Etude de l'évolution du potentiel génétique de populations bactériennes dégradant l'atrazine
Atrazine, one of the most used herbicide to control the development of weeds in crop, has led to the contamination of the environment. Repeated exposure to this herbicide resulted in the emergence of microbial populations able to degrade atrazine and to use it as a nitrogen source for its growth. These microbial populations are responsible for accelerated biodegradation of atrazine (BDA), a key ecosystemic service diminishing the persistence of this herbicide in the environment. The aim of this PhD work was to study genetic and physiological mechanisms responsible for functioning and improving of this ecosystemic service. We applied an experimental approach starting from genes to communities degrading atrazine in order to identify processes of adaptation involved in the evolution of accelerated biodegradation function.The first part of the PhD aimed at evaluating the importance of accumulation of single mutations in the atzA gene for the activity of AtzA transforming atrazine to hydroxyatrazine. Sequencing or atzA genes amplified from different atrazine-degrading isolates (Pseudomonas sp. ADP WT, Pseudomonas sp. ADP Ps and differents Chelatobacter heintzii) showed that atzA sequence was conserved. However, four non synonymous mutations were identified (1 for Pseudomonas sp. ADP Ps and 3 for Chelatobacter heintzii). Modeling of AtzA structure showed that three mutations were located in important regions (active site, interaction with atrazine and with the metal Fe2+). [...] The second part aimed at studying the plasticity of the atrazine-degrading pathway in two opposed conditions: (i) one aiming at evaluating the persistence of degrading capability in absence of selection pressure and (ii) a second one aiming at evaluating the evolution of degrading capability under high selection pressure exerted by atrazine. With these aims, experimental evolutions were carried out with Pseudomonas sp. ADP. (i) We showed that cyanuric acid exposure led to the selection of a newly-evolved population characterized by increased growing ability on culture medium containing this substance as nitrogen source. This population is characterized by the deletion of a 47 kb region containing atzABC genes from ADP1. We showed that increased fitness of newly-evolved population was due to the selective loss of the genetic burden represented by the 47 kb region, the cyanuric acid degrading ability remaining unchanged. (ii) Atrazine exposure led to the selection of population characterized by the insertion of ADP1 plasmid in the bacterial chromosome. [...] The third part aimed at developing a tool allowing monitoring the evolution of atrazine-degrading genetic potential at the scale of a synthetic microbial community. To do so four degrading strains among which, one was isolated in this study, were chosen. [...] Altogether, these results showed that the atrazine accelerated biodegradation function is highly versatile and under constant evolution. Furthermore, they highlight that the exposure to atrazine is the key parameter driving the evolution of degrading population