Shifts of antibiotic resistance genes across an estuarine meandering bend and dissemination risks to offshore oceans.
Meandering is a fundamental geomorphic feature of rivers that plays a critical role in regulating pollutant attenuation. To elucidate its impact on antibiotic resistance genes (ARGs) distribution in estuarine intertidal sediments, samples were collected from both the landward side (freshwater-dominated) and the seaward side (tide-dominated) of a meander bend during ebb and flood tides. The total relative abundance of ARGs was approximately 2.7 times higher on the landward side, peaking during the ebb tide. Microbial composition analysis showed that genera Acinetobacter and Pseudomonas were dominant at the landward sites, while halophilic genera such as Marinobacter and Exiguobacterium were abundant at the seaward sites. Further analysis of metagenome-assembled genomes (MAGs) demonstrated that the dominant landward genus Acinetobacter acted as a key host of ARGs, with two of four MAGs encoding more than ten ARGs. Notably, the total relative abundance of mobile genetic elements was high but consistent between sides and tidal cycles (p > 0.05). Given this high dissemination risk, we further forecasted the ARGs transfer scenarios to oceanic settings based on a set of offshore MAGs (n = 3626). Three ARGs, i.e., acrA, vanSL, and AAC(2')-Ia, were inferred to have transfer potential, supported by neighboring MGEs detected in marine microorganisms. Analysis of the genomes of predicted recipients in the SRA database confirmed the predicted mobilizations. Together, this study highlights that the meandering planform may serve as a significant barrier, attenuating the discharge of ARGs from terrestrial sources into the marine environment.