ORIGINAL ARTICLE Simazine treatment history determines a significant herbicide degradation potential in soils that is not improved by bioaugmentation with Pseudomonas sp. ADP A.C. Mora ´n 1 , A. Mu ¨ ller 2 , M. Manzano 1 and B. Gonza ´ lez 1 1 Laboratorio de Microbiologı´a, Departamento de Gene ´tica Molecular y Microbiologı´a, and Center for Advanced Studies in Ecology and Biodiver- sity, Facultad de Ciencias Biolo ´ gicas, Pontificia Universidad Cato ´ lica de Chile, Santiago, Chile 2 Institut fu ¨ r Mikrobiologie, Universita ¨ t Stuttgart, Stuttgart, Germany Introduction Atrazine and simazine are s-triazine herbicides used worldwide in agriculture, since the 1950s. These herbi- cides interact with the quinone-binding protein in photo- system II, thus preventing the electron transport in susceptible weeds (Wackett et al. 2002). Atrazine half-life in soils ranges between 4 and 57 weeks (Best and Weber 1974). This relatively long time of persistence and its possible threats for health lead to extensive investigations on atrazine degradation in contaminated soils, which is mostly performed by micro-organisms. Several atrazine- degrading microbial associations and monocultures able to utilize atrazine as nitrogen and/or carbon source have been isolated from treated soils (Behki et al. 1993; Man- delbaum et al. 1995; Struthers et al. 1998; Topp et al. 2000; Rousseaux et al. 2001, 2002; Ghosh and Philip 2004; Singh et al. 2004). Among them, Pseudomonas sp. ADP has become a useful model to elucidate the aerobic catabolic pathway, encoded by the atzA, -B, -C, -D, -E and -F genes (Ralebitso et al. 2002; Wackett et al. 2002). These genes are located on the 109-kb, self-transmissible, broad host-range plasmid pADP-1 (de Souza et al. 1998b; Martı´nez et al. 2001). Although several works about Keywords bioremediation, Pseudomonas sp., simazine, terminal restriction fragment length polymorphism. Correspondence Bernardo Gonza ´ lez, Laboratorio de Microbiologı ´a, Departamento de Gene ´ tica Molecular y Microbiologı´a, Facultad de Ciencias Biolo ´ gicas, Pontificia Universidad Cato ´ lica de Chile, Casilla 114-D, Santiago, Chile. E-mail: bgonzalez@bio.puc.cl 2005/1010: received 6 September 2005, revised and accepted 9 December 2005 doi:10.1111/j.1365-2672.2006.02990.x Abstract Aims: To study biological removal of the herbicide simazine in soils with dif- ferent history of herbicide treatment and to test bioaugmentation with a sima- zine-degrading bacterial strain. Methods and Results: Simazine removal was studied in microcosms prepared with soils that had been differentially exposed to this herbicide. Simazine removal was much higher in previously exposed soils than in unexposed ones. Terminal restriction fragment length polymorphism analysis and multivariate analysis showed that soils previously exposed to simazine contained bacterial communities that were significantly impacted by simazine but also had an increased resilience. The biodegradation potential was also related to the pres- ence of high levels of the atz-like gene sequences involved in simazine degrada- tion. Bioaugmentation with Pseudomonas sp. ADP resulted in an increased initial rate of simazine removal, but this strain scarcely survived. After 28 days, residual simazine removals were the same in bioaugmented and not bioaug- mented microcosms. Conclusions: In soils with a history of simazine treatment bacterial communi- ties were able to overcome subsequent impacts with the herbicide. The success of bioaugmentation was limited by the low survival of the introduced strain. Significance and Impact of the Study: Conclusions from this work provided insights on simazine biodegradation potential of soils and the convenience of bioaugmentation. Journal of Applied Microbiology ISSN 1364-5072 26 Journal compilation ª 2006 The Society for Applied Microbiology, Journal of Applied Microbiology 101 (2006) 26–35 ª 2006 The Authors