Atrazine degradation previously has been shown to be carried out by individual bacterial species or by relatively simple consortia that have been isolated using enrichment cultures. Here, the degradative pathway for atrazine was examined for a complex 8-membered enrichment culture. The species composition of the culture was determined by PCR-DGGE. The bacterial species included Agrobacterium tumefaciens, Caulobacter crescentus, Pseudomonas putida, Sphingomonas yaniokuyae, Nocardia sp., Rhizobium sp., Flavobacterium oryzihabitans, and Variovorax paradoxus. All of the isolates were screened for the presence of known genes that function for atrazine degradation including atzA,-B,-C,-D,-E,-F and trzD,-N. Dechlorination of atrazine, which was obligatory for complete mineralization, was carried out exclusively by Nocardia sp., which contained the trzN gene. Following dechlorination, the resulting product, hydroxyatrazine was further degraded via two separate pathways. In one pathway Nocardia converted hydroxyatrazine to N-ethylammelide via an unidentified gene product. In the second pathway, hydroxyatrazine generated by Nocardia sp. was hydrolyzed to N-isopropylammelide by Rhizobium sp., which contained the atzB gene. Each member of the enrichment culture contained atzC, which is responsible for ring cleavage, but none of the isolates carried the atzD,-E, or -F genes. Each member further contained either trzD or exhibited urease activity. The enrichment culture was destabilized by loss of Nocardia sp. when grown on ethylamine, ethylammelide, and cyanuric acid, after which the consortium was no longer able to degrade atrazine. The analysis of this enrichment culture highlights the broad level bacterial community interactions that may be involved in atrazine degradation in nature.

Carbon and N dynamics may be particularly important for selective enrichment of microorganisms that are capable of using xenobiotics as sources of N for growth. To investigate this hypothesis in relation to s-triazines, a soil microcosm study was performed to determine the effect of organic amendments differing in complexity and C/N ratio, and the effects of inorganic N addition on atrazine mineralization in a soil having a 15-yr history of prior exposure to this herbicide. When the soil was spiked with 100 mg/kg of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) in the absence of organic amendments, 73% of the atrazine was mineralized after 11 wk. Soils amended with rice hulls, starch, and compost yielded mineralization rates of 88, 75, and 59% in the same period, respectively. In contrast, <10% of the atrazine was mineralized in soils amended with glucose, Sudan hay, or sodium citrate. All treatments receiving supplemental inorganic N had a considerably lower rate of atrazine mineralization than corresponding treatments without N addition. However, the different effects of the organic matter supplements suggested there was no relationship between the C/N ratio of the soil and atrazine mineralization. An atrazine-degrading consortium was subsequently isolated for further characterization. The results demonstrate that while atrazine mineralization is suppressed under high N conditions in this soil, the mineralization rate also is influenced by poorly understood population dynamics related to the nutrient composition and complexity of specific organic amendments.A RAZINE is one of the most commonly used herbicides in the world. Because of its wide use and persistence in the environment, atrazine is causing increased problems with groundwater contamination (6, 19, 20). Rates

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Plant rhizosphere effects on atrazine degradation were examined in soil inoculated with an atrazine-mineralizing bacterial consortium. The consortium, consisting of three bacterial species, was isolated from an agricultural soil having previous long-term exposure to the herbicide. Atrazine mineralization and metabolite formation were monitored by measuring 14CO2 evolution from microcosms amended with radiolabeled atrazine and by HPLC of soil extracts. In noninoculated soil, ca. 11% of 14C-chain-labeled atrazine was N-dealkylated, while only 2.4% of the ring-labeled atrazine was mineralized after 5 weeks. Corn plants had no effect on atrazine mineralization or ethyl-side-chain N-dealkylation in noninoculated soils, but the formation of hydroxyatrazine was significantly enhanced in planted soil. Growth of corn in sterilized soil suggested that hydroxyatrazine formation was caused by plant metabolism of atrazine. Introduction of the atrazine-mineralizing consortium into the soil significantly increased the rate of atrazine mineralization in comparison to noninoculated soil. After 4 weeks, 71% of the atrazine was mineralized in nonplanted soil, whereas 84% of the atrazine was mineralized in soil with corn plants. There was no significant difference in the rate of atrazine mineralization by the consortium in nonplanted and planted soil. However, atrazine-mineralizing populations at the end of the incubation were higher in the planted soil, which contained 8.1 × 104 degraders g-1 of soil versus 2.7 × 103 degraders g-1 in soil without plants. The results demonstrated that bioaugmentation with the atrazine-mineralizing consortium greatly enhanced the rate of atrazine mineralization. Long-term survival of the consortium and degradation of atrazine to hydroxyatrazine were both enhanced in rhizosphere soil, but corn seedlings had no significant effect on the rate of atrazine mineralization, either by the indigenous microflora or in soil inoculated with atrazine-mineralizing bacteria.