Influence of substrate exposure history on biodegradation in porous media by Ralstonia pickettii PKO1

An experimental study was undertaken with Ralstonia pickettii PKO1 to explore the stability and induction of biodegradation in a TCE (trichloroethylene)-degrading toluene monooxygenase bacterium in response to (i) varying lengths of carbon starvation period and (ii) varying levels of organic contaminant exposure. Preculturing at high TCE concentrations (above 110 mg/l) resulted in a surviving population that tolerated TCE degradation-dependent toxicity under growing conditions. Further examination of the effect of TCE preexposure indicated that solvent stress (rather than toxic TCE degradation intermediates) was the likely dominant factor in cell selection. In a column experiment designed to examine the influence of fluctuating toluene concentrations on aerobic degradation, toluene deprivation resulted in the decline of biodegradative activity (deactivation) in attached-growth cells. A lag-phase was observed prior to subsequent reactivation, leading to a delayed response of column degradation. The traditional Michaelis-Menten-type biodegradation equation, based upon suspended-batch-measured-biodegradation kinetic parameters for toluene-grown PKO1 cells, could not predict the observed column degradation behavior because it could not incorporate deactivation and lag. Subsequent batch experiments further revealed that the deactivation observed in the toluene-deprived environments could be attributed to carbon starvation rather than a lack of inducer. The experiments also indicated that the lengths of lag and transient phases for the subsequent induction of T3MO activity depended upon (i) the length of the previous carbon starvation period and (ii) the concentration level of the inducer. In contrast, the steady-state biodegradation rate was not affected by the length of the previous carbon starvation period, but only by inducer concentration. Additional experiments were conducted to examine the effect of oxygen limitation on toluene degradation. These revealed that toluene could not induce toluene degradation in attached-growth and suspended-growth cells when deprived of oxygen. Based upon these batch and column experimental observations, a quantitative framework (mathematical model/correlations), refining the traditional Michaelis-Menten model, was proposed to better describe the kinetics of induction of biodegradation under fluctuating substrate/inducer concentration conditions, representative of natural attenuation or in situ bioremediation scenarios.