RESEARCH NEEDS FOR BIOAUGMENTATION - Bioaugmentation for Groundwater Remediation - page 341

Table 12.1. Effects of Enrichment Conditions on Activity of Culture “WL”

Culture and Electron Donor/Acceptor Pairs

Culture Parent Subculture Subculture Subculture

“WL” TCA/EtOH TCA/DCA/EtOH DCA/EtOH DCA/H 2

1,1,2-TCA + + +

1,2-DCA + + + +

PCE + +

TCE + + +

cis-DCE + +

VC + +

Organisms present Dhb and Dhc Dhb only (multiple

strains)

Dhb and Dhc Dhb only (fewer

strains)

Note: cis-DCE-cis-dichloroethene; EtOH-ethanol; PCE-perchloroethene

organisms is capable of using this substrate. Enrichment on 1,2-DCA and hydrogen, a more
stringent condition, resulted in a pure population of Dehalobacter that could no longer
dechlorinate TCA or trichloroethene (TCE). The original enrichment thus contained at least
two distinct Dehalobacter strains with different catabolic activities, as well as at least one strain
of Dehalococcoides . These strains were competing for electron acceptors, and hence different
enrichments could lead to pure strains, but at the expense of the dechlorination substrate range
of the culture.
Many sites contain multiple contaminants and other reactive molecules and this mixed
contamination and chemistry can lead to complex inhibition dynamics. This inhibition can
be acting on the native bacterial communities as well as on the bioaugmented organisms.
A bioremediation system therefore must be robust to the presence of inhibitory cocontaminants,
or, ideally, capable of degrading the full spectrum of contaminants present at a site.
For example, the reductive dechlorination of TCE to ethene by Dehalococcoides is slowed
in the presence of elevated concentrations of 1,1,1-TCA. Similarly, the reductive dechlorination
of 1,1,1-TCA to chloroethane by Dehalobacter is slowed by the presence of chlorinated ethenes,
and neither Dehalococcoides nor Dehalobacter can dechlorinate all of the substrates present in
such a system. Mixed together, the combined enrichment culture not only has a broader
substrate range, it also accelerates the rate of total dechlorination by alleviating cross inhibition
between the chlorinated ethenes and ethanes (see Table 12.2 ) (Grostern et al., 2009 ). This is an
example of a general concept that can be applied for bioaugmentation - mixing of enrichment
cultures maintained on defined substrates can allow a bioaugmentation effort to target a larger
range of contaminants.
The examples above illustrate the importance of two different concepts: (1) functional
diversity and (2) functional redundancy. Diversity is needed to tackle real sites with complex
mixtures, so that the bioaugmentation inocula contain the metabolic capacity to degrade all (or
most) of the contaminants present at a site. Redundancy is needed to provide robustness.
Functional redundancy can be selected for within the biodegrading population (e.g., multiple
Dehalobacter or Dehalococcoides strains with overlapping substrates). Additionally, the
functional redundancy of the supporting organisms (methanogens and acetogens) in higher-
complexity consortia allows rapid adjustment to perturbations in the system, including