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Plastisphere-mediated nitrogen cycling and N2O emissions in inland waters: A systematic review
Summary
Researchers synthesized evidence that microplastic biofilms in freshwater systems selectively enrich nitrifying and denitrifying bacteria while suppressing the gene responsible for converting nitrous oxide to nitrogen gas, creating conditions that amplify N₂O emissions — with biodegradable plastics posing an unexpectedly complex greenhouse gas risk.
Plastic pollution in inland aquatic ecosystems fosters unique microbial biofilms, termed the “plastisphere”, act as a potent mediator of biogeochemical cycles. This systematic review synthesizes evidence that the plastisphere disrupts nitrogen (N) cycle and amplifies emissions of nitrous oxide (N 2 O). By creating stratified microenvironments with sharp oxygen gradients, microplastics selectively enrich microbial guilds responsible for N transformations. Conventional polymers (polyethylene, polyvinyl chloride) enrich nitrifying bacteria ( Nitrosomonas and Nitrospira ), increasing the abundance of functional marker genes for ammonia oxidation ( amoA ) and nitrite oxidation ( nxrB ). Simultaneously, these and other polymers (polystyrene) promote denitrifying taxa ( Dechloromonas , Thauera , and Flavobacterium ), elevating genes for nitrite reduction ( nirK, nirS ), a key step in N 2 O production. The gene responsible for N 2 O reduction ( nosZ ) is frequently suppressed. This imbalance is quantified by the ( nirK + nirS )/ nosZ ratio, where a higher value indicates a greater genetic potential for N 2 O to be produced rather than reduced to N 2 , is a primary mechanism for N 2 O accumulation. Biodegradable polymers introduce a complex paradox: while they may inhibit classic nitrifiers, they create anoxic microinches that favor alternative pathways like nitrifier-denitrification and support distinct denitrifier communities, resulting in substantial N 2 O yields. The direction and magnitude of these effects are critically determined by polymer chemistry and size. We identify research priorities, including long-term field studies and advanced isotopic methods, essential for developing predictive models and effective mitigating strategies. • Plastic biofilms create potent greenhouse gas hotspots in inland waters. • Key nitrogen-transformation genes are dysregulated within the plastisphere. • Biodegradable plastics can unexpectedly intensify N 2 O emissions. • Nanoparticles shift effects from microbial selection to direct toxicity. • Polymer chemistry is a primary determinant of microbial functional shifts.