Factors controlling coral P/Ca nutrient proxy: Insights from Porites culture experiments and synchrotron-based X-ray spectroscopy

Coral skeletal P/Ca ratio has been developed as an indicator of temporal seawater dissolved inorganic phosphorus (DIP). The use of coral P/Ca proxy helps to assess oceanographic and climatic impacts such as upwelling, circulation, and continent runoffs on marine biogeochemical cycles. However, factors controlling skeletal P incorporation and elemental partitioning between seawater and coral skeletons remain elusive. We conducted temperature-controlled (similar to 21 to 29 degrees C) aquaria culture experiments using two colonies of Porites australiensis corals (here refer to B and C) with the only difference in zooxanthellae density (B > C). The coral growth rate ranges from 9.4 to 19.4 mg/day (B) and 0.7 to 14.1 mg/day (C). Only the growth rate of colony C significantly correlates to temperature, potentially reflecting physiological controls on the two colonies given the difference in the zooxanthellae density. We measured coral P/Ca ratios by Inductively coupled plasma mass spectrometry and determined skeletal P speciation through a synchrotron-based spectroscopic approach. Coral P/Ca ratio ranges from 6.5 to 18.6 mu mol/mol (B) and 7.2 to 19.8 mu mol/mol (C). The dominance of organic-P is confirmed, and the presence of inorganic-P cannot be excluded. Only colony C has a strong P/Ca dependence on temperature and both colonies show strong correlations between P/Ca and growth rate. Although growth rate and temperature are intercorrelated, the growth rate is more likely the direct controlling factor on coral P/Ca in our experiments. Combined laboratory data with field observations, we suggest that the validity of the Porites P/Ca proxy may be influenced by seawater DIP, coral species, and growth rate.

Automated summary

The coral skeletal P/Ca ratio can be used as an indicator of temporal seawater dissolved inorganic phosphorus (DIP), allowing for assessments of oceanographic and climatic impacts on marine biogeochemical cycles. Factors influencing P incorporation in coral skeletons and the partitioning of elements between seawater and coral skeletons remain unclear. In temperature-controlled aquarium culture experiments, it was found that the growth rate of corals may be the direct controlling factor on coral P/Ca ratio.