Question

A free living non-symbiotic anaerobic bacterium is:

• A. Chromatium sp.
• B. Beijerinchia sp
• C. Klebsiella pnevmonae
• D. Clostridium pasteurianum

Hint:

To remember free-living soil nitrogen fixers: - Aerobic: Azotobacter, Beijerinckia. - Anaerobic: Clostridium pasteurianum.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Free-living, non-symbiotic nitrogen-fixing bacteria live independently in the soil without forming direct physical associations (like root nodules) with host plants.
These bacteria can be classified as aerobic, anaerobic, or photosynthetic based on their oxygen requirements and metabolic pathways.

Step 2: Detailed Explanation:
Let us analyze the physiological traits of the listed bacteria:
- Chromatium sp.: This is a photosynthetic, anaerobic bacterium, but it is not a primary model for free-living, heterotrophic anaerobic nitrogen fixation in agricultural soils.
- Beijerinckia sp (written as Beijerinchia): This is a free-living, but strictly aerobic nitrogen-fixing bacterium.
- Klebsiella pneumoniae (written as pnevmonae): This is a facultative anaerobe that can fix nitrogen under anaerobic conditions, but it is not an obligate anaerobe.
- Clostridium pasteurianum: Discovered by Sergei Winogradsky, this is the classic example of a free-living, non-symbiotic, obligately anaerobic nitrogen-fixing bacterium in the soil.
It reduces atmospheric nitrogen ($N_2$) to ammonia ($NH_3$) under anaerobic conditions using organic compounds as energy sources.
Therefore, Clostridium pasteurianum is the correct free-living anaerobic bacterium.

Step 3: Final Answer
The correct option is (D).