Could cyanobacteria have made the salinity transition during the late Archean?

Modern molecular evolutionary studies suggest the freshwater origin of cyanobacteria during the late Archean, about 2.7 Ga ago, with an even earlier evolution of oxidative photosynthesis. The large amount of oxygen required to oxygenate the Earth’s atmosphere during the Great Oxygenation Event (GOE) ~2.4 Ga is thought to have been produced by large cyanobacterial blooms in the open ocean. This then poses the question as to whether ancient lineages of cyanobacteria would have survived the salinity transition. This study investigates the effect of increasing salinity on the photosynthetic efficiency of two modern day descendants of ancient cyanobacteria, Chroococidiopsis thermalis PCC7203 and the root species, Gloeobacter violaceus PCC7421. Organisms were cultured in fresh, brackish or sea water analogous media under a present atmospheric level (PAL) atmosphere or an atmosphere with reduced O2 and elevated CO2 (rO2eCO2). The net photosynthesis (NP) rates were determined in liquid cultures, while the O2 and redox profiles were determined in pseudomats. While C. thermalis PCC7203 was able to grow under increasing salinities under both atmospheres tested, G. violaceus PCC7421 could not make the salinity change to sea water. NP rates were reduced for C. thermalis under increasing salinities, as were the levels of dissolved O2 in the media. A gene screen indicated that C. thermalis genome carries genes for both sucrose and trehalose synthesis, whereas G. violaceus has only the later genetic component, suggesting a mechanism for their differing salt tolerances. This study supports the hypothesis of Cyanobacterial evolution in freshwater environments and their transition into increasingly salty environments during the late Archaean, prior to the GOE.