Degredation of toxic substances
This section contains 5 questions:
- What happens to chemicals once they have been added to the soil?
- What is the difference between metabolic breakdown and breakdown by co-metabolism?
- Why are some chemicals easier to degrade than others?
- Can miroorganisms adapt to the presence of a new chemical in soil?
- How are microorganisms used to test chemicals for their toxicity?
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 chemicals added to soil can have very complex structures and these are converted to several intermediate organic compounds before finally being converted to carbon dioxide and water.
Question 2. What is the difference between metabolic breakdown and breakdown by co-metabolism?
Metabolic breakdown and breakdown by co-metabolism are two ways that microorganisms can break down chemicals added to the soil. It is possible to distinguish between them by the effect that adding the chemical has on the number of microorganisms in soil. Metabolic breakdown of chemicals is when the microorganisms involved gain energy and carbon from the oxidation of 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. Breakdown of chemicals in soil by co-metabolism is when 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.
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.
Question 4. Can microorganisms adapt to the presence of a new chemical in 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.
Question 5. How are microorganisms used to test chemicals for their toxicity?
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.
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