Cross-scale synergistic detoxification mechanisms of electric field in alleviating the inhibitory effects of oxytetracycline on anammox consortia

Anaerobic ammonium oxidation (anammox) is gaining increasing attention due to its economic and environmental advantages in nitrogen removal processes. However, anammox bacteria (AnAOB) are highly susceptible to inhibition by antibiotics. Therefore, maintaining high nitrogen removal performance of AnAOB in antibiotic-containing wastewater is critical for the broader application of this technology. In this study, a 103-day long-term experiment was conducted to investigate the alleviating effect of 0.5 V low-intensity direct current electric field (DCF) on oxytetracycline (OTC)-induced inhibition of AnAOB activity at different OTC concentrations. The results showed that under the same OTC concentration, the 0.5 V DCF application significantly (p < 0.01) enhanced the nitrogen removal rate, increased the relative abundance of Candidatus Kuenenia, upregulated the expression of functional genes, and furthermore mitigated the irreversible inhibition caused by OTC. This is attributed to a synergistic detoxification mechanism ranging from microscale (nitrogen metabolic compensation, replacement of intracellular binding sites, and OTC degradation) to macroscale (extracellular polymeric substances (EPS)-mediated blockage of OTC penetration and reduction of AnAOB's OTC exposure). Short-term batch experiments were performed to examine the inhibition of anammox activity by OTC with/without 0.5 V DCF in short term. A non-competitive inhibition model showed the electric field group resulted in a 525 % increase in the inhibition constant compared to the single OTC group. The molecular docking simulation results demonstrate at the atomic level that a well-defined and functionally rational representative extracellular protein can readily form a stable and energetically favorable binding conformation with OTC molecules through hydrogen bonds, hydrophobic interactions, and van der Waals forces. This provides a logical mechanistic explanation for our experimental observation that DCF induced an increase in EPS (particularly its protein content), thereby potentially strengthening the interaction with OTC, which in turn led to the recovery of system performance.