Degredation of Toxic Substances

This section contains six questions:

  1. What happens to chemicals once they have been added to the soil?
  2. What is the differece between metabolic breakdown and breakdown by co-metabolism? How do the numbers of organisms involved change with these two processes?
  3. Why are some chemicals easier to degrade than others?
  4. What does adaption of the population of microorganisms mean and how can it influence the rate of breakdown of chemicals added to the soil?
  5. How are microorganisms used to test chemicals for their toxicity before their introduction into the environment?

 

Question 1: What happens to chemicals once they have been added to the soil?

A variety of chemicals are added to soil by different parts of society. For example, industry can be responsibe for the addition of oil and hydrocarbons into the soil and agriculture may cause the addition of pesticides. Chemicals added to the soil are broken down in a similar way to organic matter. They are oxidised by the microorganisms that produce the appropriate enzymes. They generally have very complex structures and most are converted to several intermediate organic compounds before finally being converted to carbon dioxide and water.

back to top

Question 2: What is the difference between metabolic breakdown and breakdown by co-metabolism? How do the numbers of organisms involved change with these two processes?

When chemicals are degraded metabolically the microorganisms involved gain energy and carbon from the oxidation if the chemical. This allows the microorganisms to grow and multiply and the size of the microbial population initially increases as more of the chemical is degraded. During co-metabolism of chemicals in the soil the microorganisms that degrade the chemical do not gain energy or carbon from the reaction. Thus the population of microorganisms is not increased by the presence of the chemical and remains constant while the concentration of the chemical in soil decreases.

back to top

Question 3: Why are some chemicals easier to degrade than others?

The ease with which a chemical is degraded depends on the complexity of its structure. More complex structures are more difficult to degrade because fewer microorganisms in the soil produce enzymes capable of degrading them.

back to top

Question 4: What does adaption of the population of microorganisms mean and how can it influence the rate of breakdown of chemicals added to the soil?

Adaption of the population of microorganisms occurs when a chemical is added to the soil and the microorganisms capable of degrading it grow and multiply until there is a higher proportion of microorganisms in the soil that are capable of degrading the chemical. When a community of microorganisms in the soil adapts in this way the chemical can bebroken down more rapidly than it would have previously. Adaption of a community of microorganisms can be a problem when it adapts to degrade a pesticide. The pesticide may be degraded quickly preventing it from killing the pest.

back to top

Question 5: How are microorganisms used to test chemicals for their toxicity before their introduction into the environment?

Microorganisms can be used to test the toxicity of a chemical before it is introduced into the environment by determining if the chemical is able to be broken down. If the microorganisms cannot break down the chemical it will persist in the environment for a long time. A scale is used to identify the toxicity level of the chemical against standard microorganisms as representatives of the microbial community in soil.

back to top

return to the Soil Biological Processes Menu