Environmental transformation and ecological implications of surface and subsurface microplastics in marine...

Nearly 67% of plastics entering the oceans remain unaccounted for a phenomenon termed the "missing plastic paradox," likely driven by vertical transport of suspended plastics to deeper waters. Yet, the transformation and contaminant-binding potential of these submerged microplastics (MPs) remain poorly understood. This study explores how depth influences the physicochemical and biological evolution of six common polymer types (HDPE, LDPE, PP, PS, PET, and OTHERs) during five months of exposure in a coastal marine system. FTIR, contact angle, and surface energy analyses revealed significant weathering and oxidation in all polymers, with surface MPs exhibiting greater hydrophilicity and polar surface energy due to intense UV-induced oxidative stress. Biofilm characterization showed distinct depth-related patterns: compact, autotroph-dominated biofilms at the surface and thicker, EPS-rich, heterotrophic biofilms at depth. These biological modifications substantially increased the sorption capacity of MPs for metals. Batch sorption experiments demonstrated that Pb and Cd distribution coefficients (K_d) rose significantly in subsurface MPs compared to pristine MPs (about 600 fold in case of Pb uptake by LDPE). The results highlight the pivotal role of depth-dependent aging and biofilm development in enhancing contaminant affinity, thereby influencing the vertical redistribution, ecological risks, and long-term fate of MPs in marine environments.