Blue Green Algae
Blue-green algae, properly called cyanobacteria, are tiny living things that make their own food from sunlight, just like plants. Despite the name, they are not true algae at all: they are bacteria, and they are among the oldest and most important life forms on Earth.
Scientific Classification & Quick Facts
| Common name | Blue-green algae |
| Scientific name | Cyanobacteria |
| Kingdom | Bacteria |
| Type | Bacteria that photosynthesise (cyanobacteria) |
| Diet | Makes its own energy from sunlight |
| Habitat | Oceans, fresh water, soil, rocks, hot springs |
| Special feature | Can form scummy blooms and fix nitrogen |
Bacteria, Not Algae
The name cyanobacteria comes from the Greek word for dark blue, “kyanos,” because many of them have a blue-green color. That color comes from a mix of green chlorophyll and a blue pigment called phycocyanin.
For a long time, scientists grouped them with green algae and other algae. But cyanobacteria are prokaryotes, which means their cells have no nucleus. True algae are eukaryotes: their cells have a nucleus and other tiny compartments wrapped in membranes, just like plant and animal cells. That is why many scientists say “blue-green algae” is a misleading name.
The Microbes That Changed the Air
Cyanobacteria were among the first living things to carry out the kind of photosynthesis that releases oxygen. Over hundreds of millions of years, the oxygen they produced built up in the oceans and air.
Around 2.4 billion years ago, oxygen levels rose sharply in an event called the Great Oxidation Event. Iron in the oceans reacted with the oxygen, forming rusty layers of rock that can still be seen today. The new oxygen-rich world was deadly to many microbes that could not survive oxygen, but it eventually made complex life, including animals and people, possible.
Cyanobacteria also gave plants their power. Scientists believe the chloroplasts inside plant and algae cells, where photosynthesis happens, are the descendants of cyanobacteria that were swallowed by other cells long ago and ended up living inside them.
Habitat
Cyanobacteria are found almost everywhere on Earth, at every latitude. They live in oceans, lakes, ponds, rivers and damp soil, and they can survive in extreme places such as hot springs, salty lagoons, desert rocks that are only wet now and then, and even rocks in Antarctica.
Some float in the water as plankton. Others form slimy mats or films on rocks and mud, and some live just inside the surface of rocks. Many live in partnership with other organisms. They team up with fungi to form some lichens, and live inside certain plants, sponges and other hosts, sharing food or nutrients with them. The shaggy fur of sloths can even host tiny algae and cyanobacteria, giving the animals a greenish tint that may help them hide in the trees.
Blooms and Toxins
When water is warm, still and full of nutrients, for example from fertilizer runoff, cyanobacteria can multiply quickly and form a bloom. A bloom can look like green scum, spilled paint or pea soup on the surface of a lake or pond.
Some blooms produce toxins that can make people sick and can be deadly to dogs, livestock and wildlife. When a harmful bloom is found, beaches and lakes are often closed. If water looks scummy or discolored, stay out, keep pets out and never let them drink it.
Nitrogen Fixation and Use in Paddy Fields
All living things need nitrogen to build proteins and DNA. Air is mostly nitrogen gas, but most living things cannot use it in that form. Many cyanobacteria can “fix” nitrogen, turning it from the air into compounds that plants can use.
Cyanobacteria come in different shapes. Some are single cells, while others form colonies shaped like sheets, balls or long chains called filaments. Some filament-forming kinds can make several special types of cell:
- Ordinary photosynthetic cells that grow and make food when conditions are good.
- Heterocysts, thick-walled cells that fix nitrogen. Their thick walls keep oxygen out, because the enzyme that fixes nitrogen stops working when oxygen is around.
- Akinetes, tough resting cells that can survive cold, drought or other harsh conditions.
Farmers in parts of Asia have long benefited from nitrogen-fixing cyanobacteria in rice paddies. They live freely in the water and soil, and some grow on other water plants and algae. They act as a natural fertilizer for the rice. Some live in partnership with a small floating fern called Azolla, which farmers grow alongside rice.
Form, Structure and Locomotion
Cyanobacteria have no flagella, the whip-like tails some microbes use to swim. Many cells are wrapped in a slimy, jelly-like sheath.
Even so, some can move. Many filament-forming kinds can glide slowly over surfaces. Some produce short, moving filaments called hormogonia, which break away from the main colony and glide off to start new colonies elsewhere. Filaments of Oscillatoria are famous for swaying back and forth, which is how they got their name.
Other cyanobacteria float up and down in the water using tiny gas-filled pockets called gas vesicles. By changing how buoyant they are, they can move to the depth where light and nutrients are best.
Blue-green algae FAQs
Is blue-green algae really algae?
Not exactly. Blue-green algae are actually bacteria called cyanobacteria. True algae have a cell nucleus, but cyanobacteria do not.
Where does blue-green algae live?
Cyanobacteria live almost everywhere: oceans, fresh water, damp soil, desert rocks, hot springs and even Antarctic rocks. A few live inside plants, lichens and sponges.
How do blue-green algae get energy?
They use photosynthesis, which means they make energy from sunlight. As a by-product they release oxygen.
Can blue-green algae be harmful?
Sometimes. They can form big blooms that look like bluish-green scum or paint on the water, and some blooms turn toxic. Swimming areas are often closed when one is found.
Why is blue-green algae important to rice farms?
Some cyanobacteria can take nitrogen from the air and turn it into a form plants can use. Rice farmers use them as a natural fertiliser in rice paddies.
Randi Jones is the owner and founder of the Nature Gift Store, an online retailer specializing in nature-themed gifts, educational science projects, live animal nature kits and real butterfly artwork. A biology graduate who fell in love with insects, she founded the company in 2001 to make active learning about nature fun, accessible and engaging for children and adults. Her store offers real butterfly frames, home and garden decor and educational live animal kits, including ant farms, worm farms, tadpoles, ladybugs and hermit crabs, and she runs a bug museum in Washington featuring live animals like Eddie the Iguana alongside other reptiles and insects.
