Cyanobacteria released oxygen that helped drive Earth’s Great Oxidation Event.
Cyanobacteria are photosynthetic bacteria that use sunlight to split water and produce oxygen as a by-product. Their oxygen-producing photosynthesis evolved long before animals and plants appeared. Over immense periods, oxygen produced by cyanobacteria accumulated after being consumed by reactions with minerals and reduced volcanic gases.
The Great Oxidation Event, usually dated to roughly 2.4 billion years ago, marks the major rise of oxygen in Earth’s atmosphere. It changed ocean chemistry, enabled new forms of metabolism, and made many oxygen-intolerant organisms retreat into protected environments or disappear.
Cyanobacteria are sometimes called blue-green algae, but that name is misleading: they are bacteria, not eukaryotic algae. Chloroplasts in modern plants are descendants of an ancient cyanobacterium acquired through endosymbiosis. Stromatolites, layered structures built by microbial communities, provide important evidence of ancient cyanobacterial activity, although not every stromatolite is exclusively cyanobacterial.