Cyanobacteria are single-celled organisms that derive energy from light, using photosynthesis to convert atmospheric carbon dioxide (CO2) and liquid water (H2O) into breathable oxygen and the carbon-based molecules like proteins that make up their cells. Cyanobacteria were the first organisms to perform photosynthesis in the history of Earth, and were responsible for flooding the early Earth with oxygen, thus significantly influencing how life evolved.
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Source: Phys.org