Visualization and modeling of the colonization dynamics of a bioluminescent bacterium in variably saturated, translucent quartz sand

An experimental and numerical investigation was conducted to study the colonization dynamics of a bioluminescent bacterium, Pseudomonas fluorescens HK44, during growth in a porous medium under steady, variably saturated flow conditions. Experiments were conducted in a thin-slab light transmission chamber filled with uniform, translucent quartz sand. Steady, variably saturated flow conditions were established using drip emitters mounted on the top of the chamber, with glucose applied through a central dripper located directly above an inoculated region of the chamber. Periodic pulses of salicylate and a dye tracer were applied to induce bioluminescence of the bacterium to monitor colony expansion and to track changes in the hydraulic and transport properties of the sand. Changes in the apparent water saturation of the sand were quantified by monitoring light transmission through the chamber with a CCD camera. The colonized region expanded laterally by about 15 cm, and upward against the flow by 7-8 cm during the 6-day experiment while apparent saturations in the colonized region decreased by 7-9% and the capillary fringe dropped by ∼5 cm. The observed data were reproduced approximately using a numerical model that accounted for the processes of water flow, solute and bacterial transport, cell growth and accumulation, glucose and oxygen consumption, and gas diffusion and exchange. The results of this study illustrate some of the complexities associated with coupled flow, reactive transport, and biological processes in variably saturated porous media, such as localized desaturation, capillary fringe lowering effects, and upstream movement of bacterial colonization, that may not readily observable using other experimental techniques.