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Plastisphere enhanced resistance genes propagation in sulfur autotrophic/heterotrophic denitrification system under mixed quaternary ammonium compounds pressure
Summary
A laboratory wastewater treatment study found that microplastic surfaces — particularly those of biodegradable polylactic acid plastics — create enriched "plastisphere" communities that accumulate and spread antibiotic resistance genes more aggressively than the surrounding biofilm, especially under the additional stress of quaternary ammonium compound disinfectants. The biodegradable plastic appeared to provide extra carbon and energy to microbes, inadvertently turbocharging resistance gene proliferation. This challenges the assumption that switching to biodegradable plastics in wastewater systems is straightforwardly beneficial.
Quaternary ammonium compounds (QACs) and microplastics (MPs) have been frequently identified as emerging contaminants in wastewater treatment plants, and their co-existence and accumulation pose substantial challenges to the stabilization of sulfur autotrophic/heterotrophic denitrification (SA/HD) system. This study established and systematically evaluated the impacts of mixed QACs on a stable operated SA/HD system performance, followed by a comparative analysis of microbial community dynamics and resistance genes (RGs) enrichments between biofilm and two representative MPs plastisphere. Over a 153-day operation, 0.5 mg/L mixed QACs slightly fluctuated the system performance, whereas 5.0 mg/L mixed QACs promoted the NO-N accumulation to achieve partial denitrification by restraining genes associated with electron and energy generation in tricarboxylic acid cycle, but barely affected the ability of sulfur metabolism. In comparison, both polyvinyl chloride MPs and polylactic acid (PLA) MPs plastisphere exhibited higher microbial metabolic activities than the biofilm, particularly in the case of PLA MPs. Reshaped microbial community structures with multiple QACs-resistant bacteria colonizing in plastisphere and higher stochasticity also strengthened the microbial resistance to mixed QACs, favoring diversity intracellular/extracellular RGs subtypes proliferation and co-selection. As biodegradable MPs, PLA induced upregulation of genes related to carbon metabolism and facilitated electron transfer and energy generation, potentially exacerbating the hazards of RGs proliferation. Mobile genetic elements were found to be the key contributor in directly/indirectly driving three-fraction RGs enrichments in SA/HD system. This study elucidated the response mechanisms of SA/HD system to mixed QACs and provided worthy insights into the pivotal role of MPs in accelerating RGs propagation.