Fe(III) Reducing Microorganisms from Iron Ore Caves Demonstrate Fermentative Fe(III) Reduction and Promote Cave Formation

The banded iron formations (BIF) of Brazil are composed of silica and Fe(III) oxide lamina, and are largely covered by a rock cap of BIF fragments in a goethite matrix (canga). Despite both BIF and canga being highly resistant to erosion and poorly soluble, >3,000 iron ore caves (IOCs) have formed at their interface. Fe(III) reducing microorganisms (FeRM) can reduce the Fe(III) oxides present in the BIF and canga, which could account for the observed speleogenesis. Here, we show that IOCs contain a variety of microbial taxa with member species capable of dissimilatory Fe(III) reduction, including the Chloroflexi, Acidobacteria and the Alpha- Beta- and Gammaproteobacteria; however, Fe(III) reducing enrichment cultures from IOCs indicate the predominance of Firmicutes and Enterobacteriaceae, despite varying the carbon/electron donor, Fe(III) type, and pH. We used model-based inference to evaluate multiple candidate hypotheses that accounted for the variation in medium chemistry and culture composition. Model selection indicated that none of the tested variables account for the dominance of the Firmicutes in these cultures. The addition of H₂ to the headspace of the enrichment cultures enhanced Fe(III) reduction, while addition of N₂ resulted in diminished Fe(III) reduction, indicating that these Enterobacteriaceae and Firmicutes were reducing Fe(III) during fermentative growth. These results suggest that fermentative reduction of Fe(III) may play a larger role in iron-rich environments than expected. Our findings also demonstrate that FeRM are present within the IOCs, and that their reductive dissolution of Fe(III) oxides, combined with mass transport of solubilized Fe(II) by groundwater, could contribute to IOC formation.