UW Spring 2021 Astrobiology Colloquium Series: Tanja Bosak
May 11, 2021 at 3:00 pm (PDT)
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Combining the Molecular and Fossil Records of Cyanobacterial Evolution
Cyanobacteria are the first microbial phylum to have evolved oxygenic photosynthesis. The evolution of this phylum plays a central role in our understanding of the early biosphere, primary production, and the oxygenation of the Earth. Two important questions remain: 1) When did Cyanobacteria and oxygenic photosynthesis evolve relative to the Great Oxidation Event (GOE), and 2) What do trends in the fossil record of Cyanobacteria tell us about environmental evolution? We answer the first question by constraining the ages of divergence of the cyanobacterial lineage by molecular clock models that are independent of geochemical assumptions such as the GOE. Chronograms generated in this manner test a number of fossil calibrations based on diagnostic Proterozoic cyanobacterial fossils. The models consistently place the divergence of Cyanobacteria several hundred million years before the GOE, show a rapid diversification of major cyanobacterial lineages around the time of the GOE, and indicate a post Cryogenian origin of extant marine picocyanobacterial diversity. Experimental silicification reveals a high preservation potential of modern cyanobacteria that are analogous to some of the most diagnostic Proterozoic cyanobacterial fossils. These organisms and their extracellular polymers, produced in response to various environmental stresses, mediate the precipitation of magnesium-rich amorphous silica in seawater that is undersaturated with respect to silica. These results provide a new model for microbial silicification in Proterozoic tidal environments, with implications for the identification of silica-rich deposits that may preserve organic matter or biosignatures on Mars.