Microbial systems analysis of naphthalene degradation in a continuous flow soil slurry reactor

Microbial systems analysis was used to probe two naphthalene degrading systems. The naphthalene reporter strain HK44 was analyzed in pure culture in order to develop a model relating bioluminescence to naphthalene degradation. Naphthalene degradation in a complex soil slurry system consisting of a manufactured gas plant (MGP) soil contaminated with an array of polycyclic aromatic hydrocarbons (PAHs) and a mixed microbial community was evaluated. Frequency response analysis was used as the method for studying the two systems. The naphthalene feed concentration was varied sinusoidally between 0 and 15 mg 1^-1 at six different cycle periods ranging from 1 to 24 hours per cycle. The system performance was monitored by measuring bioluminescence and the naphthalene offgas concentration, which was used to calculate the reactor liquid concentration and the naphthalene degradation rate. The degradative response of the two model systems to feed perturbations showed that the systems are inherently non-linear and adaptive. General trends in biodegradation rate could be modeled using a first-order rate equation relating degradation rate to reactor liquid naphthalene concentration. Degradation rate parameters from the soil slurry system were one order-of-magnitude higher than the parameters from the pure culture system, indicating that differences in the microbial environment have a significant impact upon degradation (this is after accounting for differences in abiotic fates of naphthalene in the two systems). A critical frequency was identified where degradative activity increased dramatically, indicating that more than one stable operating state exists in the soil slurry system. The response of the systems at the critical frequency were characterized, in the initial phase of the experiments, by a harmonic response (cycling at frequencies other than the perturbation frequency) in reactor liquid naphthalene concentration. The bioluminescent response to feed perturbations demonstrated similar non-linear adaptive behavior to the response in biodegradation. Harmonics in bioluminescence were detected at the critical frequency for degradation. In addition. Bode analysis identified an additional critical time constant of 2 hours associated with the relationship between reactor liquid naphthalene concentration and bioluminescence. At frequencies higher than the critical time constant, the bioluminescence response was shifted increasingly out of phase with the reactor liquid concentration response. However, a first-order, linear dynamical equation could be used to model the relationship between reactor liquid concentration and bioluminescence in the low frequency region (below the critical time constant). From this analysis, the critical time constant may be associated with the halflife of the bioluminescence enzymes. Attempts to monitor behavior in the soil slurry system using bioluminescence were unsuccessful due to environmental factors associated with the MGP soil matrix, which remain unidentified. An initial transient response in bioluminescence was followed by low level light detection at or near the level of detection. Blocking of light by soil particles could account for the lack of response observed in the soil slurry system. Further developments are needed to characterize the effects of complex environments on bioluminescence before this reporter technology can be generally applied as an on-line process monitor.