The fate and long-term toxic effects of NiO nanoparticles at environmental concentration in constructed wetland: Enzyme activity, microbial property, metabolic pathway and functional genes.

Although the potential threats of metallic oxide nanoparticles (MNPs) to constructed wetland (CW) have been broadly reported, limited information is available regarding the long-term impact of nickel oxide nanoparticles (NiO NPs) on CWs at the environmentally relevant concentrations. Here, we comprehensively elucidated the responses in the treatment performance, enzyme activities, microbial properties, metabolic pathways and functional genes of CWs to chronic exposure of NiO NPs (0.1 and 1 mg/L) for 120 days, with a quantitative analysis on the fate and migration of NiO NPs within CWs. Nitrogen removal evidently declined under the long-term exposure to NiO NPs. Besides, NiO NPs induced a deterioration in phosphorus removal, but gradually restored over time. The activities of dehydrogenase (DHA), phosphatase (PST), urease (URE), ammonia oxygenase (AMO) and nitrate reductase (NAR) were inhibited to some extent under NiO NPs stress. Furthermore, NiO NPs exposure reduced bacterial diversity, shifted microbial composition and obviously inhibited the transcription of the ammonia oxidizing and denitrifying functional genes. The results of nickel mass balance indicated that the major removal mechanism of NiO NPs in CWs was through substrate adsorption and plants uptake. Thus, the ecological impacts of prolonged NiO NPs exposure at environmental concentrations should not be neglected.