Biodegradable polymer backbone shapes plastisphere assembly, enhancing antibiotic sorption and reshaping aquatic microbial communities.

Biodegradable plastics are widely considered environmentally safer alternatives to conventional polymers, yet whether plastisphere-mediated aging amplifies or mitigates their antibiotic vector potential remains mechanistically unresolved. Using polylactic acid (PLA) and polybutylene succinate (PBS) microplastics aged in wastewater, we establish a continuous mechanistic framework linking polymer backbone structure through plastisphere assembly, biofilm-driven surface change, and contaminant sorption to ecosystem-scale microbial succession. Polymer surface structure shaped initial colonization, with PLA recruiting 63% greater biofilm biomass than PBS, while backbone hydrolytic susceptibility selectively enriched taxa reported to encode carboxylesterases (Kluyvera georgiana, Pseudomonas citronellolis) within a more connected plastisphere than on PBS. Specialist-driven degradation induced 42% crystallinity loss and surface area expansion from 8.8 to 14.3 m²/g on PLA through endo-type hydrolytic micropore generation, contrasting sharply with attenuated structural change on PBS. Critically, biodegradation-induced oxidation, not biofilm, governed ofloxacin uptake, yielding hierarchical adsorption capacity (degraded PLA > degraded PBS > colonized PLA > colonized PBS > pristine); degraded surfaces adsorbed 7.5-fold more via emergent -OH and -NH₂ groups, as confirmed by density functional theory. PLA-specific hydrolysis metabolites exerted selective pressure beyond the particle, reducing planktonic OTU richness by 64% and driving phylogenetic homogenization alongside Legionella-taxa. Biodegradability doesn't confer safety; plastisphere transformation governs vector potential.