ABSTRACT Iron-bearing minerals are common in deep continental settings and can provide energy to sustain microbial life in an otherwise energy-limited system. The Deep Mine Microbial Observatory is situated in iron-rich metabasalt, iron formation, and pyrite-rich schist. Fracture fluids here are locally iron-rich, reflecting host rock mineralogy. Although iron cycling is likely an important process given the high concentrations of iron in fracture fluids and detection of putative iron cycling taxa via marker gene surveys, a previous metagenomic survey detected no iron cycling potential at DeMMO. Here, we revisit the potential for iron cycling at DeMMO using FeGenie, a new annotation pipeline that is optimized for the detection of iron cycling genes. We annotate iron-specific functional genes from whole metagenome sequencing data across a range of depths and geochemical conditions and characterize putative iron cycling pathways and taxa in the context of energy density estimates from thermodynamic models. We find that not only is there potential for iron cycling at DeMMO, iron is likely an important source of energy in this iron-rich system. Based on our findings, we recommend the use of optimized pipelines where the detection of iron cycling genes is a major goal. Given the ubiquity of iron in deep continental settings, we posit that iron-fueled life is pervasive in the deep continental biosphere.
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