Mechanisms underlying enhanced simultaneous removal of tetracycline and nitrate by Paracoccus denitrificans...

Tetracycline (TC) and nitrate (NO₃^--N) co-contamination poses significant threats to aquatic ecosystems. In this study, an effective strain, identified as Paracoccus denitrificans XW1, was isolated and immobilized on wheat straw biochar for simultaneous TC and NO₃^--N removal. Results demonstrated a significant enhancement in TC and NO₃^--N removal by biochar-immobilized XW1 (IB), with IB exhibiting 1.29 to 8.90-fold greater TC removal and 1.03 to 2.57-fold higher NO₃^--N removal than free XW1 cells (FB) across varied environmental conditions. Mechanistic studies revealed that the enhanced TC removal was predominantly adsorption-dependent, whereas NO₃^--N elimination was primarily driven by biochar-facilitated biodegradation. Over five reuse cycles, IB sustained enhanced NO₃^--N removal (38.56% to 94.76%) but exhibited gradual TC decline (67.22% to 12.80%) from adsorption saturation, while FB rapidly lost TC degradation capacity within two cycles. Biochar promoted antioxidant enzyme production to mitigate oxidative stress induced by TC. Meanwhile, biochar stimulated extracellular polymeric substances (EPS) secretion, particularly humic acid-like substances, which concurrently enhanced TC adsorption and facilitated denitrification. Transcriptomics analyses further revealed that biochar upregulated genes associated with the tricarboxylic acid (TCA) cycle, ATP-binding cassette (ABC) transporters, and quorum sensing in XW1, consequently enhancing energy metabolism, enzymatic detoxification, and bacterial cooperation. This synergy enabled the efficient simultaneous bioremediation of TC and NO₃^--N. This work provides a sustainable biochar-microbe agent for complex water remediation.