Microalgae-mediated aggregation facilitates the vertical transport of microplastics: the role of Cylindrotheca closterium under simulated conditions.

Microplastics (MPs) released from aquaculture infrastructure represent a growing source of pollution in coastal environments, yet their fate in the presence of periphytic diatoms remains underexplored. This study investigates how the periphytic diatom Cylindrotheca closterium interacts with polyethylene (PE) MPs of two size classes (10-20 μm and 100-200 μm) under controlled laboratory conditions. Using 36-day time-series experiments, we examined cell attachment, aggregate formation, and sinking dynamics. C. closterium rapidly reached high biomass (>1.50 × 106 cells mL-1) and formed visible aggregates as early as day 11. Aggregation pathways differed by MP size: small particles became embedded within branched internal matrices, whereas large particles developed into dense, spherical flocs with external coatings of diatom cells. Large MPs served as nucleation cores, enabling higher MP incorporation per aggregate (mean 1.68-2.44 MPs) and generating attachment ratios exceeding 8.00 × 103 cells per particle in some treatments. Aggregate areas surpassed 1.7 × 105 μm2 in large-PE groups, and sinking velocities reached up to 3.70 mm s-1. Aggregate formation and downward transport occurred across all treatments, with distinct morphologies and performance depending on particle size and concentration. These results suggest that C. closterium promotes a robust pathway for MP aggregation and sedimentation through surface-mediated attachment rather than extensive extracellular matrix production. The size-dependent structural strategies highlight functional plasticity in capturing and retaining MPs. Overall, this study underscores the significant role of periphytic diatoms in regulating the environmental fate of aquaculture-derived MPs, providing new insight into biologically mediated removal processes in coastal ecosystems.