Bacterial community and molecular ecological network in response to Cr₂O₃ nanoparticles in activated sludge system

The potential environmental risks of chromium oxide nanoparticles (Cr₂O₃ NPs) have caused great concerns. However, their possible impacts on activated sludge process are very limited. In this study, we carried out long-term exposure experiments to evaluate the influence of Cr₂O₃ NPs on wastewater nutrient removal, bacterial community and molecular ecological network (MEN) in the sequencing batch reactor (SBR). It was found that 1 mg/L Cr₂O₃ NPs increased the effluent concentrations of NO₃^--N and TP from 6.5 to 15.4 mg/L, and 0.6-2.7 mg/L, indicating the serious deterioration of denitrification and phosphorus removal. Cr₂O₃ NPs significantly decreased the bacterial richness in terms of the number of different OTUs (626 OTUs in Cr₂O₃ samples and 728 OTUs in controls). Detrended correspondence analysis (DCA) showed that the overall taxonomic structure of bacterial community was altered at Cr₂O₃ NPs in activated sludge systems. Further analysis revealed that three genera related to denitrification (Desulfovibrio, Pseudomonas and Hyphomicrobium) and two genera related to phosphorus removal (Accumulibacter and Rhodobacter) decreased significantly, which was consistent with the observed influences of Cr₂O₃ NPs on denitrification and phosphorus removal. MEN analysis showed that the overall architecture of the network under Cr₂O₃ NPs was substantially alerted. β-Proteobacteria, playing an important role in nutrients removal, had less complex interactions in the presence of Cr₂O₃ NPs, which may be associated with the deterioration of denitrification and phosphorus removal. This study provides insights into our understanding of shifts in the bacteria community and their MEN under Cr₂O₃ NPs in activated sludge systems.