Microplastics alter nitrous oxide production and pathways through affecting microbiome in estuarine sediments

Increasing microplastics (MPs) pollution in estuaries profoundly impacts microbial ecosystems and biogeochemical processes. Nitrous oxide (NO), a powerful greenhouse gas, is an important intermediate product of microbial nitrogen cycling. However, how MPs regulate NO production and its pathways remain poorly understood. Here, impacts of traditional petroleum-based and emerging biodegradable MPs on microbial NO production and its pathways were studied through dual-isotope (N-O) labeling technique and molecular methods. Results indicated that both traditional petroleum-based and emerging biodegradable MPs promoted sedimentary NO production, whereas pathways varied. Biodegradable polylactic acid (PLA) MPs displayed greater promotion of NO production than petroleum-based MPs, polyvinyl chloride (PVC) and polyethylene (PE), of which PLA promoted through nitrifier nitrification (NN) and heterotrophic denitrification (HD), PE through nitrifier denitrification and HD, and PVC through NN. By combining the analysis of NO production rates with sediment chemical and microbiological properties, we demonstrated that the enrichment of nitrifying and denitrifying bacteria, as well as related functional genes directly and/or indirectly increased NO production primarily by interacting with carbon and nitrogen substrates. Different response of nitrogen cycling microbes to MPs led to the difference in NO increase pathways, of which nitrifying bacteria significantly enriched in all MPs treatments due to the niches provided by MPs. However, part of denitrifying bacteria significantly enriched in treatments containing PLA and PE MPs, which may serve as organic carbon substrates. This work highlights that the presence of MPs can promote sedimentary NO production, and the emerging biodegradable MPs represented by PLA may have a greater potential to enhance estuarine NO emissions and accelerate global climate change.