Biodegradation of glyphosate herbicide in vitro using bacterial isolates from four rice fields

Abstract

Glyphosate is a compound used as herbicide in the control and/or killing of grasses and herbaceous plants. It can be used in no-till agriculture, to prepare fields before planting, during crop development and after crop harvest. Because of its toxicity to non-target organisms, there is need to decontaminate glyphosate contaminated soils and bioremediation is a very useful alternative to conventional cleanup methods. The success of this will depend on isolating bacteria with the ability to degrade glyphosate in a changing environment. The abilities of five bacterial species (Escherichia sp, Azotobacter sp., Alcaligenes sp., Acetobacter sp. andPseudomonas fluorescens) to degrade glyphosate herbicide under varying environmental conditions were evaluated in this study. The isolates were screened for glyphosate utilization using mineral salt medium containing glyphosate as carbon and/or phosphorus source. Of the five bacterial isolates, P. fluorescens andAcetobacter sp. showed the capacity to utilize glyphosate efficiently and were therefore used for further biodegradation studies. Time course of growth of the isolates on mineral salt medium containing glyphosate showed that both grew significantly (P < 0.05). Microbial growth during the study was monitored by measuring the optical density at 660 nm. The comparative effects of glyphosate as carbon and/or phosphorus source on the growth of the isolates showed that there was significant (P < 0.05) growth in the medium containing glucose and glyphosate. The effects of different concentrations of glyphosate on the growth of the isolates (P. fluorescens and Acetobacter sp) were evaluated. Significant (P < 0.05) growth was observed at lower concentrations (7.2 - 25 mg/ml) of glyphosate. No inhibition of growth was observed at high concentrations (100 - 250 mg/ml), indicating that the isolated bacteria can tolerate up to 250 mg/ml of glyphosate. However, there was subsequent decrease in growth of both isolates as the concentration of glyphosate increased. This study showed that P. fluorescens and Acetobacter sp. exhibited a high capacity to efficiently degrade glyphosate under the environmental conditions studied. Thus, the organisms can be exploited for biodegradation of glyphosate and should be studied for their ability to degrade other organophosphates. Key words: Glyphosphate herbicide, degradation process.