Biological nitrogen fixation: which organisms most directly aid plants in acquiring nitrogen from atmospheric N2 under natural or managed systems?

Introduction / Context:
Nitrogen gas (N2) is abundant yet unusable by plants unless reduced to ammonia or related forms. Biological nitrogen fixation is carried out by specialized microorganisms (diazotrophs). Recognizing the correct biological agents is key for agronomy, ecology, and sustainable agriculture practices.

Given Data / Assumptions:

• Plants need fixed nitrogen (NH3, NH4+, NO3−), not N2.
• Diazotrophy requires nitrogenase, a complex that operates under high ATP demand and O2-sensitive conditions.
• Symbioses (e.g., Rhizobium–legume) and free-living bacteria (e.g., Azotobacter) are relevant.

Concept / Approach:
Only certain prokaryotes have nitrogenase. In soils and root nodules, bacteria reduce N2 to NH3, which plants assimilate after protonation to NH4+ and incorporation into amino acids via GS-GOGAT pathways. Algae can participate indirectly (some cyanobacteria fix N2), but the canonical agents aiding crop plants are bacteria, especially rhizobia in nodules.

Step-by-Step Solution:

Verification / Alternative check:
Field practices, inoculants, and textbooks emphasize Rhizobium/Bradyrhizobium for legumes and associative diazotrophs such as Azospirillum in grasses.

Why Other Options Are Wrong:

• B: “Algae” broadly is misleading; only certain cyanobacteria (prokaryotes) fix N2, not typical eukaryotic algae.
• C/D: Nematodes and moulds do not fix atmospheric N2.
• E: Earthworms improve soil structure and cycling but do not reduce N2.

Common Pitfalls:
Equating cyanobacteria with “algae” and assuming all algae fix nitrogen; most do not.

Final Answer:
Bacteria (free-living or symbiotic diazotrophs)