Biodegradation of chlorsulfuron and metsulfuron-methyl by Aspergillus niger in laboratory conditions

Two sulfonylurea herbicides, chlorsulfuron and metsulfuron-methyl, were studied under laboratory conditions, in order to elucidate the biodegradation pathway operated by Aspergillus niger, a common soil fungus, which is often involved in the degradation of xenobiotics. HPLC-UV was used to study the kinetic of degradation, whereas LC-MS was used to identify the metabolites structure. In order to avoid the chemical degradation induced by a decrease in pH, due to the production of citric acid by the fungus, the experiments were performed in a buffered neutral medium. No significant degradation for both compounds was observed in mineral medium with 0.2% sodium acetate. On the contrary, in a rich medium, after 28 days the degradations, chemical degradation excluded, were about 30% for chlorsulfuron and 33% for metasulfuron-methyl. The main microbial metabolites were obtained via cleavage of the sulfonylurea bridge. In addition the fungus seems to be able to hydroxylate the aromatic ring of chlorsulfuron. In the case of metsulfuron-methyl the only detected metabolite was the triazine derivative, while the aromatic portion was completely degraded. Finally, the demethylation of the methoxy group on the triazine ring, previously observed with a Pseudomonas fluorescens strain, was not observed with A. niger.