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

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.