Free- living nitrogen- fixing aerobic bacterium is

(a) Rhodospirillum

(b) Anabaena

(c) Nostoc

(d) Beijerinckia

(e) Rhizobium

To determine which of the listed options is a free-living nitrogen-fixing aerobic bacterium, we need to look at the characteristics of each organism. Nitrogen-fixing bacteria play a crucial role in converting atmospheric nitrogen into a form that plants can use, and they can be categorized into symbiotic and free-living types. Let's break down the options provided.

Examining the Options

• Rhodospirillum: This is a genus of purple non-sulfur bacteria that can perform photosynthesis. They are known to fix nitrogen and are considered free-living, making them a potential candidate.
• Anabaena: This is a genus of cyanobacteria that typically forms symbiotic relationships with plants, particularly in aquatic environments. While they can fix nitrogen, they are not classified as free-living in the same sense as others.
• Nostoc: Similar to Anabaena, Nostoc is also a cyanobacterium that can form symbiotic relationships with plants and is often found in moist environments. They are not primarily free-living nitrogen fixers.
• Beijerinckia: This genus consists of free-living nitrogen-fixing bacteria that thrive in soil. They are aerobic and can fix nitrogen independently, making them a strong contender.
• Rhizobium: These bacteria are well-known for their symbiotic relationships with legumes, where they fix nitrogen in root nodules. They are not free-living in the same way as Beijerinckia or Rhodospirillum.

Identifying the Correct Answer

Considering the characteristics and classifications of these organisms, the best answer to the question is Beijerinckia. This genus is specifically recognized for its ability to fix nitrogen independently in aerobic conditions, making it a prime example of a free-living nitrogen-fixing bacterium.

Understanding Nitrogen Fixation

Nitrogen fixation is a vital process in the nitrogen cycle, where atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds. This process is essential for plant growth, as most plants cannot utilize atmospheric nitrogen directly. Free-living nitrogen-fixing bacteria, like Beijerinckia, contribute to soil fertility by enriching it with nitrogen compounds that plants can absorb.

Real-World Applications

In agriculture, understanding and utilizing free-living nitrogen-fixing bacteria can lead to sustainable farming practices. By enhancing soil nitrogen levels naturally, farmers can reduce the need for chemical fertilizers, promoting healthier ecosystems and reducing environmental impact.

In summary, while several options were presented, Beijerinckia stands out as the free-living nitrogen-fixing aerobic bacterium due to its unique characteristics and role in the nitrogen cycle. This understanding not only highlights the diversity of nitrogen-fixing bacteria but also emphasizes their importance in ecological and agricultural contexts.