Ecosystem settings and urbanization shape microbial communities and antibiotic resistance genes on coastal microplastics.

Coastal wetlands are increasingly contaminated by microplastics that provide long-lived substrates for microbial colonization, yet the joint effects of ecosystem settings and urbanization on plastisphere communities and their resistomes remain poorly understood. Here, we used a 2 × 2 factorial design across mangrove and sandy-beach sediments under rural and urban influence, combined with metagenomic profiling, to characterize microplastic-associated microbiota and antibiotic resistance genes (ARGs). Microbial communities on microplastics showed clear separation between mangroves and sandy shores, with additional shifts along the rural-urban gradient, indicating context-dependent plastisphere assembly. Urbanization substantially increased richness in mangrove plastispheres, whereas effects on sandy beaches were weak or inconsistent and largely confined to low-abundance taxa. In situ exposure yielded a diverse ARG repertoire (>1 000 ARGs), and ARG composition showed significant ecosystem × human-impact interactions, with urban mangrove microplastics hosting the highest ARG diversity. Genus-ARG co-occurrence networks showed denser bacteria-ARG association patterns in mangrove than in sandy-beach plastispheres, with a limited number of genera statistically associated with multiple ARGs. These results suggest that plastisphere communities and resistomes varied across ecosystem settings and urbanization contexts, with urban mangrove microplastics showing relatively higher ARG diversity and stronger bacteria-ARG co-occurrence patterns. These findings highlight the need for habitat-specific monitoring of microplastic-associated resistance.