Question 1
Paper 2Hard16 marksA unicellular alga, A, and a photosynthetic prokaryote, P, were cultured separately or together. Each species present in a culture had an initial density of 100 cells mL⁻¹. Cultures were kept at the same temperature and pH, with inorganic carbon and mineral nutrients maintained at non-limiting concentrations. Light treatments received the same illumination for 12 hours each day; dark treatments received no light. The table shows mean cell densities after 10 days. Each mean is based on five replicate cultures.
| Culture | A at start / cells mL⁻¹ | A after 10 days / cells mL⁻¹ | P at start / cells mL⁻¹ | P after 10 days / cells mL⁻¹ |
|---|---|---|---|---|
| A alone, light | 100 | 800 | — | — |
| A alone, dark | 100 | 100 | — | — |
| P alone, light | — | — | 100 | 600 |
| P alone, dark | — | — | 100 | 100 |
| A and P together, light | 100 | 200 | 100 | 500 |
| A and P together, dark | 100 | 100 | 100 | 100 |
A dash indicates that the species was not present.
State the mode of nutrition shared by A and P, and explain how this mode supports population growth.
Calculate the percentage decrease in A’s mean density after 10 days when grown with P in light, compared with A grown alone in light.
Compare the changes in density of the two species when grown alone in light and in darkness. Use the data.
In the light treatments, the mean percentage of incident light reaching the base of the culture after 10 days was:
| Culture | A alone | P alone | A and P together |
|---|---|---|---|
| Light reaching base / % | 76 | 72 | 24 |
Evaluate the claim that P reduces the growth of A by competing with it for light. Use the population and light data, link the light data to photosynthetic nutrition, and identify and explain up to two limitations before reaching a conclusion.