Microorganisms capable to degrade organochlorine pesticides

Abstract

The γ-HCH is an organochlorine pesticide used in agriculture and medicine to world level. It has a big tendency to bioaccumulate into the environment. There are many reports indicating that hexachlorocyclohexane (γ-HCH) is present in soil, water, air, plants, agricultural products, animals, food, microbial environments etc. Considered a potential carcinogen and listed as a priority pollutant by the US EPA, γ-HCH is a lipophilic compound and therefore tends to accumulate and concentrate in the body fats of animals and humans. In Salí River, Tucumán, Argentina, lindane was detected 10-fold in relation to the traces permitted concentrations. Hence the development of new technologies to remediate these sites using microorganisms is every time more necessary.The actinomycetes are Gram positive bacteria with great potential to bioremediate xenobiotics. One strain, Streptomyces sp. M7, isolated from organochlorine pesticides (OPs) contaminated sediment was selected for its capacity to grow in presence of lindane as only carbon source. This microorganism was cultured in soil extract medium added of lindane 100 µg L-1, obtaining a maximal growth of 0.065 mg mL-1, similar to the control, with a highest lindane remotion of 70.4% at 30 ºC and pH 7. When different initial pesticide concentrations (100, 150, 200 and 300 µg L-1) were added in soil medium, an increment of the microbial growth was detected in all the concentrations tested. Also a diminution of the residual lindane concentration was determined in the soil samples in relation to the abiotic controls (29.1; 78.03; 38.81 and 14.42% respectively).Besides it was determined the optimum Streptomyces sp. M7 inoculum when lindane 100 µg Kg-1 soil was added to the soil sample. It was 2 g Kg-1 soil for obtaining the most efficiently bioremediation process, the lindane removal in these conditions was 67.8% at 28 days of incubation. Later it was considered necessary to know the pesticide effects on maize plants seeded in lindane-contaminated soil previously inoculated with Streptomyces sp. M7. Lindane concentrations of 100, 200, and 400 mg kg-1 soil did not affect the germination and vigor index of maize plants seeded in contaminated soils without Streptomyces sp. M7. When this microorganism was inoculated at the same conditions a better vigor index was observed and 68% of lindane was removed.These results confirm the potential lindane-contaminated soil bioremediation of Streptomyces sp. M7. There are some reports regarding aerobic degradation of γ-HCH by Gram-negative bacteria like Sphingomonas and by the white-rot fungi Trametes hirsutus, Phanerochaete chrysosporium, Cyathus bulleri and Phanerochaete sordida. However, little information is available on the ability of biotransformation of organochlorine pesticides by Gram positive bacteria and particularly by actinomycete species. It was demonstrated that Streptomyces sp. M7 possesses the LinA enzyme that catalyzes dehydrochlorination of γ-HCH to 1,3,4,6-tetrachloro-1,4-cyclohexadiene (1,4-TCDN) via γpentachlorocyclohexene (γ-PCCH). The increase of γ-PCCH was detected in the time by Gas Chromatography (GC).On the other side two actinomycetes defined consortium with lindane biodegradation capacity, were isolated from soil samples from a deposit of organochlorine pesticides in the locality of Santiago del Estero, Argentina. These consortia: Streptomyces sp.: A2-A5-M7-A11 and A2-A5-A8 produced a significant increment of the specific dechlorinase activity (SDA), compared to the individual culture with 1.66 mg L-1 of lindane as the only carbon source. Therefore actinomycete strains could be considered one of the most promising bacterial groups for lindane biodegradation in contaminated environment.