engleski

Enrichment and Isolation of Atrazine Degrading Bacteria

Several polluted and unpolluted ecosystems were studied in order to check potential presence of bacteria capable of degrading atrazine (2-chloro-4(ethylamino)-6-(isopropylamino)-1, 3, 5-triazine), a widely used selective herbicide for the control of grassy and broadleaf weeds in the cultivation of corn and other crops. The enrichment of atrazine degrading bacteria was carried out in continuous-flow units under the inflow of mineral salts medium (MSM) containing 25 mg/L of atrazine and 50 mg/L of yeast extract at a dilution rate (D) of 0.1 1/h. After 8 week cultivation period under these conditions all enriched mixed bacterial cultures originating from polluted ecosystems (atrazine production wastewaters and the soil within the herbicide factory exposed to many year repeated spills of effluent from atrazine synthesis) as well as those originating from undisturbed meadow soils (never subjected to atrazine treatment) showed a substantial atrazine degradation activity (monitored by HPLC analysis and carbon dioxide evolution test). The structure of bacterial cultures changed significantly during continuous cultivation and, in all the cases, from the initial cultures containing seven to nine morphologicaly different bacteria were enriched three or four membered associations (determined by plating of appropriate dilutions on the surface of nutrient agar and selective atrazine-agar plates). Furthermore, the initially dominant bacteria decreased in proportion or completely disappeared and new types of bacteria appeared in enriched cultures. The evolution and enrichment of structurally new mixed cultures suggested that relationships between culture members, including the transfer of genetic information between them, may be important in establishing catabolic sequences and increasing the rate of atrazine mineralization. A combination of continuous cultivation in MSM medium under carbon and nitrogen limitation and the plating on selective atrazine-agar plates with the addition of citrate or ethylammonium chloride as additional carbon source resulted in the selection and isolation of several bacteria which were further characterized for their atrazine catabolizing activity. One of these isolated bacteria (tentatively identified to belong to the genus Pseudomonas) showed a substantial atrazine mineralizing activity when grown as a pure culture (>60% of total carbon released as CO2). This bacterial strain both as a pure culture or in association with other strains in enriched cultures was additionally examined to determine the factors that govern atrazine transformation activity and to evaluate its potential effectiveness in the treatment of effluents from atrazine production.

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