Physiological and isotopic responses of scleractinian corals to ocean acidification

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.

Coral photosymbiosis: Linking phylogenetic identity to single cell metabolic activity in mixed symbiont populations

Isabelle Domart-Coulon, Maoz Fine, Béatrice Gaume, Anders Meibom

Tropical reef-building corals live in symbiosis with a wide range of micro-organisms, including unicellular Symbiodinium dinoflagellates also called zooxanthellae. These photosynthetic endosymbionts live inside the coral gastroderm cells. Although corals can feed on planktonic preys, the dinoflagellates significantly contribute to the nutrition of their host by transferring a large fraction (up to 90%) of photosynthates that are produced through the fixation of dissolved inorganic carbon (DIC) and nitrogen (nitrate or ammonium). Nine major clades (A-I) of Symbiodinium have been identified by molecular genetic analyses. Each clade presents distinct physiological features and the specific association between coral species and Symbiodinium clade determines the phenotype of the holobiont. Differences in the photosynthetic response to irradiance, rates of carbon fixation, and thermal tolerance can be attributed to symbiont clade. Several Symbiodinium clades can simultaneously exist within a single coral and the host can dynamically modify the proportion between dominant and background clades to adapt to changing environmental conditions. Investigating at the cellular level the metabolic exchanges between coral and Symbiodinium is of great interest to understand e.g. the bleaching process (loss of zooxanthellae) which often leads to coral death. We are aiming at quantitatively image the differential metabolic activity between symbiont clades in the same host, in the intact symbiosis. Previous studies have used mass bulk techniques to investigate the metabolism of symbionts at the colony scale. However, such studies cannot determine the specific contribution of the individual cells, as a function of their distribution in the coral host tissue. We have developed a SIMS-ISH method combining nanoscale secondary ion mass spectrometry (NanoSIMS) and in situ hybridization (ISH) for the simultaneous in situ identification of Symbiodinium genotype, and visualization of symbiont-host metabolic exchange at the level of individual cell. We focus on two reef-building coral species Pocillopora damicornis and Stylophora pistillata for which a large amount of complementary metabolic data exists. We designed specific fluorescent DNA probes to identify clade C Symbiodinium in P. damicornis and clade A in S. pistillata, and in mixed cultures. We combined the probes with pulse-chase experiments using isotopically labeled seawater (13C-bicarbonate and 15N-nitrate) to attribute a particular metabolism to a specific clade. This combined method enables us to phylogenetically identify metabolically active cells from a NanoSIMS isotopic/elemental image. The precise correlation between TEM and the NanoSIMS isotopic maps allows us to follow the turnover and translocation of metabolites with sub-cellular precision in both the symbionts and the host. Due to the complex nature of the coral symbiosis, the ability to discriminate the phylogenetic identity and metabolic role of specific Symbiodinium populations in situ is crucial to understand the effects of environmental stress on the coral holobiont plasticity. Moreover, analyzing symbiotic associations in situ provides a unique insight into the spatio-temporal patterns of metabolic interactions in holobionts. This analytical breakthrough promises to open entirely new areas of research focused on understanding the dynamics of interactions between animals and the microbial world.

2017

Reproductive and trans-generational effect of ocean acidification and warming on the coral Stylophora pistillata in the Gulf of Aqaba

Malika Menoud

Global warming is threatening 75 % of the world’s coral reefs. The reproduction of corals is a driver for the development of the whole reef ecosystem. Then, it is essential to better understand the transgenerational mechanisms in the response of parents and offspring to elevated temperature and lowered pH. Colonies of Stylophora pistillata from the Gulf of Aqaba during their reproduction period were exposed to a 4°C increase in temperature and a pH of 7.6 for 36 days, then a 6°C increase for six days. Planulae were counted on seven consecutive nights, two times during the experiment period. Physiological characteristics of adult and planulae were assessed on four and five sampling points respectively, as well as the behaviour of the planulae through their incubation. Results show no effect of OWA on the reproduction, parents, and planulae physiology. They suggest that the natural resistance of corals in the Gulf of Aqaba is transmitted from parent to offspring. Data on planulae quantity, survival, settlement, and metabolism provides additional and useful information to understand the biology of this coral, specially in its early-life stage. This study’s outcome is adding evidences of the future development of corals reefs in this region, unlike several other tropical reefs in the world.

2017

Autotrophic and heterotrophic nutrient fluxes in the dinoflagellate-coral symbiosis – A NanoSIMS perspective

Julia Marie Jeanne Yvonne Bodin, Isabelle Domart-Coulon, Maoz Fine, Thomas Krüger, Anders Meibom

The exchange of metabolites between photosynthetic symbionts and their coral host in shallow-water scleractinians provides the nutritional basis for their successful evolution and the existence of massive coral reefs in oligotrophic tropical waters. Autotrophic nutrition, based on carbon- and nitrogen-rich photosynthetic metabolites, is complemented by heterotrophic uptake and assimilation of organic matter to obtain additional carbon, nitrogen and phosphorus. By combining TEM ultrastructural observations with quantitative NanoSIMS isotopic imaging of tissue sections at sub-cellular resolution, we have compared the exchange of nutrients from the autotrophic assimilation of 15N-nitrate and 13C-bicarbonate with the heterotrophic uptake from 15N- and 13C-labelled brine shrimps in Stylophora pistillata from the Red Sea. Our results confirm that heterotrophy serves as the dominant source of nitrogen for the coral holobiont. On a time scale of 6 hours, heterotrophy provided approximately nine times more nitrogen to the host gastrodermal anabolism than symbiont nitrate fixation and translocation. Heterotrophic nitrogen was also used in anabolic processes in the symbionts, but at a level about 40% lower compared with nitrogen incorporation from nitrate fixation. The role of photosynthesizing symbionts the major contributors of carbon to the coral host was evident from the observation that autotrophic carbon enrichment in the gastrodermal layer was seven times higher in comparison to the heterotrophic carbon acquisition. Our observations directly demonstrate the complementing roles of heterotrophic and autotrophic food acquisition and support the notion that recycling of nutrients in symbiotic coral holobionts is critical to their vitality. The effects of stress from environmental change, including increasing water temperature and ocean acidification, are under investigation.

2015

Coral photosymbiosis: Linking phylogenetic identity to single cell metabolic activity in mixed symbiont populations

Isabelle Domart-Coulon, Maoz Fine, Béatrice Gaume, Anders Meibom

2017

Autotrophic and heterotrophic nutrient fluxes in the dinoflagellate-coral symbiosis – A NanoSIMS perspective

Julia Marie Jeanne Yvonne Bodin, Isabelle Domart-Coulon, Maoz Fine, Thomas Krüger, Anders Meibom

2015