Large scale demonstration of tree-based phytostabilization assisted by symbiotic fungi of metal-contaminated sites
Résumé
We developed an integrated bioremediation strategy that combines two poplar clones with rhizospheric microorganisms, exploiting the complex interactions evolved for the mutual benefit of both organisms, in which plant roots provide habitat, nutrients and exudates to microbial populations, whereas microbes facilitate mineral nutrition of plants The rationale behind combining tree species and mycorrhizal fungi resides in the fact that poplar species selected are fast growing, producing large amount of harvestable biomass and capable of forming arbuscular mycorrhizal and ectomycorrhizal symbiosis with fungi. Hosting different mycorrhizal types might be of functional importance for plant nutrition and has been shown to contribute to metal tolerance of host plants as fungi can reduce the metal uptake by the plant by sequestration, extracellular precipitation and biosorption to the cell walls. Indeed, a number of recent data support the hypothesis that mycorrhizal fungi play a filtering/sequestering role on plant roots enhancing root to shoot metal ratio and increasing survival rate in harsh conditions Implementations of large scale field trials of poplars (SRC) inoculated with symbiotic microorganisms have been set up. Three sites of about 1 ha have been selected on the basis of (i) contaminants pollution level/history, (ii) hydrological/soil conditions, (iii) ongoing soil remediation research. The first site is a former agricultural area contaminated for nearly a century by metals due to the deposit of waste waters from the city of Paris. The second site is an industrial tailing pond contaminated by mercury and arsenic. The third site is a former agricultural soil that received large amounts of metal-enriched sediments. Poplars were planted and inoculated or not with a consortium of mycorrhizal fungi. Mycorrhizal species were chosen from results of molecular analyses of the mycorrhizal diversity of poplar roots collected from several metal-polluted sites. Genomic analyses revealed a predominance of Glomus and Hebeloma species. Two year after planting, woody biomasses were estimated using dendrometric parameters. Mycorrhizal poplars produced significantly more biomass than non inoculated ones. Height, diameter, and basal area of inoculated trees varied among the three studied sites and were generally between 15 and 35% higher when compared to non inoculated trees. From these experimental sites, we have also developed a strategy for isolating fungal strains of interest. We have therefore isolated a set of ectomycorrhizal and endomycorrhizal isolates, as well as fungal endophytes from poplar roots. The isolated strains were first characterized and tested in metal tolerance assays and the most tolerant strains were used in a second set of experiments for their capacity to improve plant growth under controlled conditions. Four fungal strains were able to stimulate growth of their host plants. The two most promising strains, belonging to the group of dark-septate endophytes, were find to be tolerant to a set of metals and significantly improve plant growth. We also succeeded in isolating and producing in vitro large scale inocula of several endomycorrhizal isolates. The usefulness of symbiotic inocula for biomass production on metal-polluted sites, as well as their potential for improving the efficiency of phytotechnologies in the remediation process will be further discussed.