Abstract
The hyporheic zone (HZ) is an active biogeochemical region where groundwater and surface water mix and a potential reservoir for antibiotic resistance genes (ARGs). In this paper, the relative abundance and spatial distribution of ARGs in the HZ media were investigated, taking into consideration both the five speciation of six metals and the local characteristics. The samples of surface water, groundwater, and sediment were collected from Zaohe-Weihe Rivers of Xi'an City, which is a representative city with characteristics of the northwest region of China. Of 271 ARGs associated with 9 antibiotics, 228ARGs were detected, with a total detection rate of 84%. Sulfonamide and aminoglycoside ARGs were the dominant types of ARGs. The top 6 ARGs and mobile genetic elements (MGEs) in terms of abundance were tnpA-04, cepA, sul1, aadA2-03, sul2and intI1. The results of principal component analysis (PCA) showed that the distribution characteristics of ARGs were not associated with the sampling sites but with the environmental medias. Similarity in the water phases and significant differences in the water and sediment phases were found. The redundancy analysis (RDA) identified the key factors controlling ARG pollution, including dissolved oxygen (DO) in surface water, total nitrogen (TN) in groundwater, and total organic carbon (TOC) in sediment. In terms of the speciation of heavy metals, we further revealed the promotion effect between ARGs and heavy metals, especially the residual fraction of Ni. In terms of horizontal transfer mechanism, ARGs were significantly correlated with tnpA-03 in water phase and tnpA-04 in sediment. In the three media, intI1 and ARGs all show a significant correlation. These findings showed that hyporheic zone exerted a bottleneck effect on the distribution and transfer of ARGs.
Highlights
Many factors, such as extensive use, continuous discharge and incomplete degradation, have caused antibiotics to be widely distributed in a variety of environmental media (Hernando et al 2006, Shen et al 2020)
The total detection rate of 84.1% obtained in our study was higher than that in the reservoir system (78.9%) (Chen et al 2019). This result indicates that the existence of more resistant bacteria in the microbial communities of the hyporheic zone (HZ) and complex environmental incentives promoted the emergence of novel antibiotic resistance
Aminoglycosides and beta lactamase antibiotic resistance genes (ARGs) were the dominant types of ARGs, which was similar to the results found in the Wen-Rui Tang River in China (Zhou et al 2017)
Summary
Many factors, such as extensive use, continuous discharge and incomplete degradation, have caused antibiotics to be widely distributed in a variety of environmental media (Hernando et al 2006, Shen et al 2020). A large amount of antibiotics and other contaminants (i.e., metals, pesticides and nutrients) mix together in pharmaceutical factory effluents, wastewater treatment plants (WWTPs), estuary environments and livestock and aquaculture farms and is likely to cause complex pollution and lead to the area becoming an environmental reservoir for ARGs Chen et al (2019a) believed that environmental media can significantly affect the distribution and co-occurrence of ARGs in the aquatic environment. The significant correlations between heavy metals and ARGs was occurred by co-selection and cross-selection processes (Chen et al 2021). These metals were generally the total extractable fractions in sediments based on the digestion by strong acids. It is necessary to further explore how different speciation of heavy metals affect ARGs and MGEs
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