Deciphering the Impact of ZnO Nanoparticles and a Sunscreen Product Containing ZnO on Phosphorus Dynamics and Release in Chlorella pyrenoidosa in Aquatic Systems

Zinc oxide nanoparticles (ZnO NPs) expedite the conversion of organic phosphorus (OP) into PO4–P (Pi), facilitating phosphorus (P) absorption by algae. Our study explored the mechanisms of converting OP (2-aminoethylphosphonic acid (AEP) and β-glycerol phosphate (β-GP)) into Pi in Chlorella pyrenoidosa under P deficiency with sunscreen and ZnO NPs. Cell density followed the order of K2HPO4 > β-GP+ZnO > β-GP > AEP+ZnO > AEP > P-free. ZnO NPs promoted the conversion of β-GP, containing C–O–P bonds (0.028–0.041 mg/L), into Pi more efficiently than AEP, which possesses C–P bonds (0.022–0.037 mg/L). Transcriptomics revealed Pi transport/metabolism (phoB (3.99–12.01 fold), phoR (2.20–5.50 fold), ppa (4.49–10.40 fold), and ppk (2.50–5.40 fold)) and phospholipid metabolism (SQD1 (1.85–2.79 fold), SQD2 (2.60–6.53 fold), MGD (2.13–3.21 fold), and DGD (4.08–7.56 fold)) were up-regulated compared to K2HPO4. 31P nuclear magnetic resonance spectroscopy identified intracellular P as polyphosphate, orthophosphate, and pyrophosphate. Synchrotron radiation-based X-ray near-edge structure spectroscopy indicated that K2HPO4 and Zn3(PO4)2 in β-GP+ZnO were increased by 8.09% and 7.28% compared to AEP+ZnO, suggesting superior P storage in β-GP+ZnO. Overall, ZnO NPs improved photoinduced electron–hole pair separation and charge separation efficiency and amplified the ·OH and ·O2– levels, promoting OP photoconversion into Pi and algae growth.