Keywords:Phosphate, Isotope effect, Stable isotopes of oxygen, Hermatypic coral, Coastal marine ecosystem
The oxygen isotope ratio (δ18O) of dissolved orthophosphate (PO43-) has been recognized as a promising tool to evaluate the contributions of both external sources and internal recycling of phosphorus (P) to the P budget in natural aquatic ecosystems. However, coexistence of many biological processes that can significantly alter the phosphate δ18O (δ18Op) in a given system often complicates quantitative interpretation of this parameter. To use the information of δ18Op effectively in biogeochemical researches, we have to know both the magnitudes of oxygen isotope effect and the reaction kinetics of major biological processes that take part in the P cycle of the concerned ecosystem. In this study, we conducted a model incubation experiment using natural hermatypic corals to evaluate the influence of uptake process of PO43- by corals on the δ18Op. Live coral samples (Porites cylindrica, Heliopora coerulea, Acropora digitifera) were collected from coral reefs around Ishigaki Island (Okinawa) and Bolinao (northern Luzon), acclimatized in incubation aquaria for a few days, and then incubated for 3 to 5 days under natural light conditions with elevated concentrations of NO3- and PO43-. Subsamples of seawater were regularly collected and analyzed for the concentration and the δ18O of PO43-. PO43- was usually taken up by corals linearly with incubation time, and the uptake rate apparently depended on temperature. Difference in the uptake rate between coral species was not significant. The δ18Op was initially approx. 3‰ lower than the equilibrium value with regard to oxygen-isotope exchange with ambient seawater. In a few cases, the δ18Op remained unchanged during the incubation even though uptake proceeded. In the other cases, however, the δ18Op gradually increased with time, and in some cases became even higher than the equilibrium value at the end of incubation. This observation suggests that kinetic isotope fractionation rather than simple equilibration operated during the uptake of PO43- by corals and influenced the δ18Op. The magnitude of isotope effect associated with uptake seemed to depend on coral species, being the largest with A. digitifera and the smallest with H. coerulea. In natural environments where the concentration of PO43- is much lower than the incubation conditions we used, PO43- is presumably turned over much faster and the δ18Op is easily altered by corals and other major primary producers. This fact may limit the advantage of the δ18Op as an indicator of external PO43- sources.