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Dynamics of Motile Microbes and Model Development
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Accurate numerical models of microbial transport are needed to support design and evaluation of bioremediation implementations. The characteristics associated with the motion dynamics of motile bacteria is thought to play an important role in bioremediation processes. By stimulating the activity of natural microorganisms it is possible to manipulate the redox state of contaminants and greatly decrease their solubility, rendering them immobile and therefore reducing the risk of human exposure. Traditional transport models, such as Advection-Diffusion Equation (ADE), are generally found to be inadequate to describe the transport of bacteria in groundwater as bacterial motility often exhibits non-Fickian properties owing to the complex nature of their run-and-tumble motion and chemotaxis behavior. In this study, Geobacter and Pelosinus, which are motile species important to bioremediation of certain heavy metals, are used in micromodel experiments for microscopic examination of their motility characteristics. Experiments to observe two-dimensional motion in unobstructed medium were conducted at the Environmental Molecular Sciences Laboratory (EMSL) of the Pacific Northwest National Laboratory (PNNL). EMSL’s Cell Isolation and Systems Analysis (CISA) facility provides advanced capabilities for microbial cell isolation and microscopic studies. Statistical properties of bacterial transport are measured from recorded trajectories. Individual tracks on the order of several seconds to a few minutes in duration are analyzed to provide information on 1) the length (distance in microns) of microbial runs, 2) velocity distributions along individual trajectories, and 3) the angle between the directions of sequential runs. A Continuous Time Random Walk (CTRW) model to simulate transport and predict breakthrough plots of motile microbes is developed based on the investigated statistical properties of their motion dynamics. The results of the CTRW model are compared with the ADE model and experimentally obtained trajectories.