Rare resistome rather than core resistome exhibited higher diversity and risk along the Yangtze River.

As important freshwater ecosystems, the occurrence and distribution of antibiotic resistance genes (ARGs) in rivers are relevant to public health. However, studies investigating ARGs of different environmental media in river ecosystems are limited. In this study, we analyzed the ARGs of microbes in free-living setting, particle-associated setting, sediment and bank soil of the Yangtze River using metagenomics. Twenty-six ARGs were found in all samples regardless of media (core resistome) with a diversity of 8.6 %-34.7 %, accounting for 22.7 %-89.2 % of the relative abundance of the overall ARGs. The core resistome of the Yangtze River was dominated by multidrug resistance genes consisting mainly of efflux pumps and bacitracin resistance genes. The rare resistome was dominated by multidrug, sulfonamide, and aminoglycoside resistance genes. The core resistome was more prevalent in chromosomes, implying that these ARGs with low diversity and high relative abundance may be intrinsic to microbes in the Yangtze River. The rare resistome was more prevalent in plasmids, suggesting these ARGs with high diversity and low relative abundance were acquired under environmental stresses and had transfer potential. Additionally, we found that core and rare resistome were mainly carried by specific bacteria. Noteworthily, twenty-two ARGs of high clinical concern were identified in rare resistome, especially aac(6')-I, sul1, and tetM, which were plasmid-borne and hosted by clinically relevant pathogens. Both core and rare resistome hosts showed the highest niche breadths in particle-associated setting compared to other media, and particle-associated setting could provide more stable and ideal conditions for resistome hosts to survive. This study elucidated the genetic locations of ARGs and the community assembly mechanisms of ARG hosts in freshwater environments.