Which enzyme is considered the diagnostic marker for identifying nitrogen-fixing microorganisms?

Introduction:
Detection of nitrogen-fixing capability relies on identifying the core enzymatic machinery that converts atmospheric nitrogen to ammonia. This question asks for the definitive enzyme complex associated with diazotrophy.

Given Data / Assumptions:

• Nitrogen fixation requires a specialized enzyme not present in non-fixing organisms.
• Auxiliary enzymes in nitrogen metabolism do not necessarily indicate fixation.
• Assays can infer activity from reductant-dependent acetylene reduction or gene markers.

Concept / Approach:
Nitrogenase is the hallmark enzyme of diazotrophs. Its presence and activity are used to diagnose nitrogen fixation by biochemical assays (for example, acetylene reduction to ethylene) and by detecting nif genes coding for nitrogenase components. Nitrate reductase and putative nitrate oxidase relate to nitrate assimilation or respiration, not to N2 fixation.

Step-by-Step Solution:
Identify the unique reaction: N2 to NH3 requires nitrogenase. Differentiate from nitrate reduction enzymes that act on NO3-. Select nitrogenase as the diagnostic enzyme complex. Note that gene-based detection (nifH) corroborates enzymatic assays.

Verification / Alternative check:
Field and lab studies rely on nitrogenase activity measurements to quantify fixation rates, confirming its diagnostic status.

Why Other Options Are Wrong:

• Nitrate reductase: Functions in nitrate to nitrite conversion; not evidence of N2 fixation.
• Nitrate oxidase: Not the enzyme mediating N2 reduction; also rarely used term in this context.
• None of these: Incorrect because nitrogenase is definitive.

Common Pitfalls:
Assuming any nitrogen metabolism enzyme implies fixation; only nitrogenase demonstrates diazotrophic potential.

Final Answer:
nitrogenase is the diagnostic enzyme for nitrogen-fixing organisms.