Fluorescent Pseudomonas isolates from Mississippi Delta oxbow lakes: In vitro herbicide biotransformations

Fluorescent pseudomonads were a major component [log (10) 4.2 - 6.1 colony- forming units mL(-1)] of the culturable heterotrophic gram-negative bacteri oplankton observed in three Mississippi Delta oxbow lakes in this study. Pu re cultures of fluorescent pseudomonads were isolated from three Mississipp i Delta oxbow lakes (18 per lake), using selective media S-l. Classical phy siological tests and Biolog GN plates were used in criteria for taxonomic i dentification. Most isolates were identified as biotypes of Pseudomonas flu orescens 55% (I I), 7% (I I I), and 25% (V). About 7% of the isolates were identified as P. putida and 7% as non-fluorescent Pseudomonas-like. Cell su spensions of these isolates were tested for their ability to metabolize/co- metabolize six C-14-radiolabeled herbicides (2,4-dichlorophenoxyacetic acid (2,4-D), cyanazine, fluometuron, metolachlor, propanil, and trifluralin) t hat are commonly used for crop production in this geographical area. Almost all (53 of 54) isolates transformed trifluralin via aromatic nitroreductio n. Most isolates (70%) dechlorinated metolachlor to polar metabolites via g lutathione conjugation. About 60% of the isolates hydrolyzed the amide bond of propanil (a rice herbicide) to dichloroaniline, with the highest freque ncy of propanil-hydrolyzing isolates observed in the lake from the watershe d with rice cultivation. All propanil-hydrolyzing isolates were identified as P. fluorescens biotype 11. No metabolism of cyanazine or fluometuron was observed by any isolates tested, indicating little or no potential for N-d ealkylation among this group of bacterioplankton. No mineralization of 2,4- D labeled in either the carboxyl or ring position was observed. These resul ts indicate that reductive and hydrolytic pathways for herbicide cc-metabol ism (aromatic nitroreduction, aryl acylamidase, and glutathione conjugation ) are common in Mississippi Delta aquatic fluorescent pseudomonads; however , the potential for certain oxidative transformations (N-dealkylation, cyan o group oxidation) may be rare in this group of bacterioplankton. (C) 2001 by John Wiley & Sons, Inc.