Pesticide-degrading microbial populations

Pesticide-Degrading Microbial Populations. Innovative microbiological investigations have paved the way for increased understanding of the mechanisms of enhanced degradation. One of the... 

The microbial metabolic process is the major mechanism for the transformation of toxic organic chemicals in the subsurface environment. The transformation process may be the result of a primary metabolic reaction, when the organic molecule is degraded by a direct microbial metabolism. Alternatively, the transformation process may be an indirect, secondary effect of the microbial population on the chemical and physical properties of the subsurface constituents. Bollag and Liu (1990), considering behavior of pesticides, defined five basic processes involved in microbially mediated transformation of toxic organic molecules in the soil upper layer environment. These processes are described next. 

Several mechanisms have been proposed to explain how a soil microbial population becomes adapted to a particular pesticide to result in enhanced degradation and also to explain how this ability can be retained for months and even years without further pesticide applications. Although a discussion of this subject is considered beyond the scope of this paper, the topic has been extensively reviewed 3, 4, 3j), 40j. ... 

Several factors that influence enhanced microbial degradation include nutrient value of the metabolite molecule, toxicity of the metabolite to soil microorganisms, and the availability of the metabolite to soil microbes. Comparisons of several pesticides, and their respective degradation products provide insight into the question of why soil microbial populations can develop rapid degradation capabilities for some pesticides but not others. 

The size of Che microbial populations able to rapidly degrade carbofuran in soils enhanced for its degradation were enumerated by means of substrate addition and fumigation. Use of these techniques followed unsuccessful attempts to enumerate the population using plate or direct counts in the enhanced soils. Overall biomass size declined following application of carbofuran. No biomass suppression was observed in the non-enhanced soils and implies this suppression may be related to the formation of metabolites such as carbofuran-phenol or methylamine. In the enhanced soils, 6% of applied pesticide was initially incorporated into biomass carbon. This contrasts with 0.87% incorporation in the non-enhanced soils. After 15 days there was complete loss of Che pesticide at this time the biomass contained 2% of the applied material. 

Yentumi and Johnson (6) reported that frequency of pesticide applications shifted the population, and repeated applications of paraquat lowered the size of the microbial biomass as determined by plate counts. Moorman (19) approached the estimation of pesticide degrader biomass size using an MPN procedure. He found that following high rate (60 /xg g ) applications of EPTC in soils enhanced for EPTC degradation, the enhanced biomass was increased as compared to the control soil. 

One recent agricultural phenomenon that may be elucidated by such basic research Is the manner In which problem soils arise. In these soils, the microbial population degrades applied pesticides so rapidly that pesticide efficacy Is lost (30). Presumably, the repeated application of a particular pesticide acts as an enrichment procedure, constantly selecting for Increased numbers of the particular microorganisms that can metabolize the pesticide and derive some nutritional benefit from It. Several basic questions that need to be answered In order to understand this process. First, where do the genes that encode the pesticide degradation enzymes... 

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