NS34A-01 INVITED
Application of electrokinetics for stimulating microbial clean-up of contaminated soils
Given sufficient time there are few synthetic compounds that can resist microbial degradation, a fact exploited in environmental clean-up. Despite this the performance of micro-organisms in remedial technologies is often sub-optimal. There are many reasons for the failure of indigenous microbial communities to reduce contaminant concentrations, including issues of bioavailability and the inability of the contaminants to switch on genes (catabolic) responsible for contaminant degradation. Even if the presence of the required catabolic genes is confirmed, there continues to be a significant need to develop procedures to stimulate their activity. We have investigated the potential of soil electrokinetics (3-4 A m-2) to stimulate microbial degradation of organic pollutants and move the soil contaminants relative to the degradative microorganisms, so increasing contact between the two components. Using soils contaminated with pentachlorophenol as our model laboratory system, we have demonstrated that the technique is effective at causing gross and controlled movement of PCP through soils at the laboratory-scale. It can also stimulate rates (up to 25% over that of the control) by which introduced bacteria degrade the contaminant. The additional potential benefits of electrokinetics in regard to stimulating microbial activity and soil clean-up will be discussed.
NS34A-02 INVITED
Low-Frequency Electromagnetic Probes of Live Organisms
We report on measurements of the linear and nonlinear electromagnetic responses of live organisms and biological macromolecules to sinusoidal electric fields at kilohertz frequencies and below. A challenge to astrobiological investigations is to develop in situ instruments capable of distinguishing environmental samples or extracts containing life forms from those that do not. At the same time, any life-detection technology for astrobiology should not be geocentric by targeting characteristics, such as DNA sequences, limited to life on Earth. Since life throughout the Cosmos, regardless of its biochemistry and/or genetic material, must utilize a variety of charged macromolecules, we are investigating linear and nonlinear impedance spectroscopy as life- detection techniques. Recent results on suspensions of live cells, organelles, and protein complexes will be discussed, as well as more down-to-earth applications in biophysics and biomedicine.
NS34A-03 INVITED
Bacterial Nanowires: Is the Subsurface Hardwired?
Bacteria, ranging from oxygenic photosynthetic cyanobacteria to heterotrophic sulfate reducing bacteria, produce electrically-conductive appendages referred to as bacterial nanowires. Dissimilatory metal reducing bacteria, including Shewanella oneidensis and Geobacter sulfurreducens, produce electrically conductive nanowires in direct response to electron acceptor limitation and facilitate electron transfer to solid phase iron oxides. Nanowires produced by S. oneidensis strain MR-1, which served as our primary model organism, are functionalized by decaheme cytochromes MtrC and OmcA that are distributed along the length of the nanowires. Mutants deficient in MtrC and OmcA produce nanowires that were poorly conductive. These mutants also differ from wild type cells in their ability to reduce solid phase iron oxides, to produce electrical current in a mediator less microbial fuel cell, and to form complex biofilms at air liquid interfaces. Recent results obtained using direct cell counts and low frequency electrical measurements demonstrate that microbial growth correlated with real and imaginary electrical conductivity response in uncoated silica sand columns. Direct observation of packing material with environmental scanning electron microscopy (ESEM) revealed a fine network of extracellular structures that were morphologically similar to nanowires observed in metal reducing bacteria. No such structures were observed in control columns. We hypothesize that microbial nanowires may in part be responsible for the electrical response observed in the biostimulated columns.
NS34A-04 INVITED
Self-Potential (SP) and Active Electrical Geophysical Assessment of Bioremediation at a Contaminated Gasworks Plant
We have surveyed a former gasworks site in Portadown, Northern Ireland, using self-potential (SP), electrical resistivity, induced polarisation (IP), and ground conductivity (EM-31, EM-34, EM-61). Site lithology and hydrogeology were mapped in numerous trial pits, and groundwater redox conditions together with a host of associated biogeochemical and microbiological parameters have been monitored in several boreholes. A permeable reactive barrier (PRB) together with groundwater flow control (slurry wall) and monitored natural attenuation (MNA) are used for remediation of the complex site contamination, including hydrocarbon and heavy metals. The electrical geophysical surveys mapped the foundations of former infrastructure at the site and detected a formerly unknown tar well and a pit filled with mixed waste. In the contaminated regions of the site the total, measured SP signal is comprised of streaming potential and electrochemical components; in the uncontaminated regions the streaming potential is dominant and electrochemical potentials are negligible. The streaming potential coupling coefficient is estimated by relating the hydraulic potentials from borehole monitoring and groundwater flow modelling to the total SP signal measured in the uncontaminated regions. Residual SP is determined by subtracting the calculated streaming potential component from the total SP data, and the impact of spatially variable, bulk ground conductivity on streaming potential is elucidated. We investigate the relationship between residual SP and redox potential measured in several successive, contaminated aquifer layers separated by aquitards. The SP and electrical geophysical signatures of microbial processes naturally degrading the subsurface contaminants are examined. Preliminary findings from SP and electrical geophysical monitoring of artificially disturbed microbial processes and subsurface redox conditions are also presented.
NS34A-05 INVITED
Microbially Catalyzed Calcite Precipitation in Porous Media: Potential for Geophysical Mapping of Precipitate Distribution
Coprecipitation of trace metals in calcite offers a mechanism for in situ immobilization of inorganic contaminants in the subsurface. We have been investigating the potential for stimulating microbially mediated urea hydrolysis to promote the precipitation of calcium carbonate and the co-precipitation of trace metals as a method for treating 90Sr -contaminated systems. Urea hydrolysis results in an increase in both pH and carbonate alkalinity, and these factors can promote carbonate mineral precipitation. The ability to hydrolyze urea is widespread among subsurface microorganisms, and therefore remediation schemes based upon this approach could rely on indigenous organisms. In environments that favor calcite stability, which includes many aquifers in the western United States, this approach could result in long-term stabilization of the contaminants. Development of this concept into a practical remediation approach requires that we be able to control where precipitation occurs and at what rate. This requires a better understanding of the controls on the spatial distributions of mineral precipitation and the ureolysis reactions. A particular challenge is to understand how the system permeability and fluid flow changes over time, which is coupled to the precipitation rates and distribution of the precipitate. As part of our efforts to study these coupled processes, we are testing the application of complex resistivity (CR) as a means of mapping the distribution of precipitated calcite in a porous media column. CR measurements are sensitive to and are affected by chemical surface properties, porosity, grain size, and pore space distribution, and therefore we anticipate that mineral precipitation within the column will be detectable by CR. In this presentation we will report on our preliminary efforts to characterize the CR response within a porous media column where calcite precipitation is induced by extracellular ureolysis.