Oxygen first arose in Earth’s atmosphere 2.3 billion years ago, but geochemical evidence suggests that small pockets of oxygen may have arisen earlier than the atmospheric rise in oxygen. Cyanobacteria, a modern phylum of bacteria, are believed to have been the driving force behind the oxygenation of Earth’s atmosphere, and there are two basic hypotheses about how they caused this major geologic event. There is a hypothesis, called the ‘ecological’ hypothesis, that suggests cyanobacteria were unable to live in most environments initially, and thus we see the evidence for pockets of oxygen earlier than the atmospheric rise in oxygen. Specifically, the ‘ecological’ hypothesis says that cyanobacteria originally were unable to swim and couldn’t live in saline water, meaning seawater. However, the data for this only considers two possible states for the levels of salinity: freshwater and seawater. We used data from the literature and from experiments to show that the gradient of salinity matters to the ability of cyanobacteria to live in environments, and that we cannot say what salinity levels a cyanobacteria can tolerate based on where they were found alone. See supplemental file for full abstract.

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.