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Reef-building corals form symbioses with dinoflagellates from the diverse genus Symbiodinium. This symbiotic association has developed adaptations to acquire and share nutrients, which are essential for its survival and growth in nutrient-poor tropical waters. The host is thus able to prey on a wide range of organic food sources (heterotrophic nutrition) whereas the symbionts acquire most of the inorganic nutrients (autotrophic nutrition). However, nutrient fluxes between the two partners remain unclear, especially concerning heterotrophically acquired carbon and nitrogen. We combined physiological measurements and pulse-chase isotopic labeling of heterotrophic carbon and nitrogen, as well as autotrophic carbon to track nutrient fluxes in two coral species, Stylophora pistillata and Turbinana reniformis, in symbiosis with Symbiodinium clades A, and C, D respectively. We showed a rapid acquisition, exchange and a longterm retention of heterotrophic nutrients within the symbiosis, whereas autotrophic nutrients were rapidly used to meet immediate metabolic needs. In addition, there was a higher retention of heterotrophic nitrogen compared with carbon, in agreement with the idea that tropical corals are nitrogen-limited. Finally, a coupling between auto- and heterotrophy was observed in the species S. pistillata, with a higher acquisition and retention of heterotrophic nutrients under low irradiance to compensate for a 50% reduction in autotrophic nutrient acquisition and translocation. Conversely, T. reniformis conserved an equivalent heterotrophic nutrient acquisition at both light levels because this coral species did not significantly reduce its rates of gross photosynthesis and autotrophic carbon acquisition between the two irradiances. These experiments advance the current understanding of the nutrient exchanges between the two partners of a symbiotic association, providing evidence of the complexity of the host-symbiont relationship. [ABSTRACT FROM AUTHOR]

Corals live in symbiosis with dinoflagellates of the genus Symbiodinum. These dinoflagellates translocate a large part of the photosynthetically fixed carbon to the host, which in turn uses it for its own needs. Assessing the carbon budget in coral tissue is a central question in reef studies that still vexes ecophysiologists. The amount of carbon fixed by the symbiotic association can be determined by measuring the rate of photosynthesis, but the amount of carbon translocated by the symbionts to the host and the fate of this carbon are more difficult to assess. In the present study, we propose a novel approach to calculate the budget of autotrophic carbon in the tissue of scleractinian corals, based on a new model and measurements made with the stable isotope 13C. Colonies of the scleractinian coral Stylophora pistillata were incubated in H13CO -3-enriched seawater, after which the fate of 13C was followed in the symbionts, the coral tissue and the released particulate organic carbon (i.e. mucus). Results obtained showed that after 15 min, ca. 60% of the carbon fixed was already translocated to the host, and after 48 h, this value reached 78%. However, ca. 48% of the photosynthetically fixed carbon was respired by the symbiotic association, and 28% was released as dissolved organic carbon. This is different from other coral species, where <1% of the total organic carbon released is from newly fixed carbon. Only 23% of the initially fixed carbon was retained in the symbionts and coral tissue after 48 h. Results show that our 13C-based model could successfully trace the carbon flow from the symbionts to the host, and the photosynthetically acquired carbon lost from the symbiotic association. [ABSTRACT FROM AUTHOR]

Several untested aspects of the regulation of inorganic nutrient uptake were examined using nutrient depletion experiments with the symbiotic coral Stylophora pistillata. The total inhibition of phosphate uptake in artificial seawater lacking sodium indicates the involvement of a sodium/phosphate symporter for the uptake of phosphate across host membranes. Addition of ammonium or nitrate (up to 6.0 μmol l-1) did not enhance saturated phosphate uptake rates, thus indicating that corals, or their symbiotic algae, were not, or not sufficiently, nitrogen limited to modify their phosphate needs. Conversely, the saturated uptake rate of ammonium increased by 2.5-fold in the presence of 3.0 μmol l-1 of phosphate, thus indicating that the corals or their symbionts were lacking intracellular phosphate to take advantage of the inorganic nitrogen compounds dissolved in their surrounding medium. Overall, these results highlight some greater limitation in phosphate rather than in nitrogen. Finally, the rate of phosphate uptake decreased with particulate feeding of the host (organic phosphate source). Indeed, corals that were fed 1 and 3 days before the uptake experiment took up phosphate 42 and 19% slower, respectively, than corals that were fed 21 days before. This result provides additional evidence of phosphate limitation in S. pistillata. This study therefore brings new insights into the relationships between nutrients and symbiotic corals, and may provide a rapid and effective tool to investigate which nutrient is the most limiting for coral metabolism. [ABSTRACT FROM AUTHOR]

This study was designed to assess the importance of dissolved free amino acids (DFAA) as a nitrogen source for the scieractinian coral Stylophora pistillata. For this purpose, experiments were performed using 15N-enriched DFAAs, and %15N enrichment was measured both in animal tissue and zooxanthellae at different DFAA concentrations, incubation time and light levels. As previously observed for urea, which is another source of organic nitrogen, DFAA uptake exhibited a biphasic mode consisting of an active carrier-mediated transport for concentrations below 3 μmol-1 and a linear uptake for higher concentrations. The value of the carrier affinity (Km=1.23 μmol l-1 DFAA) indicated good adaptation of the corals to the low levels of DFAA concentrations measured in most oligotrophic waters. DFAA uptake was also correlated with light. The DFAA contribution to the nitrogen requirements for tissue growth was compared to the contribution of ammonia, nitrate and urea, for which uptake was also measured in S. pistillata. Inorganic sources (NH4+ and NO3-) contributed 75% of the daily nitrogen needs against 24% for organic sources. Taken altogether, dissolved organic and inorganic nitrogen can supply almost 100% of the nitrogen needs for tissue growth. [ABSTRACT FROM AUTHOR]

Scleractinian corals are essential constituents of tropical reef ecological diversity. They live in close association with diazotrophs [dinitrogen (N2)-fixing microbes], which can fix high rates of N2. Whether corals benefit from this extrinsic nitrogen source is still under debate. Until now, N2 fixation rates have been indirectly estimated using the acetylene reduction assay, which does not permit assessment of the amount of nitrogen incorporated into the different compartments of the coral holobiont. In the present study, the 15N2 technique was applied for the first time on three Red Sea coral species. Significant 15N enrichment was measured in particles released by corals to the surrounding seawater. N2 fixation rates were species specific and as high as 1.6-2 ng N day-1 l-1. However, no significant enrichment was measured in the symbiotic dinoflagellates or the coral host tissues, suggesting that corals do not benefit from diazotrophic N2 fixation. [ABSTRACT FROM AUTHOR]