10. Halogenated pollutant biodegradation Flashcards
What are halogenated organic molecules?
Molecules containing Cl, F, Br molecules attached to the organic compound
Why are halogenated compounds harder to degrade?
Halogenated compounds are almost always harder to degrade than their non-halogenated counter bonds, because C-Cl bonds are strong and difficult to break!
What determines the biodegradability of halogenated phenolic compounds?
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)
What are the three major steps in the biodegradation of halogenated organic compounds?
- Debranching and Ring-breakage
- Dehalogenation and Hydroxylation
- Ring-cleavage and Oxidation/Reduction
Which step of the three steps of the biodegradation of halogenated compounds is the key step?
Dehalogenation
What is PCP (pentachlorophenol)? (4)
- 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
What are the two ways that PCP can be biodegraded? And which was is it usually done?
Aerobically and anaerobically.
Its usually done anaerobically by reductive dechlorination
How is biodegradation of PCP done? (2)
- 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.
What are PCBs
- Two benzene rings attached together by a line
- They can have many Cl attached around
What are two ways that PCBs are biodegraded?
- By anaerobic reductive dehalogenation by bacteria.
- By aerobic PCB degradation by biphenyl-oxidising bacteria
What is the issue with aerobic PCB degradation by biphenyl-oxidising bacteria?
It results in accumulation of chlorobenzoates (CBAs) which are difficult to biodegrade further.
What is 2,4D
- 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
Can you biodegrade 2,4D?
- Yes! There are biodegradablel by a wide range of bacteria in soil and water
- There half life in soil is 2-16 days
What is DDT? (3)
- 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.
What are DDT’s metabolites and what is the issue with them?
It’s major metabolites DDD and DDE are more toxic and recalcitrant than the parent compound.
How is TNT degraded?
- 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
The characrerisitcs of the elusive superbug (7)
- 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
Genetic engineering of a hydrocarbon degrading superbug
How?
- 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
What is the issue with creating genetically engineered superbugs?
- “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.
Creating of genetically modified strain to mineralize PCBs
- 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
Did this genetically modified strain to metabolize PCBs without getting CBAs work?
Yes, but basically it just worked aster than the other regular strains. So useful if you’re on a time crunch.
