Microenvironment-driven interactions between mobile genetic elements and defense systems modulate the plastisphere...

Antimicrobial resistance (AMR) within the aquatic plastisphere has emerged as a critical environmental concern, while the microbial processes underlying the amplification and dissemination of antibiotic resistance genes (ARGs) in this microenvironment remain poorly understood. Here, we investigate the interplay between mobile genetic elements (MGEs) and defense systems (DSs) and their collective impact on the riverine plastisphere resistome through in situ cultivation. The resistome risk index in biodegradable plastisphere (i.e., corn starch (CS) and polylactic acid (PLA)) was higher than that in conventional plastisphere (i.e., polypropylene (PP) and polyethylene (PE)). Random forest model revealed that the elevated resistome risk was driven by rich nutrient and high oxidative stress within the CS plastisphere, where MGEs proliferation was promoted by 2.50-, 2.49-, and 0.95-folds than PP, PE, and PLA plastispheres, while horizontal gene transfer (HGT) events was intensified by 1.27-, 1.75-, and 1.14-folds relative to the PP, PE, and PLA plastispheres, respectively. Moreover, phage-carried auxiliary metabolic genes (AMGs) putatively enhanced the environmental adaptation of antibiotic-resistant bacteria (ARB). Higher levels of DSs collide with intensified HGT events in the biodegradable plastisphere relative to the conventional plastisphere. Such synergistic interplay between MGEs and DSs resulted in that DSs and ARGs were both carried by ARB, which actively participated in HGT (i.e., 24.6% of all HGT events). Overall, our findings elucidate the overlooked high AMR risk associated with biodegradable plastisphere in aquatic environments and elucidate how the synergy between DSs and MGEs drives this elevated risk, with important implications for water security and microbial safety.