Siderite-based anaerobic iron cycle driven by autotrophic thermophilic microbial consortium

Using a sample from a terrestrial hot spring (pH 6.8, 60 °C), we enriched a thermophilic microbial consortium performing anaerobic autotrophic oxidation of hydrothermal siderite (FeCO3), with CO2/bicarbonate as the electron acceptor and the only carbon source, producing green rust and acetate. In order to reproduce Proterozoic environmental conditions during the deposition of banded iron formation (BIF), we incubated the microbial consortium in a bioreactor that contained an unmixed anoxic layer of siderite, perfectly mixed N2/CO2-saturated liquid medium and microoxic (2% O2) headspace. Long-term incubation (56 days) led to the formation of magnetite (Fe3O4) instead of green rust as the main product of Fe(II) oxidation, the precipitation of newly formed metabolically induced siderite in the anoxic zone, and the deposition of hematite (Fe2O3) on bioreactor walls over the oxycline boundary. Acetate was the only metabolic product of CO2/bicarbonate reduction. Thus, we have demonstrated the ability of autotrophic thermophilic microbial consortium to perform a short cycle of iron minerals transformation: siderite–magnetite–siderite, accompanied by magnetite and hematite accumulation. This cycle is believed to have driven the evolution of the early biosphere, leading to primary biomass production and deposition of the main iron mineral association of BIF.

Life before Oxygen

This chapter discusses the nature of life on ancient Earth before the evolution of oxygen production. It suggests that the Earth enjoyed an active and diverse biosphere well before the evolution of oxygen-producing cyanobacteria. This biosphere was fueled, mainly, by chemical compounds liberated during volcanism, underscoring again the importance of plate tectonics in shaping life on our planet. Geological evidence indicates that many of the processes that we have imagined were part of the early biosphere that was in place 3.5 billion years ago. These processes include methanogenesis, sulfate reduction, and decomposition of dead organic biomass, which was likely aided by a host of different fermenting bacteria. It seems likely, though, that this early biosphere was much less active than what we enjoy at present.