Chapter 20
Microorganisms and Organic Pollutants
Raina M. Maier
1. Why is there concern about the presence of organic contaminants in the environment?
The presence of organic contaminants in soil and water leads to human, animal and ecosystem
exposures that can have profound effects. For humans, contaminants in drinking water
and food can lead to diseases such as cancer which may not occur until decades following
exposure. Similarly contaminants are taken up by animals and plants that may serve as
food sources for humans.
2. Describe the different factors that can limit biodegradation of organic contaminants in
the environment.
Biodegradation can be limited by a number of factors including:
A) The absence of appropriate degrading genes in the indigenous microbial community.
B) Toxicity of the organic contaminant which inhibits cellular metabolism.
C) Low bioavailability which is the combined effect of limited water solubility and
sorption to soil surfaces. Low bioavailability will effectively reduce uptake of the
contaminant by a microbial cell.
D) Contaminant structure including both steric and electronic effects. Steric effects are
imposed by substituent groups on a contaminant molecule which hinder recognition
or access of the degrading enzyme to the active site on the contaminant molecule.
Electronic effects are imposed by substituent groups that alter the energy required to
break critical bonds in the molecule.
E) Biodegradation can be limited by low numbers of microbes in the environment.
F) Biodegradation can be limited by insufficient oxygen.
G) Biodegradation can be limited by low nutrient availability (nitrogen or phosphorus).
H) Biodegradation can be limited by suboptimal temperatures.
I) Biodegradation can be limited by suboptimal water availability.
J) Biodegradation can be limited by suboptimal pH.
3. Draw and name an aliphatic, alicyclic, and aromatic structure, each with 6 carbons.
Hexane (aliphatic)
Cyclohexane (alicyclic)
Benzene (aromatic)
4. Outline the biodegradation pathway for each of the structures that you just drew under
aerobic conditions.
5. Why are aerobic conditions usually preferred for biodegradation of organic
contaminants? Under what conditions might anaerobic biodegradation be
preferred?
Aerobic metabolism generates more energy for growth than does anaerobic metabolism. Thus,
degradation is more rapid and efficient. One condition under which anaerobic
biodegradation is preferred is for highly chlorinated compounds such as perchloroethene.
Perchloroethene is not degraded aerobically but can serve as a terminal electron acceptor
under anaerobic conditions.
6. Compare the advantages and disadvantages of intrinsic, ex situ, and in situ
bioremediation.
Intrinsic bioremediation is the use of activity that is occurring within the site. The advantage of
this approach is that it does not require any effort or engineering to implement the
process. The disadvantage is that intrinsic activity may be too slow to prevent a
contaminant plume from impacting a nearby water supply or other valuable resource.
Ex situ bioremediation is the treatment of a contaminated water or soil sample off-site. This can
allow very efficient removal of the contaminant because the material (water or soil) can
be mixed and efficiently contacted with the biodegrading organism. The disadvantage is
that this approach requires removal of the contaminated material which can be expensive
and result in exposure of workers to toxic materials.
In situ bioremediation is the active or engineered treatment of a contaminant in place. The
advantage of this approach is that the site is not disturbed and worker exposure is
minimized. Rapid rates of remediation can be achieved to protect valuable nearby
resources. The disadvantage of this approach is that it can be difficult to accurately
locate the zone of contamination without digging and extensively characterizing the site.
Further, most sites are very heterogeneous and this can make the success of in situ
treatment inconsistent.
7. You have been hired to bioremediate a site in which the groundwater is contaminated
with petroleum. Groundwater samples have a strong sulfide smell and gas
chromatographic analysis of the samples show negligible biodegradation of the
petroleum has occurred. What is your recommendation?
The strong smell of sulfide suggests that the site is anaerobic and sulfate-reduction is occurring.
Since petroleum is not efficiently degraded under sulfate-reducing conditions, the
recommendation would be to aerate the site. A calculation could be performed based on
the amount of petroleum spilled to determine the amount of air or pure oxygen that would
be required for complete degradation.
8. Kleen Co. is in charge of a site in Nevada that was used for pesticide preparation. As a
result of years of operation, the groundwater below this site has elevated levels of
pesticides (up to 20 mg/1). Your initial investigation shows that 1) the pesticidecontaining plume is neither growing nor shrinking in size, 2) there are pesticide
degraders in the plume, 3) and the dissolved oxygen levels in the plume range from 2
to 4 mg/1. This site is not being used presently, and the groundwater is not used for
drinking water purposes. What is your best recommendation based on these site
characteristics and on your knowledge of cost of remediation?
Since the pesticide plume is not growing it does not pose a threat to nearby groundwater sources.
The fact that there are pesticide degraders in the plume and that the oxygen level is somewhat
reduced from that of oxygen saturated water (approx. 9 mg/l) suggests that pesticide degradation
is slowly occurring. Based on these observations the recommendation would be to allow
intrinsic bioremediation to continue and to monitor the site to ensure that the plume does not start
growing.