From atmospheric nitrogen to ammonium The biological process that reduces atmospheric nitrogen gas (N2) directly to ammonium (NH4+) is called:

Introduction / Context:
Nitrogen flows through ecosystems via specialized processes. Distinguishing fixation, assimilation, and transformations such as nitrification or denitrification is foundational to environmental microbiology and biogeochemistry.

Given Data / Assumptions:

• Starting substrate: atmospheric N2 (a very stable triple-bonded molecule).
• Product of interest: NH4+ incorporated into biological systems.
• Enzymology: nitrogenase complex in diazotrophs.

Concept / Approach:
Nitrogen fixation is the ATP-intensive, reductive conversion of N2 to NH3/NH4+ catalyzed by nitrogenase in certain bacteria and archaea (for example, Azotobacter, Rhizobium, cyanobacteria). Assimilatory nitrate reduction begins with nitrate, not N2. Transamination and deamination exchange or remove amino groups within organic molecules; they do not capture atmospheric nitrogen.

Step-by-Step Solution:
Identify process acting on N2: nitrogen fixation.Note enzyme system: nitrogenase (Fe protein and MoFe protein) under low oxygen conditions.Recognize product: NH3/NH4+ for incorporation into glutamate/glutamine pools.

Verification / Alternative check:
Ecological observations show legume–rhizobia symbioses supplying fixed nitrogen to plants, a practical demonstration of fixation.

Why Other Options Are Wrong:

• Assimilatory nitrate reduction: reduces NO3− to NH4+, not N2.
• Transamination/deamination: intra-organic nitrogen transfers/removals, not capture of N2.
• Nitrification: oxidizes NH3 to nitrite/nitrate, the opposite direction.

Common Pitfalls:
Equating all nitrogen “reductions” with fixation; only N2 → NH4+ is fixation.

Final Answer:
Nitrogen fixation