6 - Harnessing the Power of Microbes

Question 1

Processes microbes are responsible for

Answer 1

• Primary Production
• Decomposition
• Fixing nitrogen
• Methane production
• Industrial processes

Question 2

Great Atlantic Sargassum belt

Answer 2

• On land it decomposes and emits toxic gases
• Decomposition processes are aerobic and anaerobic
• Consumes O2 via aerobic respiration
• Creates anoxic areas (O2 does not penetrate underneath)
• Anaerobic decomposition generates H2S gas and NH3
• Gases have human health impacts

Question 3

Industrial processes microbes are involved in

Answer 3

• Mining (assist in extracting metals from low grade ores)
• Bioremediation (degrade environmental contaminants)
• Wastewater treatment (render water safe)

Question 4

Microbial leaching

Answer 4

• Process of concentrating metals in low-grade ore using microbes (useful when concentrations of metal are low)
• Minerals that are most readily oxidised are most amenable to microbial leaching (e.g. iron)
• May also be used for uranium and gold bioleaching

Question 5

Process for copper leaching

Answer 5

1. Low grade ore is dumped in a large pile
2. Dilute sulfuric acid (pH 2; rich in Fe3+) percolated through the pile
3. Resulting liquid is rich in dissolved metals of interest
4. Liquid transported to precipitation plant
5. Metal is precipitated and purified
6. Liquid recycled and returned to top of pile
7. Rinse and repeat

Question 6

Low grade ore

Answer 6

Rocks with low concentration of ore of interest

Question 7

Two reactions that involve microbes in microbial leaching

Answer 7

• Acidothiobacillus ferooxidans oxidises the sulfide in CuS to SO4
  2-, releases Cu2+
• Spontaneous oxidation of sulfide in CuS by Fe3+ (Fe3+ is generated by the bacterial oxidation of Fe2+ by different bacteria)

Question 8

Composition of microbial community changing with
temperature in microbial leaching

Answer 8

• A. ferrooxidans: mesophile, outcompeted when >30°C
• Leptospirillum ferrooxidans: mildly thermophilic ~40°C
• Archaea (e.g. Sulfolobus) dominate at 60-80°C:

Question 9

Acid mine drainage

Answer 9

• Caused by same microbial processes as bioleaching, but where mining operations are not done correctly
• Exposed sulfur rich ore + water + o2 + microbes
• Results from oxidation of sulfide minerals (bacterial or spontaneous)
• Problem in abandoned mines
• Acidic water leaches into surrounding waterways

Question 10

Two important reactions in acid mine drainage

Answer 10

• initiator: development of acidic conditions (SO4
  2- + H+)
• propagation: FeS2 is oxidised; leads to more H2
  SO4

Question 11

Bioremediation

Answer 11

• Use of microbes to clean up or detoxify contaminants (e.g. petrol, herbicide)
• For natural materials, often achieved by stimulating activities of indigenous microorganisms

Question 12

Three outcomes for the pollutant molecule in bioremediation

Answer 12

• Minor change in molecule
• Fragmentation
• Complete mineralisation (organic –> inorganic)

Question 13

Bioremediation of hydrocarbons

Answer 13

• Organic pollutants
• Microbes have already been exposed to, or have been in contact with these compounds
• Have naturally evolved ways to degrade them

Question 14

Petroleum degradation

Answer 14

• Rich source of organic carbon
• Can be completely degraded to CO2
• Readily degraded by microbes where water and air are also present
• Degraded in both oxic and anoxic conditions

Question 15

Oxic degradation of petroleum

Answer 15

• Oxygenase enzymes are important
• Allows degradation on a reasonable time scale

Question 16

Anoxic degradation of petroleum

Answer 16

Very slow rate

Question 17

Large petroleum spills

Answer 17

• Volatile compounds evaporate
• non-volatile compounds remain (80% or more are oxidised within one year)
• Branched-chain and polycyclic hydrocarbons remain longer (difficult to biodegrade)
• Oil that gets into sediments is degraded very slowly with long term environmental impacts

Question 18

Oil oxidising microbes

Answer 18

• Develop rapidly on oil films and slicks
• Bacteria attach to oil droplets and decompose the oil
• Some bacteria grow only on hydrocarbons
• Some bacteria produce detergent that breaks up oil

Question 19

Factors that influence the rate of degradation of petroleum

Answer 19

• Temperature
• Nutrients (N + P)
• Predation
• Release of biosurfactants

Question 20

Xenobiotics

Answer 20

• Compounds not seen by microbes before
• Still organic compounds (contain C), just completely artificial
• Have chemical bonds that are “foreign” to microbes and therefore degraded very slowly if at all
• Highly chlorinated compounds are the most resistant to microbial degradation
• Some are co metabolised

Question 21

Examples of xenobiotics

Answer 21

Pesticides, dyes, chlorinated solvents, munitions, PCBs

Question 22

Co-metabolism

Answer 22

• Two substrates degraded simultaneously
• e.g. pesticide (secondary substrate) degraded only when organic matter (primary
  substrate) is present
• Sometimes co-metabolism generates toxic metabolites which kills the microbes

Question 23

DDT

Answer 23

• Highly chlorinated
• Organochloride
• Developed as a pesticide in the 1940s
• Bioaccumulates

Question 24

Aerobic dechlorination

Answer 24

Catalysed by dioxygenase enzyme

Question 25

Reductive dehalogenation

Answer 25

• Occurs in anoxic environment
• Uses chlorinated organic compound as electron acceptor
• Hydrogenolysis
• Dihaloelimination
• Yields organic compounds that can enter the citric acid cycle

Question 26

Hydrogenolysis

Answer 26

Halogen atom replaced by hydrogen

Question 27

Dihaloelimination

Answer 27

Two halogen atoms removed from adjacent carbon atoms, double bond formed between carbon atoms

Question 28

Plastics

Answer 28

• Polymers, most xenobiotics
• Polyethylene (plastic shopping bags) and polystyrene have carbon-carbon backbones (challenging to degrade)

Question 29

PET (pol(ethyleneterephthalate)

Answer 29

• Contains ester bonds
• Enzymes exist that degrade these bonds

Question 30

Ideonella sakaiensis

Answer 30

• Gram negative organism discovered in 2016
• Isolated from a consortium
• Uses two enzymes to degrade PET (PETase, MHETase)

Question 31

Wastewater health risks

Answer 31

• chemical intoxication / poisoning
• Spread of disease (cholera, hepatitis A)

Question 32

Wastewater treatment uses

Answer 32

• Physical and chemical methods, and
• Industrial scale use of microbes

Question 33

BOD

Answer 33

• Biological oxygen demand
• Efficiency of nutrient removal expressed as BOD
• Relative amount of dissolved O2 consumed by microbes to completely oxidise the organic matter

Question 34

COD

Answer 34

• Chemical oxygen demand
• Measures everything that can be chemically oxidised

Question 35

Primary wastewater treatment

Answer 35

• Physical separation methods
• Water passes through gates and screens
• Solids allowed to settle and effluent from top removed for further treatment

Question 36

Anaerobic secondary treatment

Answer 36

• Used for wastewater containing large quantities of
  insoluble organic matter (high BOD)
• Anaerobes digest suspended insoluble solids into soluble
  components
• These are fermented to fatty acids, H2, Co2
• Fatty acids fermented by syntrophic bacteria to acetate, CO2, H2
• Used by methanogens to make CH4, CO2
• CH4 burned or used as power

Question 37

Aerobic secondary treatment

Answer 37

• Used for wastewater with low levels of organic
  matter (low BOD)
• Uses oxidative degradation reactions
• Wastewater is mixed and aerated in large tanks
• Masses of bacteria called flocs form in water

Question 38

Flocs

Answer 38

• Oxidation of organic matter occurs on the floc as it is
  mixed and exposed to air
• Wastewater then pumped into holding tank, flocs
  settle
• Some of the settled flocs (activated sludge) returned to the aerator (as “inoculum”)

Question 39

Floc properties

Answer 39

• Oxic, anoxic, anaerobic zones
• Range of different microbial processes
• Range of metabolites produced