Divergence of bacterial and fungal communities in hydrocarbon-contaminated soils is related to the phylogeny of introduced willows

Background/Question/Methods

The introduction of hardy plants to polluted soils can enhance the bioremediation of these sites.  Certain plants will bioremediate directly by sequestering and/or transforming pollutants, but plants may also encourage bioremediation by promoting soil microorganisms that degrade contaminants such as petroleum.  Although many microorganisms can use petroleum as a carbon and energy source, not all plant-promoted microbial communities exceed the bioremediation performance of communities found in bulk soil.  The GenoRem project, funded by Genome Canada and Genome Quebec, aims to produce phytoremediation treatments with increased effectiveness, by understanding and harnessing the synergy of plant, bacterial, and fungal communities in polluted soils.  As a component of this project, we aimed to understand how fungal and bacterial communities are shaped by the introduction of 11 different varieties of fast-growing willows (Salix spp.) under different contaminant scenarios.  We used high-throughput sequencing of bacterial 16S rRNA and fungal ITS genes to compare the community composition of 72 samples taken from the rhizosphere of planted willows and from unplanted control plots at the site of a former petrochemical plant. 

Results/Conclusions

Bacterial and fungal intercommunity similarity was significantly correlated in uncontaminated and moderately contaminated soils, but these communities diverged at high hydrocarbon concentrations (> 2000 mg/kg hydrocarbon).  Dissimilarity between fungal communities increased in the highly contaminated blocks, but did not for bacterial communities.  Divergence was related partly to high fungal sensitivity to hydrocarbon contaminants as demonstrated by reduced Shannon Diversity, but also to the fact that different willow cultivars promoted distinct fungal communities.  While bacterial community structure was not obviously related to any willow characteristics, the abundance of the fungal class Pezizomycetes in highly contaminated soils was directly related to willow phylogeny.  Pezizomycetes dominated the rhizosphere from a cluster of cultivars, including those native to North America, but were nearly absent in other soils.  This has implications for plant selection in phytoremediation, as fungal associations may affect the health of introduced plants and the success of co-inoculated or naturally co-occurring microbial strains.