Smaller sizes of polyethylene terephthalate microplastics mainly stimulate heterotrophic NO production in aerobic granular sludge systems.

Widespread polyethylene terephthalate microplastics (PET MPs) have played unintended role in nitrous oxide (NO) turnovers (i.e., production and consumption) at wastewater treatment plants (WWTPs). Mainstream aerobic granular sludge (AGS) systems possess potentially strong NO-sink capability, which may be reduced by PET MPs stress through altering NO-contributing pathways, electron transfer, and microbial community structures. In this study, the effects of PET MPs with two common particle sizes of effluent from WWTPs (0.1 and 0.5 mm) on NO turnovers, production pathways and NO-sink capability were systematically disclosed in AGS systems by a series of biochemical tests and molecular biological means to achieve the goal of carbon neutrality. The results indicated that 0.1 mm PET MPs could more significantly stimulate NO production in AGS systems by inhibiting denitrifying metabolism, compared with control and 0.5 mm PET MPs systems. Specifically, 0.1 mm PET MPs slightly increased the relative abundance of , reducing NO yields via promoting the hydroxylamine (NHOH) oxidation pathway during nitrification. Also, 0.1 mm PET MPs inhibited the electron transport system activities and the relative abundance of NO reductase, hindering NO reduction during denitrification. Most importantly, 0.1 mm PET MPs more apparently reduced the NO-sink capability based on the ratio of NO reductase gene and nitrite reductase gene.