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 data illustrates that as the concentration of ammonium chloride increases, nitrogenase activity decreases significantly. Specifically, at higher concentrations (e.g. 80 µg cm⁻³), the nitrogenase activity drops to zero, indicating a clear inhibition effect from ammonia. Therefore, the student's conclusion is supported by the evidence provided in the table, which shows a direct correlation between high levels of ammonium chloride and reduced nitrogenase activity.

However, it must be noted that the inhibition observed could also be due to other factors at play and should be validated with further experiments, perhaps including controls without ammonium chloride.

When ammonia is present in high concentrations, it may provide a readily available source of nitrogen for the bacteria. This means that the energy and resources normally allocated for nitrogen fixation via nitrogenase can be redirected towards growth, reproduction, or other metabolic processes. Consequently, inhibiting nitrogenase activity in the presence of sufficient ammonia can be beneficial, allowing the bacteria to conserve ATP and other resources.