How did Cyanobacteria survive increased atmospheric O2levels during the Great Oxygenation Event? The role of Superoxide Dismutases (SOD)

Cyanobacteria are able to conduct oxygenic photosynthesis and are thought to have been responsible for the Great Oxygenation Event (GOE). The effect of increasing levels of atmospheric O2 on the physiology of Cyanobacteria is unknown. Cyanobacteria produce toxic superoxide ions during photosynthesis through the hydrolysis of water. In this project, we investigate the expression of the Superoxide Dismutase (SOD) enzyme, which is responsible for eliminating the superoxide ion, in an ancestral marine species, Pseudanabaena sp. PCC7367. Growth curves based on Chlorophyll a and protein content were conducted under an anoxic atmosphere representing the ‘Archean’, and one representing Present Atmospheric Levels (PAL) of CO2 and O2. Expression of SOD genes was monitored over a day: night cycle, in conjunction with measuring oxygen release. The activity of the enzymes was assessed using native gel assays. The growth rate for Pseudanabaena sp. PCC7367 was highest for cultures grown under the anoxic atmosphere suggesting that modern levels of atmospheric O2 impair the growth of Cyanobacteria compared to the ‘Archean’ atmosphere. SOD gene expression was highest during the day when O2 levels were at their highest. Relative gene expression under both atmospheres was not significantly different, suggesting that the expression of SOD depends on cellular O2 production rather than atmospheric O2. Enzyme activity assays confirmed the synthesis of the SODs. In conclusion, this study suggests that increased atmospheric O2 levels would not have restricted the spread of Cyanobacteria as they would have required SODs once they acquired the ability to conduct oxygenic photosynthesis.