Deciphering the fate, mobility, and bacterial hosts of antibiotic resistomes in full-scale ballast water management systems by metagenomics

Ballast water acts as a significant vector for the transoceanic transfer of aquatic microorganisms, encompassing emerging pollutants such as antibiotic resistance genes (ARGs). Ballast water management systems (BWMS) represent the last barrier in preventing the biological invasions, yet their impacts on ARGs remain poorly understood. Herein, we examined the fate, mobility, and bacterial hosts of ARGs in full-scale BWMS (electrolytic chlorination and ultraviolet irradiation, EC and UV) using metagenomics. The results demonstrated that EC significantly increased the total relative abundance of ARGs by 89 % (from 0.082 to 0.155 copies/16S rRNA), selectively enriching multidrug and bacitracin resistance genes ( mex F, bac A > 150 %) while reducing β-lactam resistance genes by 28 %. In contrast, UV maintained the overall abundance of ARGs while selectively promoting β-lactam resistance genes by over 116 % and concurrently reducing bacitracin resistance genes by 20 %. Both treatments enhanced horizontal gene transfer potential, and the proportion of bacterial hosts carrying ≥3 ARG subtypes rose from 16 % to 26 % and 20 % to 44 % after EC and UV, respectively. Notably, putative-resistant pathogens such as Pseudomonas , Marinobacter , and Acinetobacter persisted post-treatment, with some strains harboring up to 15 ARG subtypes. Quantitative risk assessment revealed that both EC and UV increased the ecological resistance risk of ARGs by at least 25 %, respectively. Overall, this study offers a scientific foundation to support the International Maritime Organization in developing strategies to control the spread of antimicrobial resistance through global shipping.