A comparison of SNARF-1 and skeletal δ11B estimates of calcification media pH in tropical coral

Funding: SIMS analyses were supported by the Natural Environment Research Council, UK (IMF689/0519).Coral skeletal boron geochemistry offers opportunities to probe the pH of the calcification media (pHCM) of modern and fossil specimens, to estimate past changes in seawater pH and to explore the biomineralisation response to future ocean acidification. In this research we grew 2 Stylophora pistillata coral microcolonies over glass coverslips to allow analysis of the pH sensitive dye SNARF-1, in the extracellular calcification medium at the growing edge of colonies where the first aragonite crystals are formed, under both light and dark conditions. We use secondary ion mass spectrometry (SIMS) to measure the boron isotopic composition (δ11B) of the skeleton close to the growth edge after 2 to 3 days of additional calcification had enlarged the crystals until they joined, generating a continuous sheet of aragonite. Mean skeletal δ11B-pHCM estimates are higher than those of by SNARF-1 by 0.35 to 0.44 pH units. These differences either reflect real variations in the pH of the calcification media associated with each measurement technique or indicate other changes in the biomineralisation process which influence skeletal δ11B. SNARF-1 measures directly the pH of the extracellular calcification medium while skeletal δ11B analyses aragonite potentially formed via both extracellular and intracellular biomineralisation pathways. Analysis of a third coral specimen, also growing on a glass slide but with a 5 cm long branch, indicated good agreement between the δ11B value of the apex of the branch and the skeletal growth edge. The tissues overlying both these regions were transparent indicating they had low symbiont densities. This suggests that the biomineralisation process is broadly comparable between these sites and that studies growing corals over glass slides/coverslips provide representative data for the colony apex.Publisher PDFPeer reviewe

Coral bleaching under thermal stress: putative involvement of host/symbiont recognition mechanisms

<p>Abstract</p> <p>Background</p> <p>Coral bleaching can be defined as the loss of symbiotic zooxanthellae and/or their photosynthetic pigments from their cnidarian host. This major disturbance of reef ecosystems is principally induced by increases in water temperature. Since the beginning of the 1980s and the onset of global climate change, this phenomenon has been occurring at increasing rates and scales, and with increasing severity. Several studies have been undertaken in the last few years to better understand the cellular and molecular mechanisms of coral bleaching but the jigsaw puzzle is far from being complete, especially concerning the early events leading to symbiosis breakdown. The aim of the present study was to find molecular actors involved early in the mechanism leading to symbiosis collapse.</p> <p>Results</p> <p>In our experimental procedure, one set of <it>Pocillopora damicornis </it>nubbins was subjected to a gradual increase of water temperature from 28°C to 32°C over 15 days. A second control set kept at constant temperature (28°C). The differentially expressed mRNA between the stressed states (sampled just before the onset of bleaching) and the non stressed states (control) were isolated by Suppression Subtractive Hybridization. Transcription rates of the most interesting genes (considering their putative function) were quantified by Q-RT-PCR, which revealed a significant decrease in transcription of two candidates six days before bleaching. RACE-PCR experiments showed that one of them (<it>PdC-Lectin</it>) contained a C-Type-Lectin domain specific for mannose. Immunolocalisation demonstrated that this host gene mediates molecular interactions between the host and the symbionts suggesting a putative role in zooxanthellae acquisition and/or sequestration. The second gene corresponds to a gene putatively involved in calcification processes (<it>Pdcyst-rich</it>). Its down-regulation could reflect a trade-off mechanism leading to the arrest of the mineralization process under stress.</p> <p>Conclusion</p> <p>Under thermal stress zooxanthellae photosynthesis leads to intense oxidative stress in the two partners. This endogenous stress can lead to the perception of the symbiont as a toxic partner for the host. Consequently, we propose that the bleaching process is due in part to a decrease in zooxanthellae acquisition and/or sequestration. In addition to a new hypothesis in coral bleaching mechanisms, this study provides promising biomarkers for monitoring coral health.</p

Epigenome-associated phenotypic acclimatization to ocean acidification in a reef-building coral

There are increasing concerns that the current rate of climate change might outpace the ability of reef-building corals to adapt to future conditions. Work on model systems has shown that environmentally induced alterations in DNA methylation can lead to phenotypic acclimatization. While DNA methylation has been reported in corals and is thought to associate with phenotypic plasticity, potential mechanisms linked to changes in whole-genome methylation have yet to be elucidated. We show that DNA methylation significantly reduces spurious transcription in the coral Stylophora pistillata. Furthermore, we find that DNA methylation also reduces transcriptional noise by fine-tuning the expression of highly expressed genes. Analysis of DNA methylation patterns of corals subjected to long-term pH stress showed widespread changes in pathways regulating cell cycle and body size. Correspondingly, we found significant increases in cell and polyp sizes that resulted in more porous skeletons, supporting the hypothesis that linear extension rates are maintained under conditions of reduced calcification. These findings suggest an epigenetic component in phenotypic acclimatization that provides corals with an additional mechanism to cope with environmental change.This publication is based on work supported by the KAUST Office of Sponsored Research under award no. FCC/1/1973- 22-01. Part of this study was conducted as part of the Centre Scientifique de Monaco Research Program, which is supported by the Government of the Principality of Monaco

Analyse protéique de la matrice organique chez le genre Corallium, approche biochimique comparative et mécanistique

Les espèces du genre Corallium représentent un modèle d étude de la biominéralisation très intéressant : elles sont capables d élaborer deux structures squelettiques, un axe squelettique et des sclérites. Malgré des différences morphologiques et morphométriques importantes, ces structures sont toutes les deux composées d une fraction minérale de carbonate de calcium et d une fraction organique appelée matrice organique (MO). Cette MO interviendrait dans les différentes étapes du processus de biominéralisation. L étude comparée de cette MO représente donc une formidable opportunité pour déterminer les mécanismes de biominéralisation chez ces espèces. Par des approches comparatives intra- et inter-espèces, nous avons mis en évidence les caractéristiques biochimiques des protéines de MO chez différentes populations et espèces de Corallium. Nos résultats montrent à la fois des caractéristiques communes et des spécificités. Nous avons également établi que la caractérisation biochimique des protéines de MO est un critère d identification taxonomique chez ces espèces. De manière plus spécifique, la comparaison des protéines de MO des deux structures squelettiques chez C. rubrum a montré que certaines protéines présentent des caractéristiques biochimiques communes mais également des caractéristiques biochimiques différentes. Par la combinaison d approches biochimiques avec une approche transcriptomique, nous avons caractérisé la première protéine de MO chez l Octocoralliaire de Méditerranée, Corallium rubrum. Nos résultats montrent que le processus de formation du squelette chez les espèces du genre Corallium se fait par un mécanisme général commun aux différentes espècesCorallium genus species are very interesting biomineralization models: they build two skeletal structures with different size and form: an axial skeleton and sclerites. Despite their important morphological and morphometrical differences, both are composed of a mineral fraction of calcium carbonate and an organic fraction called organic matrix (OM). This OM is supposed to be involved in several biomineralization steps: nucleation, growth and orientation of crystals, final nano- and macro-architecture of biominerals. Comparative study of this OM thus represents a great opportunity to determine the fundamental mechanisms of biomineralization within these species. The aim of this PhD research project was to characterize OM proteins and inform on the skeletogenesis mechanisms among the Corallium genus. By comparative intra- and interspecies approaches, we have found OM proteic differences and similarities in different Corallium populations and species. Our results confirm a common growth mechanism and the possibility to use OM as a taxonomic criterion for these species. Moreover the comparison of OM proteins between the two skeletal structures in C. rubrum reveals proteins with identical biochemical characteristics and other with differences. Finally by the combinaison of these biochemical approaches with a transcriptomic one, we have characterized the first OM protein in the octocorallian species from the Mediterranean Sea, Corallium rubrumPARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

A comparison of SNARF-1 and skeletal δ¹¹B estimates of calcification media pH in tropical coral

Coral skeletal boron geochemistry offers opportunities to probe the pH of the calcification media (pHCM) of modern and fossil specimens, to estimate past changes in seawater pH and to explore the biomineralisation response to future ocean acidification. In this research we grew 2 Stylophora pistillata coral microcolonies over glass coverslips to allow analysis of the pH sensitive dye SNARF-1, in the extracellular calcification medium at the growing edge of colonies where the first aragonite crystals are formed, under both light and dark conditions. We use secondary ion mass spectrometry (SIMS) to measure the boron isotopic composition (δ11B) of the skeleton close to the growth edge after 2 to 3 days of additional calcification had enlarged the crystals until they joined, generating a continuous sheet of aragonite. Mean skeletal δ11B-pHCM estimates are higher than those of by SNARF-1 by 0.35 to 0.44 pH units. These differences either reflect real variations in the pH of the calcification media associated with each measurement technique or indicate other changes in the biomineralisation process which influence skeletal δ11B. SNARF-1 measures directly the pH of the extracellular calcification medium while skeletal δ11B analyses aragonite potentially formed via both extracellular and intracellular biomineralisation pathways. Analysis of a third coral specimen, also growing on a glass slide but with a 5 cm long branch, indicated good agreement between the δ11B value of the apex of the branch and the skeletal growth edge. The tissues overlying both these regions were transparent indicating they had low symbiont densities. This suggests that the biomineralisation process is broadly comparable between these sites and that studies growing corals over glass slides/coverslips provide representative data for the colony apex

Carbonic anhydrase in the scleractinian coral Stylophora pistillata - Characterization, localization, and role in biomineralization

Carbonic anhydrases (CA) play an important role in biomineralization from invertebrates to vertebrates. Previous experiments have investigated the role of CA in coral calcification, mainly by pharmacological approaches. This study reports the molecular cloning, sequencing, and immunolocalization of a CA isolated from the scleractinian coral Stylophora pistillata, named STPCA. Results show that STPCA is a secreted form of alpha-CA, which possesses a CA catalytic function, similar to the secreted human CAVI. We localized this enzyme at the calico-blastic ectoderm level, which is responsible for the precipitation of the skeleton. This localization supports the role of STPCA in the calcification process. In symbiotic scleractinian corals, calcification is stimulated by light, a phenomenon called "light-enhanced calcification" (LEC). The mechanism by which symbiont photosynthesis stimulates calcification is still enigmatic. We tested the hypothesis that coral genes are differentially expressed under light and dark conditions. By real-time PCR, we investigated the differential expression of STPCA to determine its role in the LEC phenomenon. Results show that the STPCA gene is expressed 2-fold more during the dark than the light. We suggest that in the dark, up-regulation of the STPCA gene represents a mechanism to cope with night acidosis

Bicarbonate transporters in corals point towards a key step in the evolution of cnidarian calcification

The bicarbonate ion (HCO3-) is involved in two major physiological processes in corals, biomineralization and photosynthesis, yet no molecular data on bicarbonate transporters are available. Here, we characterized plasma membrane-type HCO3- transporters in the scleractinian coral Stylophora pistillata. Eight solute carrier (SLC) genes were found in the genome: five homologs of mammalian-type SLC4 family members, and three of mammalian-type SLC26 family members. Using relative expression analysis and immunostaining, we analyzed the cellular distribution of these transporters and conducted phylogenetic analyses to determine the extent of conservation among cnidarian model organisms. Our data suggest that the SLC4 gamma isoform is specific to scleractinian corals and responsible for supplying HCO3- to the site of calcification. Taken together, SLC4 gamma appears to be one of the key genes for skeleton building in corals, which bears profound implications for our understanding of coral biomineralization and the evolution of scleractinian corals within cnidarians