Microplastics promote N2O emissions by enhancing nitrification via ammonia-oxidizing bacteria in estuarine and coastal sediments

Estuarine and coastal ecosystems are critical interfaces between land and ocean, serving as sinks for anthropogenic pollutants such as ammonium and microplastics. However, the impact of microplastic pollution on nitrification processes in these environments remains largely unexplored. This study investigates the coastal region of the Yangtze River to examine how different microplastic types (polyethylene terephthalate, polypropylene, and polyethylene) affect nitrous oxide (NO) emissions and the dynamics of nitrifiers, including ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), complete ammonia-oxidizing (comammox) Nitrospira, and nitrite-oxidizing Nitrospira. Results from incubation experiments show that all microplastic types significantly increase NO emissions across sediment samples. The reconstructed representative metagenome-assembled genomes revealed that AOA belong to group I.1a, while AOB are classified within the Nitrosomonas genus. Microplastics were found to have a stronger stimulatory effect on AOB, which are linked to higher NO production, than on AOA, which are associated with low NO production, thereby enhancing NO emissions during nitrification. Furthermore, AOB genomes encode a range of putative plastic-degrading enzymes, which may partially explain their enrichment in microplastic-contaminated environments, although other factors such as differential tolerance to ammonium or oxidative stress cannot be ruled out.