The effect of alternate nitrogen sources on carbofuran degradation in soil and in pure culture

Donald Philip Breakwell, Purdue University

Abstract

The phenomenon of enhanced biodegradation of carbofuran has been widely observed. However, efforts to describe the in situ microbial ecology of this phenomenon have been limited. Characterization of bacteria that develop the ability to degrade pesticides, both on the physiological and genetic levels, will aid in our understanding of enhanced soils. Carbofuran-degrading bacteria can use the methylcarbamate moiety of carbofuran as a nitrogen (N) source. Alternate N sources (ammonium, urea, and methylamine) were used to observe carbofuran degradation in soil degradation studies. A simple logistic model and a deterministic three-half-order model were used to describe degradation studies. Using the first and second derivatives of the fitted models, maximum degradation rates, and the length of the lag phase have been calculated. Although, the three-half-order model fit the data better, it tended to overestimate the instanteous degradation rate defined by its first derivative. Using the simple logistic model, we were able to determine that methylamine significantly decreased the maximum degradation rate and increased the length of the lag phase. Thus mathematical characterization permits quantitative analysis of pesticide degradation curves. Four different Gram negative rod bacteria, designated MAC1, MAC2, MAC3, and MAC4, were isolated from an enhanced soil. These organisms were classified as strains of the genus Pseudomonas based on their phenotypic similarities. Using random amplified polymorphic DNA (RAPDs) and denaturing gradient gel electrophoresis (DGGE), genetic differences were shown between the isolates. Pseudomonas sp. strains MAC2 and MAC3 were grown on carbofuran and NH$\sb4$Cl as sole N sources. Carbofuran-grown cells were able to degrade carbofuran within 7 h and 11 h for strains MAC2 and MAC3, respectively. However, when strains MAC2 and MAC3 were grown on ammonium, cells required 72 hand 96 h for the cabofuran to be degraded. This indicates that carbofuran degradation was repressed by ammonium, and suggests that expression of carbofuran hydrolase may be governed by global N regulation. The prolonged induction lag by these cells may help in explaining the degradation lag in soil because carbofuran is applied just below the root at planting and when soil nitrogen is plentiful.

Degree

Ph.D.

Advisors

Turco, Purdue University.

Subject Area

Agronomy

COinS