Biofilm-induced microplastic transport under subcritical shear stress

Microplastics are pervasive in aquatic environments and are frequently colonized by biofilms, yet how these biofilms influence microplastic transport remains unclear. While previous studies suggest that biofilms hinder particulate transport through aggregation and adhesion, our microfluidic experiments show that biofilms can instead promote microplastic transport under subcritical flow conditions. Combining microfluidic experiments with computational fluid dynamics simulations, we reveal that motile Pseudomonas aeruginosa preferentially form biofilms downstream of microplastic beads, where vertical convergent flow velocity exceeds cell swimming speed to concentrate cells, while streamwise flow velocity remains low to prevent cells from being swept away. Once the biofilm coverage reaches a critical threshold, the biofilm-induced lift force overcomes the submerged weight of the bead and initiates transport. This lift-induced mobilization was also observed for other bacterial species (Escherichia coli, Pseudomonas putida), indicating the generality of the mechanism. These findings uncover a previously unrecognized biological pathway for particle mobilization and establish a quantitative framework for predicting biofilm-modulated particulate transport in aquatic and porous environments.