BIOAUGMENTATION WITH DEHALOCOCCOIDES: A DECISION GUIDE - Bioaugmentation for Groundwater Remediation - page 123

Dehalococcoides Biomarkers

Sulfospirillum
Desulfitobacterium
Dehalobacter
Dehalococcoides (Dhc)
Others?

Dhc 16S rRNA

Some Dhc strains

PCE TCE cis -DCE VC ETHENE

pceA
PCE RDase
Figure 4.2. Pathway for reductive dechlorination and identification of the key Dehalococcoides
biomarkers. Biomarkers include the 16S subunit of the ribosomal RNA that is characteristic of
Dehalococcoides spp. as well as portions of reductive dehalogenase genes (RDases) acting on
each of the chloroethenes along the metabolic pathway.

tceA
TCE RDase

vcrA, bvcA
Vinyl chloride reductases

detect and quantify Dhc populations with low detection limits (typically 10 2 -10 3 cells/L in
groundwater). Although it cannot discriminate between Dhc strains that do or do not have the
ability to gain energy from VC reductive dechlorination, it seems reasonable that the numbers
should correlate with both the rate and extent of dechlorination.
Probes for VC reductase genes could provide more specific information on the ability to
completely dechlorinate chloroethenes. However, there are numerous VC reductase sequences
(H¨lscher et al., 2004 ) and so far there are probes for only two ( vcrA and bvcA ). These probes
have proven very useful for tracking introduced cultures and for detecting indigenous VC
dechlorinators at some sites. Not surprisingly, VC reductase levels can be a better indicator of
the dechlorination capacity than the Dhc numbers (van der Zaan et al., 2010 ). But at this point,
false negatives are certainly possible (i.e., neither vcr or bvc may be present, but reductive
dechlorination of VC can still occur). The tceA levels do not correlate well with dechlorination
rate or extent (Da Silva and Alvarez, 2008 ).
There are other biomarkers and techniques that may improve decision making in the future.
For example, it also may be possible to measure the in situ activity, and not just the genetic
potential, by measuring the mRNA levels directly (Johnson et al., 2005 ; Lee et al., 2008 ; Rahm
and Richardson, 2008 ). Direct measurement of key proteins such as VC reductases also is
possible, and may prove to be a powerful monitoring technique in the future (Morris et al.,
2007 ; Werner et al., 2009 ). Finally, deoxyribonucleic acid (DNA) microarrays make it possible
to characterize the entire microbial community at a site in extraordinary detail (L¨ffler and
Edwards, 2006 ; Johnson et al., 2008 ).
Interpreting MBT Results. MBT analyses can be powerful tools for characterizing and

monitoring sites. For example, Lu et al. ( 2006 ) showed that the 16S rRNA gene analysis for
Dhc could be used to decide if monitored natural attenuation (MNA) is a viable option.
Specifically, Dhc numbers 10 7 cells/L groundwater were correlated to “generally useful”

rates of reductive dechlorination. In another example, Scheutz et al. ( 2008 ) demonstrated the
value of a VC reductase biomarker ( vcrA ) to track the growth of a bioaugmentation culture and
the onset of ethene generation at a site previously stalled at cis -DCE.