In the very early days of life, Earth was populated only by anaerobic bacteria that didn’t need oxygen to survive. When cyanobacteria first made their appearance and started engaging in their photosynthetic reactions, large amounts of oxygen were suddenly released in the atmosphere. This lead to what is known as the Great Oxygenation Event, which took place around 2.5 billion years ago.
As far as we know, the Great Oxygenation Event induced by cyanobacteria’s photosynthesis has probably been the largest extinction of life forms to ever take place on the planet. Trillions of anaerobic bacteria were suddenly asphyxiated by the presence of oxygen and wiped off the face of the Earth. http://www.fastbleep.com/blog/2012/04/26/cyanobacteria-and-smarties/ without cyanobacteria (or something with similar capabilities) earth would be an anaerobic world incapable of harvesting the sun's energy. Cyanobacteria infused the atmosphere with oxygen, provided photosynthesis to plants through endosymbiosis, and increased the amount of usable nitrogen by fixing atmospheric N2 into ammonia. Many of these little photosynthetic factories can undertake conflicting metabolic processes within a single cell, such as photosynthesis and nitrogen fixation. With the need for renewable sources of energy and industrial products, cyanobacteria present themselves as an excellent self-contained platform for metabolic engineering and subsequent green chemical production.
April 11, 2011 Cyanobacteria: Growing a Green Future Around the Clock by Spencer Diamond and Britt Flaherty