Cometabolism of 17?-ethynylestradiol by nitrifying bacteria depends on reducing power availability and leads to elevated nitric oxide formation

17?-ethynylestradiol (EE2) is a priority emerging contaminant (EC) in diverse environments that can be cometabolized by ammonia oxidizing bacteria (AOB). However, its transformation kinetics and the underlying molecular mechanism are unclear. In this study, kinetic parameters, including maximum specific EE2 rate, half-saturation coefficient, capacity of AOB were obtained using model strain, Nitrosomonas europaea 19718. The relationship between cometabolism oxidation was divided into three phases according to reducing power availability, namely “activation”, “coupling”, “saturation”. Specifically, there universal lag after initiated, suggesting sufficient (approximately 0.95 ± 0.06 mol NADH/L) required activate cometabolism. Interestingly, nitric oxide emission increased 12 2% during cometabolism, along with significantly upregulated nirK cluster genes. findings importance understanding cometabolic behavior natural engineered environments. Maintaining relatively high stable supply from potentially improve removal other ECs wastewater nitrification processes.