Antibiotic-driven mechanisms in endogenous partial denitrification (EPD): Nitrite accumulation, microbial adaptation,...

The mechanisms by which antibiotics affect Endogenous Partial Denitrification (EPD) systems remain unclear. This study investigated the relationships between antibiotic type and pollutant removal, along with the underlying microbial metabolic mechanisms in an EPD system exposed to three antibiotics. The results showed that sulfadiazine (SD) and sulfamethoxazole (SMZ) exhibited higher chemical oxygen demand removal efficiency than tetracycline (TC). In the SD system, NO₃⁻-N removal was 84.13%, whereas other systems achieved 97%, which was associated with a higher abundance of the n arG and nirS. Proteobacteria and Chloroflexi demonstrated strong adaptability to the antibiotics. TC inhibited the tricarboxylic acid cycle and organic matter degradation may be related to the reduced mdh. SD induced microorganisms to convert carbon sources into polyhydroxybutyrate instead of utilizing them directly, while SMZ optimized nitrogen metabolism by increasing the abundance of nirS and nirK, leading to higher NO₂⁻-N accumulation in these two EPD systems. SD stress reduced the abundance of ppk2/ppx gene, inhibited the energy production and internal carbon reserve of denitrifying polyphosphate-accumulating organisms in anaerobic stage. Although the microorganisms had stronger phosphorus uptake potential by up-regulating ppk1 gene, the lack of ATP necessary to drive this process eventually led to the reduction of PO₄³ ⁻-P removal ability. The abundance of resistance genes peaked in the SD system, significantly increasing the risk of antibiotic resistance. This study deciphers the metabolic mechanisms of the EPD system in response to three types of antibiotics and provides a scientific basis for its application in environments with fluctuating antibiotic stress.