Biological nitrogen fixation requirements Nitrogen fixation (conversion of N2 to ammonia by nitrogenase) requires which of the following conditions?

Introduction / Context:
Biological nitrogen fixation is carried out by certain bacteria and archaea using the nitrogenase enzyme complex. The process is ecologically crucial but biochemically demanding. Knowing its requirements explains why only select microbes can fix nitrogen and why they often have special adaptations to manage oxygen exposure.

Given Data / Assumptions:

• Nitrogenase reduces N2 to NH3.
• ATP consumption during electron transfer is substantial.
• Nitrogenase is irreversibly inactivated by molecular oxygen.

Concept / Approach:

Nitrogenase requires a large input of energy (ATP) and reducing power. Because oxygen damages key metal centers in nitrogenase, fixation typically proceeds under anaerobic or microaerobic conditions. Aerobic diazotrophs (for example, Azotobacter) use high respiration rates or protective proteins to keep nitrogenase protected, effectively creating a low O2 microenvironment.

Step-by-Step Solution:

Verification / Alternative check:

Typical stoichiometry shows high ATP cost per N2 reduced; legumes provide an O2-buffering protein (leghemoglobin) in root nodules to maintain an anaerobic environment around rhizobial nitrogenase.

Why Other Options Are Wrong:

• Energy only or anaerobic only: Incomplete; both are required.
• Aerobic environment: Free O2 inactivates nitrogenase.
• Neither: Contradicts well-established biochemistry of nitrogenase.

Common Pitfalls:

• Confusing organism-level aerobiosis with enzyme-level protection; even “aerobic” fixers protect nitrogenase from O2.

Final Answer:

both (a) and (b)