Explain how bacteria are used in bioremediation of soil.

a. bioremediation uses bacteria to remove contamination;
b. suitable bacteria may already be present in the environment;
c. bacteria may use contaminants in metabolism/as an energy source;
d. end products (of metabolism) are less toxic than (inorganic) contaminants;
e. bacteria may concentrate/isolate it within the microbial cells;
f. can be carried out in situ or ex situ;
g. suitable example (eg degradation of solvents / oil spill cleanup / oxidation of selenium / can be used to deal with pesticides);
h. second suitable example;
i. useful bacterial strains obtained by genetic modification;

This was the least popular of the HL options but it was encouraging to note some schools studying it with some good standards seen. 

Many candidates knew that bioremediation used bacteria to remove contamination from soil, such as happens during an oil spill. Only better candidates were able to actually explain how bacteria used the contaminants in their own metabolism and what the source of these bacteria was. Many were incorrectly describing the nitrogen cycle. Perhaps candidates had been taught bioremediation of high nitrate content ground water by bacteria but expressed it, incorrectly, as the nitrogen cycle.