The scientists used units of µg g−1 for the concentration of ammonia in soil.

Suggest why, in this investigation, the scientists used these units.

µg

g−1

Calculate the difference in the rate of breakdown of ammonia per day between day 0 and day 2 in soil A and soil B.

Show your working and the units for your answer.

Difference in rate = ______________

The scientists concluded that the soil mixture experiment showed there were different communities of bacteria in soils A and B.

What evidence from Figure 6 supports their conclusions? Give reasons for your answer.

The oxidation of ammonia by nitrifying bacteria involves the enzyme ammonia monooxygenase. Each species of nitrifying bacteria has its own specific amoA gene that codes for production of ammonia monooxygenase.

In a second investigation, the scientists determined the expression of the amoA gene in two species of bacteria, S and T. Species S was from acid soil and species T was from soil with a neutral pH.

The scientists grew cultures of each species separately in soils of different pH. They determined the amount of mRNA from the amoA gene in each culture.

Their results are shown in Figure 7.

In which species was the number of copies of mRNA more affected by changes in soil pH from 4.9 to 7.5? Use a calculation to support your answer.

This method allowed the scientists to estimate the expression of the amoA gene in each culture but not the growth of the bacterial population in each culture.

Explain why.

The scientists set up their cultures in sterile glass bottles.

Suggest one suitable method for sterilising the bottles and explain why it was necessary to sterilise them.