Influence of the sunflower rhizosphere on the biodegradation of 1 PAHs in soil

10 11 Reduced bioavailability to soil microorganisms is probably the most limiting factor in the 12 bioremediation of polycyclic aromatic hydrocarbons PAH-polluted soils. We used sunflowers 13 planted in pots containing soil to determine the influence of the rhizosphere on the ability of soil 14 microbiota to reduce PAH levels. The concentration of total PAHs decreased by 93% in 90 days 15 when the contaminated soil was cultivated with sunflowers, representing an improvement of 16% 16 compared to contaminated soil without plants. This greater extent of PAH degradation was 17 consistent with the positive effect of the rhizosphere in selectively stimulating the growth of 18 PAH-degrading populations. Molecular analysis revealed that the increase in the number of 19 degraders was accompanied by a dramatic shift in the structure of the bacterial soil community 20 favoring groups with a well-known PAH-degrading capacity, such as Sphingomonas (α21 Proteobacteria), Commamonas and Oxalobacteria (β-Proteobacteria), and Xhanthomonas (γ22 Proteobacteria). Other groups that were promoted for which degrading activity has not been 23 reported included Methylophyllus (β-Proteobacteria) and the recently described phyla 24 Acidobacteria and Gemmatimonadetes. We also conducted mineralization experiments on 25 creosote-polluted soil in the presence and absence of sunflower root exudates to advance our 26 understanding of the ability of these exudates to serve as bio-stimulants in the degradation of 27 PAHs. By conducting greenhouse and mineralization experiments, we separated the chemical 28 impact of the root exudates from any root surface phenomena, as sorption of contaminants to the 29 roots, indicating that sunflower root exudates have the potential to increase the degradation of 30 xenobiotics due to its influence on the soil microorganisms, where sunflower root exudates act 31