IRFP: Constraining the role of photosynthetic organisms in deposition of Banded Iron Formations (BIF) on early Earth

Swanner Elizabeth D, Denver CO

Investigators

Abstract

The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award will support a twenty-four-month research fellowship by Dr. Elizabeth D. Swanner to work with Prof. Dr. Andreas Kappler at Eberhard Karls Universität Tübingen, Germany. This project addresses two fundamental questions about the co-evolution of Earth's bio- and geochemistry. The first hypothesis proposes that the lag between the evolution of O2-producing cyanobacteria and the irreversible oxidation of the Earth's atmosphere (the Great Oxidation Event, GOE) resulted from physiological limitations of the earliest cyanobacteria that curtailed their initial growth and O2 production. Cyanobacterial growth and O2 production will be assessed under the range of physical and chemical conditions possible in the Archean ocean prior to the GOE (temperature, nutrient, trace metal and light supply) to determine whether they were physiologically constrained. The second hypothesis proposes that an increase in the abundance of cyanobacteria following their adaptation to prevailing ocean conditions explains the disappearance of alternating Fe-Si microbands in Fe(III)-containing banded iron formations (BIF) deposited after the GOE. Prior to the GOE, Fe(III)-bearing BIF deposition is attributed to the activity of Fe(II)-oxidizing photosynthetic bacteria (photoferrotrophs), which generate alternating Fe-Si layers when incubated under the fluctuating temperatures that are predicted for the Archean ocean. If competition for nutrients and light existed between cyanobacteria and photoferrotrophs, as cyanobacteria adapted to prevailing conditions the photoferrotrophs would have been marginalized to less productive depths in the ocean. In that case the primary Fe-oxidation and deposition mechanism would be oxidation by cyanobacterially-produced O2 and not Fe-Si microbands. To evaluate this hypothesis, the growth and activity of cyanobacteria and photoferrotrophs in co-culture will be tracked, and the deposited Fe minerals will be compared to those formed by each organism living alone. These experiments will contribute to our understanding of how the development the biosphere is recorded in the geologic record, and also how the geochemistry of the early oceans influenced microbial evolution. This project will integrate geochemical techniques with physiological measurements, specifically through the use of voltammetric microelectrodes to acquire real-time aqueous Fe, O2 and metal speciation and concentration data, and synchrotron-based X-ray absorption spectroscopy to map the distribution and concentration of cells and metals in precipitated Fe-oxides. In addition, the project will facilitate international collaboration and exchange of ideas between the postdoc and scientists and students of all levels in the host laboratory, as well as with collaborators in Europe. Finally, the results of the project will be used to create written outreach material for primary and secondary students.