Unraveling the dynamics of organic phosphorus and its role in internal phosphorus feedback during algal blooms senescence.

Algal blooms (ABs) intensify the internal transformation of phosphorus, yet limited understanding of sedimentary organic phosphorus (SPo) and algal Po (APo) has constrained insight into phosphorus feedback in eutrophication. In this study, we comprehensively characterized SPo and APo compositions during an incubation experiment using solution 31P nuclear magnetic resonance (31P-NMR), Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and complementary techniques. A marked decrease in bioavailable orthophosphate monoester and diester in APo (1223.7 mg/kg, 25.3%), contrasted with minimal variation in SPo (22.7 mg/kg, 1%), highlighted the high biodegradability of algal-derived compounds. Moreover, FT-ICR-MS results showed that NaHCO3-extracted one-phosphorus atom (NaHCO3-1P) exhibited a higher degradation rate than NaOH-extracted 1P (NaOH-1P) with both fractions being dominated by aromatic compounds (condensed aromatic and tannin-like). In contrast, aliphatic compounds (lipid-like and protein-like) were primarily generated both in NaHCO3- two-phosphorus atoms (NaHCO₃-2P) and NaOH-2P compounds. The net loss of 100 molecular labile boundary (MLB) units in NaHCO3-1P, coupled with a net gain of 54 MLB units in NaHCO3-2P points to the synthesis of 2P-bearing compounds via phosphorylation or pyrophosphorylation reactions. These changes were accompanied by decrease in the weighted-average modified aromaticity index (AI mod, wa), double bond equivalence (DBEwa) and nominal oxidation state of carbon (NOSCwa). The transformation dynamics of APo and SPo by microbial activities (enzymatic hydrolysis and community succession) increased dissolved Po (DPo) levels and enhanced the fluorescence and autochthonous character of dissolved organic matter in the overlying water. Overall, ABs enhanced internal organic phosphorus transformation, with 17.8% of APo partitioning into 13.3% DPo and 4.5% SPo, indicating a positive internal phosphorus feedback mechanism in lakes. This study elucidates the internal phosphorus cycling driven by frequent ABs and provides a basis for targeted eutrophication mitigation.