Earth’s atmosphere underwent an irreversible, and geologically sudden, change approximately 2.5 billion years ago from oxygen free, to oxygenated, called the Great Oxidation Event (GOE). This change was driven by the evolution of a new form of photosynthesis which produced molecular oxygen as a byproduct. The group of bacteria in which this evolved, Cyanobacteria, are the only organisms to independently harness this form of photosynthesis. While we know that by the time of the GOE, Cyanobacteria were present, we do not know if they were present before the GOE. It has been proposed that Cyanobacteria were restricted to freshwater environments for hundreds of millions of years before the GOE, and only when they were able to inhabit the oceans did the GOE occur. We address this hypothesis by surveying the literature to understand how modern cyanobacteria respond to changes in salinity, as well as running a 1000 generation evolution experiment. We find evidence that just because a cyanobacterial species is found in freshwater does not mean it cannot live in marine salinities, and vice versa. Additionally, we find that prolonged exposure to a different salinity does not result in loss of ability to grow in the ancestral salinity.