Microbial Fe cycling during the genesis of Banded Iron Formations: influence on Fe mineral (trans)formation and nickel and phosphate mobility

• Applicant: Professor Dr. Andreas Kappler
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448371684

Banded Iron Formations (BIFs) are sedimentary Fe- and Si-rich rocks that formed mainly 3.8-1.8 Ga. The primary minerals forming BIFs were most probably Fe(III) oxyhydroxides formed either by (1) anoxygenic phototrophic Fe(II)-oxidizing bacteria (photoferrotrophs) (probably before the Great Oxidation Event when O2 was largely absent from the atmosphere) or (2), after the evolution of cyanobacteria, by chemical oxidation of Fe(II) by O2 produced by cyanobacteria. However, the main Fe minerals found in BIF outcrops today are the Fe(III) mineral hematite (Fe2O3) and the Fe(II)-containing minerals siderite (FeCO3) and magnetite (Fe3O4). The transformation of the Fe(III) oxyhydroxides into these secondary minerals was partly attributed to abiotic diagenetic and metamorphic processes and also to microbially catalyzed diagenetic processes e.g. microbial dissimilatory Fe(III) reduction forming magnetite and siderite. Evidence for these latter processes comes from iron isotope and BIF trace metal and nutrient analysis. It is unknown, however, to what extent microbial Fe cycling, i.e. repeated alternations of Fe(II) oxidation and Fe(III) reduction, might have progressed in such an ancient sedimentary environment, what secondary minerals were formed during microbial Fe cycling under these conditions, and how this influenced the trace metal and nutrient content of the secondary minerals. Therefore, we propose to sequentially cultivate combinations of the O2-producing cyanobacterium Synechococcus PCC 7002 with Fe(III)-reducing bacteria (3 redox cycles in total for each combination) in comparison to purely abiotic Fe(II) oxidation by O2 followed by microbial Fe(III) reduction in a medium imitating ancient ocean conditions to determine which secondary minerals are formed during repeated Fe-cycling. We further propose to experimentally examine the mobility of the bioessential trace metal Ni and nutrient phosphate during this Fe cycling. Comparing our results to the BIF rock record will ultimately allow us to determine how microbial Fe cycling influenced the secondary mineralogy and Ni and phosphate preservation in BIFs.

• DFG Programme: Research Grants