The rhizosphere of naturally selected plants shows the potential to sustain natural attenuation processes in an historical PCB polluted soil

Scientific Question and Context. The SIN Caffaro is an extended industrial polluted site of national priority located in Italy. Here the soil presents a mixed contamination of halogenated pollutants (PCBs, dioxins and furans), heavy metals and metalloids, uneven distributed and often largely exceeding the safety values. After the closure of the production plant, in the last 50 years only the process of natural attenuation occurred in the surrounding agricultural soils. We aimed to explore the natural attenuation potential of the SIN Caffaro soil microbiota, focusing on the interactions between bacteria and the plants naturally selected in the area. Methods. We studied the phylogenetic and functional diversity of the microbiome associated to three plant species growing in the most contaminated area of the SIN Caffaro. The bacterial community in the rhizosphere and the surrounding soil was described by Illumina sequencing of the 16S rRNA gene and qPCR quantification of the bphA gene, proxy of PCB aerobic degradation. A collection of cultivable rhizobacteria was tested for plant growth promotion (PGP) traits and biodegradation potential. Results. Both the soil niche and the plant species shaped the composition of the resident microbiota. The bacterial communities were more strongly influenced by the plant species in the rhizosphere than in the soil surrounding roots, leading to speculate that the “rhizosphere effect” overlapped the high pollutant concentrations in shaping the community structure. The microbiota selected in all the soil niches showed nevertheless degrading potential. The cultivable microbiota in all the rhizospheres showed similar phylogenetic and functional composition. A large fraction of the strains harboured the bphA gene and displayed catechol 2,3-dioxygenase and ACC deaminase activities, the latter considered a PGP trait contributing to plant tolerance to environmental stress. Conclusions. Overall, this work highlighted that spontaneous plant species differentially affected the composition of the rhizosphere bacterial community and harboured the intrinsic potential to sustain natural attenuation processes, further exploitable for rhizoremediation interventions at the SIN Caffaro.