Cyanobacteria oxygenated Earth's atmosphere billions of years ago, allowing oxygen-using creatures to evolve. Cyanobacteria are also credited with devising photosynthesis and multicellularity. Today they provide 20% of Earth's oxygen. These photosynthesizing bacteria are found in many environments, including freshwater ecosystems. This program was produced in collaboration with Jared Leadbetter, Professor of Environmental Microbiology at the California Institute of Technology.