Search

Marine animal forests (MAFs) are benthic ecosystems characterised by biogenic three‐dimensional structures formed by suspension feeders such as corals, gorgonians, sponges and bivalves. They comprise highly diversified communities among the most productive in the world's oceans. However, MAFs are in decline due to global and local stressors that threaten the survival and growth of their foundational species and associated biodiversity. Innovative and scalable interventions are needed to address the degradation of MAFs and increase their resilience under global change. Surprisingly, few studies have considered trophic interactions and heterotrophic feeding of MAF suspension feeders as an integral component of MAF conservation. Yet, trophic interactions are important for nutrient cycling, energy flow within the food web, biodiversity, carbon sequestration, and MAF stability. This comprehensive review describes trophic interactions at all levels of ecological organisation in tropical, temperate, and cold‐water MAFs. It examines the strengths and weaknesses of available tools for estimating the heterotrophic capacities of the foundational species in MAFs. It then discusses the threats that climate change poses to heterotrophic processes. Finally, it presents strategies for improving trophic interactions and heterotrophy, which can help to maintain the health and resilience of MAFs. [ABSTRACT FROM AUTHOR]

Corals engage in symbioses with micro‐organisms that provide nutrients and protect the host. Where the prokaryotic microbes perform their symbiotic functions within a coral is, however, poorly understood. Here, we studied the tissue‐specific microbiota of the coral Corallium rubrum by dissecting its tissues from the skeleton and separating the white polyps from the red‐coloured coenenchyme, followed by 16S rRNA gene metabarcoding of the three fractions. Dissection was facilitated by incubating coral fragments in RNAlater, which caused tissues to detach from the skeleton. Our results show compartmentalisation of the microbiota. Specifically, Endozoicomonas, Parcubacteria and a Gammaproteobacteria were primarily located in polyps, whereas Nitrincolaceae and one Spirochaeta phylotype were found mainly in the coenenchyme. The skeleton‐associated microbiota was distinct from the microbiota in the tissues. Given the difference in tissue colour and microbiota of the polyps and coenenchyme, we analysed the microbiota of three colormorphs of C. rubrum (red, pink, white), finding that the main difference was a very low abundance of Spirochaeta in white colormorphs. While the functions of C. rubrum's symbionts are unknown, their localisation within the colony suggests that microhabitats exist, and the presence of Spirochaeta appears to be linked to the colour of C. rubrum. [ABSTRACT FROM AUTHOR]

Octocorals represent a major alternative group in the benthic community of reefs that have diverged from hexacoral dominance. Despite their phototrophic symbionts, supplementing their diet with heterotrophic sources may promote their growth, particularly when compared to hexacorals in bleaching conditions. However, limited comprehensive data exists on octocorals' trophic ecology, especially regarding their ability to feed on dissolved organic matter (DOM), which comprises the largest pool of organic matter in reefs. This study aims to investigate the ability of two octocorals (Sarcophyton glaucum and Lobophytum sp.) to feed on DOM and compare this ability to that of hexacorals, such as Stylophora pistillata and Turbinaria reniformis. The study measured the net fluxes of DOM under varying DOM concentrations and under heat stress. The results demonstrate that all coral species were net producers of DOM at ambient concentrations, but became net consumers once seawater was supplemented with DOM. Furthermore, our study highlights a relationship between coral physiological responses to heat stress and DOM uptake. Corals that increased (S. pistillata) or maintained (S. glaucum and Lobophytum sp.) their DOM uptake rates at high temperature were the most resilient to heat stress. In contrast, T. reniformis exhibited lower DOM uptake rates at high temperatures, which was associated with significant bleaching. Understanding the ability of corals to feed on DOM may, therefore, provide insight into the resilience of species under ocean warming conditions. [ABSTRACT FROM AUTHOR]

Climate change‐related increase in seawater temperature has become a leading cause of coral bleaching and mortality. However, corals from the northern Red Sea show high thermal tolerance and no recorded massive bleaching event. This specific region is frequently subjected to intense dust storms, coming from the surrounding arid deserts, which are expected to increase in frequency and intensity in the future. The aerial dust deposition supplies essential bioelements to the water column. Here, we investigated the effect of dust deposition on the physiology of a Red Sea coral, Stylophora pistillata. We measured the modifications in coral and Symbiodiniaceae metallome (cellular metal content), as well as the changes in photosynthesis and oxidative stress status of colonies exposed during few weeks to dust deposition. Our results show that 1 mg L−1 of dust supplied nanomolar amounts of nitrate and other essential bioelements, such as iron, manganese, zinc and copper, rapidly assimilated by the symbionts. At 25°C, metal bioaccumulation enhanced the chlorophyll concentration and photosynthesis of dust‐exposed corals compared to control corals. These results suggest that primary production was limited by metal availability in seawater. A 5°C increase in seawater temperature enhanced iron assimilation in both control and dust‐enriched corals. Temperature rise increased the photosynthesis of control corals only, dust‐exposed ones having already reached maximal photosynthesis rates at 25°C. Finally, we observed a combined effect of temperature and bioelement concentration on the assimilation of molybdenum, cadmium, manganese and copper, which were in higher concentrations in symbionts of dust‐exposed corals maintained at 30°C. All together these observations highlight the importance of dust deposition in the supply of essential bioelements, such as iron, to corals and its role in sustaining coral productivity in Red Sea reefs. [ABSTRACT FROM AUTHOR]

Mesophotic coral ecosystems (30–150 m depth) present a high oceanic biodiversity, but remain one of the most understudied reef habitats, especially below 60 m depth. Here, we have assessed the rates of photosynthesis and dissolved inorganic carbon (DIC) and nitrogen (DIN) assimilation by Symbiodiniaceae associated with four soft coral species of the genus Sinularia and two stony coral species of the genus Leptoseris collected respectively at 65 and 80–90 m depth in the Gulf of Eilat. Our study demonstrates that both Leptoseris and Sinularia species have limited autotrophic capacities at mid‐lower mesophotic depths. DIC and DIN assimilation rates were overall ~ 10 times lower compared to shallow corals from 10 m depth in the same reef. While Leptoseris symbionts transferred at least 50% of the acquired nitrogen to their host after 8‐h incubation, most of the nitrogen was retained in the symbionts of Sinularia. In addition, the host tissue of Sinularia species presented a very high structural carbon to nitrogen ratio (C : N) compared to Leptoseris or to the shallow coral species, suggesting nitrogen limitation in these mesophotic soft corals. The limited capacity of soft coral symbionts to acquire DIN and transfer it to the coral animal, as well as the high C : N ratios, might explain the scarcity of symbiotic soft corals at mid‐lower mesophotic depths compared to their prevalence in the shallower reef. Overall, this study highlights the significance of DIN for the distribution of the Cnidarian‐ Symbiodiniaceae association at mesophotic depth. [ABSTRACT FROM AUTHOR]

Symbiotic octocorals are important reef inhabitants, from shallow to mesophotic depths. However, information regarding their nutritional ecology along the depth gradient, and in particular the changes in the autotrophic and heterotrophic acquisition of nutrients, is limited, despite the fact that nutrient acquisition is a fundamental process in explaining the distribution of reef organisms. Here, the abundance of three lipid biomarkers, specific for autotrophy and heterotrophy, was investigated in the tissue of four octocoral species and one scleractinian sampled in shallow and upper mesophotic reefs of the oligotrophic Northern Red Sea. Our findings show functional mixotrophy for all dinoflagellate‐bearing species, with a significant input of heterotrophic feeding on herbivorous crustacean zooplankton at both depths. The relatively high level of the heterotrophic marker in all symbiotic species in shallow conditions does not corroborate the common idea that corals rely mostly on autotrophy in shallow waters. The increase in heterotrophic capacity with depth was however species‐specific, likely related to physiological and morphological characteristics. In addition, octocorals maintained similar concentrations of autotrophic markers with increasing depth, while the scleractinian species exhibited a decrease in these markers with depth. These results are in agreement with previous measurements of photosynthetically‐fixed carbon acquisition, suggesting that autotrophy remains stable in Red sea soft corals, whereas it decreases in scleractinian corals. This study highlights the importance of heterotrophy across the euphotic‐upper mesophotic depth gradient and brings relevant advances on our understanding of the ecological significance of feeding for reef corals. [ABSTRACT FROM AUTHOR]

Copyright of Functional Ecology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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

Summary: Dinitrogen (N2) fixing bacteria (diazotrophs) are an important source of new nitrogen in oligotrophic environments and represent stable members of the microbiome in tropical corals, while information on corals from temperate oligotrophic regions is lacking. Therefore, this study provides new insights into the diversity and activity of diazotrophs associated with the temperate coral Oculina patagonica from the Mediterranean Sea by combining metabarcoding sequencing of amplicons of both the 16S rRNA and nifH genes and 15N2 stable isotope tracer analysis to assess diazotroph‐derived nitrogen (DDN) assimilation by the coral. Results show that the diazotrophic community of O. patagonica is dominated by autotrophic bacteria (i.e. Cyanobacteria and Chlorobia). The majority of DDN was assimilated into the tissue and skeletal matrix, and DDN assimilation significantly increased in bleached corals. Thus, diazotrophs may constitute an additional nitrogen source for the coral host, when nutrient exchange with Symbiodinium is disrupted (e.g. bleaching) and external food supply is limited (e.g. oligotrophic summer season). Furthermore, we hypothesize that DDN can facilitate the fast proliferation of endolithic algae, which provide an alternative carbon source for bleached O. patagonica. Overall, O. patagonica could serve as a good model for investigating the importance of diazotrophs in coral recovery from bleaching. [ABSTRACT FROM AUTHOR]

Abstract: Massive coral bleaching events result in extensive coral loss throughout the world. These events are mainly caused by seawater warming, but are exacerbated by the subsequent decrease in nutrient availability in surface waters. It has therefore been shown that nitrogen, phosphorus or iron limitation contribute to the underlying conditions by which thermal stress induces coral bleaching. Generally, information on the trophic ecology of trace elements (micronutrients) in corals, and on how they modulate the coral response to thermal stress is lacking. Here, we demonstrate for the first time that heterotrophic feeding (i.e. the capture of zooplankton prey by the coral host) and thermal stress induce significant changes in micro element concentrations and isotopic signatures of the scleractinian coral Stylophora pistillata. The results obtained first reveal that coral symbionts are the major sink for the heterotrophically acquired micronutrients and accumulate manganese, magnesium and iron from the food. These metals are involved in photosynthesis and antioxidant protection. In addition, we show that fed corals can maintain high micronutrient concentrations in the host tissue during thermal stress and do not bleach, whereas unfed corals experience a significant decrease in copper, zinc, boron, calcium and magnesium in the host tissue and bleach. In addition, the significant increase in δ65Cu and δ66Zn signature of symbionts and host tissue at high temperature suggests that these isotopic compositions are good proxy for stress in corals. Overall, present findings highlight a new way in which coral heterotrophy and micronutrient availability contribute to coral resistance to global warming and bleaching. [ABSTRACT FROM AUTHOR]

In shallow tropical waters, ultraviolet radiation (UVR) occurs at high intensity simultaneously with high water temperatures, and both stressors are predicted to increase in the future and to have a major impact on reef coral survival. The poor knowledge of the interactive effects of those two factors, however, prevents a good estimation of the coral resistance to climate change pressure. The results obtained in this study on two genetically distant scleractinian coral species, Pocillopora damicornis and Turbinaria reniformis, highlight an interaction of temperature and UVR on the corals' physiology as well as a species-specific response. Overall, increase in UVR level exacerbated the negative impact of thermal stress. Whereas stressors did not reduce the autotrophic capacity (symbiont density, net photosynthetic rates) of T. reniformis, they significantly lowered it in P. damicornis, which initially contained twice more symbionts in its tissue. Only grazing rates (heterotrophy) were significantly decreased in T. reniformis under thermal stress. For both corals, calcification was slowed by the combination of thermal and UVR stress. Calcification was impaired likely due to a decrease in autotrophic energy supply in P. damicornis, and in heterotrophic energy supply in T. reniformis. This study confirms that the response of corals to global change needs to be studied using multifactorial approaches and a combination of different environmental factors. [ABSTRACT FROM AUTHOR]

Phosphorus (P), an essential structural and functional component of all living organisms, is considered to be the ultimate limiting nutrient in marine ecosystems. To optimize its acquisition, marine species such as protozoa, sponges, foraminifera, clams, and reef corals, among others, have entered symbiotic relationships with algae, which recycle waste products of the animal host and transform dissolved inorganic nutrients into organic molecules, making them bioavailable to their host. Such associations provide a competitive edge in an environment where ambient nutrient availability is low. The aim of this review is to summarize the current knowledge on the P sources available to reef organisms with algal symbionts, to discuss the means by which P is taken up and stored within the symbiosis, and to assess the effects of eutrophication-induced phosphate enrichment on the algal and host physiology. Finally, we give an overview of knowledge gaps and open questions that should be addressed to better understand the role of phosphorus in reef symbioses functioning. [ABSTRACT FROM AUTHOR]

Coral aquaculture is an activity of growing interest due to the degradation of coral reefs worldwide and concomitant growing demand for corals by three industries: marine ornamental trade, pharmaceutical industry and reef restoration. Although captive breeding and propagation of corals is a well-known activity among aquarium hobbyists and public aquariums, the link between coral science and aquaculture is still poorly developed. Research on coral biology has increased in the past decades and resulted in abundant scientific information that is pivotal to further advance coral aquaculture. This review presents a holistic overview of coral aquaculture in relation to coral biology, with particular focus on ex situ aquaculture. Success factors for commercial coral aquaculture are outlined, which include qualitative aspects, such as shape, coloration and natural product content, and quantitative parameters such as growth and volumetric productivity. Manipulation of environmental factors to maximize coral quality and volumetric productivity is thoroughly discussed, and a comprehensive overview of current propagation techniques is provided. Knowledge gaps are pinpointed to indicate directions for future research. [ABSTRACT FROM AUTHOR]

Worldwide increase in seawater temperature represents one of the major threats affecting corals, which experience bleaching, and thereafter a significant decrease in photosynthesis and calcification. The impact of bleaching on coral physiology may be exacerbated when coupled with eutrophication, i.e., increasing plankton, inorganic nutrient concentrations, sedimentation and turbidity due to coastal urbanization. Whereas zooplankton provision (heterotrophy) may alleviate the negative consequences of thermal stress, inorganic nutrient supply may exacerbate them, which creates a paradox. Our experimental study aims to disentangle the effects of these two components of eutrophication on the physiological response of Turbinaria reniformis subject to normal and to a short-term temperature increase. Additionally, three different inorganic nutrient ratios were tested to assess the influence of nutrient stoichiometry on coral physiology: control (ambient SW 0.5 μM N and 0.1 μM P), N only (ambient + 2 μM N) and N + P (ambient + 2 μM N and + 0.5 μM P). Our results show a deleterious effect of a 2 μM nitrate enrichment alone (N) on coral photosynthetic processes under thermal stress as well as on calcification rates when associated with heterotrophy. On the contrary, a coupled nitrate and phosphorus enrichment (N + P) maintained coral metabolism and calcification during thermal stress and enhanced them when combined with heterotrophy. Broadly, our results shed light on the tight relationship existing between inorganic nutrient availability and heterotrophy. Moreover, it assesses the relevance of N: P stoichiometry as a determining factor for the health of the holobiont that may be adapted to specific nutrient ratios in its surrounding environment. [ABSTRACT FROM AUTHOR]

A general decline in coral calcification has been observed in response to ocean acidification. It has recently been shown that heterotrophic feeding (the acquisition of organic nutrients) enables one massive coral species to maintain its calcification rates while exposed to ocean acidification but the consequences of higher seawater partial pressure of CO2 ( [ABSTRACT FROM AUTHOR]

Thermal stress affects organism performance differently depending on the ambient temperature to which they are acclimatized, which varies along latitudinal gradients. This study investigated whether differences in physiological responses to temperature are consistent with regional differences in temperature regimes for the stony coral Oculina patagonica. To resolve this question, we experimentally assessed how colonies originating from four different locations characterized by >3 °C variation in mean maximum annual temperature responded to warming from 20 to 32 °C. We assessed plasticity in symbiont identity, density, and photosynthetic properties, together with changes in host tissue biomass. Results show that, without changes in the type of symbiont hosted by coral colonies, O. patagonica has limited capacity to acclimatize to future warming. We found little evidence of variation in overall thermal tolerance, or in thermal optima, in response to spatial variation in ambient temperature. Given that the invader O. patagonica is a relatively new member of the Mediterranean coral fauna, our results also suggest that coral populations may need to remain isolated for a long period of time for thermal adaptation to potentially take place. Our study indicates that for O. patagonica, mortality associated with thermal stress manifests primarily through tissue breakdown under moderate but prolonged warming (which does not impair symbiont photosynthesis and, therefore, does not lead to bleaching). Consequently, projected global warming is likely to cause repeat incidents of partial and whole colony mortality and might drive a gradual range contraction of Mediterranean corals. [ABSTRACT FROM AUTHOR]

Herbivory in corals, especially for symbiotic species, remains controversial. To investigate the capacity of scleractinian and soft corals to capture microalgae, we conducted controlled laboratory experiments offering five algal species: the cryptophyte Rhodomonas marina, the haptophytes Isochrysis galbana and Phaeocystis globosa, and the diatoms Conticribra weissflogii and Thalassiosira pseudonana. Coral species included the symbiotic soft corals Heteroxenia fuscescens and Sinularia flexibilis, the asymbiotic scleractinian coral Tubastrea coccinea, and the symbiotic scleractinian corals Stylophora pistillata, Pavona cactus and Oculina arbuscula. Herbivory was assessed by end-point PCR amplification of algae-specific 18 S r RNA gene fragments purified from coral tissue genomic DNA extracts. The ability to capture microalgae varied with coral and algal species and could not be explained by prey size or taxonomy. Herbivory was not detected in S. flexibilis and S. pistillata. P. globosa was the only algal prey that was never captured by any coral. Although predation defence mechanisms have been shown for Phaeocystis spp. against many potential predators, this study is the first to suggest this for corals. This study provides new insights into herbivory in symbiotic corals and suggests that corals may be selective herbivorous feeders. [ABSTRACT FROM AUTHOR]

The dual character of corals, that they are both auto- and heterotrophs, was recognized early in the twentieth Century. It is generally accepted that the symbiotic association between corals and their endosymbiotic algae (called zooxanthellae) is fundamental to the development of coral reefs in oligotrophic tropical oceans because zooxanthellae transfer the major part of their photosynthates to the coral host (autotrophic nutrition). However, numerous studies have confirmed that many species of corals are also active heterotrophs, ingesting organisms ranging from bacteria to mesozooplankton. Heterotrophy accounts for between 0 and 66% of the fixed carbon incorporated into coral skeletons and can meet from 15 to 35% of daily metabolic requirements in healthy corals and up to 100% in bleached corals. Apart from this carbon input, feeding is likely to be important to most scleractinian corals, since nitrogen, phosphorus, and other nutrients that cannot be supplied from photosynthesis by the coral’s symbiotic algae must come from zooplankton capture, particulate matter or dissolved compounds. A recent study showed that during bleaching events some coral species, by increasing their feeding rates, are able to maintain and restore energy reserves. This review assesses the importance and effects of heterotrophy in tropical scleractinian corals. We first provide background information on the different food sources (from dissolved organic matter to meso- and macrozooplankton). We then consider the nutritional inputs of feeding. Finally, we review feeding effects on the different physiological parameters of corals (tissue composition, photosynthesis and skeletal growth). [ABSTRACT FROM AUTHOR]

We assessed the uptake rates of nitrate by the scleractinian coral Stylophora pistillata by following [sup15]N from seawater into the coral tissue. Two sets of corals were first prepared, with "nitrate-enriched" corals grown in 5 µmol L[sup-1] NO[sup-, sub3] and control corals grown in ≤ 1 µmol L[sup-1] NO[sup-, sub3]. Uptake rates at 0.3 and 3 µmol L[sup-1][[sup15]N]NO[sup-, sub3] were then measured. Most of the %[sup15]N enrichment occurred in the zooxanthellae fraction. Uptake rates were not significantly different between nitrate-enriched and control corals, suggesting that they were not dependent on a nitrate acclimation. These rates increased with the in situ nitrate concentration and varied from 1.2 ± 0.2 ng h[sup-1] cm[sup-2] N to 6.1 ± 1.1 ng h[sup-1] cm[sup-2] N in the algal fraction at 0.3 and 3 µmol L[sup-1] [[sup15]N]NO[sup-, sub3], respectively. In a second experiment, two sets of corals were prepared, with "ammonium-enriched" corals grown in 5 µmol L[sup-1] NH[sup+, sub4] and control corals grown in < 1 µmol L[sup-1] NH[sup+, sub4]. Uptake rates at 3 µmol L[sup-1] [[sup15]N]NO[sup-, sub3] were measured. These rates were significantly lower with high NH[sup+, sub4] concentrations in seawater. In the algal fraction, they ranged from 0.1 to 0.6 ng h[sup-1] cm[sup-2] N in NH[sup+, sub4]-enriched corals and from 2.2 to 4.5 ng h[sup-1] cm[sup-2] N in control corals. Nitrate can therefore be considered as an important source of nitrogen for corals, at least when ammonium concentrations are low in seawater. [ABSTRACT FROM AUTHOR]