10. Halogenated pollutant biodegradation

Question 1

What are halogenated organic molecules?

Answer 1

Molecules containing Cl, F, Br molecules attached to the organic compound

Question 2

Why are halogenated compounds harder to degrade?

Answer 2

Halogenated compounds are almost always harder to degrade than their non-halogenated counter bonds, because C-Cl bonds are strong and difficult to break!

Question 3

What determines the biodegradability of halogenated phenolic compounds?

Answer 3

Primarily, molecules with a greater degree of halogenation are usually more recalcitrant, but the general complexity of the molecule may also be important (more complex = more recalcitrant to biodegradation)

Question 4

What are the three major steps in the biodegradation of halogenated organic compounds?

Answer 4

1. Debranching and Ring-breakage
2. Dehalogenation and Hydroxylation
3. Ring-cleavage and Oxidation/Reduction

Question 5

Which step of the three steps of the biodegradation of halogenated compounds is the key step?

Answer 5

Dehalogenation

Question 6

What is PCP (pentachlorophenol)? (4)

Answer 6

• Ring with 5 Cl, 1 OH
• It is a herbicide, insecticide, fungicide, algaecide, and disinfectant
• Ingredient in antifouling paint, used as a wood preservative, used in telephonr poles, in railways
• Has harmful effects on the liver, kidneys, blood, lungs, nervous system and ecosystem health

Question 7

What are the two ways that PCP can be biodegraded? And which was is it usually done?

Answer 7

Aerobically and anaerobically.
Its usually done anaerobically by reductive dechlorination

Question 8

How is biodegradation of PCP done? (2)

Answer 8

• Microorganisms at the contaminated site provide H2 as a natural byproduct of various fermentation reactions.
• The dechlorinating bacteria use this H2 as their electron donor, ultimately replacing chlorine atoms in the chloroethenes with hydrogen atoms via hydrogenolytic reductive dechlorination.

Question 9

What are PCBs

Answer 9

• Two benzene rings attached together by a line
• They can have many Cl attached around

Question 10

What are two ways that PCBs are biodegraded?

Answer 10

• By anaerobic reductive dehalogenation by bacteria.
• By aerobic PCB degradation by biphenyl-oxidising bacteria

Question 11

What is the issue with aerobic PCB degradation by biphenyl-oxidising bacteria?

Answer 11

It results in accumulation of chlorobenzoates (CBAs) which are difficult to biodegrade further.

Question 12

What is 2,4D

Answer 12

• A common pesticide/herbicide used to control broadleaf weeds
• One of the most widely used herbiceds in the world
• Is toxic → carcinogenic, neurtoxicity, liver damage, sometimes contaminated with dioxins which are highly toxic

Question 13

Can you biodegrade 2,4D?

Answer 13

• Yes! There are biodegradablel by a wide range of bacteria in soil and water
• There half life in soil is 2-16 days

Question 14

What is DDT? (3)

Answer 14

• DDT is an organochlorine pesticide used extensively since the 1940s for the control of agricultural pests and vector-borne diseases like malaria and typhus.
• DDT is toxic and very recalcitrant to degradation with the half-life of 4–30 years.
• As priority persistent organic pollutants (POPs) and endocrine-disrupting chemicals (EDCs), exposure to DDT can cause a wide range of acute and chronic effects including carcinogenesis, estrogenic action, and endocrine disruption, posing a serious risk to environmental and human health.

Question 15

What are DDT’s metabolites and what is the issue with them?

Answer 15

It’s major metabolites DDD and DDE are more toxic and recalcitrant than the parent compound.

Question 16

How is TNT degraded?

Answer 16

• Each NO2 is reduced to NH2
• Each reductive step is less complete than the previous one
• The first one may be catalyzed by aerobic microbes
• The last one can only be catalyzed by strict anaerobes
• The reduction sequence does not lead to mineralization but rather to polymerization or binding of the residues

Question 17

The characrerisitcs of the elusive superbug (7)

Answer 17

• Penetration power = can get into difficult substrates, e.g. clat
• Highly motile = moves towards the pollutant and munches it away
• Produces biosurfactant = good for solubilizing the organic compound that usually isn’t very soluble
• Nutrient reserves
• Tough surface = can survive its toxic environment
• Multifuctional respiration - can breathe different compounds, is not limited by oxygen, can biodegrade aerobically and anaerobically
• Voracious appetite for the contaminants at hand

Question 18

Genetic engineering of a hydrocarbon degrading superbug
How?

Answer 18

• Combination of plasmids (Camphor plasmid and Octanol plasmid)
• Putting plasmids in one bacterium (Xylene plasmid and Naphthalene plasmid into one bacterium)
• Combine both of the above structures to make the superbug now containing 4 plasmids

Question 19

What is the issue with creating genetically engineered superbugs?

Answer 19

• “Oh dear! I didn’t realize ‘in the field’ would be like this! We should habe stayed in the laboratory!”
• Basically, there is a reason that the already present microorganisms aren’t biodegrading, so we should fix that issue rather than add in superbugs that are likely to encounter the same issues that will impede them from biodegrading.

Question 20

Creating of genetically modified strain to mineralize PCBs

Answer 20

• The genetically modifed C. necator strain JMS34 was able to mineralize PCBs without accumulation of CBAs
• This was done by putting two genes together B. xenovorans LB4000 and C. necator JMP134-X3
• This put together two degradation pathways to degrade the PCBs and remove the CBAs

Question 21

Did this genetically modified strain to metabolize PCBs without getting CBAs work?

Answer 21

Yes, but basically it just worked aster than the other regular strains. So useful if you’re on a time crunch.