Effet de l’herbicide sulfonylurée tribenuron méthyle sur la croissance de souches telluriques d’actinobactéries.

Abstract

Granstar® 75 DF is one of the most used sulfonylurea herbicide on cereal crops; it contains 75 % of tribenuron-methyl (TBM). Assessing the changes on soil microbiota, particularly on the most abundant bacterial groups, will be a useful approach to determine the impact of Granstar® herbicide. For this purpose, Actinobacteria were analysed. They are known for their abundance and aptitude to resist to xenobiotic substances. Using a selective medium for Actinobacteria, forty two (42) strains were isolated from both untreated and Granstar® treated soils. The number of isolates recovered from the treated agricultural soil 2 was fewer than that isolated from the corresponding untreated soil, suggesting a negative effect of Granstar® herbicide on Actinobacteria community. Among the isolates, eleven (11) resistant strains, tolerating high doses of Granstar® ranging from 0.3 to 0.6% (v/v), were selected. Morphological characterization of strains indicated their affiliation to Streptomyces genus. The phylogenetic analysis, based on 16S rRNA genes sequencing, clustered five strains (SRK3, SRK4, SRK9, SRK11 and SRK18) to Streptomyces rubrocyanodiastaticus sp. piger type strain, with high bootstrap rates varying from 69 to 98%. While SRK13 and SRK14 were closely related to Streptomyces lavendulocolor and SRK17 to Streptomyces africanus, with high bootstrap values of 92; 89 and 76% respectively. SRK12 and SRK16 strains were associated to four Streptomyces type strains (S. vinaceusdrappus, S. mutabilis, S. ghanaensis and S. enissocaesilis) with a percentage of 70% of bootstrap. Strain SRK15 was related to Streptomyces diastaticus sp. ardesiacus with high bootstrap level of 92%. The two most resistant strains SRK12 and SRK17 did not grow on TBM as the sole carbon source, so they were incapable of degrading it under these conditions. However, in the presence of glucose, both tolerated its presence, but their growth was affected, compared to that on glucose, used alone. At the final pH of 6.78-6.79, the TBM was already predominantly and chemically degraded to an average rate of 76.18%. By decreasing the pH to 6.72-6.73, the TBM dissipation increased significantly to an average of 89.36%. Furthermore, at lower acid values of 4.38 and 4.23; obtained with a similar medium composition supplemented with glucose, the disappearance of the TBM was further improved by 2.26 and 5.56% respectively with the strains SRK12 and SRK17. Therefore, as the pH decreased, the TBM was more degraded probably by acid hydrolysis. In all the tests carried out, as the TBM disappeared, two compounds appeared in the medium and seemed to do not be affected by the acidity. On the basis of these observations, it was suggested that the acid hydrolysis of the herbicide may be the result of a co-metabolic process, due to the bacterial activity on another easily metabolizable substrate. Such a process would be interesting to consider in the bioremediation of alkaline soils polluted by TBM. The products of the TBM transformation were not degraded chemically nor by the studied strains and had a negative effect on the growth of these strains. This observation leaded to the need of more investigations on these compounds, in particular their identification and the determination of their fate in the soil.