Mechanistic insights into the impact of multi-dimensional microplastic stress on nitrogen removal by heterotrophic...

Heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria possess considerable potential for treating high-ammonia wastewater; however, their denitrification characteristics and response mechanisms under microplastics (MPs) stress remain inadequately understood. This study systematically investigated the effects of typical MPs on the denitrification performance of HN-AD bacteria strain TAC-1 through batch experiments, metatranscriptomic and ultrastructural analysis. The findings demonstrated that hydrophobic nature of polyvinyl chloride (PVC) disrupted the intermolecular interactions among lipid molecules, reducing cell membrane density and forming permeable channels. This structural damage decreased the expression of the sulfate/sulfonate transport system (cysW/cysP/cysU/cysA), impairing bacterial protein synthesis. In response to survival pressure, the strain activated an immune evasion mechanism by upregulating the expression of fimbriae synthesis genes (fimA and fimD). ompared to PVC, polyethylene (PE), due to its high chemical stability, induced the disorganization of membrane lipids without significantly compromising membrane integrity. Notably, 100 nm PE particles enhanced the iron acquisition capability of the strain, leading to increases of 24.63 % in ammonia nitrogen (NH₄⁺-N) removal rates. However, this promotional effect declined following prolonged exposure (>6 days) due to the accumulation of intracellular toxic substances. The cationic surface characteristics of polystyrene (PS) induced severe oxidative stress, leading to the most pronounced structural damage to the membrane. Although PS impaired denitrification efficiency by disrupting membrane integrity, it maintained NH₄⁺-N conversion capacity through compensatory metabolic reorganization mediated by the glutamine synthetase and glutamate dehydrogenase pathways. These findings provide a theoretical foundation for enhancing the anti-interference resilience of biological wastewater treatment systems.