Microplastics enhance the denitrification of glycogen-accumulating organisms by regulating electronic transport in carbon-nitrogen coupling

The increasing presence of microplastics (MPs) in wastewater treatment systems profoundly impacts microbial metabolism and process performance. However, the effects of MPs on the denitrification process of glycogen-accumulating organisms (GAOs) remain unclear. Herein, various types and concentrations of MPs were introduced into the activate sludge of GAOs to assess their impact on denitrification processes and to investigate the underlying mechanisms. Our findings revealed that adding 100 μm PVC increased the denitrification efficiency of GAOs by 14.6 %, whereas adding 100 nm PVC decreased efficiency by 8.4 %. Additionally, 100 nm PVC inhibited polyhydroxybutyrate (PHB) degradation, while 100 μm PVC promoted it. Furthermore, 100 nm and 100 μm PVC differently influenced metabolic functions, including reactive oxygen species (ROS) levels, electron transport chain (ETC) activity, and intracellular nicotinamide adenine dinucleotide (NADH) content. Metatranscriptome analyses revealed differential expression of genes such as phaC, CS, nuoL, CYC1, and nisK, which are involved in carbon-nitrogen metabolism and oxidative phosphorylation. Consequently, 100 μm PVC enhanced the denitrification rate in GAOs by promoting PHB decomposition, increasing NADH electron-donating capacity, and ultimately enhancing the denitrification rate of GAOs. Our findings reveal a novel mechanism on regulating the carbon-nitrogen coupling in activated sludge under the different particle size of MPs.