RESEARCH NEEDS FOR BIOAUGMENTATION - Bioaugmentation for Groundwater Remediation - page 338

In this context, it is clear why bioaugmentation has not worked, as there is little selective
advantage for the introduced organism. Significant breakthroughs in increasing the specificity
of hydrocarbon degradation will arise from identifying novel organisms with obligate
hydrocarbon-degrading activity and novel niches for such activity, or in engineering a strain
to have a competitive advantage in a niche that can be created in situ .
Recently, the genome sequence of a hydrocarbon-degrading marine bacterium, Alcani-
vorax borkumensis , has revealed that this organism is highly adapted to utilizing only oil
hydrocarbons (alkanes), and does not possessthegenesforusingsugarsoraminoacidsas
carbon substrates (Schneiker et al., 2006 ). The obligate-hydrocarbon consuming nature of the
organism, along with a variety of other niche-specific genes (including genes conferring
the ability to scavenge for nitrogen, to form biofilms at the oil-water interface and to
produce biosurfactants), gives this strain a competitive edge in oil-polluted environments
and points to design strategies for oil spill remediation. This particular organism and the
associated niche (marine oil spills) appear to provide the optimal scenario for bioaugmenta-
tion. Not surprisingly, Alcanivorax borkumensis is found enriched in marine oil spills around
the globe (Schneiker et al., 2006 ), much as Dehalococcoides is found enriched in chlorinated
solvent sites around the globe. Similarly, specific hydrocarbon-degrading microbes, related to
psychrophilic Oleispira have been found enriched in the Deepwater Horizon blowout in the
Gulf of Mexico (Hazen et al., 2010 ).
Are biostimulation and bioaugmentation approaches likely to be more successful when the
contaminant is the electron acceptor, rather than the electron donor in an organism's metabo-
lism? There is a notion that the diversity of electron acceptors (electron poor compounds)
used by microbes may be lower than the diversity of electron-donating (electron rich) sub-
strates. For example, in considering microbial ecosystems, it is common to describe various
redox zones on the basis of the predominant TEAP, ranging from oxygen, through nitrate,
nitrite, iron, manganese, sulfate and finally to carbon dioxide (CO 2 ). Certainly other electron
acceptors are known, including common contaminants such as halogenated organics, perchlo-
rate and uranium. However, given the widely different energetic requirements associated with
the reduction of these electron acceptors, each particular organism typically has evolved to use
a relatively limited set of acceptors. Conversely, electron donors, at least the most common
carbon-based donors, all tend to feed rapidly into central metabolism, and thus, a given
organism can easily harbor the potential to degrade a myriad of different organic compounds,
with relatively little energetic or evolutionary penalty.
Based on this apparently greater microbial diversity in anaerobic environments, researchers
seeking obligate contaminant-degrading organisms to create conditions for optimum bioaug-
mentation may have more luck when the contaminant is an electron acceptor. There may be
fewer alternatives for the microorganisms, and thus it may be easier to create an environment
where the desired electron acceptor (the contaminant) is a specific organism's only option.

Niche Concept and Necessity for Successful Bioaugmentation
The goal is to create conditions that are selective for the desired contaminant-degrading activity
in situ . Experience suggests that this goal is easier to achieve if:

The contaminant degradation is linked to microbial growth (i.e., metabolic)

The microbes are obligate contaminant-degraders

The contaminants are electron acceptors (because there are fewer alternatives, compared to
electron donors)

The site is contaminated only by a narrow range of pollutants.