Scleractinian coral morphogenesis

Metamorphosis is a crucial step in the life cycle of a scleractinian coral: The swimming coral larva, the planula, settles, and metamorphoses into a sessile calcifying primary polyp, which subsequently grows into an adult colony. Importantly, morphogenesis into a primary polyp involves the establishment of the skeletogenic cell layer, the so-called calicodermis, but the cellular mechanisms that support this process are not well understood. In the work presented here, calicodermis establishment was studied in two scleractinian corals, Pocillopora damicornis and Stylophora pistillata using cell proliferation and apoptosis assays, electron microscopy, molecular biology tools, and isotopic imaging using NanoSIMS. Cell proliferation during coral metamorphosis was evaluated with BrdU (5-bromo-2â-deoxyuridine) incuba-tion pulses (24 h). In the primary polyp stage, cell migration was observed in the chase phase for several days (48 to 64 h) after the incubation pulse. A number of key observations were made: 1) Right after settlement, during the earliest metamorphosis stage that includes initial skeletal formation and therefore requires a fully functional calicodermis, cell proliferation rate in this tissue layer is in fact not significantly higher than in the preceding planula larval stage. This indicates that calicodermis establishment is primarily happening through transdifferentiation of cells from the aboral pseudostratified epithelium of the planula into calicodermis cells. 2) Later, in the growing primary polyp local cell proliferation in the calicodermis remains low, indicating that these cells do not originate in the calicodermis itself. 3) At the same time, it is observed in S. pistillata that the pharynx is the most proliferative area with up to 19% of cells dividing, and that cells migrate from this area through pseudostratified epithelium and/or mesenterial filaments to reach the expanding calicodermis, which is actively forming skeleton at this stage. 4) In order to identify potential stem cells and calicodermis precursor cells locations, biological markers where developed. Potential stem cells expressing piwi genes were localized in the mesenteries and below tentacle tips of P. damicornis. These sites contain proliferative areas involved in gametogenesis and nematogenesis, respectively. 5) Pdcyst-rich is a protein localizedin the adult calicodermis and potentially involved in skeleton formation. The related gene was observed to be expressed at the early metamorphosis stage, when skeleton deposition commences. Preliminary results indicate that Pdcyst-rich proteins are also located in the calicodermis of the forming polyp and skeleton of the primary polyp stage. This suggests a role of Pdcyst-rich in skeletogenesis but it is not clear if it acts as a protein of the skeletal organic matrix or as an adhesion protein.

Physiological and isotopic responses of scleractinian corals to ocean acidification

Maoz Fine, Anders Meibom

Uptake of anthropogenic CO2 by the oceans is altering seawater chemistry with potentially serious consequences for coral reef ecosystems due to the reduction of seawater pH and aragonite saturation state (Omega(arag)) The objectives of this long-term study were to investigate the viability of two ecologically important reef-building coral species, massive Ponies sp. and Stylophora pistillata, exposed to high pCO(2) (or low pH) conditions and to observe possible changes in physiologically related parameters as well as skeletal isotopic composition. Fragments of Ponies sp. and S. pistillata were kept for 6-14 months under controlled aquarium conditions characterized by normal and elevated pCO(2) conditions, corresponding to pH(T) values of 8.09, 7.49, and 7.19, respectively. In contrast with shorter, and therefore more transient experiments, the long experimental time-scale achieved in this study ensures complete equilibration and steady state with the experimental environment and guarantees that the data provide insights into viable and stably growing corals. During the experiments, all coral fragments survived and added new skeleton, even at seawater Omega(arag) < 1, implying that the coral skeleton is formed by mechanisms under strong biological control. Measurements of boron (B), carbon (C), and oxygen (O) isotopic composition of skeleton, C isotopic composition of coral tissue and symbiont zooxanthellae, along with physiological data (such as skeletal growth, tissue biomass, zooxanthellae cell density, and chlorophyll concentration) allow for a direct comparison with corals living under normal conditions and sampled simultaneously. Skeletal growth and zooxanthellae density were found to decrease, whereas coral tissue biomass (measured as protein concentration) and zooxanthellae chlorophyll concentrations increased under high pCO(2) (low pH) conditions. Both species showed similar trends of delta B-11 depletion and delta O-18 enrichment under reduced pH, whereas the delta C-13 results imply species-specific metabolic response to high pCO(2) conditions. The skeletal delta B-11 values plot above seawater delta B-11 vs. pH borate fractionation curves calculated using either the theoretically derived alpha(B) value of 1.0194 (Kakihana et al. (1977) Bull. Chem. Soc. Jpn. 50, 158) or the empirical alpha(B) value of 1.0272 (Klochko et al. (2006) EPSL 248, 261). However, the effective alpha(B) must be greater than 1.0200 in order to yield calculated coral skeletal delta B-11 values for pH conditions where Omega(arag) >= 1. The delta B-11 vs. pH offset from the seawater delta B-11 vs. pH fractionation curves suggests a change in the ratio of skeletal material laid down during dark and light calcification and/or an internal pH regulation, presumably controlled by ion-transport enzymes. Finally, seawater pH significantly influences skeletal delta C-13 and delta O-18. This must be taken into consideration when reconstructing paleo-environmental conditions from coral skeletons. (C) 2010 Elsevier Ltd. All rights reserved.

2010

Rapid Recruitment of Symbiotic Algae into Developing Scleractinian Coral Tissues

Thomas Bockel

While the early acquisition of Symbiodiniaceae algae into coral host tissues has been extensively studied, the dynamics of the migration of algal cells into rapidly expanding coral tissues still lacks a systematic study. This work examined two Red Sea branching coral species, Pocillopora damicornis and Stylophora pistillata, as they were growing and expanding their tissue laterally on glass slides (January-June, 2014; 450 assays; five colonies/species). We measured lateral tissue expansion rates and intratissue dinoflagellate migration rates. Tissue growth rates significantly differed between the two species (with Stylophora faster than Pocillopora), but not between genotypes within a species. Using a "flow-through coral chamber" under the microscope, the migration of dinoflagellates towards the peripheral edges of the expanding coral tissue was quantified. On a five-day timescale, the density of the endosymbiotic dinoflagellate cells, presenting within a 90 mu m region of expanding coral tissue (outer edge), increased by a factor of 23.6 for Pocillopora (from 1.2 x 10(4) cells cm(-2) to 2.4 x 10(5) cells cm(-2)) and by a factor of 6.8 for Stylophora (from 3.6 x 10(4) cells cm(-2) to 2.4 x 10(5) cells cm(-2)). The infection rates were fast (5.2 x 10(4) and 4.1 x 10(4) algal cells day-1 cm(-2), respectively), further providing evidence of an as yet unknown pathway of algal movement within coral host tissues.

2019

Effect of environmental conditions and skeletal ultrastructure on the Li isotopic composition of scleractinian corals

Anders Meibom

The lithium isotope compositions (Li-7/Li-6) and Li/Ca ratios of shallow-water and deep-sea corals (Porites lutea, Cladocora caespitosa, Lophelia pertusa and Desmophyllum cristagalli) were measured using a Cameca ims 1270 ion microprobe. The two C. caespitosa samples were grown under controlled conditions at CO2 partial pressures (pCO(2)) of 416 +/- 29 mu atm and 729 +/- 30 mu atm, respectively. In situ analyses show that all samples are isotopically homogeneous (within analytical precision, +/- 1.1 parts per thousand, 1 sigma) and display significantly lower delta Li-7 values relative to seawater. indicating a significant isotope fractionation during aragonite formation. In contrast to all other elements analysed so far, there is no relationship between the Li isotopic compositions and the skeletal ultrastructure. However, Li/Ca does show variation correlated with ultrastructure, albeit with significant differences between species. This implies that the bio mineralization mechanisms, which are supposed to be different for the different skeletal components, do not influence the Li isotopic composition in corals. In particular, the model of Rayleigh fractionation in a semi-enclosed calcifying fluid is incompatible with the homogeneity of the Li isotope compositions at the micrometer scale We also. show that changes in pCO(2) (and pH) do not significantly affect the Li isotope signature. Nevertheless, a small but significant and systematic difference in Li isotopic composition is observed between deep-sea azooxanthellate and shallow-water zooxanthellate corals. The lack of dependence on pH and pCO(2) and on skeletal ultrastructure indicates that the U isotopic signature of corals could be used as a proxy for reconstructing the paleo-delta Li-7 of seawater and, potentially, for deconvolving past continental weathering rates. (C) 2009 Elsevier B.V. All rights reserved.