Intracellular phosphorus-pool reorganization and contrasting physiological tolerance in bloom-forming phytoplankton under coastal phosphate depletion.

Coastal eutrophication can drive phosphate depletion during late-stage blooms, yet relationships between intracellular phosphorus (P) responses and physiological tolerance in bloom-forming phytoplankton remain poorly resolved. We conducted controlled phosphate-depletion experiments with the dinoflagellate Karenia mikimotoi and the diatom Skeletonema marinoi; bloom-associated particulate biomass from a Prorocentrum shikokuense bloom in the East China Sea provided ecological context. Both taxa showed significant decreases in cellular P quota under phosphate-depleted (Low-P) conditions. At the shared day-8 sampling point after dissolved inorganic phosphorus (DIP) depletion in Low-P cultures, NaOH-EDTA-extractable total cellular P was 43.4% lower in K. mikimotoi and 67.9% lower in S. marinoi than in phosphate-replete (High-P) cultures. After prolonged phosphate depletion (day 14), this pool in K. mikimotoi was 56.3% lower under Low-P than under High-P. Pooled solution 31P NMR profiles indicated lower concentration-corrected estimates for the orthophosphate region and the operationally combined pyrophosphate/polyphosphate-related region (Total polyP), together with higher relative contributions of organic P regions under Low-P. K. mikimotoi maintained comparatively stable photochemical efficiency (Fv/Fm) during prolonged Low-P exposure and showed continued changes in extractable P-pool composition, whereas S. marinoi exhibited marked photophysiological decline by day 8. Field samples from two stations differing in ambient DIP contained heterogeneous extractable P spectral regions but did not reproduce the laboratory relative organic-P enrichment. The results show that phosphate depletion contracts and reorganizes extractable intracellular P pools and support an association between P-pool management and species-specific physiological tolerance. This species-level contrast generates a testable hypothesis concerning differences in P-use strategies among bloom-forming functional types.