogenic under elevated temperatures, raises concerns about coral vulnerability during increasingly frequent Mediterranean marine heatwaves. These taxa should be further investigated to evaluate their pathogenic potential. Overall, our results expand current knowledge of coral-fungal associations, providing a foundation for future work on their ecological significance, role in coral resilience and potential applications in biotechnology.

Carbon (C) and nitrogen (N) are essential nutrients for coral–Symbiodiniaceae associations, yet global change can disrupt C and N acquisition by corals, affecting their resilience under stress. We investigated how two octocorals (Sarcophyton glaucum, Lobophytum sp.) and two hexacorals (Stylophora pistillata, Turbinaria reniformis) assimilate nitrogen from three 15N-labelled sources—dissolved free amino acids (DFAAs), Synechococcus (picoplankton), and Artemia salina nauplii (microplankton)—supplied at 1 µM N, under control (26 °C) and heat stress (30 °C) conditions. Corals were also incubated with natural pico-nanoplankton assemblages, with concentrations measured via flow cytometry. In addition, we measured the rates of photosynthesis and respiration, to estimate the relative contribution of autotrophy to the corals’ respiratory needs. Across all species, Synechococcus was the most efficiently assimilated N source, with uptake increasing under heat stress. Estimates of heterotrophic carbon assimilation (using C:N ratio) coupled with respiratory measurements showed that Synechococcus can provide 30–70

Plastic pollution poses an increasing threat to coral reef ecosystems, yet the physiological impacts of small-sized microplastics (MPs; ∅ 2.1 μm) and nanoplastics (NPs; ∅ 30 nm) at low mass concentrations (MPs: 5.25 × 10-4 mg L-1; NPs: 2.4 × 10-2 mg L-1) comparable to those found in situ (<0.01 mg L-1) remain largely unknown. In this study, the effects of chronic exposure to MPs and NPs on two symbiotic scleractinian coral species, Stylophora pistillata and Turbinaria reniformis, were investigated over 5 and 10 weeks under controlled laboratory conditions. We evaluated symbiont physiology, photosynthetic performance, respiration, and energy reserve content of the holobiont. The results show that S. pistillata was highly sensitive to MPs, with progressive bleaching, reduced photosynthesis, and significant depletion of lipids, proteins, and carbohydrates. Despite transient metabolic adaptations after 5 weeks, prolonged exposure resulted in physiological decline. In contrast, T. reniformis maintained stable symbiotic parameters, but still exhibited a reduction in net photosynthesis and energy reserves, indicating sublethal physiological costs. NPs elicited milder and delayed effects in both species, with significant effects in S. pistillata occurring only after 10 weeks, possibly due to the low NP mass concentration used. Species-specific responses were likely influenced by differences in morphology, polyp size, heterotrophic capacity, and the dominant Symbiodiniaceae clade. These findings demonstrate that low mass concentrations of plastics can nonetheless disrupt coral physiology and energy balance over time. This can affect coral fitness and their resilience to additional stressors such as ocean warming.

Coral reefs are increasingly threatened by elevated seawater temperatures associated with heatwaves and El Niño events. These phenomena challenge conventional management practices, highlighting an urgent need for innovative interventions to enhance reef resilience. Investigating the role of oxidative stress-one of the main explanations for coral bleaching-is central to these efforts, as excessive production of reactive oxygen species can impair coral physiology, disrupt the coral-algal symbiosis, and ultimately lead to mortality. Here, we investigated an antioxidant-rich food web approach by feeding the reef-building coral Stylophora pistillata with Artemia that had been pre-fed with (i) low-cost and in-house-produced pellets containing curcumin, fucoxanthin, astaxanthin, vitamins C or E, or (ii) the phytoplankton species Pavlova lutheri, Symbiodinium sp., Nannochloropsis sp., Dunaliella salina or Synechococcus sp. Curcumin and the microalga P. lutheri offered the best protection to corals against oxidative stress, and represent the best candidates for potential scalable coral conservation interventions involving targeted feeding of antioxidants. Curcumin pellets offer a streamlined alternative to P. lutheri by eliminating the need for multi-stage microalgal culturing. Direct enrichment of Artemia with antioxidant-rich pellets simplifies production and may provide a cost-effective, scalable strategy to enhance coral resilience to oxidative stress, bleaching and potential mortality.

Abstract The global coral crisis continues to intensify the urgent need for conservation and restoration interventions. Heatwaves are the primary driver of coral declines, triggering coral bleaching by disrupting the symbiosis with the dinoflagellates that provide corals with essential nutrients and energy. Since oxidative stress and nutrient disruption are key processes associated with coral bleaching, we developed a novel, low-cost bioengineering approach to deliver antioxidant support to corals. We used the reef-building model coral Stylophora pistillata to test microparticle feeds containing antioxidants and phytoplankton cells during a simulated heatwave. Among five phytoplankton species, Pavlova lutheri, known to produce antioxidants de novo, offered the best protection to S. pistillata. Moreover, the encapsulated antioxidants curcumin and fucoxanthin outperformed P. lutheri, supporting corals by showing higher photosynthetic efficiency and total antioxidant capacity as well as lower lipid peroxidation impacts. Considering cost-effectiveness for upscaling, curcumin emerges as a promising, scalable intervention for coral conservation, allowing a direct delivery of antioxidant protection to fight oxidative stress associated with warming waters.

The world is currently experiencing the fourth global coral bleaching event, which intensifies the urgency to create interventions that enhance coral thermal tolerance. Manganese (Mn) had been previously shown to offer benefits to corals under heat stress; however, the protective pathway remains unclear, and this uncertainty hampers moving forward in designing effective conservation and restoration strategies to protect coral reefs. For this reason, the scleractinian coral Stylophora pistillata, associated with dinoflagellate symbionts (Symbiodiniaceae) clade A, was exposed to thermal stress with and without Mn enrichment to determine the role of this metal in protecting the coral holobiont under a heatwave scenario. Our results showed a significant decline in superoxide in the ambient seawater with Mn addition, suggesting a protective action against ambient oxidative stress. Additionally, Symbiodiniaceae showed a higher photosynthetic efficiency, and the activity of the enzyme glutathione peroxidase (GPx) in corals decreased significantly with Mn supplementation, suggesting a reduced activity in neutralising reactive oxygen species (ROS) and a decrease in the ambient concentration of ROS. In contrast, superoxide dismutase (SOD) remained unaltered despite the expectation of a higher activity by its manganese-dependent component, MnSOD, to neutralise superoxide radicals produced by the stressed symbionts. Consistently, MnSOD gene expression was significantly downregulated with Mn supplementation, suggesting that MnSOD does not participate in the endogenous antioxidant defence of corals under thermal stress. Mn oxides (MnOx) were detected in Symbiodiniaceae with increasing Mn supplementation, with clade D showing the highest production compared with clades A and C. Together with previous findings, our results indicate that Mn concentrations of 6.4 to 75 nM support corals under heat stress via two possible pathways (i) by Mn uptake that leads to the production of MnOx by Symbiodiniaceae that function as antioxidants, mimicking the coral endogenous antioxidant defence, and (ii) by scavenging of superoxide radicals in seawater.

Coral reefs support immense biodiversity and human well-being, yet accelerating environmental change demands new strategies to strengthen reef resilience. Across ecological systems, balancing nutritional supply with organismal demand is fundamental to performance and persistence, but this principle has been rarely applied to corals. We propose a nutritional ecology framework that links coral physiological requirements with environmental nutrient supply and quality. By systematically addressing critical knowledge gaps of coral physiological requirements - including nutrient balance, quality, acquisition, and allocation - and integrating these with environmental nutrient data, a nutritional ecology framework can identify mismatches between nutritional requirements and availability that impact coral performance under stress. These insights provide a foundation for advancing restoration practices, from site selection to propagation effectiveness, while opening opportunities for interventions such as targeted nutrient supplementation or microbiome-based nutrient enhancement.

Mediterranean corals living in coastal habitats are subjected to natural fluctuations in temperature and nutrient availability, including substantial iron (Fe) inputs via terrestrial runoff (up to 14.5 nM). While Fe is essential for coral and symbiont metabolism, the assimilation rate, physiological thresholds, and spatial allocation of Fe within coral compartments, and its interactive effects with warming, remain poorly understood. Here, we provide the first characterization of oxygen (O2) dynamics, trace metal content, and microbial community composition in two Mediterranean corals, Cladocora caespitosa and Eunicella singularis, exposed to chronic warming (18-24°C) and Fe(III) supplementation (20 nM day-1). We show that although these corals are not Fe-limited, increased temperature enhanced the Fe uptake in the algal symbionts of C. caespitosa. In C. caespitosa, Fe supplementation reduced the O2 availability within the gastrovascular cavity (GVC) and altered the composition and diversity of GVC microbial communities. In E. singularis, interactive effects of Fe and warming reduced GVC O2 availability within the GVC, and warming increased metal content, while the microbiome resembled the surrounding seawater. These intraspecific differences in the sensitivity of the coral holobiont to warming and Fe supplementation could have important implications for the resilience of Mediterranean corals to ongoing climate stress, underscoring the importance of considering coral compartments in ecophysiological research.

The gastrovascular cavity of corals plays a central role in internal circulation, digestion, reproduction, and symbiont acquisition. However, despite its importance, our understanding of the physic-chemical characteristics and dynamic properties of this internal microenvironment remains limited. Here, we employ high-resolution microsensor measurements to investigate the vertical distribution of O2 within the coral gastrovascular cavity. By combining microsensor analysis with time-lapse imaging, we show that O2 levels inside corals are strongly affected by slow, synchronized tissue movements, suggesting a mechanism of active ventilation through modulation of cavity volume and exchange with the surrounding seawater. Together with cilia beating, these movements reduce O2 accumulation under light conditions and alleviate O2 depletion in darkness, thereby stabilizing internal O2 availability. These findings highlight a hitherto overlooked role of behavior in actively regulating the internal microenvironment and O2 status of the coral holobiont and provide critical insight into the organism-environment interactions of reef-building corals.

Climate change and coastal eutrophication increasingly threaten coral reefs, yet their combined impact on coral holobionts remains poorly understood. This study examined the physiological response and carbon budget of two holobionts (Galaxea fascicularis in symbiosis with Cladocopium; Heteroxenia fuscescens in symbiosis with Durusdinium) exposed to nitrate–phosphate enrichment and thermal stress (30 °C). In G. fascicularis, individual stressors severely reduced photosynthate translocation (−90%) due to a significant increase in symbiont respiration, and this species suffered significant bleaching (86% symbiont loss) after the heat-stress phase. Conversely, H. fuscescens was resilient to individual stressors, showing no bleaching and increased carbon translocation under nutrient enrichment or thermal stress alone. Both species exhibited an “energy saving” response following heat stress exposure, significantly increasing lipid and carbohydrate stores. Combined stressors temporarily boosted photosynthetic rates and carbon translocation in G. fascicularis, before a collapse in these parameters after the stress. In contrast, under combined stress H. fuscescens suffered severe bleaching but maintained high rates of carbon translocation to the host and accumulated substantial energy reserves. These findings suggest that different strategies in carbon allocation dictate competitive success under environmental stress: while G. fascicularis prioritizes symbiont maintenance, H. fuscescens maintains or enhances translocation to preserve host metabolism. This study highlights the importance of assessing holobiont carbon budgets and energy reserves to predict coral resilience in a changing ocean.

Abstract Mass coral bleaching events, driven by rising ocean temperatures, are pushing reef ecosystems toward collapse on a global scale. Because oxidative stress is an early driver of coral bleaching, strategies that enhance coral antioxidant defenses may improve coral resilience under thermal stress. Here, we tested a targeted antioxidant supplementation designed to enhance oxidative stress regulation in three representative Red Sea scleractinian coral species subjected to a thermal challenge. While responses varied among species and physiological metrics, supplemented corals consistently maintained higher photophysiological performance under heat stress. In Stylophora pistillata , antioxidant supplementation was associated with enhanced catalase activity, maintenance of glutathione redox homeostasis, and lower intracellular reactive oxygen species levels. In contrast, non-fed corals exhibited oxidative imbalance, increased lipid peroxidation, and impaired photophysiological recovery, while corals receiving a non-enriched heterotrophic diet showed an intermediate response characterized by increased catalase activity but persistent glutathione oxidation and elevated ROS during recovery. Together, the dietary treatments revealed a gradient in oxidative regulation, ranging from insufficient antioxidant protection in autotrophic corals to enhanced oxidative homeostasis in antioxidant-supplemented corals. Our findings demonstrate the potential of targeted nutritional antioxidant supplementation to enhance coral oxidative regulation and physiological performance under elevated temperatures, highlighting a promising complementary approach for coral conservation and restoration efforts.

Abstract The nutritional symbiosis between corals and their photosynthetic dinoflagellate partners underpins the ecological success of reef-building corals in nutrient-poor environments. Although coral holobionts can assimilate the abundant yet highly variable environmental nitrate, direct insight into how nitrate reductase is regulated in these symbiotic algae has been lacking. Here, we provide the first characterization of nitrate reductase protein and gene expression in cultured Symbiodiniaceae exposed to different nitrogen sources and light regimes, revealing the multifactorial nature of its regulation. We demonstrate that nitrate reductase operates as a substrate-induced enzyme: nitrate stimulates protein synthesis in nitrogen-starved cultures, whereas ammonium actively suppresses its expression in a concentration-dependent manner. Light availability and photosynthetic electron transport further modulate protein abundance, suggesting that while nitrate reductase synthesis depends on nitrate availability, its stability may rely on photosynthesis. We also show that nitrate reductase is synthesized within hours of nitrate exposure by symbionts from nitrogen-starved corals, demonstrating that nitrate reduction can occur within the host environment. However, this response is transient and diminished relative to free-living cells, indicating that nitrate reduction is a facultative pathway activated when preferred nitrogen sources are limited. Finally, gene expression measurements and pharmacological inhibition confirm that nitrate reductase regulation is predominantly post-transcriptional, enabling this rapid and reversible control of nitrate assimilation. Together, these findings reveal a tightly regulated and responsive nitrate reduction system in coral symbionts that provides a flexible mechanism contributing to nitrogen homeostasis under fluctuating nutrient regimes.

This study examined how carbon and nitrogen assimilation and exchange shift under normal and elevated temperatures in three cnidarian-dinoflagellate symbioses involving Exaiptasia diaphana (Aiptasia) with either the native symbiont Breviolum minutum or the non-native symbionts Breviolum psygmophilum or Durusdinium trenchii. Carbon was provided as 13C-bicarbonate and nitrogen as 15N-ammonium, nitrate, dissolved free amino acids and Artemia salina nauplii. At control temperature, symbionts differed in carbon fixation—B. minutum achieved the highest rate of population-level carbon fixation, D. trenchii showed higher per-cell photosynthesis but low population-level production due to its low density, while B. psygmophilum had similar per-cell photosynthesis to B. minutum, but lower population-level production. Artemia salina feeding was associated with a higher rate of photosynthetic carbon fixation relative to incubation with dissolved nitrogen. Across the different host-symbiont pairings, photosynthesis unexpectedly increased at 32 °C, while nitrogen pathways shifted: for the hosts and their symbionts, ammonium assimilation rose substantially and dissolved free amino acid uptake stayed stable or declined, while the hosts relied more on heterotrophic nitrogen. The C:N ratio of B. minutum significantly increased, suggesting signs of increased nitrogen limitation, while the C:N ratio of D. trenchii decreased, likely due to an increased reliance on heterotrophic nitrogen. B. psygmophilum decreased carbon transfer to the host, whereas D. trenchii maintained carbon translocation. Notably, host nitrogen retention increased with temperature across all nitrogen treatments in B. minutum. These species-specific patterns highlight how thermal stress reshapes nutrient dynamics differently across symbiont types, with implications for long-term host-symbiont compatibility.

Terrestrial runoff from tropical volcanic islands impacts coral reefs by increasing turbidity and sedimentation. During explosive volcanic eruptions, large amounts of fragmented volcanogenic rock (tephra) are deposited, exacerbating sediment runoff for long periods of time. Nevertheless, tephra is an important, yet underestimated, source of the essential trace metal manganese (Mn), which promotes coral photosynthesis. Here, we show Mn leached from pristine and remobilised tephra increases resilience to sedimentation stress. Using coral culture experiments, microcolonies of Stylophora pistillata exposed to four tephra samples all showed rapid and sustained increases in photosynthetic efficiency (ΦPSII, rETR, Pn and Pgross), even under reduced light conditions. Photosynthetic efficiency is logarithmically correlated to seawater Mn concentration, with large increases at values < 3 µg Mn L− 1, and negligible changes at values > 10 µg Mn L− 1. Tephra exposure has a crucial role in coastal Mn cycling and potentially benefits stressed corals following environmental disturbances.

The climate crisis poses a serious threat to octocorals in the Mediterranean Sea as marine heatwaves (MHWs) not only impair coral metabolism but also disrupt the complex symbiosis between the coral host and its microbiome. Since octocorals are the foundation species of the Mediterranean animal forests, understanding their resilience, i.e. ability to recover and survive to MHWs, is crucial to predict their viability under future climatic conditions. Using amplification of 16 S and 18 S rRNA genes for metabarcoding and qPCR analyses to follow the changes in bacterial microbiome and eukaryome as well as host response under stress and recovery conditions, this study provides the first comprehensive assessment of the resilience of an iconic Mediterranean octocoral (the red coral Corallium rubrum) to a mild (19 °C) and more severe (23 °C) heat stress. The results of this work indicate a stress response of the host to elevated temperatures, even under mild temperature. The eukaryome was highly sensitive to heat stress and underwent rapid structural changes among the dominant microeukaryotes. In contrast, the relative and absolute abundance of the major bacterial symbionts remained stable throughout the stress. However, heat stress led to a significant increase in the abundance of some taxa such as Vibrionaceae that persisted after a week of recovery. While the host recovered from the stress, and the microbiome largely returned to its original composition during recovery, the results highlight the persistent presence of some taxa that might compromise the short-term resilience of octocoral holobionts. This study provides new information on how octocoral holobionts respond to MHWs in the Mediterranean Sea. This knowledge is crucial for the development of effective, science-based strategies for coral protection and restauration.

The precipitous decline in global coral cover over the past several decades necessitates a comprehensive understanding of the energetic underpinnings and fundamental biology governing coral growth, reproduction, and acclimation. Quantifying the flow of energy in corals is a critical component of the knowledge needed to predict, manage, and mitigate the coming future for these sentinel species in an era of rapid global change. In this review, we synthesize nearly 100 yr of the literature on carbon budgets, which are closely linked to coral energetics. To this end, this review is organized into energetic gains and losses, with a focus on detailed mechanistic processes, their historical foundations, and our current understanding. We highlight how the level of energetic reserves and trophic flexibility governs the fate of scleractinian corals, particularly following a bleaching event. We also observe a high degree of species specificity in energy acquisition, storage, and use, specifically in the ability to leverage heterotrophy to buffer energy loss during bleaching. We note that current energetic research has prioritized only 20 species out of at least 1,500 known scleractinians and is focused on taxa with wide ranging geography. Finally, we identify research priorities for addressing the precipitous loss of coral cover in the “Anthropocene.” Key research avenues include (1) taking a species-driven approach to quantify the contribution of autotrophy and heterotrophy to energy and carbon acquisition, (2) assessing trophic flexibility under environmental stress, and (3) generating robust models to explain and predict energetic status. Broadening the scope of studied scleractinians to include corals at range edges, endemics, and understudied taxa with alternative mechanisms for energy utilization, will be key to understanding the flow of energy in corals now and in the coming future.

Plastic pollution is an increasing stressor adding pressure on coral reefs. Microplastic (MP) affects photosynthetic performance and growth of corals and may lead to bleaching. When corals are bleached and autotrophy is impaired, corals are more dependent on heterotrophic feeding to complement energy acquisition. While MP pollution is suspected to interfere with heterotrophic feeding, the effect of food availability on the tolerance of corals to MP pollution is still unknown. Here, we investigated how food availability affects the physiology of corals under MP pollution, by measuring coral growth, photophysiology, and tissue composition. Finally, we also investigated how the physiological response to MP and feeding are connected to bleaching susceptibility of the corals. To this end, we conducted a six-week aquarium experiment followed by a short-term heat stress phase with the coral species Pocillopora verrucosa and Stylophora pistillata exposed to three treatments: MP-free control and twice-weekly feeding (Control), MP-treatment and twice-weekly feeding (MP), and MP-treatment and daily feeding (MP + HF). Coral growth was similar across all treatments. However, MP treatment significantly decreased the tissue energy content of P. verrucosa, although it increased photosynthesis and respiration. High food availability partially mitigated the loss of tissue energy content observed in the MP treatment while maintaining photosynthesis and respiration rates comparable to control conditions. S. pistillata was not affected by MP exposure alone, but when combined with high feeding, photosynthesis decreased below that of the Control. When exposed to short-term heat stress, all corals bleached severely, however, both species bleached less in the MP treatment. These findings highlight that food availability and MP exposure elicit complex responses that influence the effect of other stressors such as heat stress. As MP continues to accumulate in rapidly warming oceans, further research is needed to understand the interactions between food availability and multivariate stressors on coral stress tolerance.

BACKGROUND:Octocoral gorgonians are the engineer species of the Mediterranean coralligenous assemblages, but they are threatened with collapse due to recurring marine heat waves. These extreme events disrupt their symbiotic relationship with their associated microbes, promoting pathogen proliferation and tissue-degrading diseases. While the effects of seawater warming on microbial taxonomic diversity have been extensively studied, the functional response of bacterial symbionts and opportunists to thermal stress in Mediterranean octocorals has not yet been investigated. To fill this gap, we investigated a unique and very stable symbiosis between the emblematic red coral Corallium rubrum and its Spirochaetota symbionts. Although the relative and absolute abundances of Spirochaetota are not affected by heat stress, these symbionts may lose their functions within the coral holobiont. RESULTS:Our results infer that the Spirochaetota bacterial symbionts of C. rubrum underwent only limited functional changes in response to thermal stress, consistent with their stable abundance in coral tissue. These symbionts may play a role in enhancing the tolerance of C. rubrum to temperature fluctuations by maintaining essential amino acid and vitamin biosynthesis. However, thermal stress affected other groups of bacteria, with Gammaproteobacteria showing reduced functionality (with the exception of Vibrionales, which may contribute to the deterioration of coral health) and Alphaproteobacteria showing increased opportunistic activity. In addition, many differentially expressed genes were associated with the sulfur cycle, highlighting its key role in shaping coral-associated bacterial communities under thermal stress. CONCLUSIONS:The stability of the bacterial symbionts of C. rubrum, especially Spirochaetota, despite thermal stress, is consistent with their constant presence in octocoral tissues. These symbionts contribute to coral resilience by maintaining essential biosynthetic processes. However, the increased activity of opportunistic and pathogenic bacteria such as Vibrio suggests that C. rubrum may be susceptible to the recurring heat waves of the summer season.

Volcanic ash is a significant source of micronutrients including iron (Fe), copper (Cu), and zinc (Zn) in oligotrophic tropical waters. These bioactive metals enhance primary productivity, influencing local and global biogeochemical cycles. This study explores how volcanic ash exposure affects trace metal uptake and photophysiological response, and how redox-sensitive metal stable isotope measurements in the tissues of the scleractinian coral Stylophora pistillata can provide crucial information on coral health. Controlled coral culture experiments were conducted in which coral nubbins were exposed to varying intensity and duration of volcanic ash. Throughout the experiment, coral symbionts showed enhanced photosynthetic performance irrespective of intensity or duration of ash exposure. Stable isotopes, such as δ65Cu and δ56Fe, in the coral tissue are marked by systematic variations, not associated with intensity or duration of ash exposure. Instead, we suggest biologically modulated redox-sensitive fractionation associated with ash exposure, linked to the coral host's oxidative stress state. This is evidenced by significant correlations between δ65Cu in coral hosts and photophysiology, with lighter Cu isotope ratios associated with higher photosynthetic performances. Hence, we propose that δ65Cu, and more generally redox-sensitive isotopic ratios (i.e. δ56Fe), in coral hosts serves as an indicator of the physiological state of symbiotic corals.