Effects of unmodified and amine-functionalized polystyrene nanoplastics on nitrogen removal by Pseudomonas stutzeri: strain characteristics, extracellular polymers, and transcriptomics†

Nanoplastics (NPs) are emerging pollutants that widely exist in wastewater treatment systems, but their potential effects and mechanism of action on aerobic denitrifying bacteria remain unclear. This study investigated the effects of different concentrations (1, 10, 20, 50, 100 mg L⁻¹) of polystyrene nanoplastics with/without amino group (PS NPs and PS-NH₂ NPs) on a typical aerobic denitrifier, Pseudomonas stutzeri (P. stutzeri). The results indicated that NPs were abundantly adsorbed on the surface of P. stutzeri, which have hindered oxygen transfer and favored denitrification. The expression of genes related to nitrogen metabolism were both downregulated under the stress of NPs. However, PS NPs promoted the nitrogen removal performance by enhancing energy metabolism and biosynthesis, notably through the upregulation of the tricarboxylic acid (TCA) cycle and ribosome pathways. In contrast, PS-NH₂ NPs impaired the sdhC involved in the TCA cycle, leading to an obstruction of cellular energy metabolism. Additionally, PS-NH₂ NPs triggered more severe membrane damage and oxidative stress, leading to a significant upregulation of genes related to EPS secretion, biofilm formation, and ROS scavenging to alleviate cellular stress. However, this did not overcome the negative effects caused by the downregulation of nitrogen metabolism, energy metabolism, and biosynthesis, resulting in a decline in the nitrogen removal performance of P. stutzeri. This study provides an understanding of the potential mechanisms underlying changes in P. stutzeri to different NPs stressors.