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Individual and combined effects of silver nanoparticles and polyvinyl chloride microplastics on the activity and abundance of ammonia-oxidizing microorganisms from a wastewater treatment system
Summary
Silver nanoparticles (used in some antibacterial products) can shut down helpful bacteria in wastewater treatment plants that normally break down ammonia into less harmful substances, and this damage gets worse at higher concentrations. Interestingly, microplastics alone didn't cause harm, but when microplastics had already settled in the water, they actually softened the silver nanoparticles' toxic effects, showing that pollutants can interact in unexpected ways. This matters because if wastewater treatment doesn't work properly, ammonia and other pollutants could end up back in our water supply.
Silver nanoparticles (AgNPs) and microplastics (MPs) are the contaminants commonly found in wastewater and water resource recovery facilities. Here, we investigated the individual and combined effects of AgNPs and polyvinyl chloride (PVC) MPs on the activity and amoA gene abundance of ammonia-oxidizing microorganisms (AOMs). AgNPs exhibit concentration-dependent toxicity, with no inhibitory effect at 0.1 mg/L, partial inhibition at 0.5 and 1.0 mg/L (59.8 and 80.5% inhibition, respectively), and near-complete inhibition (≥ 90%) at higher concentrations. Interestingly, qPCR revealed that different AOM groups exhibited distinct tolerance levels: ammonia-oxidizing bacteria (AOB) tolerated AgNPs up to 1 mg/L, while ammonia-oxidizing archaea (AOA) exhibited better tolerance than the other AOMs at concentrations≥2.5 mg/L. Notably, complete ammonia oxidizers (comammox) showed the lowest tolerance, with inhibition at concentrations as low as 0.5 mg/L. PVC MPs alone (500 mg/L) had no discernible impact on both the activity and amoA gene numbers, but their interaction with AgNPs revealed that pre-settled MPs reduced AgNP toxicity, whereas freshly introduced MPs that remained afloat did not.