RESEARCH NEEDS FOR BIOAUGMENTATION - Bioaugmentation for Groundwater Remediation - page 337

Thus, bioaugmentation with strain KC at high pH and acetate/nitrate conditions would
appear to be a case that fits the above optimal scenario. However, this bioaugmentation
strategy fails when other acetate/nitrate-consuming organisms with a different siderophore
for scavenging iron at high pH displace strain KC. Thus, while the existence of strain KC in this
niche solely depends on the production of the siderophore, its ability to dechlorinate is not part
of its selective advantage in this case. Since the contaminant-transforming organism may be
outcompeted, even within its selective niche, bioaugmentation with strain KC is more difficult
and less predictable than bioaugmentation with Dehalococcoides .
In general, cometabolic transformations of all kinds are less efficient precisely because of
the lack of feedback between the contaminant transformation process and microbial growth,
and the competitive advantage that this feedback would provide to the bioaugmented
organism(s). Thus, while cometabolic bioaugmentation schemes can be effective, it is a short-
term effect, until the ecosystem adapts to the change in state brought about by the addition of
donors and acceptors that are required to create the niche for the desired organism. An optimal
strategy therefore would be to select for organisms that can gain a selective advantage when the
contaminant is transformed, even in a cometabolic situation.
This is not to say that cometabolic transformations should be discounted. These transfor-
mations have a very important role to play in cases where the contaminant concentrations are
too low to support the growth of organisms (Nalinakumari et al., 2010 ; Sharp et al., 2005 ), or
when there are diverse mixtures of contaminants and the transformation of some can be
cometabolic. Nonetheless, a bioaugmentation strategy should rely on finding that combination
of niche and organism(s) that provides the greatest selective advantage to the desired process.
In the case of metabolic transformations, there is the opportunity for long-term inoculation
of an ecosystem with sustainable contaminant transformation ability, provided the niche still
exists. It is clear that biological processes often continue at sites long after the active remedial
stage, and that they play an important role in dampening rebound after more aggressive
remedial efforts (chemical and thermal treatments, for example) (McGuire et al., 2006 ). This
extended treatment reflects the ability to establish a metabolic transformation activity for long
periods of time in an ecosystem where it was not previously found.

12.3.2 Hydrocarbons and Other Reduced Contaminants

Many bioremediation efforts have focused on petroleum hydrocarbons, and with just
cause - these are ubiquitous pollutants and many microbes can degrade them, under a wide
variety of conditions. This fact would suggest that bioaugmentation should be easy. But these
same features make bioaugmentation - specifically to accelerate remediation or to target a
particular constituent of petroleum - a difficult challenge.
For example, benzene is a major toxic constituent of petroleum, and a frequent driver at
contaminated sites. Can one imagine an environment that can be created in the subsurface and
populated with organisms whose existence in that niche depends on benzene transfor-

mation? The answer, with the current state of knowledge, is no. All known benzene-degrading
organisms, regardless of whether they are aerobic or anaerobic respirers, also use other
substrates - derivatives of natural organic matter (sugars, amino acids, fatty acids) and other
petroleum hydrocarbons - and do not solely rely on benzene for growth. Moreover, the niche
for hydrocarbon degraders overlaps with so many other organisms that metabolic diversity is
their key strategy for survival.
Due to this metabolic flexibility, creating sufficient selective pressure to result in
specifically enhanced biodegradation of a particular petroleum contaminant has proven to be
very difficult. Instead, the approach has been to stimulate all microbial activity, in the hopes of
also accelerating benzene degradation. This approach works, though it is highly non-specific.

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