DEHALOCOCCOIDES AND REDUCTIVE DECHLORINATION OF CHLORINATED SOLVENTS - Bioaugmentation for Groundwater Remediation - page 75

biomarker targets, such integrated approaches will transform bioremediation from an empirical
practice to a technology with predictable outcomes.
2.15 IMPLICATIONS FOR BIOREMEDIATION PRACTICE:
TAKE HOME MESSAGES
1. Dehalococcoides ( Dhc ) are naturally occurring, strictly anaerobic, specialized bacteria
that require hydrogen as electron donor and certain halogenated organic compounds
as electron acceptors.
2. Dhc are often detected in chlorinated solvent-contaminated, anoxic subsurface environ-
ments but may be present at low abundances, with prevailing environmental conditions
limiting dechlorination activity.
3. Dechlorination activity can be initiated or rates increased by biostimulation, which can
be combined with bioaugmentation.
4. Some Dhc strains reductively dechlorinate toxic dichloroethenes ( cis -DCE, trans -DCE,
1,1-DCE) and VC to environmentally benign ethene. To date, no other bacteria have
been found that detoxify these compounds via reductive dechlorination. Note that not
all Dhc strains dechlorinate DCEs and VC to ethene and some Dhc strains do not
dechlorinate chlorinated ethenes at all.
5. Dhc isolates are difficult to obtain and grow in pure culture; however, Dhc are more
easily maintained and exhibit robust dechlorination activity in consortia.
6. Dhc have small genomes but possess many (up to 36) reductive dehalogenase genes,
whose functions are largely unknown (i.e., the range of chloroorganic compounds that
Dhc can dechlorinate has not been explored).
7. Quantitative PCR assays that enumerate the Dhc 16S rRNA gene are available to
monitor the Dhc population size in environmental samples.
8. The bvcA and vcrA genes serve as biomarkers for complete reductive dechlorination of
chlorinated ethenes to ethene. Although the knowledge of biomarker genes for
monitoring the chlorinated ethene reductive dechlorination process is incomplete, the
combined analysis of Dhc 16S rRNA genes and the tceA , bvcA and vcrA genes
provides valuable information for site assessment and bioremediation implementation.
9. Under conditions of limited hydrogen flux or when the chlorinated contaminants have
been consumed, Dhc growth and activity cease and the Dhc population declines.
REFERENCES
Adrian L, Szewzyk U, G¨risch H. 2000a. Bacterial growth based on reductive dechlorination of
trichlorobenzenes. Biodegradation 11:73-81.
Adrian L, Szewzyk U, Wecke J, G¨risch H. 2000b. Bacterial dehalorespiration with chlorinated
benzenes. Nature 408:580-583.
Adrian L, Hansen SK, Fung JM, G¨risch H, Zinder SH. 2007a. Growth of Dehalococcoides
strains with chlorophenols as electron acceptors. Environ Sci Technol 41:2318-2323.
Adrian L, Rahnenfuhrer J, Gobom J, Holscher T. 2007b. Identification of a chlorobenzene
reductive dehalogenase in Dehalococcoides sp. strain CBDB1. Appl Environ Microbiol
73:7717-7724.