Scleractinian Coral Pocillopora Damicornis Research Articles

Potential adaptation of scleractinian coral Pocillopora damicornis during hypo-salinity stress caused by extreme pre-flood rainfall over south China

Global warming intensifies the water cycle, resulting in significant increases in precipitation and river runoff, which brings severe hypo-salinity stress to nearshore coral reefs. Ecological investigations have found that some corals exhibit remarkable adaptability to hypo-salinity stress during mass-bleaching events. However, the exact cause of this phenomenon remains unclear. To elucidate the potential molecular mechanism leading to high tolerance to hypo-salinity stress, Pocillopora damicornis was used as a research object in this study. We compared the differences in transcriptional responses and symbiotic microbiomes between bleaching and unbleaching P. damicornis during hypo-salinity stress caused by extreme pre-flood rainfall over South China in 2022. The results showed that: (1) Under hypo-salinity stress, the coral genes related to immune defense and cellular stress were significantly upregulated in bleaching corals, indicating more severe immune damage and stress, and the Symbiodiniaceae had no significant gene enrichment. Conversely, metabolic genes related to glycolysis/gluconeogenesis were significantly downregulated in unbleaching corals, whereas Symbiodiniaceae genes related to oxidative phosphorylation were significantly upregulated to meet the energy requirements of coral holobiont; (2) C1d was the dominant Symbiodiniaceae subclade in all samples, with no significant difference between the two groups; (3) The symbiotic bacterial community structure was reorganized under hypo-salinity stress. The abundance of opportunistic bacteria increased significantly in bleaching coral, whereas the relative abundance of probiotics was higher in unbleaching coral. This may be due to severe immune damage, making the coral more susceptible to opportunistic infection and bleaching. These results suggest that long-term hypo-salinity acclimation in the Pearl River Estuary enhances the tolerance of some corals to hypo-salinity stress. Corals with higher tolerance may reduce energy consumption by slowing down their metabolism, improve the energy metabolism of Symbiodiniaceae to meet the energy requirements of the coral holobiont, and alter the structure of symbiotic bacterial communities to avoid bleaching.

Acute hypoxia induces reduction of algal symbiont density and suppression of energy metabolism in the scleractinian coral Pocillopora damicornis

Loss of oxygen in the ocean is accelerating and threatening the coral reef ecosystem. In this study, the impacts of hypoxia on the scleractinian coral Pocillopora damicornis were explored. The algal symbiont density, chlorophyll a + c2 content, energy consumption of corals, as well as energy available and consumption of their symbionts, decreased significantly post hypoxia stress. Meanwhile, the malondialdehyde contents in corals and symbionts, together with the caspase-3 activation level in corals, increased significantly in response to hypoxia stress. Furthermore, it was revealed that activities such as coral cell division and calcification were inhibited under hypoxia. These results collectively suggest that acute hypoxia stress reduces symbiont density and chlorophyll a + c2 content in the coral P. damicornis by elevating intracellular oxidative pressure and apoptotic level, which further suppresses energy metabolism in the symbiotic association and negatively affects a series of activities such as coral cell division and calcification.

The scleractinian coral Pocillopora damicornis relies on neuroendocrine regulation to cope with polycyclic aromatic hydrocarbons under heat stress

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic environmental pollutants and are threatening scleractinian corals. In this study, PAHs treatment did not induce significant physiological responses of the coral Pocillopora damicornis and its algal symbionts, but biological processes including response to toxin, drug metabolic, and oxidation reduction were triggered at the mRNA level. These results implied that PAHs could be a group of slow-acting environmental toxicants, whose effects were moderate but persistent. Besides, it was interesting to find that PAHs activated the neuroendocrine system in the coral by triggering the expression of monoaminergic and acetylcholinergic system related genes, indicating that PAHs might function as environmental hormones. Moreover, the combined treatments of PAHs and heat caused a much obvious effect on the coral and its algal symbionts by elevating antioxidant activity and suppressing photosynthesis in the symbionts. Results from the transcriptome data further indicated that corals might perform stress responses upon PAHs and heat challenges through the TNF and apoptosis pathways, which perhaps was modulated by the neuroendocrine system of corals. Collectively, our survey demonstrates that the PAHs can function as environmental hormones and activate the neuroendocrine regulation in scleractinian corals, which may contribute to the stress responses of symbiotic association by modulating photosynthesis, antioxidation, and apoptosis.

Evaluation of fluorescence-based viability stains in cells dissociated from scleractinian coral Pocillopora damicornis

The application of established cell viability assays such as the commonly used trypan blue staining method to coral cells is not straightforward due to different culture parameters and different cellular features specific to mammalian cells compared to marine invertebrates. Using Pocillopora damicornis as a model, we characterized the autofluorescence and tested different fluorescent dye pair combinations to identify alternative viability indicators. The cytotoxicity of different representative molecules, namely small organic molecules, proteins and nanoparticles (NP), was measured after 24 h of exposure using the fluorescent dye pair Hoechst 33342 and SYTOX orange. Our results show that this dye pair can be distinctly measured in the presence of fluorescent proteins plus chlorophyll. P. damicornis cells exposed for 24 h to Triton-X100, insulin or titanium dioxide (TiO2) NPs, respectively, at concentrations ranging from 0.5 to 100 µg/mL, revealed a LC50 of 0.46 µg/mL for Triton-X100, 6.21 µg/mL for TiO2 NPs and 33.9 µg/mL for insulin. This work presents the approach used to customize dye pairs for membrane integrity-based cell viability assays considering the species- and genotype-specific autofluorescence of scleractinian corals, namely: endogenous fluorescence characterization followed by the selection of dyes that do not overlap with endogenous signals.

Involvement of the ammonium assimilation mediated by glutamate dehydrogenase in response to heat stress in the scleractinian coral Pocillopora damicornis

Involvement of the ammonium assimilation mediated by glutamate dehydrogenase in response to heat stress in the scleractinian coral Pocillopora damicornis

Physiological and transcriptomic analyses reveal the threat of herbicides glufosinate and glyphosate to the scleractinian coral Pocillopora damicornis

The amino acid metabolism-related herbicides glufosinate and glyphosate are used worldwide and have flowed into the oceans, threatening the marine organisms. In the present study, physiological activities and transcriptomic profiles of the scleractinian coral Pocillopora damicornis and symbiotic Symbiodiniaceae were determined during a 48 h-exposure to the two herbicides with the final concentration of 10 μmol L−1. Coral samples were collected at 0, 12, 24, and 48 h after exposure to determine symbiont density, chlorophyll content, as well as activities of superoxide dismutase (SOD), catalase (CAT), nitric oxide synthetase (NOS) and phenoloxidase (PO), and the caspase-3 levels, and the samples collected at 24 h were employed in the transcriptomic analysis. Specifically, the symbiont densities did not change significantly in response to the two herbicides, while the chlorophyll content increased significantly at 24 h post glufosinate exposure. SOD and CAT activities in the coral host increased significantly at 12 h after glufosinate and glyphosate exposure, while the activity of NOS in symbionts decreased significantly at 48 h after glufosinate exposure. Caspase-3 levels in the coral host declined significantly at 24 h after exposure to the two herbicides. In the transcriptomic analysis, glufosinate triggered the expression of genes related to the response to stimuli and immunoregulation in the coral host, and suppressed the expression of genes related to coral nitrogen-related metabolism, symbiont cell cycle, and response to nutrient levels. Furthermore, glyphosate activated the expression of genes involved in coral calcification and symbiont nutrient export and suppressed the expression of genes involved in coral meiosis and symbiont cell communication. These results suggest that although the coral-Symbiodiniaceae symbiosis is not disrupted, short-term glufosinate and glyphosate exposures alter several essential physiological processes including metabolism, calcification, and meiosis in the coral host, as well as the cell cycle and nutrient export in the symbiont. SummaryGlufosinate and glyphosate herbicide exposures can disturb several essential physiological processes, including metabolism, calcification, and meiosis in the coral host as well as the cell cycle and nutrient export in the symbiont, threating the survival of scleractinian corals.

Low levels of ultra-violet radiation mitigate the deleterious effects of nitrate and thermal stress on coral photosynthesis

Reef ecosystems are under increasing pressure from global and local stressors. Rising seawater temperature and high ultraviolet radiation (UVR) levels are the main drivers of the disruption of the coral-dinoflagellate symbiosis (bleaching). Bleaching can also be exacerbated by nitrate contamination in coastal reefs. However, the underlying physiological mechanisms are still poorly understood. Here, we assessed the physiological and oxidative state of the scleractinian coral Pocillopora damicornis, maintained eight weeks in a crossed-factorial design including two temperatures (26 °C or 30 °C), and two nitrate (0.5 and 3 μM-enriched), and UVR (no UVR and 25/1.5 Wm−2 UVA/B) levels. Nitrate enrichment, and high temperature, significantly impaired coral photosynthesis. However, UVR alleviated the nitrate and temperature-induced decrease in photosynthesis, by increasing the coral's antioxidant capacity. The present study contributes to our understanding of the combined effects of abiotic stressors on coral bleaching susceptibility. Such information is urgently needed to refine reef management strategies.

Nanopore long-read RNAseq reveals regulatory mechanisms of thermally variable reef environments promoting heat tolerance of scleractinian coral Pocillopora damicornis

Some scleractinian corals exhibit high thermal adaptability to climate changes, although the mechanism of their adaptation is unclear. This study investigated the adaptability of scleractinian coral Pocillopora damicornis to thermally variable reef environments by applying a nanopore-based RNA sequencing method to characterize different transcription responses that promote heat tolerance of P. damicornis. We identified 1414 novel genes and optimized 6256 mis-annotated loci. Based on full-length transcriptome data, we identified complex alternative polyadenylation and alternative splicing events, which can improve our understanding of the genome annotation and gene structures of P. damicornis. Furthermore, we constructed differentially expressed lncRNA-mRNA co-expression networks, which may play a crucial role in the P. damicornis thermal adaptive response. KEGG function enrichment analysis revealed that P. damicornis from the high-temperature pool had a lower metabolic rate than that from the low-temperature pool. We hypothesize that metabolic readjustment, in the form of a lower metabolic rate, positively correlated with increased heat tolerance in P. damicornis in thermally variable reef environments. Our study provides novel insights into lncRNAs that promote thermally tolerance of scleractinian corals in the thermally variable reef environment, suggesting potential mechanisms for their adaptation to global warming in the future.

Increased Ammonium Assimilation Activity in the Scleractinian Coral Pocillopora damicornis but Not Its Symbiont After Acute Heat Stress

Ammonium is the main nitrogen resources for scleractinian coral-Symbiodiniaceae symbiotic association, and there is urgent need to investigate the involvement of ammonium assimilation in the heat acclimation of the symbiotic association to heat stress. In the present study, symbiont density and chlorophyll content, as well as redox and ammonium assimilation enzyme activities in two symbiotic partners of the scleractinian coral Pocillopora damicornis were firstly investigated after acute heat stress (32 °C). Symbiont density and chlorophyll content decreased significantly at 24 h (1.81×105 cell cm-2, p < 0.05) and 36 h (23.25 pg cell-1, p < 0.05) after heat stress, respectively. We observed significant activity increases of coral nitric oxide synthase, superoxide dismutase and catalase, and symbiont superoxide dismutase after heat stress, but no alterations in the activities of symbiont nitric oxide synthase and catalase during the whole experiment period. The activities of coral glutamine synthetase and glutamine oxoglutarate aminotransferase began to increase significantly at 24 h, and reached the peak at 36 h after heat stress. As for glutamate dehydrogenase activity, a significant increase was observed only at 36 h after heat stress. The activities of symbiont glutamine synthetase and glutamine oxoglutarate aminotransferase did not change significantly after heat stress. Secondly, symbiont density, chlorophyll content, apoptosis rate, coral total antioxidant capacity, and caspase3 activation level were determined after glufosinate (glutamine synthetase inhibitor) treatment. Symbiont density decreased significantly at 6 h (1.25×106 cell cm-2, p < 0.05) after glufosinate treatment, and symbiont chlorophyll content also decreased significantly during 12-24 h after glufosinate treatment, with the lowest level at 12 h (16.69 pg cell-1, p < 0.05). Furthermore, the coral total antioxidant capacity and caspase3 activation level both increased significantly at 12 h (0.57 U mg-1, 2.08-fold, p < 0.05) after glufosinate treatment, while no significant change was observed for the symbiont apoptosis rate. These results suggest that the ammonium assimilation activity in the coral host P. damicornis, not its symbiont, was induced by acute heat stress, which might contribute to the acclimatization of the symbiotic association to high temperature through regulating coral antioxidant capacity and apoptosis.

Community composition of Symbiodiniaceae among four scleractinian corals in the eastern Gulf of Thailand

Symbiotic algae in the family Symbiodiniaceae are essential for scleractinian corals’ survival and resilience. Understanding the community composition of Symbiodiniaceae species within coral colonies can provide insight into how corals respond to environmental changes. We examined Symbiodiniaceae community composition in four scleractinian coral hosts, Porites lutea, Platygyra daedalea, Pavona decussata and Pocillopora damicornis at six sites (n = 3-5 colonies/species/site) in the eastern Gulf of Thailand using Denaturing Gradient Gel Electrophoresis (DGGE) to resolve polymorphisms of the Internal Transcribed Spacer-2 (ITS2) DNA. The results showed that the Symbiodiniaceae genus Durusdinium was the most common clade in all coral hosts in all areas, whereas Cladocopium was common in P. lutea. We observed at least five prevalent Symbiodiniaceae species: Cladocopium goreaui (C1); two Cladocopium spp. (C3u and C15, C15v1); and at least two Durusdinium spp. (D1), which include Durusdinium sp. (D1-6 in P. damicornis), and at least one other Durusdinium sp. (D1) in all host coral species. Cladocopium sp. (C15) was highly abundant in P. lutea. Durusdinium spp. (D1) were the dominant species in P. decussata, P. daedalea and P. damicornis. PERMANOVA revealed the effects of site, host coral species and interaction between site and host coral species on the variation of Symbiodiniaceae species composition. Cluster analysis based on Bray–Curtis similarity index suggested four large groups. The first group, consisting of all coral host species (mainly P. daedalea, P. decussata and P. damicornis), had Durusdinium spp. (D1) as dominant species; the second group, consisting of P. lutea, had Cladocopium sp. (C15) as the dominant species; the third group, consisting of P. damicornis had D. glynnii (D1-6) as the dominant species; and the last group, consisting of P. daedalea had Cladocopium sp. (C3u) as the dominant species. Our results suggested complex interactions among Symbiodiniaceae species composition, coral species and environmental conditions.

Altered Immune Landscape and Disrupted Coral-Symbiodinium Symbiosis in the Scleractinian Coral Pocillopora damicornis by Vibrio coralliilyticus Challenge.

Vibrio coralliilyticus is known to cause coral diseases, especially under environmental perturbation, but its impact on coral physiology and underpinning mechanism is poorly understood. In the present study, we investigated cytological, immunological, and metatranscriptomic responses of the scleractinian coral Pocillopora damicornis to V. coralliilyticus infection. The density and chlorophyll content of symbiotic zooxanthellae decreased significantly at 12 and 24 h after Vibrio challenge. The activities of antioxidant enzymes such as superoxide dismutase and catalase, nitric oxide synthase, phenoloxidase (PO), and the activation level of caspase3 all rose significantly in P. damicornis after Vibrio challenge. In the metatranscriptomic analysis, we found 10 significantly upregulated genes in the symbionts at 24 h after the challenge, which were mostly involved in the metabolism of nucleic acid and polysaccharide, and 133 significantly down-regulated symbiont genes, which were mainly related to amino acid catabolism and transport. Meanwhile, 1432 significantly upregulated coral genes were revealed, highly overrepresented in GO terms that are mostly related to the regulation of immune response, the regulation of cytokine production, and innate immune response. Furthermore, at 24 h after Vibrio challenge, 890 coral genes were significantly downregulated, highly overrepresented in four GO terms implicated in defense response. These results in concert suggest that V. coralliilyticus infection triggered the innate immune response including the redox, PO, and apoptosis systems, but repressed the response of the complement system in the scleractinian coral P. damicornis, accompanied by symbiont density decrease and symbiosis collapse through disordering the metabolism of the symbionts. These findings shed light on the molecular regulatory processes underlying bleaching and degradation of P. damicornis resulting from the infection of V. coralliilyticus.

Temperature regulates the recognition activities of a galectin to pathogen and symbiont in the scleractinian coral Pocillopora damicornis

Lectins serve as essential pattern recognition receptors, and play important roles in the recognition of non-self and mediation of innate immune response in metazoans. Scleractinian corals are vulnerable to pathogen infection and endosymbiosis disruption under heat stress that can finally lead to coral bleaching. In this study, a cDNA sequence encoding one galectin was cloned in scleractinian coral Pocillopora damicornis (PdGLT-1). The deduced PdGLT-1 protein shared highest amino acid sequence similarity (99%) with galectin from Stylophora pistillata (XP_022806650.1), and was composed of one signal peptide, one Collagen domain and one Gal-Lectin domain. PdGLT-1 recombinant protein (rPdGLT-1) was expressed and purified in vitro. Binding activities of rPdGLT-1 to bacteria and symbiont were determined using western blotting method. Results showed that rPdGLT-1 was able to bind to gram-positive bacterium Streptococcus mutans, gram-negative bacteria Vibrio coralliilyticus and Escherichia coli, with the highest activity for V. coralliilyticus, and further agglutinated them. The bound rPdGLT-1 to Symbiodinium (10-104 cells mL−1) was detectable, and its binding ability was concentration-dependent. Furthermore, dual binding activities were determined under different temperatures (20, 25, 30 and 35 °C), and the optimal temperatures were found to be 25 and 30 °C for V. coralliilyticus and Symbiodinium, respectively. Results suggested that PdGLT-1 could recognize pathogenic bacteria and symbiotic dinoflagellates Symbiodinium. However, their recognition activities were repressed under high temperature (>30 °C). This study provided insights into the underlying mechanism of lectin modulation to heat bleaching through its pathogen and Symbiodinium recognition in the scleractinian coral P. damicornis.

Planulation in the Scleractinian Coral Pocillopora damicornis in Singapore Waters

Planulation in the Scleractinian Coral Pocillopora damicornis in Singapore Waters

Effects of ultraviolet radiation and nutrient level on the physiological response and organic matter release of the scleractinian coral Pocillopora damicornis following thermal stress.

Understanding which factors enhance or mitigate the impact of high temperatures on corals is crucial to predict the severity of coral bleaching worldwide. On the one hand, global warming is usually associated with high ultraviolet radiation levels (UVR), and surface water nutrient depletion due to stratification. On the other hand, eutrophication of coastal reefs increases levels of inorganic nutrients and decreases UVR, so that the effect of different combinations of these stressors on corals is unknown. In this study, we assessed the individual and crossed effects of high temperature, UVR and nutrient level on the key performance variables of the reef building coral Pocillopora damicornis. We found that seawater warming was the major stressor, which induced bleaching and impaired coral photosynthesis and calcification in all nutrient and UVR conditions. The strength of this effect however, was nutrient dependent. Corals maintained in nutrient-depleted conditions experienced the highest decrease in net photosynthesis under thermal stress, while nutrient enrichment (3 μM NO3- and 1 μM PO4+) slightly limited the negative impact of temperature through enhanced protein content, photosynthesis and respiration rates. UVR exposure had only an effect on total nitrogen release rates, which significantly decreased under normal growth conditions and tended to decrease also under thermal stress. This result suggests that increased level of UVR will lead to significant changes in the nutrient biogeochemistry of surface reef waters. Overall, our results show that environmental factors have different and interactive effects on each of the coral’s physiological parameters, requiring multifactorial approaches to predict the future of coral reefs.

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

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

Acute microplastic exposure raises stress response and suppresses detoxification and immune capacities in the scleractinian coral Pocillopora damicornis

Microplastics are widespread emerging contaminants that have been found globally in the marine and freshwater ecosystem, but there is limited knowledge regarding its impact on coral reef ecosystem and underpinning mechanism. In the present study, using Pocillopora damicornis as a model, we investigated cytological, physiological, and molecular responses of a scleractinian coral to acute microplastic exposure. No significant changes were observed in the density of symbiotic zooxanthellae during the entire period of microplastic exposure, while its chlorophyll content increased significantly at 12 h of microplastic exposure. We observed significant increases in the activities of antioxidant enzymes such as superoxide dismutase and catalase, significant decrease in the detoxifying enzyme glutathione S-transferase and the immune enzyme alkaline phosphatase, but no change in the other immune enzyme phenoloxidase during the whole experiment period. Transcriptomic analysis revealed 134 significantly up-regulated coral genes at 12 h after the exposure, enriched in 11 GO terms mostly related to stress response, zymogen granule, and JNK signal pathway. Meanwhile, 215 coral genes were significantly down-regulated at 12 h after exposure, enriched in 25 GO terms involved in sterol transport and EGF-ERK1/2 signal pathway. In contrast, only 12 zooxanthella genes exhibited significant up-regulation and 95 genes down-regulation at 12 h after the microplastic exposure; genes regulating synthesis and export of glucose and amino acids were not impacted. These results suggest that acute exposure of microplastics can activate the stress response of the scleractinian coral P. damicornis, and repress its detoxification and immune system through the JNK and ERK signal pathways. These demonstrate that microplastic exposure can compromise the anti-stress capacity and immune system of the scleractinian coral P. damicornis, despite the minimal impact on the abundance and major photosynthate translocation transporters of the symbiont in the short term.

Winogradskyella pocilloporae sp. nov. isolated from healthy tissue of the coral Pocillopora damicornis

A novel Gram-stain-negative, rod-shaped, strictly aerobic, and orange-yellow pigmented bacterium, designated strain AFPH31T, was isolated from internal tissues of the scleractinian coral Pocillopora damicornis, cultured in a marine aquarium system at the Justus Liebig University Giessen, Germany. Phylogenetic analyses based on 16S rRNA gene sequences placed the strain within the monophyletic cluster of the genus Winogradskyella and showed highest sequence similarity to type strains of the species Winogradskyella eximia (96.6 %), Winogradskyella wandonensis (96.4 %), and Winogradskyella damuponensis (96.4 %). The strain grew well at 15-37 °C (optimum 25 °C), in the presence of 0.5-8.5 % NaCl (optimum 2 %), and at pH 5.5-8.5 (optimum pH 6.0-7.5). The major cellular fatty acids of strain AFPH31T were iso-C15 : 0 (22.0 %), iso-C15 : 1 G (16.9 %), iso-C17 : 0 3-OH (14.9 %), and anteiso-C15 : 0 (11.9 %). The major compound in the polyamine pattern was sym-homospermidine. The quinone system contained predominantly menaquinone MK-6. The polar lipid profile contained predominantly phosphatidylethanolamine, one unidentified aminolipid, and two unidentified lipids lacking a functional group. The genomic DNA G+C content was 36.8 mol%. According to the phylogenetic, chemotaxonomic, and phenotypic analyses we propose a novel species of the genus Winogradskyella named Winogradskyella pocilloporae sp. nov. The type strain is AFPH31T (=CCM 8816T=CIP 111546T).

Thermal regime and host clade, rather than geography, drive Symbiodinium and bacterial assemblages in the scleractinian coral Pocillopora damicornis sensu lato

BackgroundAlthough the term holobiont has been popularized in corals with the advent of the hologenome theory of evolution, the underlying concepts are still a matter of debate. Indeed, the relative contribution of host and environment and especially thermal regime in shaping the microbial communities should be examined carefully to evaluate the potential role of symbionts for holobiont adaptation in the context of global changes. We used the sessile, long-lived, symbiotic and environmentally sensitive reef-building coral Pocillopora damicornis to address these issues.ResultsWe sampled Pocillopora damicornis colonies corresponding to two different mitochondrial lineages in different geographic areas displaying different thermal regimes: Djibouti, French Polynesia, New Caledonia, and Taiwan. The community composition of bacteria and the algal endosymbiont Symbiodinium were characterized using high-throughput sequencing of 16S rRNA gene and internal transcribed spacer, ITS2, respectively. Bacterial microbiota was very diverse with high prevalence of Endozoicomonas, Arcobacter, and Acinetobacter in all samples. While Symbiodinium sub-clade C1 was dominant in Taiwan and New Caledonia, D1 was dominant in Djibouti and French Polynesia. Moreover, we also identified a high background diversity (i.e., with proportions < 1%) of A1, C3, C15, and G Symbiodinum sub-clades. Using redundancy analyses, we found that the effect of geography was very low for both communities and that host genotypes and temperatures differently influenced Symbiodinium and bacterial microbiota. Indeed, while the constraint of host haplotype was higher than temperatures on bacterial composition, we showed for the first time a strong relationship between the composition of Symbiodinium communities and minimal sea surface temperatures.ConclusionBecause Symbiodinium assemblages are more constrained by the thermal regime than bacterial communities, we propose that their contribution to adaptive capacities of the holobiont to temperature changes might be higher than the influence of bacterial microbiota. Moreover, the link between Symbiodinium community composition and minimal temperatures suggests low relative fitness of clade D at lower temperatures. This observation is particularly relevant in the context of climate change, since corals will face increasing temperatures as well as much frequent abnormal cold episodes in some areas of the world.

The effect of changing seawater Ca and Mg concentrations upon the distribution coefficients of Mg and Sr in the skeletons of the scleractinian coral Pocillopora damicornis

The skeletal composition of calcifying organisms, in particular Mg/Ca and Sr/Ca ratios, have been widely used to understand fluctuations in seawater chemistry throughout the Phanerozoic. While the success of applying these data to the geologic record depends on a knowledge of the distribution coefficients for these elements (DMg and DSr), there are scarcely any studies which have described how these values vary as a result of changing seawater Mg/Ca ratios. To address this, we have cultured the scleractinian coral, Pocillopora damicornis, in seawater with ranges of Mg and Ca concentrations. Here, we demonstrate that Mg/Ca and Sr/Ca ratios of coral skeletons correlate with total seawater Mg/Ca and Sr/Ca molar ratios, but that apparent DMg and DSr values do not remain constant across the range of experimental seawater treatments, with DMg values significantly increasing with seawater Mg/Ca ratios and DSr values significantly increasing with seawater Ca concentrations. These trends are not rate dependent and may be best explained by a Rayleigh distillation model, in which the calcifying space is semi-isolated from seawater during skeletogenesis (i.e. leaky). As there is a slight increase in DMg and decrease in DSr values between our “Jurassic” and “Modern” seawater treatments, the application of a constant distribution coefficient to estimate changes in ancient seawater chemistry may underestimate seawater Mg/Ca ratios and overestimate Sr/Ca throughout the Mesozoic and Cenozoic. We suggest that interpretations of seawater chemistry from fossil corals may be improved by using the relationships derived for skeletal and seawater Mg/Ca and Sr/Ca ratios established by our experiments, as they incorporate the effect of seawater Mg/Ca ratios on skeletal Mg/Ca and Sr/Ca ratios.

Seabirds supply nitrogen to reef-building corals on remote Pacific islets

Seabirds concentrate nutrients from large marine areas on their nesting islands playing an important ecological role in nutrient transfer between marine and terrestrial ecosystems. Here we investigate the role of guano on corals reefs across scales by analyzing the stable nitrogen isotopic (δ15N) values of the scleractinian coral Pocillopora damicornis on fringing reefs around two Pacific remote islets with large seabird colonies. Marine stations closest to the seabird colonies had higher nitrate + nitrite concentrations compared to more distant stations. Coral and zooxanthellae δ15N values were also higher at these sites, suggesting that guano-derived nitrogen is assimilated into corals and contributes to their nitrogen requirements. The spatial extent of guano influence was however restricted to a local scale. Our results demonstrate that seabird-derived nutrients not only spread across the terrestrial ecosystem, but also affect components of the adjacent marine ecosystem. Further studies are now needed to assess if this nutrient input has a positive or negative effect for corals. Such studies on remote islets also open fresh perspectives to understand how nutrients affect coral reefs isolated from other anthropogenic stressors.