Through selective enrichment of atrazinemetabolizing microorganisms, a microbial community was selected from agricultural soil. Bacterial isolates, identified by their closest similarity with 16S rDNA sequences stored in NCBI GeneBank, belonged to the genera: Massilia, Stenotrophomonas, Klebsiella, Sphingomonas, Ochrobactrum, Arthrobacter, Microbacterium, Xanthomonas and Ornithinimicrobium. From these strains, only the first six used atrazine as nitrogen and carbon source. The microbial community attached to a non-porous support was evaluated for its atrazine biodegradation rate and removal efficiency under aerobic conditions in two types of packed-bed biofilm reactors fed with a mineral salt medium containing glucose plus atrazine, or atrazine as the sole carbon and nitrogen source. Removal efficiencies near 100% were obtained at loading rates up to 10 mg l -1 h -1 . After long periods of continuous operation, the richness of microbial species in biofilm reactors diminished to only three bacterial strains; Stenotrophomonas sp., Ochrobactrum sp. and Arthrobacter sp. By PCR analysis of their DNA, the presence of atzABC genes codifying for the enzymes of the upper catabolic pathway of atrazine, was confirmed in the three strains. The gene atzD that encodes for the cyanuric acid amidohydrolase enzyme was detected only in Stenotrophomonas sp.

Using a successive transfer method on mineral salt medium containing simazine, a microbial community enriched with microorganisms able to grow on simazine was obtained. Afterwards, using a continuous enrichment culture procedure, a bacterial community able to degrade simazine from an herbicide formulation was isolated from a chemostat. The continuous selector, fed with a mineral salt medium containing simazine and adjuvants present in the commercial herbicide formulation, was maintained in operation for 42 days. Following the lapse of this time, the cell count increased from 5 x 10(5) to 3 x 10(8) CFU mL(-1), and the simazine removal efficiency reached 96%. The chemostat's bacterial diversity was periodically evaluated by extracting the culture's bacterial DNA, amplifying their 16S rDNA fragments and analyzing them by thermal gradient gel electrophoresis. Finally, a stable bacterial consortium able to degrade simazine was selected. By PCR amplification, sequencing of bacterial 16S rDNA amplicons, and comparison with known sequences of 16S rDNA from the NCBI GenBank, eight bacterial strains were identified. The genera, Ochrobactrum, Mycobacterium, Cellulomonas, Arthrobacter, Microbacterium, Rhizobium and Pseudomonas have been reported as common degraders of triazinic herbicides. On the contrary, we were unable to find reports about the ability of the genus Pseudonocardia to degrade triazinic compounds. The selected bacterial community was attached to a porous support in a concurrently aerated four-stage packed-bed reactor fed with the herbicide. Highest overall simazine removal efficiencies eta (SZ) were obtained at overall dilution rates D below 0.284 h(-1). However, the multistage packed bed reactor could be operated at dilution rates as high as D = 3.58 h(-1) with overall simazine removal volumetric rates R (v,SZ) = 19.6 mg L(-1) h(-1), and overall simazine removal specific rates R (X,SZ) = 13.48 mg (mg cell protein)(-1) h(-1). Finally, the consortium's ability to degrade 2-chloro-4,6-diamino-1,3,5-triazine (CAAT), cyanuric acid and the herbicide atrazine, pure or mixed with simazine, was evaluated in fed batch processes.

BACKGROUND: Using a suspended or immobilized microbial community obtained through chemostat selection from agricultural soils formerly treated with triazinic herbicides, the atrazine biodegradation kinetics in continuous aerobic reaction systems was studied.