Smaller sizes of polyethylene terephthalate microplastics mainly stimulate heterotrophic N2O production in aerobic granular sludge systems

Widespread polyethylene terephthalate microplastics (PET MPs) have played unintended role in nitrous oxide (N₂O) turnovers (i.e., production and consumption) at wastewater treatment plants (WWTPs). Mainstream aerobic granular sludge (AGS) systems possess potentially strong N₂O-sink capability, which may be reduced by PET MPs stress through altering N₂O-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 N₂O turnovers, production pathways and N₂O-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 N₂O 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 <i>Nitrosomonas</i>, reducing N₂O yields via promoting the hydroxylamine (NH₂OH) oxidation pathway during nitrification. Also, 0.1 mm PET MPs inhibited the electron transport system activities and the relative abundance of N₂O reductase, hindering N₂O reduction during denitrification. Most importantly, 0.1 mm PET MPs more apparently reduced the N₂O-sink capability based on the ratio of N₂O reductase gene and nitrite reductase gene.