Cyanobacteria releasing oxygen. Timelapse footage of bubbles of oxygen gas being released by photosynthesising cyanobacteria. It was this process that first led to the oxygenation of the Earth's atmosphere around 2.4 billion years ago. Cyanobacteria (formerly known as blue-green algae) are a photosynthesising form of bacteria. They were among the first lifeforms to appear on Earth. They are found worldwide in most habitats, including oceans, fresh water, and soil.