1. Introduction to microbial degradation of xenobiotic compounds

2. Fate of a chemical compound in the environment
Evaporation
Photo degradation
Chemical compound
Chemical degradation
Absorption
Sorption
Microbial
Degradation
Leaching

3. The importance of microbiological degradation
% herbicide
still left in
the soil
Sterilized soil
Non sterilized soil
From Helweg, 1988
Days

4. Who are involved in bio-degradation?
Bacteria
0.5-1 μm
Fungi
2-10 μm
(diameter)
Protozoa
3-100 μm

5. Principle diagram for degradation of a xenobiotic compound
Microbial and chemical
degradation
CO2
Micro-
organisms
Humus
Salts
Water
Bound
residues
CO2
Xenobiotic
compound
Metabolites

6. Example of total mineralization of a xenobiotic compound
2,4-D is a herbicide
From Helweg, 1988

7. Detoxication/Activation
Presence of the insecticide aldrin and it´s main metabolite
Dieldrin in soil:
An example of “activation” of a xenobiotic compound.
One if the metabolites are far more persistent and toxic
than the original molecule
From Alexander, 1999

8. Environmental factors influencing microbial degradation of xenobiotic compounds ?
Chemical composition
Soil type
Microbial biomass (acclimatisation)
Temperature pH
Water availability
Oxidising agents (Redox potentials)

9. Biodegradation vs Chemical composition
Water solubility?
Sorption (Kd)?
Molecule size?
Aliphatic vs cyclic (aromatic)?
Unsaturated (C=C ..) vs saturated?
Cl, Br, I ?
high water solubility 
Low sorptio : (same reason as above)
Small molecules: better
Aliphatic better than branbched
Halogenes:  (and nitro groups…)

10. Same substance – but what’s different? History – previous exposure
1. addition
2. addition
Concentration
of MCPA in
soil water (mg/l)
Days
Degradation of the herbicide MCPA in soil
From Helweg, 1988

11. Variation in degradation rate caused by different soil types
Typical soil profile
in West Denmark:
Chemical compound
Days until 5% is mineralised
Topsoil
Subsoil
(1 meter)
MCPA 11
100
TCA 16
300
2,4-D 9
sand, clay
and organic
matter
Topsoil
(approx
50 cm)
mainly sand
Subsoil
OBS: nutrients (N & P) !!
(C/N/P : 100/10/1)

12. Effect of temperature on the degradation rate
Mineralization of the herbicide
propyzamid in soil at different
temperatures:
Temperature variation over the year in
Danish soil at different depths:
Temperature (°C)
Days until 50%
is mineralized 23 29 8
120 3
245 °C
Q10: 2 - 3
From Helweg, 1988

13. Effect of water availability in top soil on the degradation rate
Degradation of the herbicide simazin in the same soil, at different water
contents. From Walker 1978.
Water content (%)
Days until 50% is mineralised 16 23 4 40
Normally: always enough water in the soil

14. Electron acceptors…?

15. Effect of O2 availability on the degradation
Level of microbial biomass: high
O2 availability: high
Topsoil
Subsoil
unsaturated
Level of microbial biomass: lower
O2 availability: Primarily aerobic
conditions
Groundwater level
Subsoil
saturated
Level of microbial biomass: low
O2 availability: anaerobic conditions

16. Aerobic >< Anaerobic degradation
Generally, less activity is seen under anaerobic conditions
Most compounds are degraded fastest under aerobic conditions. Exceptions are some chlorinated solvents, benzenes or alifatic componds