Degradation of toluene by a mixed population of archetypal aerobes, microaerophiles, and denitrifiers: laboratory sand column experiment and multispecies biofilm model formulation

An experiment was conducted in a saturated sand column with three bacterial strains that have different growth characteristics on toluene, Pseudomonas putida F1 which degrades toluene only under aerobic conditions, Thauera aromatica T1 which degrades toluene only under denitrifying conditions, and Ralstonia pickettii PKO1 has a facultative nature and can perform nitrate-enhanced biodegradation of toluene under hypoxic conditions (DO <2 mg/L). Steady-state concentration profiles showed that oxygen and nitrate appeared to be utilized simultaneously, regardless of the dissolved oxygen concentration and the results from fluorescent in-situ hybridization (FISH) indicated that PKO1 maintained stable cells numbers throughout the column, even when the pore water oxygen concentration was high. Since PKO1's growth rate under aerobic condition is much lower than that of F1, except under hypoxic conditions, these observations were not anticipated. Therefore these observations require a mechanistic explanation that can account for localized low oxygen concentrations under aerobic conditions. To simulate the observed dynamics, a multispecies biofilm model was implemented. This model formulation assumes the formation of a thin biofilm that is composed of the three bacterial strains. The individual strains grow in response to the substrate and electron acceptor flux from bulk fluid into the biofilm. The model was implemented such that internal changes in bacterial composition and substrate concentration can be simulated over time and space. The model simulations from oxic to denitrifying conditions compared well to the experimental profiles of the chemical species and the bacterial strains, indicating the importance of accounting for the biological activity of individual strains in biofilms that span different redox conditions.