Tissue and skeletal changes in the scleractinian coral Stylophora pistillata Esper 1797 under phosphate enrichment

Long-term phosphate enrichments (0, 0.5, and 2.5 μmol L− 1; 4 to 11 weeks) were used to assess a possible limitation in phosphorus of zooxanthellae and to complement data on the effect of phosphate enrichment on calcification and elemental composition of the tissue in the scleractinian coral Stylophora pistillata. Phosphate addition mainly affected the coral symbionts. Indeed, at 2.5 μmol L− 1 P-enriched, zooxanthellae had a greater photosynthetic efficiency, their intracellular carbon and nitrogen contents increased by 70% and their phosphorus content by 190%, while their specific growth rate increased by 18%. C:P and N:P ratios in zooxanthellae were much higher than the Redfield ratios advocated for nutrient-repleted phytoplankton, and decreased with phosphate enrichment. Collectively, these results suggest a phosphorus limitation of the zooxanthellae growth in hospite. However, the increase in zooxanthellae specific growth rate did not lead to the building of a higher symbiont density, as zooxanthellae growth just matched the tissue and skeletal growth of the enriched corals. Benefits of phosphate supplementation were thus not substantial enough to lead to the building of higher zooxanthellae density and to their balanced growth, which suggests that symbiont growth was likely limited by another nutrient as well, probably nitrogen. At the host level, there were no changes in the elemental composition or in the protein levels, while skeletal growth rate increased by 31% between unenriched and 2.5 μmol L− 1 P-enriched corals. Phosphate-enriched corals also incorporated 1.7 times more phosphorus into their skeleton than did unenriched corals. These results evidenced that zooxanthellae and the skeleton are the two accumulation sites of inorganic phosphorus within the symbiotic association.

► We examined P enrichment's impact on calcification and tissue composition in corals.
► We assessed a possible phosphorus limitation in symbiotic zooxanthellae.
► Photosynthetic efficiency, CNP contents, and specific growth of symbionts increased.
► Results indicated a phosphorus limitation of zooxanthellae growth in hospite.
► Skeletal growth rates and phosphorus incorporation into the skeleton also increased.