Biodegradation of sulfonamide antibiotics by a soil bacteria enrichment and the impacts of soil organic matter.

Biodegradation plays a crucial role in the removal of sulfonamides (SAs) from soils; however, the biodegradation pathways in soils and the impacts of soil organic matter (SOM) on SA biodegradation remain unclear. Here, we used [phenyl-U-14C]-labeled SAs to investigate the degradation of sulfadiazine (SDZ), sulfamonomethoxine (SMM), and sulfamethoxazole (SMX) in a soil-free enrichment culture derived from an SDZ-degrading soil microbial community in the absence or presence of soil humic acids and artificial root exudates. The culture utilized the individual SAs as the sole carbon source and mineralized 60.4%-65.4% of the phenyl ring within 156 h, which was not inhibited by the antifungal actidione, suggesting a predominant bacterial contribution to the degradation. Several typical SA-degrading genera, including Achromobacter, Brevundimonas, Leucobacter, Microbacterium, Pseudomonas, and Rhodococcus, were enriched, and 16, 14, and 10 metabolites of SDZ, SMM, and SMX were identified, respectively. Twelve primary transformation pathways were proposed, including sulfonamide bond cleavage, desulfonylation, para-amino group modification, and heterocyclic moiety modification. Notably, the downstream transformation pathways of two desulfonylation products were elucidated, revealing their contributions to SA mineralization. The presence of additional organic matter, especially humic acids, significantly promoted the degradation and mineralization via covalent binding or co-metabolism, and substantially altered the dynamics and amounts of SA metabolites. Though biodegradation of SAs in soil can be much lower than in bacterial enrichment culture, our results provide insights into the complex SA transformation by soil microbial communities and the regulatory effects of SOM, with new implications for managing SA-contaminated environments.