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Difficulty: MediumNutrient and Biogeochemical Cycles

In an undisturbed grassland ecosystem, free-living aerobic soil bacteria continuously fix atmospheric nitrogen gas into organic compounds without forming symbiotic associations with plant roots. Which of the following bacterial genera is responsible for this process?

  1. A
    Rhizobium
  2. AzotobacterAnswer
  3. C
    Nitrosomonas
  4. D
    Pseudomonas

Answer

Azotobacter is the free-living aerobic bacterium responsible for nonsymbiotic nitrogen fixation.
Azotobacter is an aerobic, free-living soil bacterium that directly fixes atmospheric nitrogen into soil organic compounds without requiring host plant tissue or root nodule symbiosis.

Step-by-Step Solution

1
Identify the ecological process described in the stem
Nonsymbiotic (free-living) aerobic atmospheric nitrogen fixation.
The stem specifies that nitrogen fixation occurs in soil bacteria without forming symbiotic host-plant relationships.
2
Distinguish between the biological roles of the given bacteria in the nitrogen cycle
Azotobacter is a free-living aerobic fixer. Rhizobium is a symbiotic fixer. Nitrosomonas is a nitrifying bacterium. Pseudomonas is a denitrifying bacterium.
Matching each organism to its specific chemical pathway resolves the correct genus.

Key Concept

Biological Nitrogen Fixation and Bacterial Functional Roles
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Topics

Nutrient and Biogeochemical Cycles

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