THE ROLE OF MICROAEROPHILIC FE-OXIDIZING MICROORGANISMS IN PRODUCING BANDED IRON FORMATIONS

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe-oxidizing microorganisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder if similar organisms played a role in producing BIFs. To evaluate this, we review current knowledge of modern aerobic FeOM, in the context of BIF paleoenvironmental studies, and also present new work demonstrating the presence of marine planktonic FeOB in the Chesapeake Bay. In modern environments wherever Fe(II) and O₂ co-exist, aerobic FeOM proliferate. These organisms grow in a variety of environments, including the water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O₂ concentrations, measured as low as 2 µM to date, though lower concentrations have not been tested. While some can tolerate up to ~100 µM O₂, many aerobic FeOM appear to prefer and thrive at low O₂ concentrations (~4 to 25 µM). These are similar to concentrations estimated during the “Great Oxidation Event” (GOE), and possibly in the few hundred million years prior. We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O₂, and show that aerobic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. A commonly postulated model of BIF formation involves Fe(II)-rich water upwelling onto continental shelves, bringing Fe(II) into the vicinity of oxygenic photosynthesizing cyanobacteria. The Fe(II)/O₂ interface would have occurred in the water column, so any FeOM would have been planktonic. In the Chesapeake Bay, during summer stratification, we used cultivation to detect Zetaproteobacteria FeOM at the oxic anoxic transition zone, and correlated this with active Fe cycling. Based on this synthesis and our new evidence of modern marine planktonic FeOM, we propose that aerobic FeOM were capable of playing a significant role in depositing the largest, most well-known BIFs associated with the GOE, as well as afterwards when global O₂ levels increased.