Question

In which of the following Azolla symbiotically associates to fix atmospheric nitrogen?

• A. Bacteria
• B. Algae
• C. Virus
• D. Fungi

Hint:

Always pair Azolla with BGA (Blue-Green Algae).
It is the most successful non-leguminous nitrogen-fixing system in paddy fields.
Think of it as a "Green Manure" that floats!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the group of organisms that forms a nitrogen-fixing symbiotic relationship with the water fern Azolla.
Detailed Explanation:

• The Azolla-Anabaena Complex:
Azolla is a small aquatic fern that floats on the surface of water in rice paddies and ponds.
It hosts a cyanobacterium named Anabaena azollae within specialized cavities in its dorsal leaves.

• Historical and Biological Nomenclature:
Cyanobacteria were traditionally and colloquially known as Blue-Green Algae (BGA).
Although modern biology classifies them as bacteria (prokaryotes), most agricultural literature and competitive exams still use the term "Algae" or "Blue-Green Algae" to describe them in this context.

• Mechanism of Nitrogen Fixation:
The Anabaena fixes atmospheric nitrogen using the enzyme nitrogenase in specialized cells called heterocysts.
The fern provides the bacteria with sugar (from photosynthesis), and in return, the bacteria provide fixed nitrogen to the fern.

• Agronomic Use:
When the fern dies and decomposes, the fixed nitrogen is released into the water, making it an excellent biofertilizer for rice crops. It can contribute $30-40$ kg N/ha per season.

Step 2: Final Answer:
Azolla associates with Anabaena, which is traditionally categorized as Blue-Green Algae.