Microplastics provide new niches for nitrifiers to maintain nitrification performance in nitrifying bioreactors

Microplastics (MPs) in various matrices have attracted increasing attention because of their potential threats to ecological safety and human health. Previous studies reported that MPs have distinct effects on the microbial conversion of nitrogen compounds, but the differences between nonbiodegradable and biodegradable MPs are not well known. In this study, polyethylene (PE), polyvinyl chloride (PVC), and polylactic acid (PLA) MPs were selected for comparative investigations of their effects on the microbial nitrification process in nitrifying bioreactors. The results revealed that PVC-MPs increased the potential nitrification rate by 17.6 %. MP exposure has a toxic effect on nitrifying microbes, resulting in increases in reactive oxygen species (ROS) and lactate dehydrogenase (LDH) levels of 25.5-38.4 % and 25.1-149.7 %, respectively. Compared with PE-MPs and PVC-MPs, biodegradable PLA-MPs more strongly inhibited the enrichment of ammonia-oxidizing bacteria Nitrospira and nitrogen metabolism in nitrifying sludge. However, the nitrogen-metabolizing microbes enriched in the MP-biofilm maintained favorable nitrogen conversion performance. The potential plastic-degrading bacteria such as Bacillus, Pseudomonas, and Achromobacter identified in the nitrifying sludge were significantly enriched in the MP-biofilm. These results suggest that the MPs present in bioreactors could provide new niches and reshape diverse nitrifiers to maintain stable microbial nitrification.