USA Biology Olympiad (USABO) Practice Exam

Which nutrient is NOT required by nitrogen-fixing organisms for growth?

1. Phosphate

2. Nitrogen

3. Nitrate

4. Iron

The correct answer is: Nitrate

Nitrogen-fixing organisms, such as certain bacteria and archaea, have the unique ability to convert atmospheric nitrogen (N₂) into ammonia (NH₃), which they can use for their growth and metabolism. In the process of nitrogen fixation, these organisms primarily benefit from the nitrogen that they are capable of fixing from the environment. Among the nutrients listed, nitrate is not required by nitrogen-fixing organisms in the same way as the others. While nitrate (NO₃⁻) is a source of nitrogen for many plants and some microorganisms, nitrogen-fixers do not need this compound because they directly convert atmospheric nitrogen into a usable form. Their growth and function do not depend on external sources of nitrate, which makes it the correct answer to the question. Other nutrients like phosphate and iron play essential roles in the growth of these organisms. Phosphate is critical for energy transfer and nucleic acid synthesis, while iron serves as an essential cofactor in various enzymatic functions, including those involved in nitrogen fixation. Thus, while nitrogen-fixers need many nutrients for optimal growth, they do not require nitrate specifically for their nitrogen acquisition, distinguishing it from the other options.