Abstract
About 2.45 billion years ago, oxygen began to accumulate in Earth's atmosphere in a phenomenon known as the Great Oxygenation Event (GOE). While the timing of this event is fairly well understood from the geological record, the timing of the origin of its hypothesized driving factor, oxygenic photosynthesis, is still uncertain. While an origin of oxygenic photosynthesis directly preceding the GOE has been proposed, there is considerable evidence for limited production of oxygen long before the GOE which suggests that the origin of oxygenic photosynthesis far preceded the GOE. As oxygenic photosynthesis only evolved in Cyanobacteria, one hypothesis is that the delay between the origin of oxygenic photosynthesis and the GOE was caused by an evolutionary transition of cyanobacteria from freshwater to saltwater environments. In this experiment, an evolutionary transition was modeled by serially transferring a freshwater strain of cyanobacteria in a saltwater medium and a euryhaline strain of cyanobacteria in a freshwater medium. Growth of the cyanobacteria was measured using a spectrometer and growth curves were fitted to this data. Although both strains can grow in media of different salinities, an improvement in growth rates over the course of this experiment was not observed, possibly because the time scale was too short or the applied stress was not high enough. However, there are signs pointing towards an eventual increase in fitness. This timescale is very rapid compared to the hundreds of millions of years that has been proposed to separate the origin of oxygenic photosynthesis and the GOE, suggesting that a freshwater to saltwater evolutionary barrier may not have been the reason for this apparent delay.