Abstract:
The precursor minerals of Banded Iron Formations (BIF) were precipitated between 3.8 and 1.85 billion years ago from the then iron (Fe)- and silicate (Si)-rich Precambrian oceans and today represent the largest iron ore deposits on almost every continent, giving them enormous industrial significance. Due to the long time span during which they were deposited, they contain valuable information on the biogeochemistry of the early oceans as well as indirect evidence for early life in these waters. They particularly covered the emergence of atmospheric oxygen during the Great Oxidation Event (GOE) from 2.45 to 2.3 Ga, which altered life on earth drastically. Chapter 1 (Dreher et al., 2021) gives an introduction into the topic, providing an overview of various precipitation mechanisms of BIF precursor minerals and the geochemistry of the Precambrian oceans. Chapter 2 summarizes the open research questions that were targeted in the thesis. The overarching research question explores whether cyanobacteria in combination with Fe(III)-reducing bacteria (DIRB) could have been responsible for the precipitation of Fe- and Si-containing precursor minerals of BIF. In the pre-GOE oxygen-free oceans, dissolved Fe(II) was presumably precipitated by phototrophic, anoxygenic Fe(II)-oxidizing bacteria as ferrihydrite, which was reduced in deeper water layers and the seafloor by DIRB to minerals containing Fe(II) and Fe(III) (e.g. magnetite, siderite or Fe(II) silicates). In this context, we aimed to understand the impact of geochemical proxies (Fe, Si, reactive oxygen species (ROS), nutrients, heavy metals) on living conditions of early bacteria. Furthermore, we were motivated by the open question why the oxygen produced by cyanobacteria increased rapidly during the GOE, instead of continuously starting with the emergence of these microorganisms at least 2.7 Ga. We hypothesized that the formation of ROS or high nickel concentrations may have had toxicity effects on the bacteria and low phosphorus concentrations may have led to nutrient limitation. Chapter 3 (Dreher et al., 2025) of this thesis experimentally investigated whether cyanobacteria in co-cultivation with DIRB can precipitate precursor minerals of BIF during three consecutive Fe(II) oxidation and reduction cycles. Using µXRD, ⁵⁷Fe-Mössbauer spectroscopy and SEM-EDS, it was shown that mainly ferrihydrite, goethite and uncharacterizable aggregates of Fe(II), Fe(III) and Si were formed. This implies that the combination of cyanobacteria and DIRB could form realistic precursor minerals of BIF. In Chapter 4 (Dreher et al., Nat. Comm., in review), the impact of ROS on cyanobacteria in early ocean analogues analyzed. It was previously assumed that the oxygen produced by cyanobacteria in the Fe(II)aq-rich oceans led to the formation of ROS, which would damage the cyanobacteria themselves and delay the release of oxygen. However, our experiments showed that no ROS formation occurred at realistic Si concentrations (2.2 mM) and under simulated day-night cycles with typical Fe(II)aq concentrations (0.5 mM). Even at elevated Fe(II)aq concentrations up to 5 mM, ROS formation was distinctly inhibited by Si. Therefore, we conclude that cyanobacteria in the photic zone did not experience toxic effects from ROS and could even survive in upwelling areas with elevated iron concentrations. Finally, in Chapter 5 (Dreher et al., Chemical Geology, submitted), we investigated how nutrient limitation (using phosphorus (P) as an example) and increased heavy metal concentrations (using nickel (Ni) as an example) affect the growth and activity of cyanobacteria and DIRB. Cultures containing as little as 0.44 µM Ni showed limited oxygen production, reduced Fe(II) oxidation and Fe(III) reduction rates and slowed growth at low phosphorus concentrations (4 µM), while elevated P concentrations (370 µM) continued to allow growth, oxygen production and cell division even at 4 µM nickel. This implies that the microbial precipitation of precursor BIF mineral remained possible even if the process was likely slower than under idealized lab conditions. Chapter 6 synthesizes the results of all chapters, emphasizing that cyanobacteria played an important role in the precipitation of BIF precursor minerals. Furthermore, our results draw implications on the ecology of the early ocean. For instance, we showed that cyanobacteria were likely protected from ROS formation by the elevated silicate concentrations (2.2 mM), even in the presence of highly elevated iron concentrations (≤5 mM) in the early ocean. Before the GOE, microbial life was limited, however possible, by elevated Ni concentrations in seawater (up to 0.4 µM) in and low P concentrations (4 µM). Higher elevated Ni concentrations, however, would have led to severe Ni toxicity. High Ni in combination with low P concentrations, could therefore have played a key role in delaying the release of free oxygen by cyanobacteria into the atmosphere.
iron cycling