1

Introduction to microbial degradation of xenobiotic

compounds

2

Photo degradation

Chemical degradation

MicrobialDegradation

Chemical compound

Leaching

Sorption

Fate of a chemical compound in the environment

Evaporation

Absorption

3

The importance of microbiological degradation

Days

% herbicidestill left in the soil Sterilized soil

Non sterilized soil

From Helweg, 1988

4

Who are involved in bio-degradation?

Fungi2-10 μm (diameter)

Bacteria0.5-1 μm

Protozoa3-100 μm

5

Principle diagram for degradation of a xenobiotic compound

Xenobiotic compound

Microbial and chemicaldegradation

Metabolites

CO2

Micro-organisms

Humus

Salts

Water

Boundresidues

CO2

6

Example of total mineralization of a xenobiotic compound

2,4-D is a herbicide

From Helweg, 1988

7

Detoxication/Activation

An example of “activation” of a xenobiotic compound. One if the metabolites are far more persistent and toxicthan the original molecule

Presence of the insecticide aldrin and it´s main metaboliteDieldrin in soil:

From Alexander, 1999

8

Environmental factors influencing microbial degradation of xenobiotic

compounds ?

9

Biodegradation vs Chemical composition

• Water solubility?

• Sorption (Kd)?

• Molecule size?

• Aliphatic vs cyclic (aromatic)?

• Unsaturated (C=C ..) vs saturated?

• Cl, Br, I ?

History – previous exposure

10

Degradation of the herbicide MCPA in soil

Days

Concentrationof MCPA insoil water (mg/l)

1. addition 2. addition

From Helweg, 1988

Same substance – but what’s different?

11

Variation in degradation rate caused by different soil types

Chemical compound

Days until 5% is mineralised

Topsoil Subsoil

(1 meter)

MCPA 11 100

TCA 16 300

2,4-D 9 100

Topsoil(approx 50 cm)

Subsoil

Typical soil profilein West Denmark:

sand, clayand organicmatter

mainly sand

OBS: nutrients (N & P) !!

(C/N/P : 100/10/1)

12

Effect of temperature on the degradation rate

Temperature (°C) Days until 50%

is mineralized

23 29

8 120

3 245

From Helweg, 1988

Mineralization of the herbicidepropyzamid in soil at differenttemperatures:

Temperature variation over the year inDanish soil at different depths:

°C

Q10: 2 - 3

13

Effect of water availability in top soil on the degradation rate

Water content (%) Days until 50% is mineralised

16 23

4 40

Degradation of the herbicide simazin in the same soil, at different water contents. From Walker 1978.

Normally: always enough water in the soil

Electron acceptors…?

14

15

Effect of O2 availability on the degradation

Topsoil

Subsoilunsaturated

Subsoilsaturated

Groundwater level

Level of microbial biomass: highO2 availability: high

Level of microbial biomass: lowerO2 availability: Primarily aerobic conditions

Level of microbial biomass: lowO2 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