Introduction to microbial degradation of xenobiotic compounds

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

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

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

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

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

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

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

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…)

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

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)

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

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

Electron acceptors…?

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

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