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Final Progress Reports: Michigan State University: Molecular Insight into Polyaromatic Toxicant Degradation by Microbial Communities

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Superfund Research Program

Molecular Insight into Polyaromatic Toxicant Degradation by Microbial Communities

Project Leader: James M. Tiedje
Grant Number: P42ES004911
Funding Period: 2000 - 2013

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Final Progress Reports

Year:   2012  2004 

Microbial hydroxylases are integral to the biodegradation of Superfund targeted pollutants, particularly dioxins, dibenzofurans, PAHs, PCBs and other hydrocarbon co-contaminants in the environment. Due to the paucity of microbes that have been isolated that grown on dioxins or dibenzofurans, Dr. Tiedje and his research team attempted to discover more in order to understand what might be limiting the degradation of these compounds. The research team used the strategy that centuries of prairie fires would have resulted in selection of microbes able to degrade these the same or similar natural products. The research team found that a pristine prairie soil yielded a bacterial community that completely degrade dibenzofuran. Subsequently, the researchers isolated a novel dibenzofuran-degrading bacterial consortium from this soil. This three-member consortium completely degrades dibenzofuran with a 10 h doubling time. The three members are phylogenetically diverse belonging to three different phyla: Firmicutes (unclassified Bacillales), Proteobacteria (unclassified Comamonadaceae), and Actinobacteria (Agromyces). It appears each member plays a required role in different steps in dibenzofuran degradation. These roles are being being confirmed. A number of studies imply that degradation of environmental pollutants occurs via consortia action, and this is direct evidence of this mechanism of degradation for the difficult to biodegrade angular polyaromatic ether hydrocarbons.

Dr. Tiedje and his research team have completed outlining differential transcriptome responses of the only known bacterium that grows on and hence degrades both dibenzofuran and dibenzo-p-dioxin, Sphingomonas wittichii strain RW1. While the dibenzofuran-specific transcriptome is robust the dibenzo-p-dioxin specific transcriptome is characterized by (cyto)toxicity and stress responses (detailed in Core E: Environmental Molecular Analysis Core report). This may contribute to the apparent rarity of dibenzo-p-dioxin degraders in environment.

In the environment dioxins are predicted to be mostly sorbed to geosorbents such as clay and chars. This is one means to sequester dioxin in contaminated soil but the question remains as to whether this sorbed dioxin would be degraded by microbes. The research team has found that strain RW1 completely degrades dibenzo-p-dioxin sorbed to clay saponite. Moreover, this growth occurs faster compared to growth on crystalline dibenzo-p-dioxin. The researchers have sequenced the transcriptomes from the microbes degrading the dioxin on the clay versus non-clay solution and are currently analyzing the data to gain insight into the environmentally relevant degradation processes, e.g. if there are any candidate genes important for facilitating attack of sorbed aromatic substrates.

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