Nitrogen cycle in microbiology: Which group of organisms is responsible for converting atmospheric nitrogen (N2) into forms that ultimately become nitrate (NO3−) in soils?

Introduction / Context:
Nitrogen moves through ecosystems via the nitrogen cycle. Atmospheric nitrogen (N2) is inert and must be converted into bioavailable forms. This question checks understanding of which organisms drive the sequence of microbial processes that ultimately yield nitrate (NO3−) in soils used by plants.

Given Data / Assumptions:

• N2 must first be fixed to ammonia/ammonium (NH3/NH4+).
• Ammonia is oxidized to nitrite (NO2−) and then to nitrate (NO3−) by specialized microbes.
• Answer should identify the group that executes these steps in nature.

Concept / Approach:
Nitrogen fixation is performed by diazotrophic bacteria (for example, Rhizobium, Azotobacter, some cyanobacteria). Nitrification is a two-step bacterial process: ammonia → nitrite by Nitrosomonas and nitrite → nitrate by Nitrobacter and related taxa. Fungi and animals do not perform these conversions from N2 to NO3−.

Step-by-Step Solution:

Verification / Alternative check:
Legume–Rhizobium symbiosis and free-living nitrogen fixers establish the NH3 pool; classical nitrifiers complete oxidation to nitrate—standard textbook pathways.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming any soil microbe can fix N2; confusing mineralization (organic N → NH4+) with fixation (N2 → NH3).

Final Answer:
Bacteria (including nitrogen-fixing and nitrifying types).