Rapid Recruitment of Symbiotic Algae into Developing Scleractinian Coral Tissues

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.

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

Differential nitric oxide synthesis and host apoptotic events correlate with bleaching susceptibility in reef corals

Thomas Krüger

Coral bleaching poses a threat to coral reefs worldwide. As a consequence of the temperature-induced breakdown in coral–dinoflagellate symbiosis, bleaching can have extensive effects on reef communities. However, our understanding of bleaching at a cellular level is limited, and this is particularly true regarding differential susceptibility among coral species. Recent work suggests that bleaching may represent a host innate immune-like response to symbiont dysfunction that involves synthesis of the signalling compound nitric oxide (NO) and the induction of host apoptotic-like cell death. In this study, we examined the activity of apoptosis-regulating enzymes alongside oxidised NO accumulation (a proxy for NO synthesis) in the reef corals Acropora millepora, Montipora digitata, and Pocillopora damicornis during experimental thermal stress. P. damicornis was the most sensitive species, suffering mortality (tissue sloughing) after 5 days at 33 °C but non-lethal bleaching after 9 days at 31.5 °C. A. millepora bleached at 33 °C but remained structurally intact, while M. digitata showed little evidence of bleaching. P. damicornis and A. millepora both exhibited evidence of temperature-induced NO synthesis and, after 5 days of heating, levels of oxidised NO in both species were fivefold higher than in controls maintained at 28.5 °C. These responses preceded bleaching by a number of days and may have occurred before symbiont dysfunction (measured as chlorophyll a degradation and oxidised NO accumulation). In A. millepora, apparent NO synthesis correlated with the induction of host apoptotic-like pathways, while in P. damicornis, the upregulation of apoptotic pathways occurred later. No evidence of elevated NO production or apoptosis was observed in M. digitata at 33 °C and baseline activity of apoptosis-regulating enzymes was negligible in this species. These findings provide important physiological data in the context of the responses of corals to global change and suggest that early events in the host may be important in the collapse of the coral–dinoflagellate symbiosis.

Springer Verlag2014

Scleractinian coral morphogenesis

Agathe Eliette Marie Lecointe

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.

EPFL2016