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Past industrial activities have generated many contaminated lands from which Mercury (Hg) escapes, primarily by volatilization. Current phytomanagement techniques aim to limit Hg dispersion by increasing its stabilization in soil. Although soil fungi represent a source of Hg emission associated with biovolatilization mechanisms, there is limited knowledge about how dead fungal residues (i.e., fungal necromass) interact with soil Hg. This study determined the Hg biosorption potential of fungal necromass and the chemical drivers of passive Hg binding with dead mycelia. Fungal necromass was incubated under field conditions with contrasting chemical properties at a well-characterized Hg phytomanagement experimental site in France. After four months of incubation in soil, fungal residues passively accumulated substantial quantities of Hg in their recalcitrant fractions ranging from 400 to 4500 μg Hg/kg. In addition, infrared spectroscopy revealed that lipid compounds explained the amount of Hg biosorption to fungal necromass. Based on these findings, we propose that fungal necromass is likely an important factor in Hg immobilization in soil.

The diversity of cultivable bacteria associated with plants from phytomanaged soils with mixed trace metal (TM) and polycyclic aromatic hydrocarbon (PAH) contamination in Pierrelaye (France) was evaluated. The emphasis was on the cultivable bacterial community since the overall objective is to obtain inoculants to improve the remediation of this type of contaminated site. Root endophytic and rhizosphere soil bacterial counts were determined, and isolates were pooled by amplified rDNA restriction analysis and identified by 16S rDNA sequencing. Isolates were further characterized for the production of plant growth-promoting (PGP) substances, and resistance to TM. The selected strains were evaluated for their ability to degrade PAHs. The potential of cell-free microbial supernatant to increase the mobilisation of PAHs from the polluted soil of Pierrelaye was also evaluated. Proteobacteria and Actinobacteria dominated the collection of isolates, and differences in taxonomic diversity were observed between plant species (Populus or Zea mays) and depending on the remediation treatment (Populus inoculation with mycorrhizae or Populus intercropping with Alnus). The majority of isolates exhibited at least one of the tested PGP traits, as well as resistance to more than one TM. Several rhizosphere, endophyte and even one bulk soil isolate showed high rates of fluoranthene and pyrene reduction. The endophyte Rhizobium strain MR28 isolated from maize and degrading pyrene produced bioemulsifying molecules capable of improving the availability of PAHs from the soil of Pierrelaye. A selection of the most interesting strains was made for further re-inoculation experiments in order to assess their potential in rhizoremediation processes., This work was supported by the Agence Nationale de la Recherche (ANR) under the PRECODD (ANR-06-ECOT-O15-01- PHYTOPOP) and the Blanc International (ANR-10-INTB-1703-01- BIOFILTREE) projects.