Cyanobacteria (Blue-Green Algae) and Nitrogen Fixation
Biology · Biological Classification · NEET
Cyanobacteria are also called blue-green algae. They are prokaryotes in Kingdom Monera. They have chlorophyll a like green plants, so they make their own food (photosynthetic autotrophs). Some of them fix nitrogen from air inside special cells called heterocysts, for example Nostoc and Anabaena. Memory hook: "Blue-green = Monera, NOT true algae. Heterocyst = nitrogen factory."
A cyanobacterial filament: green vegetative cells do photosynthesis (chlorophyll a), while the larger golden heterocyst is the special cell that fixes atmospheric nitrogen. Cyanobacteria are prokaryotes of Kingdom Monera, not true algae.
Your doubts, answered
Is cyanobacteria a plant, an alga, or a bacterium?
It is a bacterium. Even though we call it 'blue-green algae', cyanobacteria are prokaryotes and belong to Kingdom Monera, not to Plantae or Protista. The word 'algae' in the name is only historical. Real algae (like Spirogyra) are eukaryotic. For NEET, always remember cyanobacteria = Monera = prokaryote.
Why are cyanobacteria called blue-green algae if they are actually bacteria?
Because they have chlorophyll a, the same green pigment found in green plants, so they can do photosynthesis like plant-like algae. Old classification looked only at 'has a cell wall + does photosynthesis' and wrongly grouped them with plants. Whittaker's five kingdom system corrected this and placed them in Monera as prokaryotes. This is a very common NEET trap.
What is a heterocyst and what does it do?
A heterocyst is a special thick-walled cell found in some filamentous cyanobacteria. It is the site where atmospheric nitrogen (N2) is fixed (converted into a usable form). NCERT gives Nostoc and Anabaena as examples. The heterocyst keeps oxygen out so the nitrogen-fixing enzyme can work. In one word: heterocyst = nitrogen-fixing cell.
Are cyanobacteria prokaryotic or eukaryotic?
Prokaryotic. They have no true nucleus, no membrane-bound organelles, and they are placed in Kingdom Monera. NEET repeatedly asks 'which is NOT a prokaryote' and lists Nostoc or Oscillatoria (both cyanobacteria) as prokaryotes, while yeast/Saccharomyces (a fungus) is the eukaryote and the correct 'odd one out'.
Are cyanobacteria autotrophic or heterotrophic?
They are photosynthetic autotrophs. They contain chlorophyll a and make their own food using sunlight, just like green plants. NEET 2019/2022 asked this directly and the correct statement was 'Cyanobacteria are a group of autotrophic organisms classified under Kingdom Monera.'
Where do cyanobacteria live and what are blooms?
They live in freshwater, marine and terrestrial (land) habitats. Their colonies are covered by a gelatinous (jelly-like) sheath. In polluted water bodies they multiply fast and form 'blooms' - a thick coloured layer on the water surface. Remember: cyanobacteria habitat = aquatic environment (a matching-type NEET point).
⚠️ The NEET trap ✗ Cyanobacteria are eukaryotic algae because they are called 'blue-green algae' and do photosynthesis. ✓ Cyanobacteria are prokaryotic, placed in Kingdom Monera. The word 'algae' is only a name; they have no nucleus and no organelles. They photosynthesise using chlorophyll a but are still bacteria. 🧠 Name says 'algae', kingdom says Monera. Trust the kingdom, not the name.
Real NEET questions
NEET 2019 / 2022
Which of the following is a correct statement?
A · Cyanobacteria are a group of autotrophic organisms classified under Kingdom Monera. ✓
B · Bacteria are exclusively heterotrophic organisms
C · Slime moulds are saprophytic organisms classified under Kingdom Monera.
D · Mycoplasma have DNA, Ribosome and cell wall
Solution: Cyanobacteria are prokaryotic photosynthetic autotrophs placed in Kingdom Monera, so (A) is correct. (B) is wrong because bacteria show huge metabolic diversity - they can be autotrophic or heterotrophic. (C) is wrong because slime moulds are protists (Kingdom Protista), not Monera. (D) is wrong because Mycoplasma completely lack a cell wall.
NEET 2016
Which one of the following statements is wrong?
A · Cyanobacteria are also called blue-green algae
B · Golden algae are also called desmids
C · Eubacteria are also called false bacteria ✓
D · Phycomycetes are also called algal fungi
Solution: Eubacteria are called 'true bacteria', not false bacteria, so (C) is the wrong statement. The other three match NCERT: cyanobacteria = blue-green algae (A correct), golden algae = desmids (B correct), and phycomycetes = algal fungi (D correct).
NEET 2018 / ReNEET 2026
Which among the following is not a prokaryote?
A · Nostoc
B · Mycobacterium
C · Saccharomyces ✓
D · Oscillatoria
Solution: Saccharomyces (yeast) is a unicellular fungus and therefore a eukaryote, so it is NOT a prokaryote. Nostoc and Oscillatoria are cyanobacteria (blue-green algae) and Mycobacterium is a bacterium - all belong to Kingdom Monera and are prokaryotes.
Solved Biological Classification NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
