Speaker: Dr. George Luther (University of Delaware)

Abstract:

The oxidation of aqueous Fe(II) to Fe(III) solids is of great significance to Earth history including banded iron formation (BIFs) and the rise of O₂ in waters and the atmosphere. The chemical oxidation of aqueous Fe(II) in air saturated solutions is facile at circumneutral pH, with O₂ produced mainly from photosynthetic activity. There are currently three theories on how microbes could have contributed to Fe(III) precipitation in BIFs: (1) oxygenic photosynthesis, coupled to abiotic Fe oxidation, (2) aerobic (anaerobic using nitrate?) Fe oxidation by iron oxidizing bacteria (FeOB), and (3) anoxygenic photosynthesis, with Fe as an electron donor (photoferrotrophs).  Using kinetic data obtained in the field as well as in the laboratory with in situ microelectrode techniques developed in our lab, it is now possible to discriminate between hemical Fe(II) oxidation and these microbially based processes in real time. In situ techniques also provide the opportunity to search for FeOB habitats. Field data will be shown from diverse sites including Yellowstone National Park where groundwater, rich in Fe(II) and Mn(II) but with little or no O₂, enter oxygenated systems (oxygenic photosynthesis). In the case of FeOB, their importance in Fe(II) oxidation increases at low O₂ concentrations both in field and laboratory experiments. The Fe(III) solids formed with FeOB mediation typically have discrete morphologies rather than amorphous structures formed during abiotic oxidation. Thermodynamic calculations for the first electron transfer between the metal ions, Fe(II) and Mn(II),and O₂ over pH give insight to the distribution of these metals in BIFs and their biogeochemical behavior.