Simultaneous nitrogen removal and diclofenac degradation by Alcaligenes sp. FDN-09 under carbon-limited conditions.

Microorganisms capable of simultaneously achieving nitrogen removal and emerging organic contaminants (EOCs) degradation under carbon-limited conditions are highly desirable for wastewater treatment, however, such multifunctional performance remains poorly understood. Diclofenac (DCF), a frequently detected pharmaceutical with recognized environmental risks, is particularly recalcitrant to biological removal. In this study, an Alcaligenes sp. FDN-09 was identified as a strain capable of concurrent ammonium nitrogen removal and DCF attenuation under low C/N conditions. Optimal performance was achieved at C/N ratio of 4.0, with total nitrogen and DCF removal efficiencies of 63.3% and 80.2%, respectively, without detectable accumulation of NO₂⁻-N, NO₃⁻-N, or N₂O. Nitrogen balance analysis combined with genomic evidence revealed the presence of the dnfA/B/C gene cluster and the absence of conventional ammonia monooxygenase and hydroxylamine oxidoreductase genes, suggesting the involvement of alternative nitrogen transformation routes, including Dirammox-like pathways. Functional gene annotation and liquid chromatography-mass spectrometry analysis further indicated that DCF attenuation by strain FDN-09 may proceed through multiple transformation reactions, including hydroxylation, dechlorination, C-N bond cleavage, and aromatic ring opening, leading to the identification of nine major transformation products. Overall, this study provides process-level and genomic insights into the potential of Alcaligenes sp. FDN-09 for simultaneous nitrogen transformation and DCF degradation under carbon-limited conditions, supporting the feasibility of integrated nitrogen and EOCs removal in biological wastewater treatment systems.