Reconciling biological and geochemical perspectives on the production of oxygen on early Earth

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The Great Oxygenation Event (GOE) is the period during which the level of free O2 in the Earth’s atmosphere rose from zero to above ~ 10-5 times that of today. However, evidence of early pockets of localised oxygenation, the so called "whiffs", are being found in Archeal shallow water environments and subaerial crust. When considered together with recent genomic phylogenetic investigations indicating that cyanobacteria, the only bacteria able to conduct oxidative photosynthesis, arose at least 2.7 Ga, and that the oxidative splitting of water evolved even earlier, the question arises as to why it took another 0.3 Ga for the oxygenation of the Earth’s atmosphere. One hypothesis is that limitation of bioessential nutrients such as nitrates, phosphates and trace metals, meant that the cyanobacteria were unable to thrive. Another hypothesis suggests that localised oxidation of subaerial crustal minerals adsorbed O2 released by cyanobacteria. This project addresses these hypotheses by utilising liquid cultures of both marine and freshwater nitrogen fixing cyanobacteria to determine the rates of photosynthesis and biological nitrogen fixation under a reducing, anaerobic Archean atmosphere with elevated CO2. Pursuant to this are trials to assess the effects of nutrient limitation on cyanobacterial primary production during the Archean. By generating pseudomats on a solid matrix of ground rocks, we will be able to determine the exact interactions occurring at the mineral: microbe interface with respect to O2 concentrations as well as pH and Redox changes. Combining data from the individual, synchronised liquid cultures with that obtained from pseudomats, will provide insight into the growth and physiological responses of modern-day descendants of ancient cyanobacteria at the level of the individual organism AND the ecological community response in mats. The experimentally obtained data will be compared to the total N and trace element content determined for organic rich layers of preserved microbial mats and stromatolites from marine and non-marine settings. This is the first study to investigate the potential nutrient limitations on cyanobacteria during the late Archean, and their ability to source essential trace elements via weathering of their mineral substrates, leading up to the GOE.