Metabolic fate of the ¹⁴C-labeled herbicide clodinafop-propargyl in a sediment–water system

Abstract

The metabolic fate of ¹⁴C-phenyl-labeled herbicide clodinafop-propargyl (¹⁴C-CfP) was studied for 28 days in lab assays using a sediment–water system derived from a German location. Mineralization was 5.21% of applied ¹⁴C after 28 days exhibiting a distinct lag phase until day 14 of incubation. Portions of radioactivity remaining in water phases decreased at moderate rate to 18.48% after 28 days; 62.46% were still detected in water after 14 days. Soxhlet extraction of the sediment using acetonitrile released 35.56% of applied ¹⁴C with day 28, while 33.99% remained as non-extractable residues. A remarkable increase of bound ¹⁴C was observed between 14 and 28 days correlating with the distinct increase of mineralization. No correlation was found throughout incubation with microbial activity of the sediment as determined by dimethyl sulfoxide reduction. Dissolved oxygen and pH value of water phases remained almost constant for 28 days. Analyses of Soxhlet extracts of the sediment and ethyl acetate extracts of water phases by radio-TLC and radio-HPLC revealed that CfP was rapidly cleaved to free acid clodinafop (Cf), which was further (bio-) transformed. DT₅₀ values (based on radio-HPLC) were below 1 day (CfP) and slightly above 28 days (Cf). Further metabolites were not detected. Fractionation of humic and non-humic components of the sediment demonstrated that CfP's non-extractable residues were predominantly associated with fulvic acids up to 14 days of incubation (3.36%), whereas after 28 days, the majority of radioactivity was found in the humin/mineral fraction (13.30% of applied ¹⁴C). Due to high-performance size-exclusion chromatography of the fulvic acids fraction derived from assays incubated for 28 days, this portion of ¹⁴C was firmly, possibly covalently bound to fulvic acids and did not consist of CfP or Cf. Using an isolation strategy comprising preincubation of sediment with CfP and mineralization of ¹⁴C-CfP as criterion, a microorganism was isolated from the sediment examined. It grew on ¹⁴C-CfP as sole carbon source with evolution of ¹⁴CO₂. The bacterium was characterized by growth on commonly used carbon sources and 16S rDNA sequence analysis. Its sequence exhibited high similarity with that of Nocardioides aromaticivorans strain H-1 (98.85%; DSM 15131, JCM 11674).

Keywords:

• ¹⁴C-Clodinafop-propargyl
• herbicide
• fate in sediment-water system
• non-extractable residues
• mineralization
• biodegradation
• Nocardioides aromaticivorans
• high-performance size-exclusion chromatography (HPSEC)
• 16S rDNA sequence analysis

Acknowledgments

We would like to thank Dr. H.-P. Klenk (DSMZ, Braunschweig, Germany) for the 16S rDNA sequence analysis and Dr. C. Brinkmann from The German Federal Institute of Hydrology (BfG; Koblenz, Germany) for kindly providing sediment and water from river Rhine. We gratefully acknowledge technical assistance by Mrs. B. Thiede and Mrs. H. Patti.

Funding

We gratefully acknowledge Y. Yuan's fellowship funded by the China Scholarship Council of the People's Republic of China and financial assistance of the students of the Biological Department of RWTH Aachen University for purchasing ¹⁴C-CfP.