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Performance evaluation of microbe and plant-mediated processes in phytoremediation of toluene in fractured bedrock using hybrid poplars

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Title: Performance evaluation of microbe and plant-mediated processes in phytoremediation of toluene in fractured bedrock using hybrid poplars
Author: Ben-Israel, Michael
Department: School of Environmental Sciences
Program: Environmental Sciences
Advisor: Dunfield, Kari
Abstract: Efficacy of hybrid poplar trees for phytoremediation of toluene in fractured bedrock aquifers is unclear and active mechanisms require validation. This multi-year field study was conducted on a pilot phytoremediation system at an urban site, implemented to address aged toluene impacts to a shallow fractured dolostone aquifer. The study aimed to establish performance by quantifying phytoremedial activities at the site. Contaminant concentrations in groundwater, soil, and soil vapour, measured in high spatial resolution, showed the main residual toluene mass is coincident with the water table and located favourably for phyto-remedial uptake and biodegradation, with shallow groundwater concentrations approaching aqueous solubility in high-impact areas. Biodegradation occurring in the vadose zone was shown through metagenomic analyses that enumerated toluene degradation genes and gene transcripts in roots and root-associated soil, and compound-specific stable isotope analysis that showed enrichment of toluene stable carbon isotopes in soil vapour. Transpiration measurement, in planta contaminant quantification, and high-throughput sequencing of microbial taxonomic genes in roots and stem tissue were employed to measure toluene uptake through phytoextraction and resolve biodegradation influences upon uptake patterns. Though most phyto-available toluene was being degraded/attenuated prior to uptake, phytoextraction rates were quantified in a subset of trees over a two-week peak-season period. Phytoextraction was greatest in the site’s high-impact region. Trees there had distinct, more uniform root-colonizing bacterial communities, also surmised to have a greater toluene-degrading capacity compared to other locations. Stem phyllosphere microbiomes were shaped by in planta toluene presence as well, showing enrichment in predictive degradation capacity with increasing toluene exposure. Microbial diversity and richness in the phyllosphere were seasonally dynamic, increasing in the late growing season. Finally, a lab-scale DNA stable isotope probing study identified putatively novel toluene-degrading bacteria and fungi taxa in rhizosphere soil and their taxonomic gene sequences were made available for future studies. This study validated ways in which phytoremediation of toluene using hybrid poplars actively occurs in fractured bedrock systems and resolved contributing chemical and biological mechanisms of action on quantitative and qualitative scales. Techniques employed in this study broaden available field-validated methods to monitor and assess aromatic hydrocarbon attenuation in poplar phytoremediation systems.
Date: 2020-03
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