Which free-living, anaerobic, nitrogen-fixing bacterium is found in soil?

Concept

Biological nitrogen fixation is carried out by both free-living and symbiotic microorganisms. Among free-living fixers, oxygen requirement distinguishes aerobic forms from anaerobic forms.

Application

Clostridium includes free-living obligate anaerobes in soil that fix atmospheric nitrogen. Therefore, Clostridium matches all three clues: free-living, anaerobic, and soil-dwelling.

Contrast

• Azotobacter is a free-living soil nitrogen fixer but is aerobic.

• Rhizobium fixes nitrogen symbiotically in legume root nodules, not as a free-living bacterium.

• Vibrio is not the standard free-living anaerobic soil nitrogen fixer described by these clues.

Cross-check

The keyed organism must satisfy both the lifestyle and oxygen-condition clues. Clostridium satisfies both, so the answer is Clostridium.