Which of the following is a nitrogen-fixing blue-green algae of soil?

Step 1: Analyze the question The question asks for an organism that meets three conditions:

1. It fixes nitrogen.
2. It is a blue-green alga (cyanobacterium).
3. It lives in soil.

Step 2: Explain the concept Nitrogen fixation is the conversion of atmospheric nitrogen (N₂) into compounds plants can use, such as ammonia. Different groups of microbes do this:

• Symbiotic bacteria live in root nodules of legumes. Rhizobium is the example.
• Free-living bacteria fix nitrogen in the soil without a host. Azotobacter (aerobic) and Clostridium (anaerobic) are examples.
• Blue-green algae (cyanobacteria) photosynthesize and also fix nitrogen. Anabaena and Nostoc are examples. They are common in soils and in rice fields.

Step 3: Evaluate each option

• A. Rhizobium: a nitrogen-fixing bacterium living in legume root nodules. It is not an alga.
• B. Nitrosomonas: a nitrifying bacterium that converts ammonia to nitrite. It does not fix nitrogen.
• C. Clostridium: a free-living anaerobic nitrogen-fixing bacterium. It is not a blue-green alga.
• D. Anabaena: a cyanobacterium (blue-green alga) that fixes nitrogen using specialized cells called heterocysts. ✔

Step 4: Answer The correct answer is D. Anabaena.

Exam tip: Link "blue-green algae" with Anabaena and Nostoc. Link "root nodules" with Rhizobium. Link "nitrifying" with Nitrosomonas (ammonia → nitrite) and Nitrobacter (nitrite → nitrate). Remember that Nitrosomonas makes nitrite and Nitrobacter makes nitrate.