Blue-green algae, more commonly known as cyanobacteria, is one of the simplest, most common, and earliest-appearing life forms on the planet. Fossils created by colonies of cyanobacteria date back 2.8 billion years, the earliest known fossils, existing more than 2 billion years before multicellular life. Unlike other forms of "algae," like green, red, and brown algae, blue-green algae consists of bacteria -- not eukaryotic cells. Bacterial cells are about ten times smaller than eukaryotic cells and lack nuclei and complex organelles (cellular organs).
When blue-green algae first appeared on the scene about 2.8 billion years ago, it set in motion biochemical and atmospheric changes that eventually made it possible for complex life to exist. At the time blue-green algae evolved, the Earth's atmosphere was composed mostly of nitrogen (like today), but the other gas, making up about 25%, was carbon dioxide. Blue-green algae was the first life forms capable of oxygenic photosynthesis, which takes in carbon dioxide and the Sun's rays to produce energy, oxygen, and water.
After working for about 500 million years, the blue-green algae transformed the atmospheric carbon dioxide into mostly oxygen. This made much for free energy available for any subsequent organisms, but killed off all the anaerobic (air-hating) bacteria that had dominated the planet before. Because of the massive die-off, this event is known as the Oxygen Catastrophe. Evidence of this event is left behind in the form of banded iron formations, rocks consisting of bands of oxidized iron compounds alternating with iron-poor minerals such as shale. These iron compounds formed when cyanobacteria were locking up iron in compounds which then sunk to the sea floor.
Cyanobacteria at mainly found in the oceans, where they are primary producers and are eaten by many other organisms. The blue-green tinge they give the water is the source of their name, blue-green algae. The green color comes from chlorophyll in their cells. In the oceans, where there is ample nitrogen available in seawater and carbon dioxide available from the atmosphere, the main limiting factor in their growth is iron. Some scientists have begun to experiment with fertilizing the oceans with iron to boost their growth, which can sequester carbon dioxide from the atmosphere and curb global warming.
Cyanobacteria are also well-known for their ability to fix nitrogen -- or convert it from gas form into solid form -- another essential feature of any ecosystem. All organisms need fixed nitrogen sources to survive. Other organisms convert fixed nitrogen compounds into protein and nucleic acids.