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Difficulty: MediumNutrient and Biogeochemical Cycles

In nitrogen cycling within terrestrial ecosystems, distinct groups of prokaryotes fix atmospheric nitrogen (N2N_2) under specific environmental conditions. Which of the following free-living, anaerobic microorganisms is directly responsible for fixing atmospheric nitrogen gas in soil?

  1. ClostridiumAnswer
  2. B
    Rhizobium
  3. C
    Nitrobacter
  4. D
    Pseudomonas

Answer

Clostridium is the correct answer because it is a non-symbiotic (free-living) bacterium that fixes atmospheric nitrogen under anaerobic conditions in the soil.
The correct answer is the choice naming Clostridium because it is a genus of free-living (non-symbiotic) anaerobic soil bacteria capable of reducing atmospheric nitrogen gas (N2N_2) into bioavailable organic compounds.

Step-by-Step Solution

1
Identify the required ecological and biochemical criteria
The target organism must be free-living (non-symbiotic), anaerobic, and capable of nitrogen fixation (N2→N_2 \rightarrow organic nitrogen).
Nitrogen-fixing bacteria differ in their metabolic dependence on host plants (symbiotic vs. free-living) and their oxygen requirements (aerobic vs. anaerobic).
2
Evaluate the nitrogen-fixing candidates
Rhizobium requires symbiotic root nodules of legumes. Azotobacter is free-living but aerobic. Clostridium is free-living and anaerobic.
Classifying nitrogen fixers by habitat and oxygen requirement pinpoints Clostridium as the matching free-living anaerobe.
3
Differentiate from other nitrogen cycle bacterial roles
Nitrobacter participates in nitrification (NO2−→NO3−NO_2^- \rightarrow NO_3^-) while Pseudomonas carries out denitrification (NO3−→N2NO_3^- \rightarrow N_2).
Neither Nitrobacter nor Pseudomonas incorporates gaseous elemental nitrogen into soil organic matter.

Key Concept

Microbial Differentiation in Biological Nitrogen Fixation
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Topics

Nutrient and Biogeochemical Cycles

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