Mixotrophic Iron-Oxidizing<i>Thiomonas</i>Isolates from an Acid Mine Drainage-Affected Creek

Abstract

Natural attenuation of heavy metals occurs via coupled microbial iron cycling and metal precipitation in creeks impacted by acid mine drainage (AMD). Here, we describe the isolation, characterization, and genomic sequencing of two iron-oxidizing bacteria (FeOB) species: <i>Thiomonas ferrovorans</i> FB-6 and <i>Thiomonas metallidurans</i> FB-Cd, isolated from slightly acidic (pH 6.3), Fe-rich, AMD-impacted creek sediments. These strains precipitated amorphous iron oxides, lepidocrocite, goethite, and magnetite or maghemite and grew at a pH optimum of 5.5. While <i>Thiomonas</i> spp. are known as mixotrophic sulfur oxidizers and As oxidizers, the FB strains oxidized Fe, which suggests they can efficiently remove Fe and other metals via coprecipitation. Previous evidence for <i>Thiomonas</i> sp. Fe oxidation is largely ambiguous, possibly because of difficulty demonstrating Fe oxidation in heterotrophic/mixotrophic organisms. Therefore, we also conducted a genomic analysis to identify genetic mechanisms of Fe oxidation, other metal transformations, and additional adaptations, comparing the two FB strain genomes with 12 other <i>Thiomonas</i> genomes. The FB strains fall within a relatively novel group of <i>Thiomonas</i> strains that includes another strain (b6) with solid evidence of Fe oxidation. Most <i>Thiomonas</i> isolates, including the FB strains, have the putative iron oxidation gene <i>cyc2</i>, but only the two FB strains possess the putative Fe oxidase genes <i>mtoAB</i> The two FB strain genomes contain the highest numbers of strain-specific gene clusters, greatly increasing the known <i>Thiomonas</i> genetic potential. Our results revealed that the FB strains are two distinct novel species of <i>Thiomonas</i> with the genetic potential for bioremediation of AMD via iron oxidation.<b>IMPORTANCE</b> As AMD moves through the environment, it impacts aquatic ecosystems, but at the same time, these ecosystems can naturally attenuate contaminated waters via acid neutralization and catalyzing metal precipitation. This is the case in the former Ronneburg uranium-mining district, where AMD impacts creek sediments. We isolated and characterized two iron-oxidizing <i>Thiomonas</i> species that are mildly acidophilic to neutrophilic and that have two genetic pathways for iron oxidation. These <i>Thiomonas</i> species are well positioned to naturally attenuate AMD as it discharges across the landscape.