Reconciling the variability in the biological response of marine invertebrates to climate change.

As climate change increases the rate of environmental change and the frequency and intensity of disturbance events, selective forces intensify. However, given the complicated interplay between plasticity and selection for ecological - and thus evolutionary - outcomes, understanding the proximate signals, molecular mechanisms and the role of environmental history becomes increasingly critical for eco-evolutionary forecasting. To enhance the accuracy of our forecasting, we must characterize environmental signals at a level of resolution that is relevant to the organism, such as the microhabitat it inhabits and its intracellular conditions, while also quantifying the biological responses to these signals in the appropriate cells and tissues. In this Commentary, we provide historical context to some of the long-standing challenges in global change biology that constrain our capacity for eco-evolutionary forecasting using reef-building corals as a focal model. We then describe examples of mismatches between the scales of external signals relative to the sensors and signal transduction cascades that initiate and maintain cellular responses. Studying cellular responses at this scale is crucial because these responses are the basis of acclimation to changing environmental conditions and the potential for environmental 'memory' of prior or historical conditions through molecular mechanisms. To challenge the field, we outline some unresolved questions and suggest approaches to align experimental work with an organism's perception of the environment; these aspects are discussed with respect to human interventions.

Rapid, high-throughput phenotypic profiling of endosymbiotic dinoflagellates (Symbiodiniaceae) using benchtop flow cytometry.

Endosymbiotic dinoflagellates (Family Symbiodiniaceae) are the primary producer of energy for many cnidarians, including corals. The intricate coral-dinoflagellate symbiotic relationship is becoming increasingly important under climate change, as its breakdown leads to mass coral bleaching and often mortality. Despite methodological progress, assessing the phenotypic traits of Symbiodiniaceae in-hospite remains a complex task. Bio-optics, biochemistry, or "-omics" techniques are expensive, often inaccessible to investigators, or lack the resolution required to understand single-cell phenotypic states within endosymbiotic dinoflagellate assemblages. To help address this issue, we developed a protocol that collects information on cell autofluorescence, shape, and size to simultaneously generate phenotypic profiles for thousands of Symbiodiniaceae cells, thus revealing phenotypic variance of the Symbiodiniaceae assemblage to the resolution of single cells. As flow cytometry is adopted as a robust and efficient method for cell counting, integration of our protocol into existing workflows allows researchers to acquire a new level of resolution for studies examining the acclimation and adaptation strategies of Symbiodiniaceae assemblages.

Thermotolerant coral-algal mutualisms maintain high rates of nutrient transfer while exposed to heat stress.

Symbiotic mutualisms are essential to ecosystems and numerous species across the tree of life. For reef-building corals, the benefits of their association with endosymbiotic dinoflagellates differ within and across taxa, and nutrient exchange between these partners is influenced by environmental conditions. Furthermore, it is widely assumed that corals associated with symbionts in the genus Durusdinium tolerate high thermal stress at the expense of lower nutrient exchange to support coral growth. We traced both inorganic carbon (H13CO3-) and nitrate (15NO3-) uptake by divergent symbiont species and quantified nutrient transfer to the host coral under normal temperatures as well as in colonies exposed to high thermal stress. Colonies representative of diverse coral taxa associated with Durusdinium trenchii or Cladocopium spp. exhibited similar nutrient exchange under ambient conditions. By contrast, heat-exposed colonies with D. trenchii experienced less physiological stress than conspecifics with Cladocopium spp. while high carbon assimilation and nutrient transfer to the host was maintained. This discovery differs from the prevailing notion that these mutualisms inevitably suffer trade-offs in physiological performance. These findings emphasize that many host-symbiont combinations adapted to high-temperature equatorial environments are high-functioning mutualisms; and why their increased prevalence is likely to be important to the future productivity and stability of coral reef ecosystems.

Newly deceased Caribbean reef-building corals experience rapid carbonate loss and colonization by endolithic organisms.

Coral mortality triggers the loss of carbonates fixed within coral skeletons, compromising the reef matrix. Here, we estimate rates of carbonate loss in newly deceased colonies of four Caribbean reef-building corals. We use samples from living and recently deceased colonies following a stony coral tissue loss disease (SCTLD) outbreak. Optical densitometry and porosity analyses reveal a loss of up to 40% of the calcium carbonate (CaCO3) content in dead colonies. The metabolic activity of the endolithic organisms colonizing the dead skeletons is likely partially responsible for the observed dissolution. To test for the consequences of mass mortality events over larger spatial scales, we integrate our estimates of carbonate loss with field data of the composition and size structure of coral communities. The dissolution rate depends on the relative abundance of coral species and the structural properties of their skeletons, yet we estimate an average reduction of 1.33 kg CaCO3 m-2, nearly 7% of the total amount of CaCO3 sequestered in the entire system. Our findings highlight the importance of including biological and chemical processes of CaCO3 dissolution in reef carbonate budgets, particularly as the impacts of global warming, ocean acidification, and disease likely enhance dissolution processes.

A decade of study on the condition of western Cuban coral reefs, with low human impact.

Background: The long-time study of coral reefs with low human impacts can provide information on the effects of regional pressures like climate change, and is an opportunity to document how these pressures are reflected in coral communities. An example of minimal local anthropogenic impacts are the Guanahacabibes coral reefs, located in the westernmost region of Cuba. The objectives of this study were: to evaluate the temporal variability of six benthic biological indicators of coral reefs, and to explore the possible relationship between predictive abiotic variables and biological response variables. Methods: Four coral reef sites were sampled between 2008 and 2017, to analyze biological indicators (living coral cover, fleshy algae index, coral species richness, coral species abundance, coral trait groups species abundance, Functional Reef Index). Seven abiotic variables (wave exposure, sea surface temperature, degree heating week, chlorophyll-a concentration, particulate organic carbon, photosynthetically available radiation, and the diffuse attenuation coefficient) were compiled between 2007 and 2016, from remote sensing datasets, to analyze their relationship with the biological indicators. Permanova statistical analysis was used to evaluate trends in biological variables between sites and years, and Routine Analysis Based on Linear Distances (DISTLM) was used to explore some dependencies between biotic and abiotic variables. Results: We found significant variability in the temporal analysis, with a decrease in living coral cover, a decline in the predominance of the branching and massive framework reef-building species, a decline in Orbicella species abundance, and an increase in the fleshy algae index. Some abiotic variables (average of degree heating weeks, standard deviation of the diffuse attenuation coefficient, average of the sea surface temperature, among others) significantly explained the variability of biological indicators; however, determination coefficients were low. Conclusions: Certain decrease in the functionality of the coral reef was appreciated, taking into account the predominance of secondary and nom-massive framework reef-building species in the last years. A weak association between abiotic and biological variables was found in the temporal analysis. The current scenario of the condition of the coral reefs seems to be regulated by the global effects of climate change, weakly associated effects, and in longer terms.

DNA barcoding of marine teleost fishes (Teleostei) in Cebu, the Philippines, a biodiversity hotspot of the coral triangle.

A morphology-based barcoding library of market teleost fishes (Teleostei) in Cebu is built based on cytochrome c oxidase subunit I (COI) sequences and voucher specimens which aimed to establish a reliable reference of frequently traded fishes in the province, a biodiversity hotspot at the center of the Philippine archipelago. A total of 1721 specimens were collected from 18 fish markets and landing sites around the province, in which 538 specimens were sequenced belonging to 393 species from 229 genera, 86 families, and 37 orders. Most speciose families are coral reef or reef-related shallow-water species. Twelve species from 11 families are newly recorded in the Philippine waters, among which 7 species are deep-sea inhabitants, while 3 species have expanded their distribution range. Only 20 taxa could not be identified to the species level due to the difficulty in morphological examinations, absence of matched reference sequences in online databases, and/or problematic species awaiting further studies. This first comprehensive DNA barcoding survey of Cebu fishes can facilitate further taxonomic research as well as the conservation and management of fisheries in the Philippines.

Molecular docking and antibacterial activity of Sargassum fusiforme extracts against major coral pathogens.

The present study evaluates the antibacterial properties of alkaloids and the crude extracts (ethanol, n-hexane and ethyl acetate) from seaweed Sargassum fusiforme against coral pathogens (Photobacterium galatheae, Vibrio harveyi, Bordetella trematum, and Ochrobactrum pseudogrignonese) isolated from coral Porites lutea. To our knowledge, this is the first in vitro assay for such extracts on Porites lutea coral pathogens. Bacterial pathogens have been identified using 16S RNA and BankIt into gene bank and given the accession numbers (OR401000; OR401001; OR401336, and OR400998 respectively). GC-Mass profiling conducted for n-hexane compounds confirmed the presence of thirty-eight molecules, twelve of which have been previously reported for their bioactivity. The results revealed that alkaloids and n-hexane extract demonstrated eminent antibacterial activity compared to the other extracts against the tested coral pathogenic bacteria. Molecular docking was conducted to evaluate the twelve previously mentioned bioactive molecules to get a full understanding of the interaction of those bioactive molecules on vital bacterial proteins (Hemolysin protein (PDB ID: 1XEZ) and Cytoplasmic proteins (PDB ID: 3TZC)). Docked twelve molecules against hemolysin protein (PDB ID: 1XEZ) came exactly in line with the docked result of the same molecules with cytoplasmic proteins (PDB ID: 3TZC), proving the bioactivity of 6-O-Palmitoyl-L-ascorbic acid, 3TMS derivative; Glycerol monostearate, 2TMS derivative and Eicosanoic acid complexes in antibacterial activity action and score higher than reference ligand. Those three compounds will be investigated separately in future in vitro assay soon. Our conclusions align with the study's antibacterial in vitro assay results. The present study reports the novelty of different extracts of S. fusiforme as an antibacterial agent against coral pathogenic bacteria that trigger diseases in Porites lutea.

Atopic outcomes at 2 years in the CORAL cohort, born in COVID-19 lockdown.

INTRODUCTION: The CORAL study is a cohort of infants born during the first weeks of the first SARS-CoV-2 (COVID-19) lockdown. This cohort has had lower antibiotic exposure, higher breastfeeding rates and lower infection rates, especially in the first year of life. We hypothesized that the altered early-life environment of infants born during lockdown would change the incidence of allergic conditions. METHODS: This longitudinal, observational study followed 365 infants born between March and May 2020 from enrolment to the age of 2 years. Infants attended three research appointments at 6-, 12-, and 24-months and completed detailed questionnaires. At research appointments, children had skin prick testing, and atopic dermatitis (AD) assessment. Statistical analysis focused on changes within the group at different time points, the influence of specific environmental factors on allergic risk and compared the incidence of atopic conditions with a pre-pandemic Irish infant cohort, BASELINE. RESULTS: AD was more common in CORAL group at both 12 (26.5% vs. 15.5%; p < .001) and 24 months (21.3% vs. 15.9%; p = .02) compared with pre-pandemic BASELINE cohort. Within the CORAL group, those with AD at both 12- and 24-month appointments had a more severe AD phenotype associated with a higher risk of allergic sensitization. There was less milk (0% vs. 1%; p = .09), peanut (0.6% vs. 1.8%; p = .3), and egg allergy (0% vs. 2.9%; p < .001) in the CORAL group at 24 months compared with the BASELINE cohort. Aeroallergen sensitization increased between 12 and 24 months in the CORAL cohort (1.5% vs. 8.9%; p < .001), as did parent-reported wheezing episodes (9% vs. 24%; p < .001). CONCLUSIONS: Despite higher AD incidence in the CORAL cohort, the incidence of food sensitization and allergy are lower than expected pre-pandemic rates possibly reflecting the early introduction and maintenance of dietary allergens enhanced by changes in infant infections, antibiotic use, and breastfeeding in the first 2 years of life in the group. These beneficial effects of the lockdown could be outweighing the expected risk of less early-life microbial encounters outlined by the hygiene hypothesis.

Heterolactone and Heterolactams A-M, Verticillane Diterpenoids with Anti-Inflammatory and Hepatoprotective Activities from the Soft Coral Heteroxenia ghardaqensis.

Fourteen new verticillane diterpenoids, heterolactone (1) and heterolactams A-M (2-14), were isolated from the soft coral Heteroxenia ghardaqensis. They structurally share the same 6/12 bicyclic carbon skeleton that is not commonly encountered in marine organisms. The structures, including the absolute configurations, were determined by extensive spectroscopic analysis, single-crystal X-ray diffraction analysis, calculated ECD spectra, and DP4+ probability analyses. Compounds 5, 8, and 9 showed anti-inflammatory activities, and 2, 8, and 12 displayed hepatoprotective activities in zebrafish assays.

The Coral Triangle and Strait of Malacca are two distinct hotspots of mangrove biodiversity.

Knowledge of the biogeography of marine taxa has lagged significantly behind terrestrial ecosystems. A hotspot of marine biodiversity associated with coral reefs is known in the Coral Triangle of the Indo-West Pacific, but until now there was little data with which to evaluate broad patterns of species richness in the coastal fauna of ecosystems other than coral reefs. This data is critically needed for fauna with low functional redundancy like that of mangroves, that are vulnerable to habitat loss and rising sea levels. Here we show that the diversity of mangrove fauna is characterized by two distinct hotspots in the Indo-West Pacific, associated with two habitat types: fringe mangroves in the Coral Triangle, and riverine mangroves in the Strait of Malacca, between the west coast of Peninsular Malaysia and Sumatra. This finding, based on a family of slugs of which the systematics has been completely revised, illustrates an unexpected biogeographic pattern that emerged only after this taxon was studied intensively. Most organisms that live in the mangrove forests of Southeast Asia remain poorly known both taxonomically and ecologically, and the hotspot of diversity of onchidiid slugs in the riverine mangroves of the Strait of Malacca indicates that further biodiversity studies are needed to support effective conservation of mangrove biodiversity.

Long-term coral microbial community acclimatization is associated with coral survival in a changing climate.

The plasticity of some coral-associated microbial communities under stressors like warming and ocean acidification suggests the microbiome has a role in the acclimatization of corals to future ocean conditions. Here, we evaluated the acclimatization potential of coral-associated microbial communities of four Hawaiian coral species (Porites compressa, Porites lobata, Montipora capitata, and Pocillopora acuta) over 22-month mesocosm experiment. The corals were exposed to one of four treatments: control, ocean acidification, ocean warming, or combined future ocean conditions. Over the 22-month study, 33-67% of corals died or experienced a loss of most live tissue coverage in the ocean warming and future ocean treatments while only 0-10% died in the ocean acidification and control. Among the survivors, coral-associated microbial communities responded to the chronic future ocean treatment in one of two ways: (1) microbial communities differed between the control and future ocean treatment, suggesting the potential capacity for acclimatization, or (2) microbial communities did not significantly differ between the control and future ocean treatment. The first strategy was observed in both Porites species and was associated with higher survivorship compared to M. capitata and P. acuta which exhibited the second strategy. Interestingly, the microbial community responses to chronic stressors were independent of coral physiology. These findings indicate acclimatization of microbial communities may confer resilience in some species of corals to chronic warming associated with climate change. However, M. capitata genets that survived the future ocean treatment hosted significantly different microbial communities from those that died, suggesting the microbial communities of the survivors conferred some resilience. Thus, even among coral species with inflexible microbial communities, some individuals may already be tolerant to future ocean conditions. These findings suggest that coral-associated microbial communities could play an important role in the persistence of some corals and underlie climate change-driven shifts in coral community composition.

A predatory gastrula leads to symbiosis-independent settlement in Aiptasia.

The planula larvae of the sea anemone Aiptasia have so far not been reported to complete their life cycle by undergoing metamorphosis into adult forms. This has been a major obstacle in their use as a model for coral-dinoflagellate endosymbiosis. Here, we show that Aiptasia larvae actively feed on crustacean nauplii, displaying a preference for live prey. This feeding behavior relies on functional stinging cells, indicative of complex neuronal control. Regular feeding leads to significant size increase, morphological changes, and efficient settlement around 14 d postfertilization. Surprisingly, the presence of dinoflagellate endosymbionts does not affect larval growth or settlement dynamics but is crucial for sexual reproduction. Our findings finally close Aiptasia's life cycle and highlight the functional nature of its larvae, as in Haeckel's Gastrea postulate, yet reveal its active carnivory, thus contributing to our understanding of early metazoan evolution.

Single-polyp metabolomics reveals biochemical structuring of the coral holobiont at multiple scales.

All biology happens in space, and spatial structuring plays an important role in mediating biological processes at all scales from cells to ecosystems. However, the metabolomic structuring of the coral holobiont has yet to be fully explored. Here, we present a method to detect high-quality metabolomic data from individual coral polyps and apply this method to study the patterning of biochemicals across multiple spatial (~1 mm - ~100 m) and organizational scales (polyp to population). The data show a strong signature for individual coral colonies, a weaker signature of branches within colonies, and variation at the polyp level related to the polyps' location along a branch. Mapping metabolites to either the coral or algal components of the holobiont reveals that polyp-level variation along the length of a branch was largely driven by molecules associated with the cnidarian host as opposed to the algal symbiont, predominantly putative sulfur-containing metabolites. This work yields insights on the spatial structuring of biochemicals in the coral holobiont, which is critical for design, analysis, and interpretation of studies on coral reef biochemistry.

Reef structure of the Florida Reef Tract for the period 2005-2020.

Shallow-water coral reefs of the Florida Reef Tract compose the third largest reef in the world, but during the last several decades, scleractinian (stony) corals have suffered unprecedented declines from global and local stressors. A program to evaluate the effects of high-temperature bleaching events was initiated by The Nature Conservancy's Florida Reef Resilience Program in 2005 and surveys have been completed across at least some portion of the entire region every year since. The program adopted a demographic (colony-based) assessment approach, which records colony species, size (height and maximum diameter), and estimated partial mortality (percent barren skeleton). Because reef structure is critical to ecosystem functioning and services, data from 2005 to 2020 were analyzed to describe the abundance, size, and morphological complexity of stony coral colonies forming the biogenic reef. Colony height, footprint, surface area, and volume summed for 6016 transects were used to describe reef structure and averages were used to characterize the components that contributed to the structure. Nearly 150,000 colonies representing 49 species were reported during this period and results demonstrated both spatial and temporal changes for the region and for geographic subregions. Some subregions showed increasing colony density, especially for three small, hemispheric species, and declining average colony size.

Divergent recovery trajectories of intertidal and subtidal coral communities highlight habitat-specific recovery dynamics following bleaching in an extreme macrotidal reef environment.

Coral reefs face an uncertain future punctuated by recurring climate-induced disturbances. Understanding how reefs can recover from and reassemble after mass bleaching events is therefore important to predict their responses and persistence in a rapidly changing ocean. On naturally extreme reefs characterized by strong daily temperature variability, coral heat tolerance can vary significantly over small spatial gradients but it remains poorly understood how this impacts bleaching resilience and recovery dynamics, despite their importance as resilience hotspots and potential refugia. In the macrotidal Kimberley region in NW Australia, the 2016 global mass bleaching event had a strong habitat-specific impact on intertidal and subtidal coral communities at our study site: corals in the thermally variable intertidal bleached less severely and recovered within six months, while 68% of corals in the moderately variable subtidal died. We therefore conducted benthic surveys 3.5 years after the bleaching event to determine potential changes in benthic cover and coral community composition. In the subtidal, we documented substantial increases in algal cover and live coral cover had not fully recovered to pre-bleaching levels. Furthermore, the subtidal coral community shifted from being dominated by branching Acropora corals with a competitive life history strategy to opportunistic, weedy Pocillopora corals which likely has implications for the functioning and stress resilience of this novel coral community. In contrast, no shifts in algal and live coral cover or coral community composition occurred in the intertidal. These findings demonstrate that differences in coral heat tolerance across small spatial scales can have large consequences for bleaching resilience and that spatial patchiness in recovery trajectories and community reassembly after bleaching might be a common feature on thermally variable reefs. Our findings further confirm that reefs adapted to high daily temperature variability play a key role as resilience hotspots under current climate conditions, but their ability to do so may be limited under intensifying ocean warming.

Neorhizobium turbinariae sp. nov., a coral-beneficial bacterium isolated from Turbinaria peltata.

Coral reef ecosystems are facing decline due to climate change, overfishing, habitat destruction and pollution. Bacteria play an essential role in maintaining the stability of coral reef ecosystems, influencing the well-being and fitness of coral hosts. The exploitation of coral probiotics has become an urgent issue. A short-rod shaped aerobic bacterium, designated NTR19T, was isolated in a healthy coral Turbinaria peltata from Daya Bay, Shenzhen, PR China. Its cells were Gram-negative, motile with a polar flagellum. The activities of catalase and oxidase were positive. Strain NTR19T grew at 10-41 °C (optimum, 28 °C), with NaCl concentrations of 0-4 % (w/v; optimum, 0.5 %) and at pH 5.0-9.5 (optimum, pH 7.0-7.5). The predominant fatty acids (>10 %) were summed feature 8 (57.6 %), C19 : 0 cyclo ω8c (12.6 %) and C16 : 0 (12.0 %). The polar lipids were diphosphatidylglycerol, phosphatidylglycerol, phospholipid and phosphatidylcholine. The major respiratory quinone was Q-10. The draft genome was 4.68 Mbp with 61.2 mol% DNA G+C content. In total, 4477 coding sequences were annotated and there were 64 RNA genes. The average nucleotide identity (ANI) and average amino acid identity (AAI) values between strain NTR19T and the related Neorhizobium species were 78.23-79.70% and 80.26-80.50 %, respectively. This strain encoded many proteins for the activities of catalase and oxidase in the genome. Strain NTR19T was clearly distinct from its closest neighbours Rhizobium oryzicola ACCC 05753T and Neorhizobium petrolearium ACCC 11238T with the 16S rRNA gene sequence similarity values of 96.86 and 96.36 %, respectively. The results of phylogenetic analysis, as well as ANI and AAI values, revealed that strain NTR19T belongs to Neorhizobium and was distinct from other species of this genus. The physiological, biochemical and chemotaxonomic characteristics also supported the species novelty of strain NTR19T. Thus, strain NTR19T is considered to be classified as a novel species in the genus Neorhizobium, for which the name Neorhizobium turbinariae sp. nov. is proposed. The type strain is NTR19T (=JCM 35342T=MCCC 1K07226T).

Community Structure and Predicted Functions of Actively Growing Bacteria Responsive to Released Coral Mucus in Surrounding Seawater.

A direct relationship exists between diverse corals and fish farming in Keten Bay, Amami-Oshima, Japan. The release of coral mucus has a significant impact on the microbial activity of surrounding seawater. To obtain a more detailed understanding of biogeochemical cycles in this environment, the effects of coral mucus on the community structure and function of bacteria in surrounding seawater need to be elucidated. We herein used a bromodeoxyuridine approach to investigate the structures and functions of bacterial communities growing close to mucus derived from two different Acropora corals, AC1 and AC2. The alpha diversities of actively growing bacteria (AGB) were lower in mucus-containing seawater than in control seawater and their community structures significantly differed, suggesting that the growth of specific bacteria was modulated by coral mucus. Rhodobacteraceae and Cryomorphaceae species were the most dominant AGB in response to the mucus of Acropora AC1 and AC2, respectively. In contrast, the growth of Actinomarinaceae, Alteromonadaceae, Flavobacteriaceae, and SAR86 clade bacteria was inhibited by coral mucus. The results of a Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt2) ana-lysis suggested that the predicted functions of AGB in mucus-containing seawater differed from those in seawater. These functions were related to the biosynthesis and degradation of the constituents of coral mucus, such as polysaccharides, sugar acids, and aromatic compounds. The present study demonstrated that complex bacterial community structures and functions may be shaped by coral mucus, suggesting that corals foster diverse bacterial communities that enhance the ecological resilience of this fish farming area.

An Integrated Micro-Device System for Coral Growth and Monitoring.

Corals are fundamental organisms in marine and coastal ecosystems. With the advancement of coral protection research in recent years, precise control of the coral culture environment is highly in demand for coral conservation and study. Here, we developed a semi-closed coral culture micro-device system as a multi-functional platform, which can provide accurate and programmable temperature control, a sterile initial environment, long-term stable water quality, an adjustable dissolved oxygen concentration, and a customized light spectrum for corals. Owing to the modular design, the coral culture system can be upgraded or modified by installing desirable new modules or removing existing ones. Currently, under appropriate conditions and with proper system maintenance, the sample corals can survive for at least 30 days in a healthy state. Furthermore, due to the controllable and sterile initial environment, this coral culture system can support research into the symbiotic relationship between corals and associated microorganisms. Therefore, this micro-device system can be applied to monitor and investigate sea corals in a relatively quantitative manner.

Physiological response of Symbiodiniaceae to thermal stress: Reactive oxygen species, photosynthesis, and relative cell size.

This study investigates the physiological response to heat stress of three genetically different Symbiodiniaceae strains isolated from the scleractinian coral Mussismilia braziliensis, endemic of the Abrolhos Bank, Brazil. Cultures of two Symbiodinium sp. and one Cladocopium sp. were exposed to a stepwise increase in temperature (2°C every second day) ranging from 26°C (modal temperature in Abrolhos) to 32°C (just above the maximum temperature registered in Abrolhos during the third global bleaching event-TGBE). After the cultures reached their final testing temperature, reactive oxygen species (ROS) production, single cell attributes (relative cell size and chlorophyll fluorescence), and photosynthetic efficiency (effective (Y(II)) and maximum (Fv/Fm) quantum yields) were measured within 4 h and 72 h. Non-photochemical coefficient (NPQ) was estimated based on fluorescence values. Population average ROS production was variable across strains and exposure times, reaching up a 2-fold increase at 32°C in one of the Symbiodinium sp. strains. A marked intrapopulation difference was observed in ROS production, with 5 to 25% of the cells producing up to 10 times more than the population average, highlighting the importance of single cell approaches to assess population physiology. Average cell size increases at higher temperatures, likely resulting from cell cycle arrest, whereas chlorophyll fluorescence decreased, especially in 4 h, indicating a photoacclimation response. The conditions tested do not seem to have elicited loss of photosynthetic efficiency nor the activation of non-photochemical mechanisms in the cells. Our results unveiled a generalized thermotolerance in three Symbiodiniaceae strains originated from Abrolhos' corals. Inter and intra-specific variability could be detected, likely reflecting the genetic differences among the strains.

Characterization of a sperm motility signalling pathway in a gonochoric coral suggests conservation across cnidarian sexual systems.

Most stony corals liberate their gametes into the water column via broadcast spawning, where fertilization hinges upon the activation of directional sperm motility. Sperm from gonochoric and hermaphroditic corals display distinct morphological and molecular phenotypes, yet it is unknown whether the signalling pathways controlling sperm motility are also distinct between these sexual systems. Here, we addressed this knowledge gap using the gonochoric, broadcast spawning coral Astrangia poculata. We found that cytosolic alkalinization of sperm activates the pH-sensing enzyme soluble adenylyl cyclase (sAC), which is required for motility. Additionally, we demonstrate for the first time in any cnidarian that sAC activity leads to protein kinase A (PKA) activation, and that PKA activity contributes to sperm motility activation. Ultrastructures of A. poculata sperm displayed morphological homology with other gonochoric cnidarians, and sAC exhibited broad structural and functional conservation across this phylum. These results indicate a conserved role for pH-dependent sAC-cAMP-PKA signalling in sperm motility across coral sexual systems, and suggest that the role of this pathway in sperm motility may be ancestral in metazoans. Finally, the dynamics of this pH-sensitive pathway may play a critical role in determining the sensitivity of marine invertebrate reproduction to anthropogenic ocean acidification.