Long-term effects of CeO2 NPs on the biological phosphorus removal mechanism of DPR-AGS in A/O/A SBRs

The increasing use of cerium dioxide nanoparticles (CeO2 NPs) has raised concerns about their potential environmental toxicity. Denitrifying phosphorus removal-aerobic granular sludge (DPR-AGS) is a distinct form of microbial aggregates. In this study, the phosphorus removal efficiencies, the metabolic intermediates of polyhydroxyalkanoates (PHA) and glycogen, the enzyme activities of exophosphatase (PPX) and polyphosphate kinase (PPK), and the microbial community of DPR-AGS under long-term exposure to CeO2 NPs (0 (the control), 1, and 10 mg L−1) in anaerobic/oxic/anoxic (A/O/A) sequencing batch reactors (SBRs) were investigated. The results showed that chronic toxicity caused by 10 mg L−1 CeO2 NPs resulted in increased reactive oxygen species (ROS) production and lactate dehydrogenase (LDH) release, which reached 176.32% and 237.35%, respectively, when compared with the control (100%). High-throughput sequencing showed that denitrifying-phosphorus-accumulating organisms (DPAOs) were more sensitive to CeO2 NPs than phosphorus-accumulating organisms (PAOs), which was in accordance with phosphorus transformations during one cycle. However, the abundance of Dechloromonas and Defluviicoccus increased from 4.68% and 3.08% (R0) to 9.95% and 7.06% (R10), respectively, indicating that the tolerance of glycogen-accumulating organisms to CeO2 NPs was stronger than that of PAOs, which was consistent with the transformation of the metabolic pathway from polyphosphate accumulation (PAM) to glycogen accumulation (GAM).