Microalgae Enhances the Adaptability of Epiphytic Bacteria to Sulfamethoxazole Stress and Proliferation of Antibiotic Resistance Genes Mediated by Integron

The transmission of ARGs in the microalgae-associated epiphytic bacteria remains unclear under antibiotic exposure, apart from altering the microbial community structure. In this study, <i>Chlorella vulgaris</i> cocultured with bacteria screened from surface water was examined to explore the spread of ARGs in the presence of sulfamethoxazole (SMX). The extracellular polymers released by <i>Chlorella vulgaris</i> could reduce antibiotic-induced collateral damage to bacteria, thus increasing the diversity of the microalgae-associated epiphytic bacteria. The abundances of <i>sul1</i> and <i>intI1</i> in the phycosphere at 1 mg/L SMX dose increased by 290 and 28 times, respectively. Metagenomic sequencing further confirmed that SMX bioaccumulation stimulated the horizontal transfer of <i>sul1</i> mediated by <i>intI1</i> in the microalgae-associated epiphytic bacteria, while reactive oxygen species (ROS)-mediated oxidative stress induced the SOS response and thus enhanced the transformation of <i>sul1</i> in the J group. This is the first study to verify that microalgae protect bacteria from antibiotic damage and hinder the spread of ARGs mediated by SOS response, while the transfer of ARGs mediated by integron is promoted due to the bioaccumulation of SMX in the phycosphere. The results contribute to present comprehensive understanding of the risk of ARG proliferation by the presence of emerging contaminants residues in river.