Tropical Reef-building Corals Research Articles

Drivers of adaptive capacity in wild populations: Implications for genetic interventions

The unprecedented rate of environmental change in the Anthropocene poses evolutionary challenges for wild populations globally. Active human-mediated interventions are being increasingly considered to accelerate natural adaptive processes. Where experimentation is challenging, evolutionary models can evaluate how species may fare under future climate, elucidate which evolutionary processes are critical to rapid adaptation, and how active interventions may influence fitness trajectories of organisms. Here we use polygenic metapopulation adaptation models to quantify the relative importance (effect sizes) of different eco-evolutionary parameters on the rates of adaptation in wild populations i) without active interventions, and ii) under a subset of active interventions. We demonstrate that genetic diversity (heterozygosity, He), population connectivity and the effect size of additive genetic variance are the primary drivers of natural adaptation rates. We quantify the effect sizes of these parameters on population fitness across three proposed assisted evolution scenarios and identify critical thresholds for intervention effectiveness and implementation. Specifically, the interventions tested here were most effective at low levels of genetic diversity in target populations (He < 0.2) and when timed during a cold-to-warm phase of an ENSO-like oscillation. Beneficial levels of connectivity were highly dependent on desired outcomes for the meta-population. We also present a global meta-analysis of genetic diversity in tropical reef-building corals as a case study of how thresholds derived from evolutionary models can be used to guide decision making by managers. We find genetic diversity to be highly variable by coral taxon and region, highlighting how thresholds from evolutionary models can be used in conjunction with empirical data to assess intervention needs and priorities. Quantitatively characterizing these key thresholds should provide managers, conservationists, and practitioners with a starting point for evaluating the necessity, risks and benefits of genetic interventions of wild species with large populations sizes. Finally, we highlight the critical knowledge and data gaps to produce the next suite of applied models for conservation management decision-support.

The reef-building coral Galaxea fascicularis: a new model system for coral symbiosis research

Reef-building corals owe their evolutionary success to their symbiosis with unicellular algae (Symbiodiniaceae). However, increasingly frequent heat waves lead to coral mass-bleaching events and pose a serious threat to the survival of reef ecosystems. Despite significant efforts, a mechanistic understanding of coral–algal symbiosis functioning, what leads to its breakdown and what can prevent it, remains incomplete. The main obstacles are low amenability of corals to experimental handling and, owing to its obligatory nature, the difficulties of manipulating the coral–algal association. Indeed, many studies on the symbiotic partnership are conducted on other cnidarian model organisms and their results may therefore not be fully transferable to tropical reef-building corals. Here, we identify the tropical stony coral species Galaxea fascicularis as a novel candidate coral model system. Individual polyps of this species can be separated, enabling highly replicated genotype studies, and are well suited to experimental investigation of the symbiosis as they can be easily and effectively rid of their algal symbionts (bleached). We show that bleached adult individuals can reestablish symbiosis with non-native symbionts, and we report the completion of the gametogenic cycle ex situ, with the successful spawning in aquaria over multiple years. These achievements help overcome several of the major limitations to direct research on corals and highlight the potential of G. fascicularis as an important new model system for investigations of symbiosis functioning and manipulation.

Polyp loss and mass occurrence of sea urchins on bamboo corals in the deep sea: an indirect effect of fishing impact?

The ability of corals to detach their polyps (leaving behind a naked skeleton) in times of stress, allows polyps to escape from a local source of hostility (Sammarco 1982). In situ observations of this behavior (referred to as 'polyp bailout') were first documented in tropical reef-building corals. Later it was observed in some cold-water corals of Acanthogorgia (Braga-Henriques 2014), and Acanella arbuscula (Rakka et al. 2019) in aquaria.

Symbiosis maintenance in the facultative coral, Oculina arbuscula, relies on nitrogen cycling, cell cycle modulation, and immunity

Symbiosis with unicellular algae in the family Symbiodiniaceae is common across tropical marine invertebrates. Reef-building corals offer a clear example of cellular dysfunction leading to a dysbiosis that disrupts entire ecosystems in a process termed coral bleaching. Due to their obligate symbiotic relationship, understanding the molecular underpinnings that sustain this symbiosis in tropical reef-building corals is challenging, as any aposymbiotic state is inherently coupled with severe physiological stress. Here, we leverage the subtropical, facultatively symbiotic and calcifying coral Oculina arbuscula to investigate gene expression differences between aposymbiotic and symbiotic branches within the same colonies under baseline conditions. We further compare gene ontology (GO) and KOG enrichment in gene expression patterns from O. arbuscula with prior work in the sea anemone Exaiptasia pallida (Aiptasia) and the salamander Ambystoma maculatum—both of which exhibit endophotosymbiosis with unicellular algae. We identify nitrogen cycling, cell cycle control, and immune responses as key pathways involved in the maintenance of symbiosis under baseline conditions. Understanding the mechanisms that sustain a healthy symbiosis between corals and Symbiodiniaceae algae is of urgent importance given the vulnerability of these partnerships to changing environmental conditions and their role in the continued functioning of critical and highly diverse marine ecosystems.

Evidence on the impacts of chemicals arising from human activity on tropical reef-building corals; a systematic map

BackgroundTropical coral reefs cover ca. 0.1% of the Earth’s surface but host an outstanding biodiversity and provide important ecosystem services to millions of people living nearby. They are currently threatened by local stressors (e.g. nutrient enrichment and chemical pollution arising from poor land management, sewage effluents, agriculture, industry) and global stressors (mainly seawater warming and acidification, i.e. climate change). Global and local stressors interact in different ways, but the presence of one stressor often reduces the tolerance to additional stress. While global stressors cannot be mitigated solely by local actions, local stressors can be reduced through ecosystem management, therefore minimizing the impact of climate change on coral reefs. We systematically mapped the evidence of impacts of chemicals arising from anthropogenic activities on tropical reef-building corals, which are the main engineer species of reef ecosystems, to inform decision-makers on the available evidence on this topic.MethodsWe searched the relevant literature using English terms combined in a tested search string in two publication databases (Scopus and Web Of Science Core Collection). The search string combined terms describing the population (tropical reef-building corals) and the exposure (chemicals). We searched for additional literature through three search engines, three dissertations repositories, 11 specialist websites, and through a call to local stakeholders. Titles, abstracts, and full-texts were successively screened using pre-defined eligibility criteria. A database of all studies included in the map with coded metadata was produced. The evidence was described and knowledge clusters and gaps were identified through the distribution and frequency of studies into types of exposure and/or types of outcomes and/or types of study.Review findingsThe initial searches identified 23,403 articles which resulted in 15,177 articles after duplicate removal. Among them, 908 articles were retained after screening process, corresponding to 7937 studies (a study being the combination of a taxon, an exposure, and an outcome). Among these studies, 30.5% dealt with the impact of nutrient enrichment on corals while 25% concerned the impact of human activities without reference to a chemical. The most measured outcomes were those related to the chemical concentration in corals (bioaccumulation, 25.8%), to coral physiology (16.9%), cover (14%), and mortality (9%). Half of the studies (48.4%) were experimental—the exposure was controlled by the researchers—and were conducted in laboratory conditions (39.4%) and in situ (9%). The most studied taxa, exposure, and outcomes were different between experimental and observational studies.ConclusionsWe identified four well-represented subtopics that may be amenable to relevant full syntheses via systematic reviews: (1) evidence on bioaccumulation of chemicals by corals; (2) evidence on the effects of nutrient enrichment on corals; (3) evidence on the effects of human activities on corals; and (4) evidence on the ecotoxicological effects of chemicals on corals (except nutrient enrichment). The systematic map shows that corals in their natural environment can be exposed to many categories of chemicals, and that there is a complete gap in experimental research on the combined effects of more than two categories of chemicals. We therefore encourage research on this topic.

Characterizing environmental stress responses of aposymbiotic Astrangia poculata to divergent thermal challenges

Anthropogenic climate change threatens corals globally and both high and low temperatures are known to induce coral bleaching. However, coral stress responses across wide thermal breadths remain understudied. Disentangling the role of symbiosis on the stress response in obligately symbiotic corals is challenging because this response is inherently coupled with nutritional stress. Here, we leverage aposymbiotic colonies of the facultatively symbiotic coral, Astrangia poculata, which lives naturally with and without its algal symbionts, to examine how broad thermal challenges influence coral hosts in the absence of symbiosis. A. poculata were collected from their northern range limit and thermally challenged in two independent 16-day common garden experiments (heat and cold challenge) and behavioural responses to food stimuli and genome-wide gene expression profiling (TagSeq) were performed. Both thermal challenges elicited significant reductions in polyp extension. However, there were five times as many differentially expressed genes (DEGs) under cold challenge compared to heat challenge. Despite an overall stronger response to cold challenge, there was significant overlap in DEGs between thermal challenges. We contrasted these responses to a previously identified module of genes associated with the environmental stress response (ESR) in tropical reef-building corals. Cold challenged corals exhibited a pattern consistent with more severe stressors while the heat challenge response was consistent with lower intensity stressors. Given that these responses were observed in aposymbiotic colonies, many genes previously implicated in ESRs in tropical symbiotic species may represent the coral host's stress response in or out of symbiosis.

What evidence exists on the impacts of chemicals arising from human activity on tropical reef-building corals? A systematic map protocol

BackgroundTropical coral reefs cover ca. 0.1% of the Earth’s surface but host an outstanding biodiversity and provide important ecosystem services to millions of people living nearby. However, they are currently threatened by both local (e.g. nutrient enrichment and chemical pollution of coastal reefs, arising from poor land management, agriculture and industry) and global stressors (mainly seawater warming and acidification, i.e. climate change). Global and local stressors interact together in different ways, but the presence of one stressor often reduces the tolerance to additional stress. While global stressors cannot be halted by local actions, local stressors can be reduced through ecosystem management, therefore minimizing the impact of climate change on reefs. To inform decision-makers, we propose here to systematically map the evidence of impacts of chemicals arising from anthropogenic activities on tropical reef-building corals, which are the main engineer species of reef ecosystems. We aim to identify the combinations of chemical and coral responses that have attracted the most attention and for which evidence can be further summarized in a systematic review that will give practical information to decision-makers.MethodsThe systematic map will follow the Collaboration for Environmental Evidence Guidelines and Standards for Evidence Synthesis in Environmental Management. We will search the relevant literature using English terms combined in a tested search string in two publication databases (Web Of Science Core Collection and Scopus). The search string will combine terms describing the population (tropical reef-building corals) and the exposure (chemicals). We will supplement this literature with some more obtained through search engines, specialist websites, and through a call to local stakeholders. Titles, abstracts, and full-texts will then be successively screened using pre-defined eligibility criteria. A list of pre-defined variables will then be extracted from full-texts. Finally, a database of all studies included in the map with coded metadata will be produced. The evidence will be described in a map report with text, figures and tables, and a matrix showing the distribution and frequency of included study into types of exposure and types of outcomes will be computed to identify potential knowledge gaps and knowledge clusters.

Environmental drivers of distribution and reef development of the Mediterranean coral Cladocora caespitosa

Cladocora caespitosa is the only Mediterranean scleractinian similar to tropical reef-building corals. While this species is part of the recent fossil history of the Mediterranean Sea, it is currently considered endangered due to its decline during the last decades. Environmental factors affecting the distribution and persistence of extensive bank reefs of this endemic species across its whole geographic range are poorly understood. In this study, we examined the environmental response of C. caespitosa and its main types of assemblages using ecological niche modeling and ordination analysis. We also predicted other suitable areas for the occurrence of the species and assessed the conservation effectiveness of Mediterranean marine protected areas (MPAs) for this coral. We found that phosphate concentration and wave height were factors affecting both the occurrence of this versatile species and the distribution of its extensive bioconstructions in the Mediterranean Sea. A set of factors (diffuse attenuation coefficient, calcite and nitrate concentrations, mean wave height, sea surface temperature, and shape of the coast) likely act as environmental barriers preventing the species from expansion to the Atlantic Ocean and the Black Sea. Uncertainties in our large-scale statistical results and departures from previous physiological and ecological studies are also discussed under an integrative perspective. This study reveals that Mediterranean MPAs encompass eight of the ten banks and 16 of the 21 beds of C. caespitosa. Preservation of water clarity by avoiding phosphate discharges may improve the protection of this emblematic species.

Impacts of food availability and <i>p</i>CO<sub>2</sub> on planulation, juvenile survival, and calcification of the azooxanthellate scleractinian coral <i>Balanophyllia elegans</i>

Abstract. Ocean acidification, the assimilation of atmospheric CO2 by the oceans that decreases the pH and CaCO3 saturation state (Ω) of seawater, is projected to have severe adverse consequences for calcifying organisms. While strong evidence suggests calcification by tropical reef-building corals containing algal symbionts (zooxanthellae) will decline over the next century, likely responses of azooxanthellate corals to ocean acidification are less well understood. Because azooxanthellate corals do not obtain photosynthetic energy from symbionts, they provide a system for studying the direct effects of acidification on energy available for calcification. The solitary azooxanthellate orange cup coral Balanophyllia elegans often lives in low-pH, upwelled waters along the California coast. In an 8-month factorial experiment, we measured the effects of three pCO2 treatments (410, 770, and 1220 μatm) and two feeding frequencies (3-day and 21-day intervals) on "planulation" (larval release) by adult B. elegans, and on the survival, skeletal growth, and calcification of newly settled juveniles. Planulation rates were affected by food level but not pCO2. Juvenile mortality was highest under high pCO2 (1220 μatm) and low food (21-day intervals). Feeding rate had a greater impact on calcification of B. elegans than pCO2. While net calcification was positive even at 1220 μatm (~3 times current atmospheric pCO2), overall calcification declined by ~25–45%, and skeletal density declined by ~35–45% as pCO2 increased from 410 to 1220 μatm. Aragonite crystal morphology changed at high pCO2, becoming significantly shorter but not wider at 1220 μatm. We conclude that food abundance is critical for azooxanthellate coral calcification, and that B. elegans may be partially protected from adverse consequences of ocean acidification in habitats with abundant heterotrophic food.

Gene expression in the scleractinian Acropora microphthalma exposed to high solar irradiance reveals elements of photoprotection and coral bleaching.

BackgroundThe success of tropical reef-building corals depends on the metabolic co-operation between the animal host and the photosynthetic performance of endosymbiotic algae residing within its cells. To examine the molecular response of the coral Acropora microphthalma to high levels of solar irradiance, a cDNA library was constructed by PCR-based suppression subtractive hybridisation (PCR-SSH) from mRNA obtained by transplantation of a colony from a depth of 12.7 m to near-surface solar irradiance, during which the coral became noticeably paler from loss of endosymbionts in sun-exposed tissues.Methodology/Principal FindingsA novel approach to sequence annotation of the cDNA library gave genetic evidence for a hypothetical biosynthetic pathway branching from the shikimic acid pathway that leads to the formation of 4-deoxygadusol. This metabolite is a potent antioxidant and expected precursor of the UV-protective mycosporine-like amino acids (MAAs), which serve as sunscreens in coral phototrophic symbiosis. Empirical PCR based evidence further upholds the contention that the biosynthesis of these MAA sunscreens is a ‘shared metabolic adaptation’ between the symbiotic partners. Additionally, gene expression induced by enhanced solar irradiance reveals a cellular mechanism of light-induced coral bleaching that invokes a Ca2+-binding synaptotagmin-like regulator of SNARE protein assembly of phagosomal exocytosis, whereby algal partners are lost from the symbiosis.Conclusions/SignificanceBioinformatics analyses of DNA sequences obtained by differential gene expression of a coral exposed to high solar irradiance has revealed the identification of putative genes encoding key steps of the MAA biosynthetic pathway. Revealed also by this treatment are genes that implicate exocytosis as a cellular process contributing to a breakdown in the metabolically essential partnership between the coral host and endosymbiotic algae, which manifests as coral bleaching.

Ocean Acidification and Its Potential Effects on Marine Ecosystems

Ocean acidification is rapidly changing the carbonate system of the world oceans. Past mass extinction events have been linked to ocean acidification, and the current rate of change in seawater chemistry is unprecedented. Evidence suggests that these changes will have significant consequences for marine taxa, particularly those that build skeletons, shells, and tests of biogenic calcium carbonate. Potential changes in species distributions and abundances could propagate through multiple trophic levels of marine food webs, though research into the long-term ecosystem impacts of ocean acidification is in its infancy. This review attempts to provide a general synthesis of known and/or hypothesized biological and ecosystem responses to increasing ocean acidification. Marine taxa covered in this review include tropical reef-building corals, cold-water corals, crustose coralline algae, Halimeda, benthic mollusks, echinoderms, coccolithophores, foraminifera, pteropods, seagrasses, jellyfishes, and fishes. The risk of irreversible ecosystem changes due to ocean acidification should enlighten the ongoing CO(2) emissions debate and make it clear that the human dependence on fossil fuels must end quickly. Political will and significant large-scale investment in clean-energy technologies are essential if we are to avoid the most damaging effects of human-induced climate change, including ocean acidification.

Nature's sunscreen from the Great Barrier Reef, Australia

All shallow-water marine organisms have natural features and behaviours that can reduce exposure to UV radiation and limit the amount of photodamage to functional biomolecules and organelles. Protection from UV radiation often includes the production of UV-absorbing compounds which, in marine organisms, are usually mycosporine-like amino acids (MAAs) having a range of absorption maxima between 310 and 360 nm. Shallow-water corals produce large quantities of MAAs and their apparent ability to withstand long-term environmental UV exposure suggests utilization of their UV-absorbing chromophore for sunscreen applications. We present an overview of the biochemical photophysiology of tropical reef-building corals, and of the development of a new class of synthetic sunscreen, 1-alkyl-3-alkanoyl-1,4,5,6-tetrahydropyridines, based on their natural UV protection. Several sunscreen candidates have been examined for sunscreen suitability and a limited selection is being tested for potential use in human skin-care and cosmetic products.