Biofilms on microplastics across ecological systems: Formation mechanisms, community composition, environmental impacts, and ecotoxicity.

Microplastics (MPs) have emerged as pervasive environmental contaminants in aquatic, terrestrial, and atmospheric ecosystems. Once released into the environment, MPs are rapidly colonized by microorganisms, leading to the formation of complex biofilm communities collectively termed the "plastisphere." These biofilms significantly alter the physicochemical properties, transport behavior, ecological interactions, and toxicity of microplastics. This review synthesizes recent findings on the mechanisms of biofilm formation on microplastics, including the roles of polymer type, surface aging, eco-corona formation, and environmental factors such as salinity, temperature, nutrient availability, and hydrodynamics. The composition and ecological functions of plastisphere communities, including bacteria, archaea, fungi, algae, and protists, are discussed with emphasis on extracellular polymeric substances (EPS), quorum sensing, metabolic interactions, and horizontal gene transfer. The review further evaluates the role of biofilm-coated microplastics as vectors for pollutants, antibiotic resistance genes, and pathogenic microorganisms across marine, freshwater, wastewater, soil, and agricultural systems. In addition, the interactions between microplastics and co-contaminants such as heavy metals, pharmaceuticals, PFAS, and organic pollutants are examined in the context of ecotoxicological risks. Current methodological approaches, environmental implications, and regulatory challenges are also addressed. Overall, this review emphasizes the importance of adopting a biofilm-centered perspective for understanding the environmental fate and ecological impacts of microplastics, delves deeper into microplastic-associated biofilms across diverse ecosystems, including marine and freshwater environments, wastewater and urban water systems, and soils and agricultural lands, while only briefly considering less-studied compartments such as the atmosphere, integrating pathogen-specific food safety evidence, nanoplastic-EPS interactions, AMR/HGT mechanisms, and co-contaminant-derived risks, identifying the impacts of microplastics/biofilms themselves or the pathogens they carry on human health, and identifies critical knowledge gaps that require future investigation.