Spatial pattern of bacterial diversity in a site with mixed and uneven contamination, and assessment of rhizoremediation potential

The SIN Caffaro is a large polluted site of national priority located in the Northern Italy, originated by the activities of the former Caffaro s.p.a. chemical factory. The soil in the site presents a mixed contamination of halogenated Persistent Organic Pollutants, particularly polychlorinated biphenyls, and heavy metals in variable concentrations, uneven distributed in the area and often exceeding the safety values. The use of plants to extract and modify the pollutants (phytoremediation) together with root associated microbes to i) degrade or modify the pollutants (rhizoremediation) or ii) support plant growth (plant growth promotion, PGP) has recently arose as a promising approach for bioremediation. The high concentration of pollutants could therefore be considered as a gradient of environmental selection toward the resident community potentially able to support soil remediation. In this context, we collected 63 soil samples from three different areas in the site to a depth of 1 meter, which were chemically and microbiologically characterized. A DNA-based fingerprinting approach was applied to describe the bacterial community’s structure, which proved to be significantly different according to the area and depth of collection. Through a statistical approach, we tested the influence of selected environmental parameters, showing that the concentration of different classes of pollutants was significantly related to the microbiome structure. Furthermore, the rhizosphere of three autochthonous plant species was collected in the most contaminated area of the site. The overall bacterial community was studied by 16S rRNA pyrosequencing and a collection of bacterial strains was in parallel established and tested in vitro and in vivo for PGP potential. The results showed that the rhizosphere-dwelling microbiome was highly similar between the plant species, in terms of both phylogenetic diversity and PGP potential, confirming the existence of a strong selective pressure given by the pollution profile rather than the plant species. Overall, this work highlighted the occurrence of distribution patterns in bacterial populations related to gradients in soil pollution, and showed the intrinsic potential of the highly contaminated soils at the Caffaro site for rhizoremediation potential.