Contrasting seasonal dynamics of antibiotic resistance genes in tropical estuarine-coastal sediments of Hainan, China.

Antibiotic resistance genes (ARGs) in tropical estuarine-coastal sediments remain insufficiently understood, especially under contrasting urban-tourism and agriculture-aquaculture pressures. This study systematically investigated ARGs, mobile genetic elements (MGEs), bacterial communities, and sediment physicochemical properties in the Sanya River and Ningyuan River estuarine-coastal systems of Hainan, China, across dry and wet seasons. We quantified 17 target ARGs and two representative MGEs, intI1 and Tn916. Ten of the 17 target ARGs were detected, with absolute abundances ranging from 104 to 108 copies g-1 dry sediment. The detected ARG profiles were dominated by the macrolide resistance gene oleC and tetracycline resistance gene tetM. The Sanya River estuarine-coastal system showed a dry-season enrichment pattern, whereas the Ningyuan River estuarine-coastal system showed higher wet-season ARG loads. These contrasting patterns were consistent with different source structures, with urban point-source retention being more evident in Sanya during the dry season and runoff-associated non-point source input being more evident in Ningyuan during the wet season. Correlation analyses showed that intI1 and Tn916 were strongly associated with multiple ARGs, suggesting that MGE-associated mobility may contribute to ARG persistence and dissemination. Environmental variables, including EC-related dissolved ionic conditions, TCLP-extractable metals, and DOM characteristics, were associated with ARG patterns directly or indirectly through microbial community variation. These findings elucidate the divergent mechanisms driving ARGs pollution in tropical urban versus rural estuarine-coastal systems, providing essential data for ecological risk assessment and targeted management strategies in coastal ecosystems.