Bull. Environ. Contam. Toxicol. (1990)44:254-259 ~Environmental 9 1990 Springer-Verlag New York Inc. ~Contamination ~and Toxicology Application of the Microtox System to Assess the Toxicity of Pesticides and Their Hydrolysis Metabolites L. Somasundaram, J. R. Coats, K. D. Racke, and H. M. Stahr Department of Entomology and Verterinary Diagnostic Laboratory, Iowa State University, Ames, Iowa 50011, USA The soil microorganisms, bacteria in particular, play an important role in the environmental fate of soil-applied pesticides. The ability of bacteria to metabolize pesticides and/or their metabolites for their benefit has been well documented (Sudhakar- Barik et al. 1979; Racke and Coats 1987). One result of this catabolism is the failure of some pesticides to adequately control the target pests because of deereased persistence (Felsot 1989). One of the properties of pesticides and their metabolites that may influence the induction or inhibition of enhanced microbial degradation is their toxicity to the soil microbes that are responsible for degradation. Inasmuch as it would be inhibitively expensive and time consuming to determine the toxicities of pesticides and their metabolites fo ai1 possible soil bacteria involved in the enhanced degradation process, we used the Beckman Microtox system to assess their relative toxicities to a model bacterium. This system utilizes Photobacterium phosphoreum, a marine bacterium that is phylogenetically related to several genera of bacteria important in soil. The Microtox system was originally developed to assess the toxic effects of complex industrial effluents (Bulich 1984). Since its introduction in 1979, the application of the Microtox system has been extended to determine the toxicity of aquatic pollutants, wastewaters, fossil-fuel process waters, mycotoxins, and numerous other chemicals (Ribo and Kaiser 1987). Some state, provincial, and federal regulatory agencies employ the Microtox system in screening tests to monitor environmental pollutants. In this study, we used the Microtox analyzer to determine the relative microbial toxicities of some pesticides and their metabolites. Pesticides known to be susceptible to enhaneed degradation and others that have apparent resistence to enhanced degradation were included in this study. Because hydrolysis is a significant step in the chemical and microbial degradation of pesticides in soil (Matsumura 1980), the principal focus of this investigation was on hydrolytic metabolites. Send reprint requests to Joel Coats at the above address. 254