Effect of DissolvedBiochar on the Horizontal Transfer of Extracellular Antibiotic Resistance Genes by Transformation in Aquatic Environments

The coexistence of extracellular antibiotic resistance genes (eARGs) and dissolved biochar (DBC) in surface water environments has drawn widespread attention. However, the effect of DBC on the transformation-mediated horizontal transferprocess of eARGs in aquatic environment remains unknown. In this study, DBC released from reed (Phragmites australis) and reed bamboo (Arundo donax) biochar at different pyrolysis temperatures (300~600 ℃) was selected to investigate its effect on the transformation of the resistance plasmid pBR322 into Escherichia coli (E. coli) DH5α. The results showed that the effect of DBC on the transformation of pBR322 was promoted by low concentration (1 mg·L^-1) and inhibited by high concentration (10 mg·L^-1), and the inhibitory effect of DBC increased with the rising of pyrolysis temperature. DBC could inactivate the eARGs through disrupting the structure of the pBR322 plasmid. At low concentrations, soluble microbial byproduct-like material, and heavy metals in DBC can induce its SOS responsein recipient bacteria, thereby facilitating their uptake of plasmids. However, at high concentrations (5~10 mg·L^-1), DBC activated the cellular stress defense system of the bacteria, decreased cell membrane permeability, and hindered the access of plasmid to the competent cell. Meanwhile, random forest model(R²= 0.627 9, MSE=0.002 67) revealed that fulvic acid, soluble microbial byproduct-like material, pH value and heavy metals in DBC play key roles in the transformation process. In addition, structural equation modelling demonstrated that the reduction of DNA replication activity is the main factor inhibiting the transformation of eARGs.This study extends our understanding of how dissolved organic matter drives the environmental fate of emerging pollutants ARGs in aquatic environments, providing a scientific basis for the establishment of biochar-based pollution control technologies for ARGs in aquatic environments.