NO₃^- as an electron acceptor elevates antibiotic resistance gene and human bacterial pathogen risks in managed...

Managed aquifer recharge (MAR) stands out as a promising strategy for ensuring water resource sustainability. This study delves into the comparative impact of nitrate (NO₃^-) and oxygen (O₂) as electron acceptors in MAR on water quality and safety. Notably, NO₃^-, acting as an electron acceptor, has the potential to enrich denitrifying bacteria, serving as hosts for antibiotic resistance genes (ARGs) and enriching human bacterial pathogens (HBPs) compared to O₂. However, a direct comparison between NO₃^- and O₂ remains unexplored. This study assessed risks in MAR effluent induced by NO₃^- and O₂, alongside the presence of the typical refractory antibiotic sulfamethoxazole. Key findings reveal that NO₃^- as an electron acceptor resulted in a 2 times reduction in dissolved organic carbon content compared to O₂, primarily due to a decrease in soluble microbial product production. Furthermore, NO₃^- significantly enriched denitrifying bacteria, the primary hosts of major ARGs, by 747%, resulting in a 66% increase in the overall abundance of ARGs in the effluent of NO₃^- MAR compared to O₂. This escalation was predominantly attributed to horizontal gene transfer mechanisms, as evidenced by a notable 78% increase in the relative abundance of mobile ARGs, alongside a minor 27% rise in chromosomal ARGs. Additionally, the numerous denitrifying bacteria enriched under NO₃^- influence also belong to the HBP category, resulting in a significant 114% increase in the abundance of all HBPs. The co-occurrence of ARGs and HBPs was also observed to intensify under NO₃^- influence. Thus, NO₃^- as an electron acceptor in MAR elevates ARG and HBP risks compared to O₂, potentially compromising groundwater quality and safety.