Cyanobacteria
Imagine a living thing that can do what plants do — capture sunlight and make food — but is built more like a bacterium. That is exactly what cyanobacteria are. They are often called blue-green algae, but that name is misleading. They are not algae at all; they are bacteria. The "blue-green" comes from the colour they give to water when they grow in large numbers, and from the pigment that helps them photosynthesise.
You have probably seen them without knowing it. That greenish scum that forms on the surface of a stagnant pond or a slow-moving river in summer? That is a cyanobacterial bloom. They thrive in water rich in nutrients — especially phosphorus and nitrogen from fertilisers or sewage — and when conditions are right, they multiply explosively.
The Precise Statement
Cyanobacteria are a group of Gram-negative bacteria that obtain energy through oxygenic photosynthesis — the same type of photosynthesis that plants use, producing oxygen as a byproduct. They contain chlorophyll a (the same green pigment found in plants) and accessory pigments called phycobilins (which give them their blue or red hues). Some species can also fix atmospheric nitrogen — convert inert nitrogen gas (N₂) into ammonia (NH₃) — inside specialised cells called heterocysts.
6 CO₂ + 6 H₂O --(light, in the presence of cyanobacteria)--> C₆H₁₂O₆ + 6 O₂
This is the same overall reaction as plant photosynthesis. Cyanobacteria were the first organisms on Earth to perform oxygenic photosynthesis, and over billions of years, they transformed the planet's atmosphere from one rich in carbon dioxide to one rich in oxygen.
Why They Matter in Your Syllabus
You will encounter cyanobacteria in three main contexts:
1. As photosynthetic autotrophs — They are producers in aquatic ecosystems, forming the base of many food webs. Unlike true algae, they are prokaryotes (no nucleus, no membrane-bound organelles).
2. As nitrogen fixers — Some cyanobacteria (e.g., Anabaena, Nostoc) have heterocysts, thick-walled cells where the enzyme nitrogenase converts N₂ into ammonia. This is crucial because nitrogenase is destroyed by oxygen, and heterocysts provide an oxygen-free environment. These cyanobacteria often live in symbiotic relationships — for example, Anabaena lives inside the water fern Azolla, which is used as a green manure in rice paddies.
3. As indicators of pollution — Cyanobacterial blooms in lakes and ponds signal eutrophication (excess nutrients from human activity). Some species produce toxins (cyanotoxins) that can poison livestock and humans.
Do not confuse cyanobacteria with true algae. Algae are eukaryotes (have a nucleus); cyanobacteria are prokaryotes. The term "blue-green algae" is a historical misnomer — use it only if your exam question does, and then immediately clarify that they are bacteria.
A Quick Visual Summary
| Feature | Cyanobacteria | True Algae (e.g., Chlamydomonas) |
|---|
| Cell type | Prokaryotic | Eukaryotic |
| Chlorophyll | Chlorophyll a only | Chlorophyll a, b, c, etc. |
| Nucleus | Absent | Present |
| Nitrogen fixation | Some species (in heterocysts) | None |
| Habitat | Freshwater, marine, soil, extreme environments | Mostly aquatic |
The Big Picture
Cyanobacteria are not just exam facts. They are the organisms that invented photosynthesis as we know it. Every breath of oxygen you take exists because, over two billion years ago, cyanobacteria started pumping oxygen (O₂) into the atmosphere. They are still here, still photosynthesising, still fixing nitrogen — and still forming those green scums on polluted water. When you see a bloom, you are looking at a living fossil that changed the world.
Cyanobacteria is discussed in the Biological Classification chapter of NCERT Class 11 Biology, a unit that is heavily tested in NEET Biology every year. Students revising the five-kingdom system often search "Cyanobacteria class 11 notes" or "Cyanobacteria important questions for NEET".