Insights into antibiotic sulfadiazine effects on denitrification process of Paracoccus denitrificans during continuous aerobic-anaerobic shift

Nitrogen-driven eutrophication progressively depleting oxygen frequently occurs in aquatic ecosystems, where the co-occurrence of extra pollutants such as antibiotics substantially impacts microbial nitrate removal processes. However, the denitrification response to oxygen level transition under antibiotic stress remains unexplored. Paracoccus denitrificans , a nirS -type aerobic denitrifiers, was exposed to both antibiotic sulfadiazine (SDZ) and DO transition. As SDZ concentrations increased from 0 to 1,000 μg/L, nitrate reduction inhibition rates rose to ~ 39%, and electron transport system activity decreased by 0.08 ~ 0.11 (µg O 2 ·g −1 protein·min −1 ). During the aerobic-anaerobic transition, inhibition rates dropped from 53.15% to 3.01%, indicating partial adaptation of P. denitrificans to SDZ stress. Transcriptomic analyses revealed that SDZ treatment decreased the expression of genes nirS and nosZ more significantly than the other denitrifying genes. Moreover, SDZ interfered with folate metabolism, triggering the downregulation of genes involved in folate ( folK/P ) and purine ( purU ) synthesis. In response to SDZ stress, P. denitrificans initially downregulated genes in genetic information processing and energy metabolism pathways, followed by upregulating the genes in ribosome and citrate cycle pathways, and finally elevated expression of key ABC transporters (e.g., mineral and organic ion transporters) and quorum sensing (e.g., Type IV secretion system) genes. Additionally, the key gene profiles related to DO shifts were identified and enriched in pathways such as oxidative phosphorylation. This study provides important insights into the effects of antibiotics on nirS -type denitrifiers under DO changes.