Studies on the breakdown of hormone herbicides and allied compounds by soil organisms

Abstract

Experiments were carried out using pure cultures of a soil bacterium (B. globiforme) to determine optimum conditions of growth for this organism in 2:4-D culture. The effect of aeration of liquid cultures and the addition of various other substances to the media was recorded. The ability of this organism to grow on other substrates was studied. Assays of phytotoxicity in the culture media were undertaken and the effect of concentration of 2 :4-D on its decomposition studied. Isolations of fresh organisms from garden soil capable of the detoxication of 2:4-D were attempted and these organisms subsequently grown, singly and in mixtures, in 2 :4-D culture In an attempt to produce a breakdown in culture similar to that found in soil. Experiments using a soil perfusion technique and also mixtures of organisms obtained from perfusate enriched to 2:4-D were also carried out. The original isolates became ineffective in 2:4-D decomposition after culture of approximately twelve months and fresh isolations had to be made. These resulted in the eventual isolation from fresh garden soil of a strain of the Actinomycete Nocardia which proved capable of utilising 2:4-D as a carbon source In agar and in aerated liquid culture. Subsequent experiments were carried out using sub-cultures of this organism. The effects of concentration of 2:4-D and of the presence of a soil extract and agar dialysate on detoxication of the culture fluids were recorded. The growth of this organism in unaerated liquid culture on various carbon substrates was studied and the subsequent ability of organisms so cultured to decompose 2:4-D in agar recorded. The decomposition of 2 :4-D in liquid culture was followed using radioactive samples of 2:4-D labelled in either the methylene or carboxyl groups of the side chain. The changes in radioactivity and phytotoxicity were followed chromatographically and measurements of chloride and phenolic material present in the culture media made throughout the detoxication period. Measurements of radioactivity and phenolic materials absorbed in an alkali trap through which air leaving the cultures was passed were also recorded. A possible route for the early stages of breakdown of the 2:4-D molecule by this strain of Nocardia is suggested involving the formation of 2:4-dichlorophenol, o-chlorophenol and phenol.