Ecological insights into the elevational biogeography of antibiotic resistance genes in a pristine river: Metagenomic analysis along the Yarlung Tsangpo River on the Tibetan Plateau.

Presently, the prevalence of antibiotic resistance genes (ARGs) is regarded as an emerging environmental issue, and many studies have illuminated biogeographical patterns of the antibiotic resistome. However, few studies have investigated elevational biogeography and associated assembly mechanisms of ARGs in natural river systems. Accordingly, in the present study, we used metagenomics approaches to analyze the biogeographical pattern of ARGs along the pristine Yarlung Tsangpo River on the Tibetan Plateau. Our study retrieved the baseline profiles of ARGs in the pristine river and showed that the ARGs were dominated by bacA, which was resistant to bacitracin and represented more than 91% of total ARGs. The diversity and abundance of ARGs in the pristine river were lower than those in the human-impacted area, suggesting that the antibiotic resistome evolved and was promoted in a human-impacted environment. Furthermore, an elevational distance-decay relationship of ARGs was observed along the pristine Yarlung Tsangpo River, which provided strong evidence of ARG dissimilarity under the elevational variation. Elevational gradients could lead to changes in environmental variables and spatial factors, and consequently alter ARG composition. Elevational gradients could influence the assembly processes of ARGs. The deterministic and stochastic assembly processes both played critical roles and equally participated in shaping ARG composition at the watershed scale along the Yarlung Tsangpo River, and with increasing elevational variation along the river, the ecological processes of ARG assembly shifted from deterministic to stochastic. Moreover, abundant and rare ARGs were tentatively separated to investigate the difference and similarity in their distributions. Although abundant and rare ARGs presented similar distance-decay relationships, rare ARGs were more diverse and vulnerable to the dispersal process and mutation. Overall, our study provides valuable ecological insights to profile the large-scale elevational patterns of ARGs in a pristine river system, thereby providing important information for public health and environmental management.