Design an experiment, using radioactive carbon and the heavy isotope of nitrogen (15N₂), that would test whether the Rhizobia–pea plant interaction is mutualistic.

Verified step by step guidance

1. Define the hypothesis: The rhizobia–pea plant interaction is mutualistic, meaning both organisms benefit from the interaction. Rhizobia provide nitrogen to the pea plant, and the plant provides carbon compounds to the rhizobia.
2. Select the experimental groups: Prepare two groups of pea plants. One group will be inoculated with rhizobia, and the other will serve as a control without rhizobia.
3. Label the carbon and nitrogen sources: Use radioactive carbon (e.g., $C^{14}$) to label the carbon compounds provided to the plants and $N^{15}$ to label the nitrogen source available to the rhizobia.
4. Monitor and measure: Allow the plants to grow for a set period, then measure the uptake of $N^{15}$ in the plant tissues and the presence of $C^{14}$ in the rhizobia.
5. Analyze the results: Compare the levels of $N^{15}$ in the plants and $C^{14}$ in the rhizobia between the experimental and control groups. A mutualistic interaction would be indicated by significant uptake of $N^{15}$ in the plants and $C^{14}$ in the rhizobia in the experimental group compared to the control.

Key Concepts

Mutualism

Mutualism is a type of symbiotic relationship where both parties involved benefit from the interaction. In the context of rhizobia and pea plants, mutualism would mean that the rhizobia provide nitrogen to the plant, while the plant offers carbohydrates and a habitat to the rhizobia. Understanding mutualism is crucial to designing an experiment that tests the benefits each organism receives.

Nitrogen Fixation

Nitrogen fixation is the process by which certain bacteria, including rhizobia, convert atmospheric nitrogen (N2) into ammonia (NH3), a form that plants can use for growth. This process is essential for the mutualistic relationship between rhizobia and pea plants, as it provides the plants with a vital nutrient. Using the heavy isotope 15N2 can help trace nitrogen fixation and confirm the exchange of nitrogen between rhizobia and the plant.

Radioactive Tracers

Radioactive tracers are substances containing a radioactive isotope that can be used to track the movement of elements through biological systems. In this experiment, radioactive carbon can be used to trace the flow of carbon compounds from the pea plant to the rhizobia, while 15N2 can trace nitrogen fixation. This helps determine if both organisms benefit from the interaction, confirming mutualism.