Aerobic biodegradation of hexachlorociclohexane isomers by Streptomyces sp M7

Abstract

The Hexachlorocyclohexane (HCH) isomers are highly toxic and persistent organochlorine pesticides, which have caused serious environmental problems since they began to be produced at the beginning of the1940s. During the synthesis of lindane (-HCH), other isomers were also synthesized. There are seven isomers (, , , , , , ), but only -HCH possesses insecticidal activity. However, the other isomers are still toxic for humans and in the environment they cause an accumulation of the more persistent form, the β-HCH. For this reason we have focused on the ability of biotransformation of HCH isomers by Gram-positive microorganisms and particularly by actinobacteria species. Therefore the objective of this work was to study the optimum conditions for the degradation of - and - isomers and the determination of the -HCH isomer intermediaries during the first steps of the degradation by Streptomyces sp. M7. Streptomyces sp. M7, previously isolated from pesticide contaminated soils, was grown in Minimal Medium (MM), supplemented with each HCH isomer (,  and ). They were aseptically added to the autoclaved medium at different final concentrations (1.66, 8.3, and 16.6 mg L-1). The strain was also grown in presence of glucose as sole carbon source. All cultures were incubated on a rotatory shaker (100 rpm) at 30 ºC, for 96 h. The biomass produced was measured by dry weight, washing the pellets with 25 mMTris-EDTA buffer (pH 8.0) and drying to constant weight at 105 ºC. Streptomyces sp. M7 has the ability to use α- and β-HCH isomers as carbon source, and to remove them from the culture medium, at 30 ºC and an initial pH of 7, and 30 ºC and initial pH of 9, respectively, until the maximum concentration studied. The removal capacity of the α-HCH was not affected to the highest concentration tested, while the β-HCH removal capacity was affected at a concentration of 16.6 mg L-1. When tested with isomers mixtures (α-, β-, -HCH) the removal capacity decreased to 40% less than in studies with individual isomers. The metabolic intermediates detected by Gas Chromatograph-Mass Spectrometry in cell free extracts from cultures of Streptomyces sp. M7 with α-HCH as carbon source were: 1,2-dichlorobenzene; 1,3 - or 1,4-dichlorobenzene and 1,2,3-, 1,2,4- or 1,3,5-trichlorobenzene compounds that were also detected in Sphingobium. These results represent a significant advance in the study of the growth of actinobacteria in presence of HCH-isomers. For this reason Streptomyces sp. M7 could be considered as promising strains for HCH isomers degradation from contaminated areas. Biodegradation, Streptomyces, isomers HCH, Lindane