12    METABOLISM DECHLORINATION OF
TRICHLOROETHYLENE BY NITRIFYING BACTERIA
IN AN ENRICHED BATCH CULTURE
Lei Yang, Associate Professor
Department of Marine Environment
National Sun Yat-sen University
Kaohsiung, Taiwan
INTRODUCTION
Trichloroethylene (TCE) is used widely in industry and laundry as solvent for degreasing and
dry cleaning. TCE migrates quickly and exhibits slow breakdown rates in subsurface to make it
become the most widely distributed organic groundwater and soil pollutant in the United
States.12 According to the report from the USEPA, this chlorinated hydrocarbon is the most
prevalent hazardous organic compound at 246 out of 1,035 Superfund sites.3 TCE has been
found to be highly potentially carcinogenic and extremely toxic.4 Due to its suspected carcinogenicity and toxicity, an extensive study of its fate in the environment is necessary to reduce the
risk to human health and to apply remediation techniques successfully.
Biological processes, instead of chemical or physical methods, have received much attention
to remove organic compounds from contaminated soil and groundwater, principally because the
costs of bioremediation are potentially low. Thus, a variety of biological treatment technologies
for the destruction of chlorinated hydrocarbons, such as TCE, are now under development. Although the microorganism which can grow on TCE as a sole carbon or energy source has not yet
been isolated, it was found that through cometabolism, alternative approaches have been
achieved by several physiological types of bacteria which can cometabolically dechlorinate and
then partially or fully degrade TCE.5"9 Vogel and McCarty10 found that TCE could be cometabolically transformed as a nongrowth-supporting electron acceptor by methanogens in an anaerobic condition, in which the reduction of TCE resulted in dechlorination to generate products with
progressively decreasing levels of chlorination. As to aerobic biodegradation of TCE, it was first
reported by Wilson and Wilson" in their study of stimulating soil with natural gas. They found
that methanotrophic (methan-oxidation) bacteria in soil were in charge of this biological reaction. The mechanism of this reaction was found to be triggered from cometabolic dechlorination
of TCE catalyzed by the enzyme of methane monooxygenase (MMO) generated in the bacterial
cells initially to catalyze the oxidation of methane as its primary substrate to sustain growth
while gaining no benefit from TCE degradation.12 Since then, research on microbial processes
capable of degrading TCE and other chlorinated hydrocarbons has been focused largely on organisms that oxidize compounds by monooxygenase under aerobic conditions.413 Up to the present, aerobic oxidation of TCE has been demonstrated for pure and mixed cultures, including
bacteria grown on methane,14 toluene,15 phenol,7 propane,1* propylene,6 cumene,17 isoprene,18
and ammonia.919"22 Also due to broad substrate specificities of monooxygenase enzymes, the
bacteria possessing the enzymes potentially could be used in a wide variety of bioremediation
situations. However, because it goes on a cometabolic pathway, we have to make sure that there
are sufficient primary substrates provided in the treatment systems to sustain the bacterial
growth, which can stimulate more enzymes generated for cometabolism. This is usually the
major limitation of using cometabolism to remove TCE or other chlorinated organic compounds
out of the contaminated sites in in-situ bioremediation.
The applications of such bioremediation technologies are based on the study of pure and
mixed cultures of aerobic bacteria that cometabolize chlorinated hydrocarbons. Methanotrophic
bacteria, for both pure and mixed cultures, and their ability to cometabolize TCE have been
52nd Purdue Industrial Waste Conference Proceedings, 1997, Ann Arbor Press, Chelsea, Michigan 48118. Printed in
U.S.A.
105