Variation in oxygen isotope ratio of dissolved orthophosphate induced by uptake process in natural coral holobionts
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A model incubation experiment using natural zooxanthellate corals was conducted to evaluate the influence of phosphate uptake by coral holobionts on oxygen isotope ratio of dissolved PO4 3− (δ18Op). Live coral samples of Acropora digitifera, Porites cylindrica, and Heliopora coerulea were collected from coral reefs around Ishigaki Island (Okinawa, Japan) and Bolinao (northern Luzon, Philippines) and incubated for 3–5 d after acclimatization under natural light conditions with elevated concentrations of PO4 3–. Phosphate uptake by corals behaved linearly with incubation time, with uptake rate depending on temperature. δ18Op usually increased with time toward the equilibrium value with respect to oxygen isotope exchange with ambient seawater, but sometimes became higher than equilibrium value at the end of incubation. The magnitude of the isotope effect associated with uptake depended on coral species; the greatest effect was in A. digitifera and the smallest in H. coerulea. However, it varied even within samples of a single coral species, which suggests multiple uptake processes with different isotope effects operating simultaneously with varying relative contributions in the coral holobionts used. In natural environments where concentrations of PO4 3– are much lower than those used during incubation, PO4 3– is presumably turned over much faster and the δ18Op easily altered by corals and other major primary producers. This should be taken into consideration when using δ18Op as an indicator of external PO4 3– sources in coastal ecosystems.
KeywordsCorals Nutrient uptake Phosphate Oxygen isotopes Fractionation
This research was conducted under the Integrated Coastal Ecosystems Conservation and Adaptive Management under Local and Global Environmental Impacts in the Philippines (CECAM) Project funded by the Japan International Cooperation Agency (JICA) and Japan Science and Technology Agency under the Science and Technology Research Partnership for Sustainable Development Program. This study was financially supported by JSPS Grants-in-Aid for Scientific Research Grant Numbers 23405002 and 25550010. We thank the Bolinao Marine Laboratory of the University of the Philippines (UP) Marine Science Institute and the Seikai National Fisheries Research Institute for providing the research facilities necessary for aquarium incubations. Coral samples were collected and imported under the permission of the Philippine and Japan Governments and local government units (Bolinao and Okinawa Prefecture). We are sincerely grateful to the UP staff and researchers for extending help during the study and to the JICA Philippines office and the CECAM coordinating office for all logistical arrangements. We also acknowledge the three anonymous reviewers whose comments and suggestions greatly improved the manuscript.
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