Hidden link between microplastic biofilms and aquatic nitrogen transformation: From regulator to diverse ecological effects

As a vital carrier for the colonization and succession of diverse microbial communities, microplastics (MPs) surface form unique microplastic biofilms (MPBs) ecosystems. MPBs can enrich microorganisms involved in core nitrogen transformation processes, and the localized anoxic environment within these biofilms regulates nitrogen removal efficiency. Consequently, biofilms establish novel nitrogen transformation microenvironments within aquatic systems. Numerous studies have revealed that MPBs play a crucial role in key pathways of the nitrogen transformation. However, the potential regulatory role of MPBs in nitrogen transformation, along with the ecological risks and environmental applications arising from this process, warrants further review. A dual effect of MPBs has been identified in aquatic nitrogen transformation, presenting both ecological risks and environmental applications. MPBs promote multi-pathway coupling processes, such as nitrification and denitrification, by enriching functional microbial communities and constructing oxygen-gradient microenvironments. Incomplete denitrification and an imbalance in functional genes may significantly increase the risk of N₂O emissions from MPBs. Furthermore, biofilm succession under stress from environmental factors often accelerates the directional selection of functional communities and may alter the nitrogen balance in aquatic systems. Factors such as dissolved oxygen, DOM, and antibiotics can alter the final products of nitrogen transformation by regulating the expression of functional genes and the supply of electron donors. This paper reviews the general patterns of structural succession in MPBs, summarizes their primary ecological functions in the nitrogen transformation, and explores their underlying mechanisms from the perspective of regulation by environmental factors. The aim is to elucidate the potential impacts of MPs, as an emerging ecological niche, on nitrogen transformation processes in aquatic environments. This review deepens our understanding of nitrogen transformation functions and their environmental regulation mechanisms in MPBs, providing scientific support for aquatic environmental management and pollution control.