High phosphate uptake requirements of the scleractinian coral Stylophora pistillata

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.