| dc.description.abstract | Polycyclic aromatic hydrocarbons (PAHs) are environmentally significant
because of their ubiquity and the toxicity of some. Their recalcitrance and persistence
makes them problematic environmental contaminants. Microbial degradation is
considered to be the primary mechanism of PAH removal from the environment.
Biodegradation kinetics of individual PAHs by pure and mixed cultures have been
reported by several researchers. However, contaminated sites commonly have complex
mixtures of PAHs whose individual biodegradability may be altered in mixtures.
Biodegradation kinetics for fluorene, naphthalene, 1,5-dimethylnaphthalene and 1-
methylfluorene were evaluated in sole substrate systems, binary and ternary systems
using Sphingomonas paucimobilis EPA505. The Monod model was fitted to the data
from the sole substrate experiments to yield biokinetic parameters, (qmax and Ks). The
first order rate constants (qmax/Ks) for fluorene, naphthalene and 1,5-
dimethylnaphthalene were comparable, although statistically different. However, affinity
constants for the three compounds were not comparable. Binary and ternary experiments
indicated that the presence of another PAH retards the biodegradation of the co-occurring PAH. Antagonistic interactions between substrates were evident in the form of competitive inhibition, demonstrated mathematically by the Monod multisubstrate
model. This model appropriately predicted the biodegradation kinetics in mixtures using
the sole substrate parameters, validating the hypothesis of common enzyme systems.
Competitive inhibition became pronounced under conditions of: Ks1 << Ks, S1 >> Ks1
and S1 >> S. Experiments with equitable concentrations of substrates demonstrated the
effect of concentration on competitive inhibition. Ternary experiments with naphthalene,
1,5-dimethylnapthalene and 1-methylfluorene revealed preferential degradation, where
depletion of naphthalene and 1,5-dimethylnapthalene proceeded only after the complete
removal of 1-methylfluorene. The substrate interactions observed in binary and ternary
mixtures require a multisubstrate model to account for simultaneous degradation of
substrates. However, developing models that account for sequential degradation may be
useful in scenarios where PAHs may not be competitive substrates. These mixture
results prove that substrate interactions must be considered in designing effective
bioremediation strategies and that sole substrate performance is limited in predicting
biodegradation kinetics of complex mixtures. | en |
