Comprehensive assessment of toxicity and environmental risk associated with sulfamethoxazole biodegradation in sulfur-mediated biological wastewater treatment.

Incomplete mineralization of sulfamethoxazole (SMX) in wastewater treatment systems poses a threat to ecological health. The toxicity and environmental risk associated with SMX biodegradation in the sulfur-mediated biological process were examined for the first time through a long-term (180 days) bioreactor study and a series of bioassays. The results indicated that the sulfur-mediated biological system was highly resistant and tolerant to SMX toxicity, as evidenced by the enrichment of sulfate-reducing bacteria (SRB), the improved microbial metabolic activity, and the excellent performance on pollutants removal under long-term SMX exposure. SMX can be effectively biodegraded by the cleavage and rearrangement of the isoxazole ring, hydrogenation and hydroxylation reactions in sulfur-mediated biological wastewater system. These biodegradation pathways effectively reduced the acute toxicity, antibacterial activity, and ecotoxicities of SMX and its biotransformation products (TPs) in the effluent of the sulfur-mediated biological system. The TPs produced via hydrogenation (TP1), hydroxylation, and isoxazole ring cleavage (TP3, TP4, TP5, TP8, and TP9) exhibited lower toxicity than SMX. Under SMX stress, although the abundance of sulfonamide resistance genes increased, the total abundance of ARGs decreased due to the extrusion of some intracellular SMX by the efflux pump genes and the inactivation of some SMX through the biodegradation process. Efflux pump and inactivation, as the main resistance mechanisms of antibiotics in the sulfur-mediated biological system, play a crucial role in microbial self-defense. The findings of this study demonstrate the great potential of the sulfur-mediated biological system in SMX removal, detoxication, and ARGs environmental risk reduction.

Stress-responses of activated sludge and anaerobic sulfate-reducing bacteria sludge under long-term ciprofloxacin exposure.

The activated sludge (AS) and sulfate-reducing bacteria (SRB) sludge systems were continuously operated for 200 days in laboratory to investigate the stress-responses of these two sludge systems under ciprofloxacin (CIP) exposure. It was found that CIP was effectively removed by SRB sludge via adsorption and biodegradation, but little biodegradation in AS system. The CIP biodegradation by SRB sludge made the SRB sludge system more sustainable and tolerant to long-term CIP exposure than AS system with significant (p < 0.05) CIP desorption and decrease of CIP removal. CIP shaped the microbial communities in AS and SRB sludge, and significantly (p < 0.05) inhibited the family Nitrosomonadaceae (ammonia-oxidizing bacteria (AOB)) and genus Nitrospira (nitrite-oxidizing bacteria (NOB)/complete ammonia oxidizer(comammox)) and the nitrogen removal in AS system. Moreover, CIP posed the increase of genus Zoogloea-like organisms and the non-filamentous bulking of AS, e.g. 313 ±â€¯12 mL/g of sludge volume index (SVI) at phase V (influent CIP = 5000 µg/L). The genus Desulfobacter was enriched in SRB sludge system under long-term CIP exposure, and stimulated chemical oxygen demand (COD) removal and sulfate reduction. The increase of genera Zoogloea, Acinetobacter and Flavobacterium in AS, and Zoogloea and Acinetobacter in SRB sludge systems under CIP exposure promoted extracellular polymeric substances (EPS) production and CIP adsorption for self-protection of microbes against CIP toxicity. The functional groups of N-H, O-H, C-O-C and C=O in EPS of AS and SRB sludge provided adsorption sites for CIP and impeded CIP impact on microbial cells. The findings of this study provide an insight into the stress-responses of AS and SRB sludge under long-term CIP exposure, and exhibit the great potential of treating CIP-laden wastewater by SRB sludge system.