Biomedical Engineering Reference
In-Depth Information
Boon et al., 2000 ). Each ecosystem is unique, and there are no well-established methods for
predicting inoculum survival.
The type of bioaugmentation agent used also can play a major role in the survival of the
inoculum. A recurrent theme in strain selection is that bacteria from the site itself often make
the best inoculant (Singer et al., 2005 ; Thompson et al., 2005 ). Enrichments from the site itself
may have a higher chance of survival than commercial inocula, since they are already accli-
mated to the site parameters (El Fantroussi and Agathos, 2005 ). Gene bioaugmentation may be
an even better choice in the future, as the survival of the inoculum itself is not necessary.
The introduced organisms only have to survive long enough to transfer their MGEs. Direct gene
bioaugmentation without the use of bacterial hosts would improve on this technology.
However, the technique to deliver naked DNA that would encourage uptake by indigenous
bacteria rather than its destruction has yet to be perfected. Predicting gene transfer frequencies
also is difficult, and therefore performance cannot be evaluated easily.
1.5.4 Pollutant Bioavailability
Once the inoculum is in place, the introduced bacteria must obtain sufficient nutrients to
survive and also must have access to the pollutant. Pollutant bioavailability can be a major factor
in the time-scale of the treatment and thus the cost of remediation. Bioavailability is a serious
concern for bioremediation of contaminants - such as chloroethenes - that formNAPLs because
they are slowly released into the aqueous solution. If the pollutant is only slightly soluble,
its concentration might not be high enough to induce the degradation pathways in microbes
(Cases and de Lorenzo, 2005 ). One way to improve the bioavailability of the pollutant is to use
surfactants to mobilize the pollutant (El Fantroussi and Agathos, 2005 ). Pollutants trapped in
DNAPLs that would ordinarily take years for natural dissolution may be more quickly dislodged
using surfactants that act either by forming micelles that encapsulate the pollutant or by
reducing the interfacial tension between the pollutant and water. The combination of a surfac-
tant foam with a bioaugmentation inoculum potentially can combine enhanced bioavailability
and degradation capacities to speed up bioremediation (Rothmel et al., 1998 ).
1.5.5 Potential Undesirable Side-Effects
All possible impacts of bioaugmentation cannot be predicted. Certainly, bioaugmentation
involves some potential risks, though to date experience has indicated the risks are minimal, and
any such risks must be weighed against the benefits of pollutant removal (Gentry et al., 2004 ).
Table 1.4 lists some examples of unanticipated side effects of bioaugmentation. The
Table 1.4. Examples of Unexpected Side-Effects of Bioaugmentation (adapted from Sayre and
Seidler, 2005 )
Microorganism
Use
Effect
Reference
Pseudomonas SR3
Biodegrades
pentachlorophenol
Inhibits nodule number and
size in Lotus corniculatus
Inhibits substrate induced
respiration
Pfaender et al., 1997
Pseudomonas putida
PPO301 (pRO103)
Degrader of herbicide
2,4-D
Metabolic byproduct causes
significant decreases in soil
fungi
Short et al., 1991
Pseudomonas cepacia
AC1100
Degrader of 2,4,5-T Causes change in taxonomic
diversity of soil microbiota
Bej et al., 1991
Note: 2,4-D - 2,4-dichlorophenoxyacetic acid; 2,4,5-T - 2,4,5-trichlorophenoxyacetic acid.
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