Nitrogen-fixing bacteria such as Azotobacter chroococcum use the enzyme nitrogenase to produce ammonia from nitrogen gas in the air - AQA - A-Level Biology - Question 3 - 2022 - Paper 2

The student’s conclusion is valid as the results indicate that nitrogenase activity decreases with higher concentrations of ammonium chloride. From the data, we can see that at 0 µg cm⁻³ of ammonium chloride, nitrogenase activity was at its peak (45 arbitrary units). However, as the concentration of ammonium chloride increased to 20 µg cm⁻³ and above, the nitrogenase activity dropped significantly, eventually reaching zero at 80 µg cm⁻³. This suggests a clear inhibitory effect of ammonia on nitrogenase activity, supporting the student's conclusion.

When ammonia is present in high concentrations, it reduces the need for nitrogen fixation by the bacteria, as ammonium chloride serves as an easily accessible nitrogen source. This allows the nitrogen-fixing bacteria to conserve energy and resources that would otherwise be spent on nitrogen fixation processes, such as synthesizing ATP, which requires significant energy.