Dynamics of aerobic and denitrifying bacterial strains during the biodegradation of toluene in a porous medium: Experimental quantification and mathematical simulation

Heterotropic bacteria can degrade organic substrates utilizing different terminal electron acceptors. The sequence of electron acceptor utilization depends on the energy yield of the individual reaction pathway, which decreases as the redox potential decreases. However, the dynamics of hydrocarbon biodegradation by mixed bacterial populations, and the distribution of different hydrocarbon degraders along the sediment redoxcline are poorly understood, and the measurements from the field or laboratory experiment frequently produce results that are different from the theoretical expectation.; For investigation of these questions, laboratory sand column experiment with toluene as carbon source were conducted. Experiments conducted with pure aerobic and denitrifying bacterial cultures showed an apparent simultaneous utilization of oxygen and nitrate as electron acceptors in a region where the oxygen concentration was significantly higher than the threshold concentration reported in the literature at which denitrification can take place. A dual biofilm model formulated to explain the overlapping electron acceptor utilization showed that depletion of oxygen in the outer aerobic biofilm can result in inner oxygen-free microlocations that allows denitrification to proceed, even though oxygen is still present in the bulk fluid.; The addition of a bacterial strain to the column that performs nitrate-enhanced degradation of toluene in the presence of oxygen resulted in the simultaneous utilization of oxygen and nitrate regardless of the dissolved oxygen concentration in the porous medium, which was not fully expected based on microbial traits of strains used. For the simulation of the observed dynamics, a multispecies biofilm model that is composed of the three bacterial strains used was formulated. Model simulation showed that the decrease in oxygen concentration in the biofilm could govern the gradual turnover of the dominant species such that PKO 1 that utilizes a strategy to save oxygen under hypoxic conditions, allowing this strain to exist as a significant fraction of the overall biomass in the biofilm.; Evaluation of physiological states of bacteria in the porous medium was made with various plate counting and staining methods. The results indicated that significant portion of bacteria was under stress from the long-term exposure to chemical compound, and competition with other strains.