[Effects of Two PPCPs on Nitrification in Sediments in the Yarlung Zangbo River]

To understand the effects of pharmaceuticals and personal care products (PPCPs) on nitrogen transformation in high-altitude rivers, we investigated the influence of the single and combined exposure of sulfamethoxazole (SMX) and 2-ethylhexyl-4-methoxycinnamate (EHMC) at different concentrations (0.01, 0.1, 1.0, 10, and 100 μg·L^-1) on nitrification in the sediments of the Yarlung Zangbo River in sediment slurry experiments. All treatments significantly reduced nitrification rates, and the maximum inhibition rates induced by SMX and EHMC (alone and in combination) were 47%, 50%, and 66%, respectively. SMX significantly inhibited ammonia monooxygenase (AMO) activity in a concentration-dependent manner, with the inhibition rates ranging from 51% to 78%. The inhibitory effect of SMX combined with EHMC on AMO activity was similar to or stronger than that of SMX alone, and the inhibition rates ranged from 55% to 84%. All EHMC exposures also significantly inhibited AMO activity, but the inhibition rates decreased with an increase in EHMC concentrations. The amoA gene abundance was significantly reduced in all the treatments, and SMX alone and in combination with EHMC induced stronger inhibitory effects than EHMC alone. SMX alone and in combination with EHMC significantly inhibited HAO activity, showing a similar concentration-effect relationship, but their combined exposure produced a stronger effect. SMX alone and in combination with EHMC also significantly decreased hao gene abundance in the sediments, and the maximum inhibition rates were 76% and 68%, respectively. Although exposure to higher concentrations of EHMC increased hao gene abundance and HAO activity, the notable decrease in nitrification rates in the sediments suggested that oxidation of ammonia to hydroxylamine catalyzed by AMO was the rate-limiting step in the nitrification process. The results also showed that PPCPs influence the activities of nitrifying communities in the sediments of high-altitude rivers and inhibit nitrification, and their coexistence further increases the pressure of nitrogen loading in aquatic ecosystems.