Mechanistic insights into riboflavin-mediated recovery of the single-stage partial nitritation-anammox system under tetracycline stress

Abstract

Tetracycline (TC) contamination poses a potential threat to the stability of anammox-based wastewater treatment systems, yet effective strategies for accelerating functional recovery remain poorly understood. This study explored the inhibitory effects of 5 mg/L TC on a single-stage partial nitritation-anammox (SPNA) system, and further unraveled its functional potential and underlying mechanisms of exogenous riboflavin for restoring system performance. TC exposure substantially impaired nitrogen removal, with total nitrogen removal efficiency decreasing from 53.39 % to 34.44 %, accompanied by a pronounced decline in anammox activity. Supplementation with 0.5 mg/L riboflavin accelerated system recovery, partially restoring total nitrogen removal efficiency to 48.15 %, by enhancing anammox activity and suppressing nitrite-oxidizing bacteria proliferation.

Riboflavin alleviated these disturbances by enhancing electron transfer, mitigating oxidative stress, and restoring nitrogen metabolism-related functional genes. Metagenomic analysis further revealed that riboflavin recovered extracellular electron transfer and amino acid and nucleotide metabolism pathways disrupted by TC stress, while promoting the enrichment of tetracycline resistance and degradation-related genes and putative tetracycline-degrading bacteria, including Sphingobacterium and Bacillus. These responses were accompanied by enhanced tetracycline removal, suggesting potential mechanisms of improved microbial adaptation to antibiotic stress.