Question

Nostoc is a _________.

• A. Virus
• B. Fungi
• C. Protozoa
• D. Cyanobacteria

Hint:

Nostoc and Anabaena are filament-forming, heterocystous Cyanobacteria (Blue-Green Algae) that perform oxygenic photosynthesis and fix atmospheric nitrogen in paddy fields.

Answer 1

The Correct Option is D

Concept:
Biological classification groups photosynthetic, prokaryotic microorganisms that produce oxygen under Cyanobacteria, commonly termed Blue-Green Algae (BGA).
These organisms possess a simple prokaryotic cellular architecture lacking membrane-bound organelles and a true nucleus.
Nostoc is an important member of this group and plays a crucial role in agricultural systems, particularly in wetland rice farming.

Step 1: Structural and Taxonomic Characteristics of Nostoc:
Nostoc is a filamentous, colonial cyanobacterium enclosed within a gelatinous sheath.
It belongs to the Domain Bacteria, Phylum Cyanobacteria, and Order Nostocales.
Its cells are photosynthetic prokaryotes containing chlorophyll-a, phycocyanin, and phycoerythrin pigments.

Step 2: Nitrogen-Fixing Capacity and Heterocysts:
Under nitrogen-limiting conditions, Nostoc develops specialized, thick-walled, non-photosynthetic cells called heterocysts.
Heterocysts create an anaerobic micro-environment essential for the enzyme nitrogenase to convert atmospheric dinitrogen ($\text{N}_2$) into bioavailable ammonia ($\text{NH}_3$).

Step 3: Agricultural Role as a Biofertilizer:
Because of its nitrogen-fixing ability, Nostoc is extensively used as an eco-friendly biofertilizer in submerged paddy fields, adding substantial nitrogen and organic carbon to the soil.

Step 4: Final Answer:
Hence, Nostoc is classified as a Cyanobacterium, which matches option (D).