- The 1,1-dichloroethene (1,1-DCE) cometabolic transformation abilities of
indigenous and bioaugmented microorganisms were compared in microcosms
constructed with groundwater and aquifer solids from the Moffett Field site, CA.
Microbial community structure in the microcosms and possible community shifts
due to 1,1-DCE transformation stress was evaluated by terminal restriction
fragment length polymorphism method (T-RFLP). An existing biotransformation
model was used to simulate the experimental data using parameter values
determined by Kim et al. (2002) and Rungkamol (2001) with small adjustments to
the parameter values.
The laboratory microcosm studies showed that both indigenous and
bioaugmented butane utilizers were capable of transforming 1,1-DCE when fed
butane as a primary substrate. A butane-grown enriched culture was bioaugmented
into the microcosms and exposed to several repeated additions of butane and/or
1,1-DCE, ranging from 7.1 to 76 μmol and from 0.17 to 1.99 μmol, respectively.
The bioaugmented butane-utilizers showed a reduced lag period compared to the
indigenous butane-utilizers. The greatest ability to transform 1,1-DCE was
observed in bioaugmented microcosms, simultaneously exposed to butane and 1,1-DCE. Very little 1,1-DCE was transformed in the bioaugmented microcosms that
were not fed butane, presumably due to lack of reductant supply and/or product
toxicity of 1,1-DCE transformation.
Microbial community analyses revealed similar results for replicate
microcosms and differences in the community structure in microcosms subjected
to different patterns of substrate addition and 1,1-DCE cometabolism. 1,1-DCE
transformation resulted in temporal fluctuations in specific bacterial groups in the
bioaugmented microcosms. It could be inferred that microorganisms, correlated
with the T-RFL of 183 base pair (bp) were generally predominant in butane-fed
bioaugmented microcosms simultaneously exposed to 1,1-DCE. Bioaugmented
microcosms that were pre-exposed to 1,1-DCE for 29 days in the absence of
growth substrate, followed by the addition of butane showed a significantly
different microbial community from bioaugmented microcosms fed butane and
1,1-DCE simultaneously. Microorganisms with T-RFL of 179 or 277.8 bp
dominated in these microcosms. These differences were possibly the result of
extensive 1,1-DCE transformation product toxicity during the pre-exposure phase
of the tests.
A model developed by Kim et al. (2002) was used to mathematically
describe the rate and extent of butane utilization and the cometabolic
transformation of 1,1-DCE in the microcosm tests. Using the kinetic parameter
values previously determined by Kim et al. (2002) and Rungkamol (2001),
heuristic fits were obtained between the experimental data and model simulations.
The model successfully predicted the trend of the butane utilization and 1,1-DCE
transformation. The model outputs were statistically quantified for their fit to the
experimental data by estimating Standard Error of Estimate (SEE). A reasonable fit
between model predictions and experimental observations was achieved.
A significant contribution of this study was developing the laboratory
methods to evaluate the microbial abilities to cometabolize 1,1-DCE and determining the communities of microorganisms correlated with those
biotransformation activities. Furthermore, the model comparison to experimental
data indicated that there was a potential in using the existing model to predict and
improve bioremediation strategies. The results showed the successful
bioaugmentation of a butane-utilizing culture to improve transformation