RESEARCH NEEDS FOR BIOAUGMENTATION - Bioaugmentation for Groundwater Remediation - page 336

“Why” this is so is a question for fundamental research, but the result of this specialization
has profound implications for bioaugmentation. Most obvious is that these organisms grow
specifically on the substrates that are contaminants of interest for bioaugmentation. Moreover,
because organisms that dechlorinate typically cannot do anything else to gain metabolic energy,
they have few competitors and their presence is strongly indicative of dehalogenation. This
linkage between presence and activity is highly useful for tracking bioaugmentation inocula,
and for assessing sites for potential dechlorination, because detectable levels of Dehalococ-
coides DNA can be considered reliable evidence that dechlorination is occurring. The only
competition of concern within a dechlorination system is the potential for competition between
dehalogenators for substrates. It is thus important to select for strains of organisms known to
catalyze the dechlorination reaction to a nontoxic end product. Dechlorinating organisms do
have to compete for electron donor (hydrogen) and other nutrients, but easily have the
advantage when there is ample halogenated organic present.
It is clear now that one of the major limitations to many early bioaugmentation schemes
was the inability of introduced organisms to survive and compete, and the challenge of
promoting the expression of contaminant-degrading activity under in situ conditions. This is
particularly true for most organisms that degrade petroleum hydrocarbons. Such organisms
can typically metabolize a wide variety of carbon-containing substrates and will only degrade
contaminants as a last resort. Thus when introduced into a contaminated site, the desired
degradation of the contaminants often is not seen. The petroleum hydrocarbon situation is
further complicated by the fact that there are multiple compounds with different functionalities
(alkanes, alkenes, arenes and combinations thereof), and it is difficult to generate an enrich-
ment culture capable of complete breakdown of the multitude of compounds present at a
petroleum-contaminated site. These observations have led to a proposed optimal scenario for
bioaugmentation (below).

Optimal Scenario for Successful Bioaugmentation
An optimal scenario for efficient and successful bioaugmentation is to create an environment that
can be populated with organisms (native or introduced) whose existence in that niche solely depends
on the transformation of the contaminants(s) of interest.

The obligate dehalogenating lifestyle of Dehalococcoides is the most clear-cut example of
a system that meets this requirement. Moreover, each strain of Dehalococcoides harbors a
selective suite of enzymes that distinguish them and further narrow their niche. Successful
bioaugmentation relies on creating an environment where reducing conditions (i.e., hydrogen),
the contaminant of interest, and organism(s) who are obligate degraders of the contaminant of
interest are co-located. Let us investigate if this optimal scenario holds for other examples of
successful bioaugmentation.

12.3.1.2 KC and Cometabolic Transformations

In the case of Pseudomonas stuzeri strain KC (see Chapter 9 ) (Dybas et al., 1995 ; Essen
et al., 2007 ; Zawadzka et al., 2006 ), the environment has to be changed to create a niche for the
introduced organism. In a high-pH environment, most organisms become severely iron-limited.
In strain KC, a higher pH triggers the production of an iron-scavenging siderophore that
fortuitously dechlorinates carbon tetrachloride (CT) rapidly. At high pH, and in combination
with a specific set of donor and acceptor (acetate/nitrate), strain KC's existence is solely
dependent on the production of the CT-degrading siderophore.

Pseudomonas stuzeri