Biodegradation of Herbicide Chlornitrofen (CNP) and mutagenicity of its degradation products

Abstract

Chlornitrofen has been widely used as a diphenyl-ether-derived herbicide for the rice paddy fields. Thus, the present study was undertaken to see whether CNP and its degradation products are mutagenic, and how CNP can be degraded in the natural environment. The following results were obtained.(1) CNP was weakly mutagenic in a new test strain YG 1029 but not mutagenic in the other strains tested here, even in both strains TA 100 and TA 98 that are conventionally used as the most sensitive strains in the Ames test. CNP was a direct-acting mutagen that does not require a metabolic activation system (S9 mix).(2) Amino-CNP, a degradation product of CNP, was potentially mutagenic in the strains YG 1029 or YG 1024 and was a moderate mutagen in the other test strains employed here. Amino-CNP required S9 mix for the mutagenesis, suggesting that a metabolite of amino-CNP is an active mutagen. Acetylamino-CNP, another degradation product of CNP, was also mutagenic with almost the same strains, although the mutagenic potency was somewhat lower than that of amino-CNP. No mutagenicity was detected for the other degradation products, hydroxy-CNP, dichlorophenol, trichlorophenol, and p-chlorophenol.(3) The diphenyl-ether-derived herbicides nitrofen and chlormethoxynil, which are structurally related to CNP, and their amino derivatives were tested for their mutagenicity in a similar way. Chlormethoxynil was not mutagenic in all strains tested, but nitrofen was highly mutagenic in strain YG 1026 and weakly in YG 1029 when tested in the presence of S9 mix. In contrast, amino-nitrofen and amino-chlormethoxynil were mutagens with quite similar properties to those of amino-CNP, respectively.(4) The biological degradation of CNP was tested using river water. It was observed that CNP degraded rapidly and was converted to amino-CNP with a high efficiency in river water. An increase in the mutagenicity of the testing solution could be seen depending upon the decay of CNP, formation of amino-CNP, and elevation of temperature. It seems that most of the mutagenicity observed may be due to the mutagenicity of amino-CNP formed from CNP by microorganisms in the river water.