Introduction
Chlorinated ethenes are a major source of soil and groundwater
contamination and, hence, represent a significant
potential threat to human and ecological health. In situ
bioremediation, the use of naturally occurring microorganisms
to destroy groundwater contaminants or to convertthem
to harmless forms, is one of the most promising methods
available for mitigating these compounds. However, our
ability to understand, monitor, and manipulate the complex
microbial communities performing bioremediation has been
limited partially by reliance upon traditional culture-based
techniques. Because nucleic acid-based techniques allow for
the investigation of complex communities without relying
onselective isolation of individual species, they are promising
methods for the task of monitoring communities performing
in situ bioremediation.
Microbial degradation of chlorinated ethenes is known
to occur under both aerobic and anaerobic conditions. Under
anaerobic conditions, chlorinated ethenes are transformed
by reductive dechlorination catalyzed by different groups of
anaerobic microorganisms (1-3). For example, a number of
pure cultures have been described in recent years that can
specifically use reductive dechlorination to gain biologically
useful energy by using perchloroethene (PCE) or trichloroethene
(TCE) as terminal electron acceptors in anaerobic
respiration (4-17). Of these, only Dehalococcoides ethenogenes
(5) dechlorinates past cis-dichloroethene (cDCE). D.
ethenogenes uses H2 as its sole electron donor, grows slowly
in pure culture, and is fastidious in its growth conditions (5).
It requires the extract of an anaerobic sludge community for
sustained growth, suggesting that it relies on biochemical
collaboration with other microorganisms and that it benefits
significantly by growth in a mixed community. In general,
the body of knowledge obtained from pure cultures capable
of reductive dechlorination of chlorinated ethenes does not
yet explain the full range of degradation patterns observed
at field sites and in laboratory enrichment cultures.
In addition to the pure cultures of dehalorespirers,
reductive dechlorination of chlorinated solvents has also been
observed by a wide variety of methanogenic and nonmethanogenic
consortia (2, 18-33). Reductive dechlorinations
catalyzed by these groups of microorganisms produce end
products that vary depending on the physiological groups of
bacteria involved and may occur via a combination of
metabolic and cometabolic processes. Despite the considerable
body of research dealing with the complete reductive
dechlorination of chlorinated ethenes by mixed communities,
relatively little is known about the diversity of microbial
communities capable of catalyzing these reactions. In
addition, in many instances, dechlorination of PCE and TCE
results in the accumulation of chlorinated daughter products,
including vinyl chloride (VC), a known human carcinogen.
Fundamental information on the ecology, physiology, and