Orthosulfamuron, a rice herbicide of the family pyrimidinyl sulfonylurea, causes ground water contamination for its moderate persistence and high water solubility. It may cause phytotoxicity and, thereby, affect the growth of succeeding crops. Keeping in view of the environmental safety, the interaction between orthosulfamuron and soil fungi was investigated. Ten different fungal species were found to survive in the soil containing orthosulfamuron at the level of 50 mg kg-1. However, based on the previous research reports, Aspergillus niger was selected for the degradation study on orthosulfamuron in the sterilized soil. It exhibited concentration-depended degradation rate for orthosulfamuron, where the rate decreased with the increasing concentration of the herbicide. The degradation pattern of the herbicide followed the first-order rate kinetics for each applied concentration. The structural elucidation of the five degradation products by liquid chromatography-mass spectroscopic analysis indicated that the fungal strain could degrade the herbicide through various biochemical processes. The major degradation of the herbicide took place through the hydrolytic cleavage of sulfonylurea bridge and cleavage of N (urea bridge)-C (pyrimidinyl ring) bond. The rate and degradation pattern of orthosulfamuron found in the present study strongly imply the presence of a recognition mechanism for the substrate and a consequent metabolic response system in the A. niger strain isolated from the agricultural soil.
Keywords: Aspergillus niger; Biodegradation; Herbicides; Orthosulfamuron; Sulfonylurea.